Miller-Joints Flashcards
What is the role of hydroxyappetite on stem fixation?
Hydroxyapatite may be used as surface coating on implants designed for cementless fixation.
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Formula is Ca10(PO4)6 (OH)2.
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Osteoconductive only
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Effect—allows more rapid closure of gaps between bone and prosthesis
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Bidirectional closure of space between prosthesis and bone
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Osteoblasts adhere to hydroxyapatite surface during implantation and then grow toward bone.
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Clinically shortens time to biologic fixation
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Success requires
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High crystallinity—amorphous areas of hydroxyapatite will dissolve.
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Optimal thickness—a thick coating will crack and shear off.
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Thickness less than 50–70 μm preferred
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Surface roughness
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Higher implant Ra provides increased metal-hydroxyapatite interface fracture toughness.
what is a proximal coated stem?
what is bone ongrowth technique?
Description
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Prosthetic surface is prepared by blasting of the surface with an abrasive grit material. Nickname is grit blast fixation.
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Grit blasting process creates microdivots—no pores, just divots. Divot diameter approximately the same size as pore hole for a porous-coated implant.
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Bone grows onto rough surface, stabilizing prosthesis.
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Surface roughness (Ra) (Fig. 5.13)
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Ra is defined as average peak to valley on the surface of the implant.
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Implant roughness determines strength of biologic fixation.
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Linear relation of Ra to fixation strength
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Technique
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Initial rigid fixation of implant is always a press fit technique.
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Femoral stem design is typically a high-angle, double-wedge taper (wedge in both coronal and sagittal planes) (Fig. 5.14).
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Grit surface is extensile. Fixation strength with grit blast fixation is significantly lower than that with porous coating, and therefore the area of surface coating is greater.
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There are very few cups designed with bone ongrowth surface coating.
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Complication
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Aseptic loosening
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Stem settling occurs when initial rigid fixation is not good enough to allow osteointegration.
what is femoral stress shielding?
Description
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Proximal femoral bone density loss observed over time in the presence of a solidly fixed implant; typically applies to cementless implants
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Etiology
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Stem stiffness is main factor.
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Problem is modulus mismatch between stem and femoral cortex.
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Factors affecting stem stiffness
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Stem diameter is most important. r4
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Stem stiffness approximates radius4 of stem.
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Larger-diameter stems are exponentially stiffer.
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Metallurgy
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Co-Cr (cobalt-chrome) alloy is stiffer than titanium alloy.
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Stem geometry
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More stiff
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Solid and round stems
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Less stiff
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Hollow stems, slots, flutes, taper designs
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Typical scenario creating stress shielding
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Large-diameter stem, of 16 mm or greater
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Co-Cr alloy stem
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Round, solid, cylindrical stem shaft
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Extensive porous coating
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Distal bone loading
Review the approaches for a total hip:
Review the surgical indications for hip surgery
(non THA procedures)
Arthroscopy
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Limited indications in patients with radiographic evidence of arthritis
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Preoperative joint space narrowing is negative predictor of a good clinical outcome.
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THA
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See Section 5, Total Hip Arthroplasty.
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Hip fusion
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Less frequently used as THA technology advances
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Classic indications
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Very young male laborer
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Unilateral hip arthritis
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Energy expenditure
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Approximately 30% increase in energy output during ambulation
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Contralateral arthritis
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Abnormal gait causes arthritis in these adjacent joints in 60% of patients.
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Lumbar spine
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Contralateral hip
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Ipsilateral knee
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Symptoms of pain typically start within 25 years of hip fusion.
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Hip fusion technique
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Preservation of abductor complex.
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Many fusions are taken down for disabling pain in adjacent joints.
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Selection of fusion technique that allows successful conversion to THA.
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Greater trochanteric osteotomy with lateral plate fixation is preferred technique.
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Care must be taken not to injure superior gluteal nerve, which innervates abductor complex.
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Fusion position
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20–25 degrees of flexion
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Neutral abduction
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Increased back and knee pain when fusion is in abduction
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Neutral or slight external rotation of 10 degrees
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Fusion conversion to THA
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Indications
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Disabling back pain—most common
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Disabling ipsilateral knee pain with instability
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Excess knee stress will cause knee ligament stretch-out if fusion position is incorrect.
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Disabling contralateral hip pain
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Function after conversion to THA
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Hip function and clinical results directly related to integrity of abductor complex
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Preoperative electromyogram of gluteus medius may be helpful.
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When hip abductor complex nonfunctional
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Severe lurching gait results
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Very high risk for instability; may require constrained acetabular component
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Resection arthroplasty
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Indications
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Incurable infection
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Patients are most often immunocompromised.
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Recurrent periprosthetic THA infection
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Failed hip fusion with infection
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Chronic destructive septic arthritis
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Noncompliant patient with recurrent THA dislocation
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Nonambulator
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Intractable pain from arthritis
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Hip fracture with open decubitus ulcers
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Significant contracture interfering with hygiene and posture
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Failed hip fusion in patient with prior major trauma to hip and/or pelvis
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Soft tissue loss to hip region precludes successful placement of THA.
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Neurologic injury to extremity precludes successful function of THA.
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Hemiarthroplasty
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Not routinely used in the treatment of arthritis and is relegated to specific limited role
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Fracture treatment in low-demand elderly patient
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Best indication—displaced subcapital hip fracture with little or no prior history of symptomatic hip arthritis
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Patient not able to comply with standard THA precautions (dementia)
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High risk for dislocation (Parkinson disease)
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Advantages
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Reduced surgical time
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Stability
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Maximizes head-neck ratio.
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Large-diameter ball requires more distance to travel before dislocation.
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Suction fit provided by labrum (may be negated if labrum and capsule resected)
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Disadvantages
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Groin pain in active individuals
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Increased risk for need for conversion to THA in active individual due to acetabular erosion
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Protrusio deformity may result, particularly if osteoporosis present
Non-operative treatment for hip arthritis
Activity modification
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Reduction of impact-loading exercises
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Reduction of weight
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Avoidance of stairs, inclines, squatting
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Physical therapy
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Nonnarcotic medications
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NSAIDs
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Evidence does not support the use of glucosamine sulfate.
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Joint injections
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Corticosteroid—antiinflammatory treatment
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Hyaluronate
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Backbone of proteoglycan chain of articular cartilage
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No strong evidence to support use in the hip
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Not approved by FDA for hip use in United States
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Assist device (cane or crutch)
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Opposite hand of affected hip
common causes of pain after THA:
Start-up pain is the most common initial presentation of loosening.
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Groin pain indicates a loose acetabular cup.
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Thigh pain indicates a loose femoral stem.
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Infection must always be ruled out as a cause of pain.
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Anterior iliopsoas impingement and tendinitis may be the cause of groin pain in THA when a prominent or malpositioned cup is present and no other causes can be found.
Review Rubash Screw Quadrants
Posterior-superior quadrant is the safe zone for acetabular screw placement. This is preferred zone for screw placement.
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Anterior-superior quadrant is considered the zone of death. Screws and/or drill that penetrate too far risk laceration of the external iliac artery and veins.
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If a major vessel injury occurs during screw placement, the hip wound should be immediately packed tight. Without closure of the hip wound, an anterior pelvic incision is made to gain proximal control of the bleeding artery. Repair of the bleeding source is then addressed.
What is the difference between cavitary and segmental loss?
Cavitary deficiency is a loss of cancellous bone without compromise of main structural bone support.
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Segmental deficiency is loss of main bony support structures.
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Acetabular rim
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Acetabular column
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Medial wall
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Combined deficiencies
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Well-fixed cementless implant with osteolytic defect
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Can be treated with débridement, bone grafting, and bearing component exchange without revision of the cup.
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Contraindications to this approach are a poorly positioned cup, poor implant design, an ongrowth fixation surface, or damaged locking mechanism.
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Significance of bone defects
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Major segmental bone deficiencies require a reconstruction cage, structural bone graft, or modular porous metal augments.
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A structural bone graft (a graft that reconstructs a segmental defect) alone without a cage has a high loosening rate.
what are the reconstruction options for acetabular revision?
Cementless porous biologic fixation is preferred.
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A cemented cup with impaction bone grafting is used more frequently outside of North America.
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Hemispheric porous cup with screws is most common solution.
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Must have at least two-thirds of rim and a reasonable initial press fit to work
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Requires at least 50% contact with host acetabular bone
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Recommended cup replacement is to re-create the native center of rotation.
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Cup placement should be inferior and medial (i.e., low and in).
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Lowest joint reactive forces
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Cup placement superior and lateral (i.e., up and out) is not recommended.
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Highest joint reactive forces
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Higher wear and component loosening
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Filling of cavitary deficiencies with particulate bone graft.
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Acetabular porous metal wedge augmentation is an acceptable adjuvant to hemispheric cup to achieve stability and fixation when necessary.
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Reconstruction cage (Fig. 5.20)
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Used when segmental bone deficiencies prevent initial rigid fixation of a hemispheric porous cup in desired position.
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Bone graft
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Cage placement is against acetabulum and pelvis. Bone graft is placed behind cage.
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Particulate graft preferred
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Bulk support allograft when needed
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Acetabular cup insertion
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Acetabular cup is cemented into reconstruction cage.
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Mid- to long-term failure rates using this technique are significant because of mechanical loosening and/or breakage of the cage as a result of lack of biologic fixation. Many surgeons have abandoned this technique in favor of porous metal augments, cup-cage constructs, and custom triflange cups.
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Modular porous metal construct (Fig. 5.21)
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Increasingly being used for cases of severe bone loss
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May allow achievement of mechanical stability and osseointegration when less than 50% host bone contact is available for a hemispherical implant.
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Can help facilitate restoration of the hip center of rotation by filling superior defects
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Different highly porous metal options available, including tantalum (75% porous by volume).
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Intraoperative flexibility to match defects
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Revision cup may be combined with a cage in a so-called cup-cage construct to improve initial stability and fixation
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Custom triflange cup (Fig. 5.22)
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Severe cases of bone loss where defect-matching techniques are limited
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Decision to use is made preoperatively as this cup is custom made for each patient on the basis of a CT scan.
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Requires several weeks to manufacture
Review the bone defects for revision of the femur
Cavitary deficiency is loss of endosteal bone. Cortical tube remains intact.
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Endosteal ectasia is a form of cavitary deficiency in which the outer cortex has increased in diameter as a result of mechanical irritation by a loose femoral stem.
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Segmental deficiency is a loss of part of the cortical tube in the form of either holes in or complete loss of a portion of the proximal femur.
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Combined deficiencies
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Significance of bone defects
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Revision femoral stem must bypass the most distal defect.
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New implant must bypass most distal cortical defect by a minimum of two cortical diameters. Otherwise there is an increased risk for fracture at the tip of the stem.
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The revision stem must prevent bending movements from passing through the region of the cortical hole, which is a weak point.
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Extensive metadiaphyseal bone loss and a nonsupportive diaphysis (Paprosky type IV classification) require a femoral replacement endoprosthesis or an allograft-prosthetic composite.
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Fixation revision of femur
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Cementless porous biologic fixation is preferred.
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Cemented revision stems without impaction bone grafting have high failure rates at intermediate term and limited indications in the revision setting.
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Extensively porous-coated cylindrical long-stem prosthesis
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Monoblock stem typically made of Co-Cr
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Achieves fixation in the diaphysis
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Longer stems may be bowed, and engagement of the stem in the canal will dictate anteversion.
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Stem should bypass defects and be long enough to achieve initial rigid fixation.
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Extensively grit-blasted stem with splines also an accepted solution
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Minimum of 4 cm of diaphyseal bone required
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Becoming less popular due to technical difficulty in use, risk for fracture, and thigh pain with large stiff implants
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Tapered fluted implant
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Monoblock or modular stem made of more flexible titanium with a roughened surface
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Achieves stability in the diaphysis
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Taper design provides axial stability, and flutes provide rotational control.
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Modular junctions allow for freedom in component anteversion and leg length but may increase the risk of breakage.
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May obtain adequate stability and fixation with less than 4 cm of diaphyseal bone
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Becoming more popular due to ease of use and ability to restore biomechanics through modularity
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Cemented revision stem
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High intermediate-term failure rate
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Reasonable consideration in patients with irradiated bone
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Acceptable for use in very elderly or very low-demand patient when immediate full weight bearing is needed
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Impaction grafting technique
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Acceptable revision technique with greater popularity outside North America
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Surgical technique
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Distal cement restrictor placed into diaphysis
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Particulate allograft bone (fresh frozen bone recommended) impacted into endosteal canal. Bone is impacted around a femoral stem trial
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Polished tapered stem cemented into impacted allograft bone
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Polished tapered stem allowed to settle slightly within cement. Mechanical load forces are transmitted as compression forces upon allograft bone.
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Allograft heals to endosteal bone.
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Cement stays interdigitated with allograft.
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Endosteal bone is restored.
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Indications
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Used to reconstitute cortical bone when there is significant cortical ectasia
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Cortical tube must be intact. Small cortical defects can be covered with an external mesh or allograft strut.
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Bone must not be devascularized during process of covering hole
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Complications
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Most common complications are fracture and subsidence.
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Choice of allograft and morcelization technique are important factors affecting success.
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Segmental bone deficiency of femur
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Cortical holes are reinforced with allograft cortical struts secured with cerclage cables (or wires).
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Proximal cortical deficiencies may be restored with modular metallic endoprosthetic segments (proximal femur replacement) or with a bulk support allograft.
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Proximal allograft technique (allograft-prosthesis composite, or APC)
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Revision stem cemented into proximal allograft
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Allograft connected to host femur with a step cut or through an intussusception (telescoping) technique.
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Allograft held to native femur with cables, plate, and/or allograft cortical strut.
what to do when intra-operative THA fracture?!?
Highest risk is with cementless implants.
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Acetabular fracture
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Most common reason for fracture is underreaming.
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Underreaming of 2 mm or more associated with higher fracture risk
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Cup may be left in place if stable, and additional screws used to enhance fixation.
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An unstable cup needs to be revised and may require a posterior column plate.
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Femoral fracture
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A longitudinal split in the calcar encountered during implantation of a tapered, proximally coated stem may be treated with stem removal, cabling, and reinsertion.
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If this procedure does not result in a stable implant, a stem that bypasses the fracture and achieves diaphyseal fixation may be needed.
Review the Vancover Fracture Classification
Nerve Injury and THA
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Involved nerves: 80% sciatic nerve, 20% femoral nerve
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Compression is most common pathologic mechanism of injury.
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Of patients who have a nerve injury after primary THA, only 35%–40% will have recovery to normal strength.
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Sciatic nerve travels closest to acetabulum at the level of ischium.
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During surgery, the most common reason for sciatic nerve injury is errant retractor placement causing excess compression to nerve.
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Peroneal nerve division is most often involved because this part of nerve is closest to acetabulum.
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Risk factors for nerve injury
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Female gender
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Posttraumatic arthritis
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Revision surgery
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Developmental dysplasia of the hip
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Risk for sciatic nerve palsy increases with leg lengthening of more than 3–5 cm.
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Postoperative functional footdrop
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Clinical scenario—patient sits in chair after surgery and experiences footdrop.
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With hip flexed 90 degrees in chair, there is too much tension on sciatic nerve.
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Treatment—patient returned to bed.
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Hip placed in extension (bed flat)
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Knee flexed on one or two pillows
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This position provides least tension on sciatic nerve.
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Postoperative hematoma
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A hip hematoma from anticoagulation can cause sciatic nerve palsy.
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Compression is mechanism of injury.
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Treatment is immediate evacuation of hematoma.
specific factors associated with THA complications
Sickle cell disease
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Associated with early prosthetic loosening
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Mechanism is extended bone infarct disease.
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Higher risk of periprosthetic joint infection.
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Psoriatic arthritis
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Associated with higher periprosthetic infection rate
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Ankylosing spondylitis
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Associated with higher risk for heterotopic ossification (HO)
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Hip hyperextension due to fixed pelvic deformity can lead to a higher anterior dislocation rate.
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Parkinson disease
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Higher dislocation rate
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Higher perioperative mortality
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Higher perioperative medical complications
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Higher reoperation rate
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Paget disease
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Increased blood loss
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Good results may still be obtained with cementless fixation.
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Dialysis
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Higher risk of infection and loosening
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Fat emboli syndrome
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Occurs with femoral stem insertion
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Fat and bone marrow emboli are pressurized into bloodstream.
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Intraoperative hypotension, hypoxia, mental status changes, and petechial rash are hallmark findings.
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Treatment is volume and respiratory support.
HO and THA
Small amounts of clinically insignificant HO are common and likely present in a majority of patients undergoing THA.
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Risk factors: male gender, ankylosing spondylitis, hypertrophic subtype of arthritis, posttraumatic arthritis, head injury, and history of HO
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Prevention
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Careful handling of soft tissues
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Prophylaxis with oral indomethacin or radiation therapy (700–800 Gy) within 24 hours prior to surgery or 72 hours after surgery
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Treatment
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No effective treatment in early postoperative period once the process has started
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Indication for surgical resection is significant loss of motion.
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Process should appear mature and stable on serial radiographs before resection is undertaken.
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Heterotopic bone may recur after operative resection.
hip psoas after THA
Underrecognized cause of groin pain after THA
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Discomfort with resisted hip flexion and straight-leg raise
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Cross-table lateral radiograph or CT may show a retroverted cup or anterior overhang of the acetabular component.
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An injection may be used to confirm the diagnosis.
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Treatment is with release or resection of the iliopsoas tendon, alone or in combination with acetabular revision for an anterior overhanging component.
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Acetabular revision is more predictable for groin pain resolution in patients with 8 mm or more of anterior acetabular component prominence.
Implant stiffness:
youngs module
Coefficient of friction:
bearing materials
review the treatment for THA hip instability
Each case of hip dislocation is unique. There is not one common treatment.
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In each case the following issues, as previously described, should be assessed:
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Component design
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Component alignment
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Soft tissue tension
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Soft tissue function
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Spinopelvic mobility
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Clinical review of dislocating event important
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Was dislocation at extreme end range or within the range for usual activities of daily living?
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Patient’s cognition—impaired versus normal
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Clinical examination
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Determination of where THA starts to lever and sublux
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Radiographic review
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Implant design and position should be scrutinized.
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Sitting lateral and standing lateral x-rays may be obtained to assess spinopelvic relationship and mobility.
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Initial treatment for dislocated THA
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Two-thirds of patients with first-time THA dislocation early in the postoperative period can be successfully treated with closed measures.
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Closed reduction
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Sedation or anesthesia preferred to minimize soft tissue trauma
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During closed reduction, hip is taken through full range to assess position of dislocation.
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Subluxation is identified as being either within patient’s activities of daily living or at extreme end range.
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Posterior hip dislocation
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In supine position, the leg lies in internal rotation, adduction, and shortened position.
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Reduction maneuver for posterior hip dislocation
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Flexion to 80–90 degrees
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Internal rotation
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Adduction
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Distraction
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Anterior hip dislocation
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In supine position, the leg lies in external rotation, slight abduction, and slightly shortened position.
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Reduction maneuver for anterior hip dislocation
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Extension
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External rotation
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Slight abduction
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Distraction
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Postreduction treatment
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Education—hip precautions
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Immobilization of joint (usually 6 weeks)
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Spica brace/cast
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Knee immobilizer—to keep patient from putting hip in a compromised position
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Physical therapy
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Focus on strength, balance, agility, coordination
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Optimization of medical conditions
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Surgical treatment
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Surgical options
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Implant revision
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Greater trochanter advancement
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Constrained acetabular socket (cup)
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Conversion to bipolar hemiarthroplasty
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Resection arthroplasty
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Rule 1 for surgical treatment: If any implant component is malaligned, it needs to be changed.
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May require complete hip revision
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Goals of component revision
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Maximizes head/neck ratio to increase primary arc range.
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No neck skirts or acetabular hoods
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Accurate component alignment
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Re-creates center of hip rotation and head offset.
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Stabilizes greater trochanter (if possible) if it is detached.
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Dual-mobility cups may allow for use of a larger femoral head and improve range of motion before impingement.
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Greater trochanter advancement (also known as Charnley tensioning) (Fig. 5.62)
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Technique is to perform trochanteric osteotomy, advance the greater trochanter distally on the lateral femur, and resecure it with claw, cables, and/or wires.
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With distal advancement of the greater trochanter, the abductor complex is tensioned more tightly, thereby increasing hip compression forces.
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Requirements
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No component malalignment
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Adequate distal bone surface for bony fixation and bone healing
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Intact superior gluteal nerve
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Constrained PE liner
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A constrained PE liner encloses the femoral head and mechanically prevents hip from distracting out of socket.
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Reserved as a last resort for the patient who has experienced multiple dislocations with soft tissue dysfunction and appropriately positioned implants.
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Best indications
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Elderly patient (i.e., low demand) with normal component alignment
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Abductor deficiency/dysfunction
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Central neurologic decline
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Revision THA with reconstruction cage (Fig. 5.63)
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Significant soft tissue dissection and potential muscle dysfunction with cage placement
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Contraindication
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Cup malposition
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Failure mechanisms
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Because a constrained cup significantly reduces primary arc range (to as low as 60–70 degrees), the cup is exposed to more frequent and more intense lever range forces. With repetitive loading, the constrained cup will fail via two different mechanisms.
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The PE deforms at the edges of the socket and the hip dislocates.
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The PE does not deform. In this case the levering forces are then transmitted to the acetabular prosthetic-bone interface, resulting in mechanical loosening of the cup.
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In the patient with failed constrained PE liner, the rate of subsequent failure is high if revision involves use of another constrained PE liner.
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Conversion to bipolar hemiarthroplasty
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Rarely done and should be reserved for unusual circumstances
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Technique: removal of acetabular component; reaming of remaining bone to a hemisphere; press fitting of bipolar ball to rim of acetabulum (minimizes risk for medial migration of head).
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Requirements
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Fully intact acetabular bone
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No segmental rim deficiencies; otherwise the bipolar ball will dislocate
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Good bone density
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Rim fit technique
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Advantages
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Maximizes fully head/neck ratio
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Bipolar construct has a little more inherent stability than monopolar ball.
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Disadvantages
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Groin pain—metal articulating on bone (Fig. 5.64)
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Medial migration of head developing into protrusio deformity
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Accelerated PE wear
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Larger overall PE wear surface area
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Resection arthroplasty
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Indications
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Nonambulatory patient
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Neurologic deficits in which stability cannot be achieved
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Recurrent/ongoing periprosthetic infection
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Drug-seeking behavior with purposeful voluntary dislocations
what is the incidence of hip dislocation
Primary THA—typically 1%–2%
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Revision THA—typically 5%–7%
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Highest incidence of dislocation
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THA in the elderly patient (>80 years) for failed ORIF of femoral neck fracture—reasons:
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Muscular weakness
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Mental compromise
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Loss of balance and coordination
What are the risk factors for dislocation
Female gender
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THA for osteonecrosis
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Posterolateral approach
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Smaller femoral head size
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Greater trochanter nonunion
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Revision THA
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Obesity
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Alcoholism
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Neuromuscular conditions
Evaluating hip instability
Assessment of a dislocating THA evaluates:
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Component design
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Component alignment
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Soft tissue tension
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Soft tissue function
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Spinopelvic alignment and postural pelvic positioning
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Component design
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Prosthetic ROM consists of two parts, primary arc range (Fig. 5.46) and lever range (Fig. 5.47).
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Primary arc range
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Primary arc range is controlled by the head/neck ratio (Fig. 5.48).
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Ratio of head diameter to neck diameter is head/neck ratio.
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Best stability is achieved by maximizing head/neck ratio (Fig. 5.49).
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Additions to acetabulum and/or femoral neck decrease primary arc range.
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Neck skirt (also known as femoral head collar on femoral stem)
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Decreases head/neck ratio
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Acetabular hoods
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Decrease primary arc range (Fig. 5.50)
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Acetabular constrained cups
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Markedly decrease primary arc range (Fig. 5.51)
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Rule—constrained cups should be avoided as much as possible.
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Lever range
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Range allowed as hip starts to lever out of socket
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Lever range is controlled by head radius (Fig. 5.52).
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A large head has higher excursion distance and is more stable.
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Most stable construct is a bipolar hemiarthroplasty (two pivot points).
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Best range in THA
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High primary arc range
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Maximized head/neck ratio
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No additions to cup or neck
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High excursion distance
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Large-diameter head
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Component alignment
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Primary arc range must be centered within patient’s functional hip range (Fig. 5.53).
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Component malalignment does not decrease primary arc range.
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Placement of components in a malaligned position results in a stable side and an unstable side of the functional hip range.
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Implant positioning in THA
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Cup anteversion: 20–30 degrees (Fig. 5.54)
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Cup theta (ϴ) angle (also known as coronal tilt): 35–40 degrees (Fig. 5.55)
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Stem anteversion: 10–15 degrees (Fig. 5.56)
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Cup malposition
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Retroversion—risk is posterior dislocation.
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Excess anteversion—risk is anterior dislocation.
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High ϴ-angle (vertical cup)—risk is posterior-superior dislocation.
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Low θ-angle (horizontal cup)—risk is inferior dislocation.
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Stem malposition
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Retroversion—risk is posterior dislocation.
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Excess anteversion—risk is anterior dislocation.
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Soft tissue tension
□
Abductor complex is key to hip stability.
•
Consists primarily of gluteus medius and gluteus minimus muscles
•
Prosthetic implant design and positioning must maintain/restore proper abductor hip tension.
□
Restoration of abductor tension achieved by the following (Fig. 5.57):
•
Restored normal hip center of rotation
•
Restored head offset
•
Restored femoral neck length
□
Problems with reduced hip offset
•
Weakened abductor complex
•
Increased joint reaction force (decreased abductor lever arm)
•
Presence of Trendelenburg sign
•
Gluteus medius lurch with walking
•
Increased risk for dislocation
□
Problems with short neck length
•
Short neck length occurs when a low neck cut is made, a short prosthetic neck length is used, or both.
•
Shortens abductor muscle length, resulting in abductor weakness
•
Decreases hip offset, which also weakens abductor complex
•
Results in bony impingement of greater trochanter against pelvis during hip range (Fig. 5.58)
•
Causes pain
•
Allows hip levering and increases risk for dislocation
•
Shortens leg length
□
Problems with restoring neck length by using a long head
•
A short femoral neck cut can be compensated with an extra-long prosthetic neck length. However, a long neck length requires a skirt (Fig. 5.59).
•
A skirt decreases primary arc range.
•
Increases risk for dislocation
•
If the prosthetic neck-shaft angle is lower than that of native hip, the addition of neck length will excessively increase neck offset.
•
This can cause trochanter bursitis and chronic lateral hip pain (Fig. 5.60).
•
Narrow-offset femoral stem design
•
A femoral stem with an offset designed with a narrower angle than the native hip will reduce hip offset. This reduces abductor tension and increases risk for hip dislocation.
•
A narrow-offset stem can be compensated for by with a longer femoral head length (i.e., neck length). This creates two potential problems.
•
Addition of neck length to restore offset will excessively lengthen the leg.
•
Addition of a long neck requires a skirt, which decreases primary arc range and increases risk for dislocation.
□
Greater trochanteric escape (Fig. 5.61)
•
Greater trochanteric escape occurs when the greater trochanter pulls away from the proximal femur.
•
Usually a result of failed trochanter fixation after revision THA
•
Can also occur from trauma (usually associated with osteolysis in greater trochanter)
•
Successful reattachment is difficult and often fails.
•
Problems
•
Because the hip abductor complex attaches to the greater trochanter, trochanteric escape results in a loss of hip compression and increases risk for hip dislocation.
•
There is greater external and internal hip rotation because the greater trochanter no longer restricts rotation range. This also increases the risk for dislocation because the hip can more easily approach and exceed lever range.
•
The greater trochanter fragment can impinge between the hip and pelvis, causing hip levering.
•
Treatment
•
Maximized head/neck ratio.
•
No neck skirts and no acetabular hoods
•
Resection of greater trochanter fragment to prevent impingement levering.
•
Constrained acetabular cup is last resort.
▪
Soft tissue function
□
The soft tissues about the hip are controlled by several body systems. All are integrated to provide hip stability. The three main factors controlling soft tissue function are:
•
Central nervous system
•
Peripheral nervous system
•
Local soft tissue integrity (surrounding hip region)
□
CNS mechanisms causing disruption to hip function and increasing risk for dislocation
•
Muscle dysfunction
•
Sensory impairment
•
Impaired coordination
•
Impaired balance
•
Cognitive loss of restraint (i.e., compliance/memory)
□
CNS conditions affecting hip function
•
Cerebral dysfunction
•
Stroke, seizure, CNS disease
•
Cerebellar dysfunction
•
Balance/coordination
•
Delirium
•
Medications, withdrawal phenomenon
•
Dementia
•
Psychiatric problems
•
Psychosis, addiction
□
Peripheral nervous system mechanisms causing disruption to hip function and thereby increasing risk for dislocation
•
Muscle dysfunction
•
Sensory impairment
•
Pain
□
Peripheral nervous system conditions affecting hip
•
Spinal stenosis
•
Radiculopathy
•
Neuropathy
•
Paralysis/paresis
□
Local soft tissue integrity mechanisms causing disruption to hip function and increasing risk for dislocation
•
Muscle dysfunction
•
Soft tissue dysfunction (other than muscle)
•
Soft tissue loss
•
Skeletal deformity
•
Example: patients with ankylosing spondylitis have increased risk for anterior dislocation.
□
Local soft tissue conditions affecting hip function
•
Trauma
•
Soft tissue loss
•
Myoligamentous disruption
•
Deconditioning
•
Poor health
•
Aging process
•
Irradiation
•
Radiation fibrosis with soft tissue contraction
•
Dysplasia
•
Musculoskeletal hypoplasia
•
Osteolysis
•
Bone loss
•
Myotendinous disruption
•
Collagen abnormalities
•
Clinical hyperelasticity
•
Myopathy
•
Malignancy
•
Infection
▪
Spinopelvic alignment
□
Increasingly recognized as a potential contributing factor to instability following THA
□
Patients with fixed spinopelvic alignment in sitting and standing positions are at higher risk of dislocation.
•
Patients with lumbar spine disease and previous lumbar spine surgery may have reduced spinopelvic motion and are at higher risk for instability.
Changing to a ceramic head with a used femoral head
Assessment of a dislocating THA evaluates:
□
Component design
□
Component alignment
□
Soft tissue tension
□
Soft tissue function
□
Spinopelvic alignment and postural pelvic positioning
▪
Component design
□
Prosthetic ROM consists of two parts, primary arc range (Fig. 5.46) and lever range (Fig. 5.47).
•
Primary arc range
•
Primary arc range is controlled by the head/neck ratio (Fig. 5.48).
•
Ratio of head diameter to neck diameter is head/neck ratio.
•
Best stability is achieved by maximizing head/neck ratio (Fig. 5.49).
•
Additions to acetabulum and/or femoral neck decrease primary arc range.
•
Neck skirt (also known as femoral head collar on femoral stem)
•
Decreases head/neck ratio
•
Acetabular hoods
•
Decrease primary arc range (Fig. 5.50)
•
Acetabular constrained cups
•
Markedly decrease primary arc range (Fig. 5.51)
•
Rule—constrained cups should be avoided as much as possible.
•
Lever range
•
Range allowed as hip starts to lever out of socket
•
Lever range is controlled by head radius (Fig. 5.52).
•
A large head has higher excursion distance and is more stable.
•
Most stable construct is a bipolar hemiarthroplasty (two pivot points).
□
Best range in THA
•
High primary arc range
•
Maximized head/neck ratio
•
No additions to cup or neck
•
High excursion distance
•
Large-diameter head
▪
Component alignment
□
Primary arc range must be centered within patient’s functional hip range (Fig. 5.53).
□
Component malalignment does not decrease primary arc range.
□
Placement of components in a malaligned position results in a stable side and an unstable side of the functional hip range.
□
Implant positioning in THA
•
Cup anteversion: 20–30 degrees (Fig. 5.54)
•
Cup theta (ϴ) angle (also known as coronal tilt): 35–40 degrees (Fig. 5.55)
•
Stem anteversion: 10–15 degrees (Fig. 5.56)
□
Cup malposition
•
Retroversion—risk is posterior dislocation.
•
Excess anteversion—risk is anterior dislocation.
•
High ϴ-angle (vertical cup)—risk is posterior-superior dislocation.
•
Low θ-angle (horizontal cup)—risk is inferior dislocation.
□
Stem malposition
•
Retroversion—risk is posterior dislocation.
•
Excess anteversion—risk is anterior dislocation.
▪
Soft tissue tension
□
Abductor complex is key to hip stability.
•
Consists primarily of gluteus medius and gluteus minimus muscles
•
Prosthetic implant design and positioning must maintain/restore proper abductor hip tension.
□
Restoration of abductor tension achieved by the following (Fig. 5.57):
•
Restored normal hip center of rotation
•
Restored head offset
•
Restored femoral neck length
□
Problems with reduced hip offset
•
Weakened abductor complex
•
Increased joint reaction force (decreased abductor lever arm)
•
Presence of Trendelenburg sign
•
Gluteus medius lurch with walking
•
Increased risk for dislocation
□
Problems with short neck length
•
Short neck length occurs when a low neck cut is made, a short prosthetic neck length is used, or both.
•
Shortens abductor muscle length, resulting in abductor weakness
•
Decreases hip offset, which also weakens abductor complex
•
Results in bony impingement of greater trochanter against pelvis during hip range (Fig. 5.58)
•
Causes pain
•
Allows hip levering and increases risk for dislocation
•
Shortens leg length
□
Problems with restoring neck length by using a long head
•
A short femoral neck cut can be compensated with an extra-long prosthetic neck length. However, a long neck length requires a skirt (Fig. 5.59).
•
A skirt decreases primary arc range.
•
Increases risk for dislocation
•
If the prosthetic neck-shaft angle is lower than that of native hip, the addition of neck length will excessively increase neck offset.
•
This can cause trochanter bursitis and chronic lateral hip pain (Fig. 5.60).
•
Narrow-offset femoral stem design
•
A femoral stem with an offset designed with a narrower angle than the native hip will reduce hip offset. This reduces abductor tension and increases risk for hip dislocation.
•
A narrow-offset stem can be compensated for by with a longer femoral head length (i.e., neck length). This creates two potential problems.
•
Addition of neck length to restore offset will excessively lengthen the leg.
•
Addition of a long neck requires a skirt, which decreases primary arc range and increases risk for dislocation.
□
Greater trochanteric escape (Fig. 5.61)
•
Greater trochanteric escape occurs when the greater trochanter pulls away from the proximal femur.
•
Usually a result of failed trochanter fixation after revision THA
•
Can also occur from trauma (usually associated with osteolysis in greater trochanter)
•
Successful reattachment is difficult and often fails.
•
Problems
•
Because the hip abductor complex attaches to the greater trochanter, trochanteric escape results in a loss of hip compression and increases risk for hip dislocation.
•
There is greater external and internal hip rotation because the greater trochanter no longer restricts rotation range. This also increases the risk for dislocation because the hip can more easily approach and exceed lever range.
•
The greater trochanter fragment can impinge between the hip and pelvis, causing hip levering.
•
Treatment
•
Maximized head/neck ratio.
•
No neck skirts and no acetabular hoods
•
Resection of greater trochanter fragment to prevent impingement levering.
•
Constrained acetabular cup is last resort.
▪
Soft tissue function
□
The soft tissues about the hip are controlled by several body systems. All are integrated to provide hip stability. The three main factors controlling soft tissue function are:
•
Central nervous system
•
Peripheral nervous system
•
Local soft tissue integrity (surrounding hip region)
□
CNS mechanisms causing disruption to hip function and increasing risk for dislocation
•
Muscle dysfunction
•
Sensory impairment
•
Impaired coordination
•
Impaired balance
•
Cognitive loss of restraint (i.e., compliance/memory)
□
CNS conditions affecting hip function
•
Cerebral dysfunction
•
Stroke, seizure, CNS disease
•
Cerebellar dysfunction
•
Balance/coordination
•
Delirium
•
Medications, withdrawal phenomenon
•
Dementia
•
Psychiatric problems
•
Psychosis, addiction
□
Peripheral nervous system mechanisms causing disruption to hip function and thereby increasing risk for dislocation
•
Muscle dysfunction
•
Sensory impairment
•
Pain
□
Peripheral nervous system conditions affecting hip
•
Spinal stenosis
•
Radiculopathy
•
Neuropathy
•
Paralysis/paresis
□
Local soft tissue integrity mechanisms causing disruption to hip function and increasing risk for dislocation
•
Muscle dysfunction
•
Soft tissue dysfunction (other than muscle)
•
Soft tissue loss
•
Skeletal deformity
•
Example: patients with ankylosing spondylitis have increased risk for anterior dislocation.
□
Local soft tissue conditions affecting hip function
•
Trauma
•
Soft tissue loss
•
Myoligamentous disruption
•
Deconditioning
•
Poor health
•
Aging process
•
Irradiation
•
Radiation fibrosis with soft tissue contraction
•
Dysplasia
•
Musculoskeletal hypoplasia
•
Osteolysis
•
Bone loss
•
Myotendinous disruption
•
Collagen abnormalities
•
Clinical hyperelasticity
•
Myopathy
•
Malignancy
•
Infection
▪
Spinopelvic alignment
□
Increasingly recognized as a potential contributing factor to instability following THA
□
Patients with fixed spinopelvic alignment in sitting and standing positions are at higher risk of dislocation.
•
Patients with lumbar spine disease and previous lumbar spine surgery may have reduced spinopelvic motion and are at higher risk for instability.
why ceramic heads are best/most common
Ceramic on highly crosslinked polyethylene (HCLPE) has become the most popular bearing option in North America.
▪
Rates of osteolysis have fallen dramatically with the widespread adoption of HCLPE.
▪
Concerns about trunnion corrosion have reduced the use of Co-Cr femoral heads in favor of ceramic heads.
▪
Although polyethylene (PE) wear debris has historically been the main culprit of osteolysis, metal debris from metal-on-metal bearings and trunnion corrosion may have been a more common reason for osteolysis in the past decade.
poly as a hip bearing
Ultra-high-molecular-weight polyethylene (UHMWPE) was introduced in the 1960s by Sir John Charnley.
▪
HCLPE was introduced in the late 1990s to reduce wear and osteolysis.
□
Cross-linking of poly chains made by irradiation of PE.
□
Disadvantage of HCLPE is reduction in mechanical properties and can lead to catastrophic failure of the implant
Review highly cross linked Polyethylene
HCLPE
Irradiation of PE ruptures PE bond, creating free radicals.
□
Free radicals can rebond via two different pathways.
•
In presence of oxygen (i.e., air), free radicals can bond with oxygen, resulting in PE chain scission. This is termed oxidized PE. Irradiation of PE must be done in an inert environment (i.e., with no oxygen).
•
In absence of oxygen, the free radical bonds with an adjacent chain to create a cross-link. This is termed cross-linked PE.
▪
Dose of irradiation
□
Requires high-dose irradiation, 5–15 Mrad (10 Mrad = 100 kGy)
□
The higher the dose of irradiation, the greater number of free radicals generated.
□
Problem—residual free radicals after cross-linking do remain. This is an oxidation risk.
▪
Free radical elimination
□
Postirradiation thermal treatment of the polymer is required to reduce remaining free radicals to ensure the oxidative stability of joint implants in the long term.
•
Annealing—heating below the melting point—preserves mechanical properties but does not completely eliminate free radicals.
•
Melting—heating above the melting point—decreases mechanical properties owing to loss of crystallinity but eliminates free radicals.
□
Vitamin E, an antioxidant, may be added to PE as another method to stabilize free radicals
what are the ways to remove free radicals after radiation?
Postirradiation thermal treatment of the polymer is required to reduce remaining free radicals to ensure the oxidative stability of joint implants in the long term.
•
Annealing—heating below the melting point—preserves mechanical properties but does not completely eliminate free radicals.
•
Melting—heating above the melting point—decreases mechanical properties owing to loss of crystallinity but eliminates free radicals.
□
Vitamin E, an antioxidant, may be added to PE as another method to stabilize free radicals
Review the physical property of UHMWPE
PE in a manufactured implant exists in two forms (i.e., phases; Fig. 5.26).
•
Crystalline phase
•
Provides mechanical strength to PE
•
Amorphous phase
•
Only amorphous regions of PE cross-link.
▪
PE properties—crystallinity
□
Optimum crystallinity 45%–65%
□
Decreased crystallinity (<45%)
•
Decreases mechanical properties
•
PE more prone to macroscopic failure (i.e., cracks)
□
Increased crystallinity (>65%)
•
The large crystalline phase leaves a very small amorphous phase.
•
The greatly reduced amorphous region is more susceptible to chain scission oxidation.
•
Creates significant increase in particulate debris
what are the three methods of poly sterization?
Ethylene oxide gas
□
Gas plasma spray (peroxide)
□
Low-dose irradiation (generally between 2.5 and 4.5 Mrad)
what are the four mechanisms of poly manufacturing?
Ram bar extrusion (Fig. 5.27)—machine component
□
Sheet molding—machine component
□
Compression molding—machine component
□
Direct compression molding—no machining (implant made directly from mold)—best wear of four techniques
□
Calcium stearate used in the manufacturing process has been shown to cause problems with PE and should be avoided
what methods are used to store poly?
Vacuum packaging
□
Oxygen-free gas packaging: uses argon or nitrogen
□
Oxygen that diffuses back into PE product can lead to on-the-shelf oxidation.
review the HCLPE clinical performance
Clinical studies demonstrate reduction in wear rates and the incidence of osteolysis compared to standard UHMWPE implants at 10+ years of follow-up.
□
Low wear rate of HCLPE appears to be independent of head diameter, with significant reductions in wear seen even with large head size.
Review trunion chrosis
There has been increasing attention on the potential for metal ion release from the modular junction between the head and stem.
□
Corrosion seen on majority of retrievals for failed metal-on-metal hip replacements
□
Large-diameter femoral heads, larger femoral component offsets, and varus stems increase the mechanical stress on the trunnion (toggle effect), although the importance of this increase in trunnion corrosion is unclear.
□
Process has been associated with ALTR and pseudotumor formation (similar in pathology to MOM THA failures).
□
Has been reported in metal on PE bearings
□
Has been reported with smaller-diameter heads (28 mm) in addition to larger-diameter implants
▪
Corrosion (trunnionosis)
□
Gradual destruction of metal by reaction with environment (oxidation)
□
Passivation—formation of protective oxide coat
□
Fretting corrosion (or mechanically assisted crevice corrosion)
•
Small cyclic motion (<100 μm
Review orthobullets poly sterilization
olyethylene sterilization
Radiation gamma radiation is the most common form of polyethylene sterilization
results in oxidized PE which wears poorly and results in osteolysis
oxidation vs. cross linking presence of oxygen determines pathway following free radical formation oxygen rich environmentPE becomes oxidized leads to early failure due to
subsurface delamination
pitting
fatigue strength/cracking
oxygen depleted environmentPE becomes cross linked
improved resistance to adhesive and abrasive wear
decrease in mechanical properties (decreased ductility and fatigue resistance)
greater risk of catastrophic failure under high loads
methods to obtain
packing via argon, nitrogen
packing in vacuum environment
Removal of free radicals thermal stabilization/remelting removes free radicals formed during the radiation sterilization process for cross-linking
most effective means of removing free radicals as it occurs above the PE melting point
changes the PE from its partial crystalline state to its amorphous state
disadvantage is that it reduces the mechanical properties of the material
annealing
maintaines its mechanical property
less effective at removing free radicals as it occurs below the PE melting point
leaves the PE more susceptible to oxidation
Solution
irradiate PE in inert gas or vacuum to minimize oxidation
Poly Manufacturing Orthobullets Review
Introduction
cutting tools can disrupt chemical bonds of PE
Fabrication methods ram bar extrusion and machining
UHMWPE powder fed into heated chamber, ram pushes powed into heated cylinder barrel, forming a cylindrical rod, cut into 10ft lengths for sale
implants are machined from the cylindrical bar stock
leads to variations in PE quality within the bar
calcium stearate additive
leads to fusion defects in PE
sheet compression molding
UHMWPE powder introduced into large 4’ x 8’ rectangular container to make sheets up to 8” thick
implants are machined from these molded sheets
direct compression molding/net shape
UHMWPE powder placed into a mold the shape of the final component, which is heated
best PE fabrication process
the net shape implant is removed and packaged
no external machining involved, implants have highly glossy surface finish
lower wear rates (50% wear rate of machined products)
slow, expensive
Cause of failure machining shear forces cause subsurface region (1-2mm) stretching of PE chains
especially in amorphous regions > crystalline regions
PE chains are more susceptible to radiation resulting in greater oxidation in this region leads to subsurface delamination and fatigue cracking
can show classic white band of oxidation in subsurface 1-2mm below articular surface
“Perfect storm” scenario for catastophic wear
metal-backed tibial baseplate with bone-conserving tibial bone cut (thin PE)
flat bearing design in coronal plane (low contact area with high contact load)
PCL retention with flat PE insert (high sliding wear)
ram bar PE with calcium stearate additive (fusion defects in PE)
gamma radiation sterilization in air (weakened mechanical properties of PE)
machined PE surface (cutting-tool stretch effect upon the PE)
Solution use direct-compression molding of PE
performed by molding directly from PE powder to the desired product
results in less fatigue crack formation and propagation compared to ram bar extrusion
avoid machining of articular surface
Review particulate wear Poly Orthobullets
Osteolysis represents a histiocytic response to wear debris.
Steps in the process include (see below)
particulate debris formation
macrophage activated osteolysis
prosthesis micromotion
particulate debris dissemination
Evaluationradiostereometric analysis
is the most accurate and precise technique to evaluate polyethylene wear
uses radiopaque tantalum beads planted in the bone to follow the position of the components relative to the beads on radiographs.
Step 1: Particulate Debris Formation
Types of wear adhesive wear
most important in osteolytic process
microscopically PE sticks to prosthesis and debris gets pulled off
abrasive wear
cheese grater effect of prosthesis scraping off particles
third body wear
particles in joint space cause abrasion and wear
volumetric wear
main determinant of number of particles created
directly related to square of the radius of the head
volumetric wear more or less creates a cylinder
V=3.14rsquaredw
V is volumetric wear, r is the radius of head, w is linear head wear
head size is most important factor in predicting particles generated
linear wear
is measured by the distance the prosthesis has penetrated into the liner
Wear leads to particulate debris formation wear rates by materialpolyethylenenon-cross linked UHMWPE wear rate is 0.1-0.2 mm/yr
linear wear rates greater than 0.1 mm/yr has been associated with osteolysis and subsequent component loosening
highly-cross linked UHMWPE generates smaller wear particles and is more resistant to wear (but has reduced mechanical properties compared to conventional non-highly cross-linked)
factors increasing wear in THA
thickness < 6mm
malalignment of components
patients < 50 yo
men
higher activity level
femoral head size between 22 and 46mm in diameter does not influence wear rates of UHMWPE
ceramics
ceramic bearings have the lowest wear rates of any bearing combination (0.5 to 2.5 µ per component per year)
ceramic-on-polyethylene bearings have varied, ranging from 0 to 150 µ.
has a unique complication of stripe wear occurring from lift-off separation of the head gait
recurrent dislocations or incidental contact of femoral head with metallic shell can cause “lead pencil-like” markings that lead to increased femoral head roughness and polyethylene wear rates.
metals
metal-on-metal produces smaller wear particles as well as lower wear rates than those for metal-on-polyethylene bearings (ranging from 2.5 to 5.0 µ per year)
titanium used for bearing surfaces has a high failure rate because of a poor resistance to wear and notch sensitivity.
metal-on-metal wear stimulates lymphocytes
metal-on-metal serum ion levels greater with cup abduction angle >55 degrees and smaller component size
Particulate Type UHMWPE
most common
PMMA
Co-Cr
Ti
third-body
Particulate size
is < 1 micron
Step 2: Macrophage Activated Osteoclastogenesis and Osteolysis
Macrophage activation
results in macrophage activation and further macrophage recruitment
macrophage releases osteolytic factors (cytokines) including
TNF- alpha
osteoclast activating factor
oxide radicals
hydrogen peroxide
acid phosphatase
interleukins (Il-1, IL-6)
prostaglandins
Osteoclast activation and osteolysis
increase of TNF- alpha increases RANK
increase of VEGF with UHMWPE inhances RANK and RANKL activationRANKL mediated bone resorption
an increase in production of RANK and RANKL gene transcripts leads to osteolysis
Step 3: Prosthesis Micromotion
Osteolysis surrounding the prosthesis leads to micromotion
micromotion leads to increase particle wear and further prosthesis loosening
N-telopeptide urine level is a marker for bone turnover and are elevated in osteolysis
Step 4: Debris Dissemination
Increase in hydrostatic pressure leads to dissemination of debris into effective joint space
increased hydrostatic pressure is result of inflammatory response
dissemination of debris into effective joint space further propagates osteolysis
circumferentially coated prosthesis limits osteolysis in the distal femur
what is a marker for bone turnover and is present in osteolysis?
N-telopeptide
Review the macrophage activation cascade for osteolyis
Macrophage activation
results in macrophage activation and further macrophage recruitment
macrophage releases osteolytic factors (cytokines) including
TNF- alpha
osteoclast activating factor
oxide radicals
hydrogen peroxide
acid phosphatase
interleukins (Il-1, IL-6)
prostaglandins
Osteoclast activation and osteolysis
increase of TNF- alpha increases RANK
increase of VEGF with UHMWPE inhances RANK and RANKL activationRANKL mediated bone resorption
an increase in production of RANK and RANKL gene transcripts leads to osteolysis
Review poly wear characteristics
Wear leads to particulate debris formation
wear rates by materialpolyethylenenon-cross linked UHMWPE wear rate is 0.1-0.2 mm/yr
linear wear rates greater than 0.1 mm/yr has been associated with osteolysis and subsequent component loosening
highly-cross linked UHMWPE generates smaller wear particles and is more resistant to wear (but has reduced mechanical properties compared to conventional non-highly cross-linked)
factors increasing wear in THA
thickness < 6mm
malalignment of components
patients < 50 yo
men
higher activity level
femoral head size between 22 and 46mm in diameter does not influence wear rates of UHMWPE
Review ceramic wear rates
ceramic bearings have the lowest wear rates of any bearing combination (0.5 to 2.5 µ per component per year)
ceramic-on-polyethylene bearings have varied, ranging from 0 to 150 µ.
has a unique complication of stripe wear occurring from lift-off separation of the head gait
recurrent dislocations or incidental contact of femoral head with metallic shell can cause “lead pencil-like” markings that lead to increased femoral head roughness and polyethylene wear rates.
review metal particulate wear rates
metal-on-metal produces smaller wear particles as well as lower wear rates than those for metal-on-polyethylene bearings (ranging from 2.5 to 5.0 µ per year)
titanium used for bearing surfaces has a high failure rate because of a poor resistance to wear and notch sensitivity.
metal-on-metal wear stimulates lymphocytes
metal-on-metal serum ion levels greater with cup abduction angle >55 degrees and smaller component size
