Ch 59 - Surgical Management of Hip Dysplasia Flashcards
What kind of “joint” is the pelvic symphysis?
A synchondrosis which transforms over time to become a synostosis
Which end of the symphysis is larger?
Ischial portion is slightly larger
What kind of cartilage makes up the pelvic symphysis?
- Hyaline cartilage - acts as the growth plate
- Fibrocartilage - gradually replaced by bone
When does the pelvis symphysis start to ossify?
When is it complete?
- Starts to ossify in a caudal to cranial direction beginning at 9-21 months
- Completely ossified within 2-6yr
List the peripheral ligaments of the pelvic symphysis
- Transverse or oblique fibrous fascia - along the dorsal surface, strengthens
- Prepubic tendon
- Arcuate pubic ligament - crosses ischial arch, often becomes ossifies
What is a Juvenile Pubic Symphysiodesis (JPS)?
Application of electrocautery to the hyaline cartilage of the pubic symphysis resulting in head-induced necrosis of the germinal chondrocytes and premature closure of the symphysis
How does a Juvenile Pubic Symphysiodesis (JPS) effect acetabular growth?
Results in external rotation of the acetabulum in a ventrolateral axial direction, increasing the acetabular coverage of the femoral head
What is the ideal age for Juvenile Pubic Symphysiodesis (JPS)?
12 - 20 weeks
25% treated dogs developed OA vs 83% of sham operated dogs
What muscles are partially elevated from the symphysis for Juvenile Pubic Symphysiodesis (JPS)?
Gracilis and adductor muscles
What are the recommended electrosurgical settings for Juvenile Pubic Symphysiodesis (JPS)?
- 500kHz current frequency
- 40W
- current applied via monopolar probe for 10-30 seconds, every 2-3mm along cranial 1/3 - 1/2 of the symphysis
What are the aims of TPO/DPO?
- Reducing joint laxity
- Normalising joint stresses
- Improving joint congruence
What vessels need to be avoided during pubic osteotomy?
Nerve?
- Deeper medial circumflex femoral artery and vein
- Obturator nerve
What muscles need to be elevated from the ischium for ischial osteotomy of TPO? What structures need to be avoided?
Dorsal
- Internal obturator muscle
- Pudendal nerve
Ventral
- Semimembranosus
- Semitendinosus
- External obtruator muscle
What range of angles are typically available for TPO/DPO plates?
20 - 45 degrees
The desired degree of rotation is generally 5 degrees greater than the measured angle of subluxation
Coverage by the DAR does not signifcantly increase over that achieved by a 20 degree plate
Rotation beyong 40 degrees is not advised (increases risks of impingement)
What is the reported complication rate after a TPO? What are the main complications?
35 - 70%
Screw loosening and pelvic canal narrowing
How have the use of locking plate and DPO effected the rates of screw loosening?
- DPO reduced screw loosening to 3.2% (from 6-36%)
- Locking plates reduced the rate of screw loosening to 0.4%
How do metallic grains effect THR implant strength?
List manufacturing methods of increasing implant strength
- The smaller the metallic grain, the stronger the implant
Manufacturing methods to srengthen implants:
- Forging
- Investment casting
- Hot isostatic pressing
- Cold working
- Heat working
What is the elastic modulus of stainless steel, cobalt chromium and titanium?
- Stainless steel and cobalt chromium = approx 200GPa
- Titanium = approx 100GPa
What is stress shielding?
Occurs when implant is stiffer than bone, preventing adequate load transfer to the bone, resulting in bone resorption due to relative disuse atrophy leading to implant loosening
What metals are in 316L stainless steel?
- Iron
- Chromium
- Nickel
- Molybdenum
What metals are in cobalt alloys?
What THR implants use this?
- Chromium
- Molybdenum
- Nickel
Implants:
- BioMedtrix BFX anf CFX
- Very hard with excellent wear and corrosion resistance
What is the most common titanium alloy?
Which THR implant uses this?
- Ti6A14V
- Stronger, good fatigue resistance
- More prone to wear debris than cobalt alloys
Implants
- Zurich Cementless
- Recently switched to Ti6A14Nb
What is tantalum?
A metal that can be fabricated with a porosity and elastic modulus similar to those of cancellous bone
What is tribology?
The study of bearing surfaces
The ideal bearing surface is low friction, low wear debris generation, biocompatible, and damage resistant
What is the acceptable linear wear rate in people?
0.1mm/year
Dogs showed a significatnyl lower volumetric wear that is seen in humans however dogs had a more severe bony reaction (osteolysis) to the debris
List the 5 main ways in which wear debris can be generated
- Adhesive wear (cold welding)
- Abrasive wear (irregularity on surface of harder material or thrid-body wear)
- Fatique wear (Cyclic loading causing cracks/microcracks)
- Erosive wear (Solid particle erosion, impingement wear)
- Corrosive wear (Galvanic corrosion - oxidation that generally results from interactions of dissimilar metals)
Under what circumstances is the greatest amount of wear debris created?
Titanium bearing surface with a cemented prosthesis
List options for coating of metallic heads
- Titanium nitride
- Diamond-like carbon (“amorphous carbon film”)
What are two forms of ceramic used in THR in humans?
What are the benefits?
- Alumina and Zirconia
- Wear products are locally inert
- Hydrophilic crating a virtually frictionless fluid-film lubrication
What is the reported loosening rate of a Helica implant?
37.5%
What strategies can be imployed to enhance the strength of the cement mantle?
- Centrifugation and application of a vacuum (Decreases porosity)
- Pressurisation (injector and intramedullary cement restrictor)
- Minimum 2mm mantle
- Femoral stem centralisers
- Collared prostheses
- Minimising blood and fat interposition
What environment is necessary for bone ingrowth?
- Pore size 50 - 400mcm
- Micromotion less than 20mcm
- Porosity of 30-40% is ideal
What is the angle of inclination?
Angle between the anatomic axis of the femoral neck and femoral shaft
- Lower Femoral neck angle: varus neck
- Higher Femoral neck angle: valgus neck
What is femoral offset?
DIstance between center of rotation of femoral head and anatomical axis of the femur
What is femoral neck anteversion?
The cranial angulation of the femoral neck in relative to the anatomical axis of the frontal plane of the femur
Increased anteversion can predispose to craniodorsal luxation
What is the goal version angle of the acetabulum?
Goal angle of lateral opening?
- 15 - 20 degrees acetabular retroversion
- 45 degrees lateral opening
What is the recommened version angle of the femoral neck?
- 15 - 25 degrees anteversion
What is the overall success rate and complication rate for THR?
- 95% success
- 5 - 22% complication rate
List some forms of mechanical and biological failure of a THR
Mechanical
- Luxation (2 - 17%)
- Femoral fracture
- Acetabular fracture
- Acetabular cup avulsion
- Femoral stem avulsion
- Subsidence (less than 4-5mm likely insignificant)
- Implant failure
- Cement failure
Biologic
- Aseptic loosening
- Septic loosening (1 - 2%)
- Stress protection
How are THR-related femoral fractures classified?
The Vancouver classification:
- Fracture at level of greater trochanter Ag
- Fracture of lesser trochanter Al
- Fracture with stable prosthesis B1 (screw fixation)
- Fracture with unstable prosthesis B2 (press-fit stem)
- Fracture with unstable prosthesis, worse prognosis B3
- C describes fractures distal to prosthesis (cemented stems)
What is the “coffin-lid” approach for removing cement?
- Creation of a bone flap starting at base of greater trochanter and extending the length of the cement mantle. Edges should be beveled and corners should be rounded
What is the primary cause of aseptic loosening?
Wear-debris mediated osteolysis
Why is the removal of periprosthetic fibrous tissue a crucial part of revision?
Contains mediators of bone lysis
- Activated macrophages
- TNF-a
- Oxygen-derived free radicals
What are the reported outcomes of an FHO?
- 38% good (no lameness)
- 20% satisfactory (some lameness)
- 42% poor (persistent mild to severe lameness)
What are the anatomical landmarks for a FHO?
From medial aspect of greater trochanter proximolaterally to lesser trochanter distomedially
What are two palliative options for hip OA?
- Hip denervation
- Pectineus myectomy
What muscles need to be elevated from the ischium for ischial osteotomy of TPO? What structures need to be avoided?
Dorsal
- Internal obturator muscle
- Pudendal nerve
Ventral
- Semimembranosus
- Semitendinosus
- External obtruator muscle
