Miller-Total knee Flashcards
Review the AAOS guidelines for knee OA treatment
Review osteotomy treatments
Best indication
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Young active patient, generally younger than 45 years, and
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Occupation precludes the use of prosthetic joint replacement owing to significant implant loading and cycles (i.e., high-load, high-stress type occupation)
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Most likely to succeed when disease affects predominantly one compartment
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For varus knee malalignment
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Treatment is valgus-producing proximal tibial osteotomy
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Reason: problem is usually due to proximal tibial varus.
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Goal of surgery: correct the deforming problem
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Osteotomy goal: maintains joint line of knee perpendicular to mechanical axis of leg
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Mechanical axis of leg defined as center of hip through center of knee to center of ankle
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For valgus knee malalignment
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Treatment is varus-producing supracondylar femoral osteotomy
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Reason: problem typically is result of lateral femoral condylar hypoplasia.
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Goal of surgery: correct the deforming problem
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Osteotomy goal: maintain joint line of knee perpendicular to the mechanical axis of leg
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Valgus-producing tibial osteotomy (for varus knee deformity)
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Selection criteria
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Clinical examination and radiographs show that other two compartments are free of arthritis.
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Clinical pain is isolated to medial knee compartment.
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Patient is physiologically young and has an occupation or activity level that makes prosthetic arthroplasty less appropriate
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Contraindications
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Inflammatory arthritis
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Lack of flexion—minimum of 90 degrees needed
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Flexion contracture more than 10 degrees
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Ligament instability
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Especially varus thrust gait (this indicates abnormal lateral compartment ligament/capsular stretch-out)
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Femoral-tibial subluxation more than 1 cm (viewed on AP radiograph)
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Note: ACL deficiency acceptable if all other criteria are met
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Medial compartment bone loss
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Lateral compartment joint narrowing
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Detected by valgus stress radiograph
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Osteotomy less successful in following conditions
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Smoking
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Age 60 years or older
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Varus deformity of 10 degrees or more
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There is just not enough bone to remove to correct deformity.
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Concomitant arthritis in other compartments
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Main problems
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Closed-wedge technique
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Patella baja deformity (most common)
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Patella baja results in loss of knee flexion
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Loss of tibial posterior slope
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Open-wedge technique
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Patella baja deformity (also most common)
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Nonunion
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Loss of valgus correction (i.e., collapse of open wedge)
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Varus-producing femoral osteotomy (for valgus knee deformity)
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Selection criteria
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Valgus deformity of 12 degrees or greater
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Clinical pain isolated to lateral knee compartment
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Clinical examination and radiographs show medial knee compartment free of arthritis.
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Patellofemoral joint should also be free of arthritis, but minimally symptomatic patellofemoral disease is acceptable (reduction of Q angle improves patellofemoral mechanics and reduces pain).
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Patient is physiologically young and has an occupation or activity level that makes prosthetic arthroplasty less appropriate.
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Contraindications
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Inflammatory arthritis
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Prior medial meniscectomy
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Lack of flexion—minimum of 90 degrees needed
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Flexion contracture more than 10 degrees
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Ligament instability
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Especially valgus thrust gait (this indicates abnormal medial compartment ligament/capsular stretch-out)
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Femoral-tibial subluxation seen on AP radiograph
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Medial compartment joint narrowing
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Detected by varus stress radiograph
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Age older than 65—relative contraindication
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Osteoporosis—relative contraindication
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Main problems
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Nonunion
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Loss of varus correction
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Seen more often in patients with osteopenia/osteoporosis
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Residual patellofemoral maltracking may require a lateral retinacular release.
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Osteotomy technique (for femoral osteotomy)
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Crescentric dome preferred
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This osteotomy produces the least bone displacement.
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Allows for femoral stem with TKA
Review unicompartment treatments of the knee
Utilized for patients in whom arthritis predominantly affects one compartment of knee
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The most common UKA, by far, is medial compartment replacement
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Advantages of UKA (medial or lateral) over TKA and knee osteotomy
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Quicker recovery
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Fewer short-term complications
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Shorter hospital stay with less postoperative pain
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Better knee function than with TKA
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ACL is preserved as it is in TKA
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Results
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High rate of short-term to mid-term satisfaction
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However, long-term survivorship is not comparable to that with TKA when measured by revision rates.
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Contraindications
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Inflammatory arthritis
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Significant fixed deformity
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Deformity must be correctable on clinical exam (e.g., resting varus attitude must be correctable to normal valgus)
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Previous meniscectomy in opposite compartment
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ACL-deficient knee
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ACL deficiency is an absolute contraindication to a mobile-bearing UKA
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Mobile-bearing UKA is utilized only for medial compartment replacement
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Flexion contracture less than 10 degrees
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Tricompartmental arthritis
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Selection criteria—important:
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Pain must be localized to the compartment being replaced
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Medial knee pain signifies medial compartment disease
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Lateral knee pain signifies lateral compartment disease
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Anterior knee pain signifies patellofemoral compartment disease
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Diffuse or global pain signifies tricompartmental disease
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Surgical technique
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Overcorrection must be avoided.
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Overcorrection puts increased load on unresurfaced compartment.
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Can result in early revision owing to accelerated progression of arthritis
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For medial UKA, correction to 1–5 degrees of clinical valgus
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Complications unique to UKA
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Stress fracture of tibia (never femur)
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Associated with heavy weight and high and early postoperative activity level
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Typical presentation: pain-free interval (usually 4–6 weeks), then spontaneous acute onset of pain with weight-bearing activity
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Aspiration of knee reveals blood
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Treatment
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If tibial fixation stable
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Relative rest and limited weight bearing
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If tibial fixation compromised
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Revision of tibial implant with or without ORIF of the medial tibia
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Conversion to TKA with tibial stem support when medial bone is compromised
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Failure mechanisms
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Overcorrection at time of surgery
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Risk is disease progression in opposite compartment
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Pain localized to arthritic compartment
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Undercorrection at time of surgery
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Risk is implant overload with subsequent failure due to accelerated polyethylene wear/failure, osteolysis, and/or mechanical loosening
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Implant subsidence
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Tibial side only
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Due to weak metaphyseal bone; factors:
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The deeper the tibial cut, the weaker the metaphyseal bone
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Undercoverage—preference is to place tibial implant on host rim bone, which is stronger.
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Osteoporosis
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Patellar impingement upon femoral implant causing pain
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Related to implant design and surgical technique
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Pain is localized anteriorly
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Requires revision to TKA
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Arthritis progression in other compartments (i.e., natural progression of disease)
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Pain in other knee compartments
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Requires revision to TKA
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Isolated patellofemoral arthritis
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TKA (not patellofemoral arthroplasty) is recommended choice in older patients (≥50 years)
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Superior functional results than those of patellectomy and patellofemoral arthroplasty
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Lateral retinacular release commonly required with isolated patellofemoral arthritis
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Reason: maltracking is usually the cause of isolated patellofemoral arthritis
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TKA: see next section
Review the end cuts on TKA
Goal of end cuts is to restore neutral mechanical alignment of the limb.
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Neutral mechanical alignment is defined as a line from hip head center, through knee center, to ankle center
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Preoperative analysis of femur (review of full-length radiographs) is used to determine the following (Fig. 5.67):
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Anatomic axis of femur (AAF)
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A line that bisects the medullary canal of the femur
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The AAF, drawn to the distal end of the femur, determines entry point for the femoral medullary guide rod for the cutting jigs
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Mechanical axis of femur (MAF)
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A line from center of distal femur (entry point hole) to center of femoral head
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Significance—distal femur is cut perpendicular to MAF.
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Allows even mechanical loading to knee implant
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Valgus cut angle
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Defined as angle between AAF and MAF
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Intramedullary guide rod is placed into femur (this defines AAF).
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Distal femoral cut jig is assembled to intramedullary guide rod.
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Surgeon selects valgus cut angle (typically between 4 and 7 degrees).
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Distal femur should end up being perpendicular to MAF.
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Valgus cut angle should always be measured in tall and short patients (Fig. 5.68).
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Hip offset remains relatively constant.
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Femur length, therefore, has more influence on valgus cut angle.
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Preoperative analysis of tibia (review of full-length AP radiograph) is used to determine the following (Fig. 5.69):
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Anatomic axis of tibia (AAT)
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A line that bisects the medullary canal of the tibia
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The AAT, drawn through the proximal tibia, determines the entry point for the tibial medullary guide.
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Both intramedullary and extramedullary cutting jigs for the proximal tibial end cut are acceptable techniques.
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Mechanical axis of tibia (MAT)
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A line from center of proximal tibia (entry point hole) to the center of ankle
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Significance—proximal tibia is cut perpendicular to MAT.
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Allows even mechanical loading to knee implant
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Tibial cut angle
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Defined as angle between AAT and MAT
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Intramedullary guide technique
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Intramedullary guide is placed into tibia.
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Proximal tibia cut jig is assembled to intramedullary guide.
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Surgeon selects tibial cut angle (usually 0 degrees).
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Proximal tibia should end up being cut perpendicular to MAT.
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Extramedullary guide technique
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The extramedullary guide technique is placed over the anterior tibia. A jig distally holds guide centered over ankle. A proximal jig holds guide centered over proximal tibia (landmark is medial one-third of tibial tubercle).
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Surgeon selects tibial cut angle.
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In most cases the AAT and MAT are coincident. Therefore tibial cut angle is zero.
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When there is a tibial deformity (e.g., fracture or bowing deformity), the AAT and MAT are divergent. The tibial cut angle is then carefully measured and selected to provide a proximal tibial end cut perpendicular to MAT.
Review the sagital plane gap deformity guide
Review the sequence of release for medial deformity
Convex side is lateral—loose
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Concave side is medial—medial compartment release needed
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Medial compartment release in sequence
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Osteophytes
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Deep medial collateral ligament (also known as meniscal tibial ligament)
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Includes medial knee capsule
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Posterior medial corner
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Capsule
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Semimembranosus
lateral release sequence
Lateral compartment release in sequence
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Osteophytes
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Lateral capsule
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Iliotibial band—key structure
Release for lateral extension tightness
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Popliteus—key structure
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Tight in flexion
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Release for lateral flexion tightness
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Release of popliteus off anterior portion of lateral epicondyle (Fig. 5.72)
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Note: the inadvertent cut of a noncontracted popliteus tendon does not significantly affect the static stability of the knee.
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Lateral collateral ligament (LCL)—last
review extra-articular bone deformity
General rules
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The closer the extraarticular coronal bone deformity is to the knee joint, the greater the mechanical malalignment at the joint line.
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For any given magnitude, the farther a deformity is from the knee, the smaller the intraarticular bone cut needed to correct the mechanical alignment.
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An extraarticular deformity within the distal one-fourth of the femur or proximal one-fourth of the tibia is the most difficult to correct if bone cuts are made only at the knee joint. Reasons:
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Large bone resections required may compromise ligament attachment sites.
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Large bone resections adversely affect implant sizing, fitting, and rotational alignment.
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Extreme releases required to balance the knee often render the ligament incompetent (Fig. 5.73).
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McPherson one-fourth rule: when coronal deformity is within the distal one-fourth of the femur or proximal one-fourth of the tibia and the deformity is 20 degrees or more, the recommended treatment is:
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Concomitant osteotomy and TKA
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Closing wedge osteotomy preferred
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Diaphyseal press fit stem with splines recommended
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Provides rotational stability and obviates the need for additional fixation at osteotomy site
treatments for flexion deformity
Concave side is posterior—posterior knee release required
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Posterior knee release procedure—in sequence is:
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Osteophytes
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Posterior capsule
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Gastrocnemius muscle origin
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Posterior releases are performed with the knee flexed (generally at 90 degrees of flexion).
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Less danger to popliteal artery
Criteria for diagnosing a periprosthethic joint infection
Review the total knee prothesthic designs
Designs are categorized according to an increasing level of mechanical constraint in knee system.
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Least constrained
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Cruciate-retaining TKA—remove ACL and keep PCL
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Cruciate-sacrificing TKA—remove ACL and PCL
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Both used for straightforward primary TKA
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Constrained
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Constrained nonhinged TKA
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Used for complex primary or revision TKA
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Highly constrained
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Hinge TKA
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Used for complex revision TKA
Review Cruciate Retaining Knee implant
PCL helps provide flexion gap stability.
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PCL tension influences femoral prosthetic rollback.
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Rollback is defined as the progressive posterior change in femoral-tibial contact point as the knee moves into flexion.
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Generally, cruciate-retaining implants have more flat PE inserts to accommodate for flexion rollback.
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Advantages
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Bone conserving
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More consistent joint line restoration
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Keeping PCL keeps flexion gap smaller
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Disadvantages
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Harder to balance with severe deformities
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Cruciate-retaining implants should be avoided if
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Varus more than 10 degrees
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Valgus more than 15 degrees
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PCL balance is critical for long-term bearing wear
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A tight PCL in flexion causes increased PE wear
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PCL in flexion must be balanced.
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Lift-off must be avoided (Fig. 5.80).
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PCL can be released off femur or tibia.
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PCL balance is sometimes hard to assess intraoperatively.
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PCL balance is sometimes hard to achieve—over-release of PCL is common.
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Excess recession (i.e., release) can result in late failure caused by flexion instability and repetitive subluxation
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Flexion instability is characterized by
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Knee effusion
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Chronic pain
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Inability to climb stairs with reciprocal gait
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Inability to arise from low chair
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Knee buckling
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Late rupture of PCL with resultant instability
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PE particle debris can cause osteolysis and result in disruption of PCL from bony attachments
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Traumatic fall onto flexed knee can cause rupture.
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Paradoxical forward sliding as knee flexes
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With ACL removed, knee kinematics are drastically altered.
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Prosthetic knee does not roll back like native knee.
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As knee flexes, there is paradoxical forward-sliding movement, which causes sliding wear on PE insert.
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Sliding wear causes significant PE wear.
Review cruciate sparing TKR implant
Spine and cam mechanism in the posterior aspect of the knee
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Also called posterior stabilized knee
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An extended anterior PE lip with a concomitant smaller posterior lip
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Also called anterior stabilized knee
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Posterior stabilized primary TKA design
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Description (Fig. 5.81)
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A cam connects between the two posterior femoral condyles.
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The cam engages a tibial PE post during flexion.
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The cam and post control rollback.
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Generally, posterior stabilized implants have more dished (i.e., congruent) PE inserts.
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Advantages
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Easier balancing in severe coronal deformities (i.e., varus/valgus) because both ACL and PCL are removed
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Controlled flexion kinematics with spine and cam, less sliding wear
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Disadvantages
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Femoral cam jump (Fig. 5.82)
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Occurs when flexion gap is left too loose
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Mechanism of cam jump
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Varus or valgus stress when knee is flexed
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Patient usually lying in bed or sitting on floor
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Flexion gap opens up, and femoral cam rotates in front of post and then comes to rest in front of tibial post.
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Closed reduction maneuver
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With use of anesthesia, knee is positioned at 90 degrees of flexion off the table (dependent dangle)
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An anterior drawer maneuver is performed
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Clunk will be felt as knee is reduced.
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Ultimate solution requires knee revision to address loose flexion gap
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Causes of loose flexion gap
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Overrelease of contracted popliteus
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Inadvertently occurs also with saw blade
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Overrelease of anterior portion of superficial MCL
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Anterior translation of femoral component
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Anterior translation increases flexion gap space
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Patella clunk syndrome
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Scar tissue (descriptively, a nodule of scar) superior to patella gets caught in box as knee moves from flexion into extension.
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Scar catches in box, then releases with a clunk.
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Clunk occurs in range between 30 and 45 degrees.
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Treatment is removal of suprapatellar scar nodule (Fig. 5.83).
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Arthroscopic removal is acceptable.
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Miniarthrotomy is also acceptable.
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Preventive treatment (Fig. 5.84)
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Synovectomy and débridement of all scar from quadriceps tendon at time of TKA
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Risk factors that cause patellar clunk are related to factors that increasequadriceps force. These include:
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Small patella implant (decreased extensor offset)
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Thin total patella height (decreased extensor offset)
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Short patella tendon (patella baja)
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Increased posterior condylar offset (patella pulled lower down)
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A result of PCL removal requiring an increase in the AP femoral size to fill the increased flexion gap
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Tibial post wear and breakage
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Tibial post is an additional PE surface that can wear and enhance risk for osteolysis.
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Aseptic loosening and osteolysis are correlated with tibial post wear and damage.
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If the knee hyperextends, the edge of the femoral box can impinge on the anterior tibial post (Fig. 5.85).
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Causes anterior post damage and fatigue
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Causes increased PE wear and osteolysis
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Anterior tibial post wear occurs when TKA components are in net hyperextension, such as with
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Flexion of femoral component on distal femur
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Excess tibial posterior slope
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Knee hyperextension (i.e., loose extension gap)
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Anterior translation of tibial component on tibia (i.e., placing tibial implant toward front of tibia rather than placing on posterior tibial rim)
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Anterior translation of femoral component has no effect on anterior tibial post impingement.
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Additional bone is removed from middle of distal femur.
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Bone removed can be substantial in a small knee.
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Flexion gap is bigger
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Flexion gap opens up when PCL is removed.
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To balance the extension gap, additional distal femur bone is removed in a posterior stabilized TKA.
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Consequence of additional distal femoral bone removal
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Joint line elevation with possible baja deformity
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The maximum joint line elevation allowed in primary TKA is 8 mm so as to maintain knee ligament kinematics.
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Ensures proper kinematic function and stability of collateral ligaments.
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PS TKA must be used in following situations
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Patellectomy
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Cruciate-retaining knee with a flat PE is prone to anterior subluxation when patella is absent.
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Inflammatory arthritis
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PCL is at risk for rupture with erosive disease process
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Trauma with PCL rupture or attenuation
Review anterior stabilizing TKA implant
A cruciate-retaining femoral component is used.
The PCL is removed (or highly recessed).
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Tibial insert is a highly congruent bearing with a raised anterior PE lip and a smaller posterior lip.
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No mechanism for rollback
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Anterior lip resists anterior translation.
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Advantages
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Easier balancing in severe deformities (i.e., varus/valgus)
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Bone conserving—no box cut needed
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Operative versatility
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Switch to posterior stabilized system not required if PCL is lost or overreleased.
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Regulated flexion kinematics
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High congruency limits sliding wear
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Knee flexion is achieved by
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Posterior placement of tibial knee flexion center; this is called posterior offset center of rotation.
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Placement of tibial component with native posterior slope; femur is less likely to impinge upon posterior tibia in flexion.
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Disadvantages
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Increased PE surface area
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Increases risk for greater PE wear debris and osteolysis
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Minimal rollback
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Surgical technique must be adjusted to attain high flexion.
•
Posterior translation of tibial component on tibia, when possible; this will place tibial center of rotation more posteriorly.
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Recreation of native tibial posterior slope.
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Flexion gap laxity causes rotational instability and pain
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A loose flexion gap will cause instability usually in midflexion and full flexion.
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Mechanism of midflexion instability
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Varus or valgus stress when knee is flexed (between 50 and 90 degrees)
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Patient usually lying in bed or sitting on floor
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Flexion gap opens up and tibia rotates anteriorly. This creates a subluxing event, but knee usually does not lock up.
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Patient experiences pain when climbing stairs with reciprocal gait, arising from a chair, or negotiating uneven surfaces; it is usually associated with a knee effusion.
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Treatment requires revision to address loose flexion gap.
Review tibial rotating platform
The tibial PE bearing rotates on a polished metal tibial baseplate.
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The rotating platform can be used with both anterior stabilized (high congruent) and posterior stabilized TKA designs.
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The PCL is removed when a tibial rotating platform is used.
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Advantages
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Better articular conformity through entire knee range
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Theoretically less PE wear
□
Equivalent in survivorship to fixed-bearing knee, but not superior
•
Wear and osteolysis still seen
▪
Disadvantages
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Bearing spinout (Fig. 5.88)
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Occurs when flexion gap is left too loose
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Mechanism of spinout
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Varus or valgus stress when knee is flexed
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Patient usually lying in bed or sitting on floor
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Flexion gap opens up, and tibia rotates behind femur.
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Femur then comes to rest in front of tibial PE bearing and locks into spinout position.
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Closed reduction maneuver
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With use of anesthesia, knee is positioned at 90 degrees of flexion off the side of the table (dependent dangle).
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Tibial bearing is manipulated by digital palpation and pressure into reduced position.
•
Ultimate solution requires knee revision to address loose flexion gap.
Review all poly insert
Implant is nonmodular
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Implant is cemented
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Economical (compared with metal tray–modular PE combo)
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Wear rate and aseptic loosening rates are similar to those of modular tibial implant.
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Disadvantages
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Cementing an APT is technically harder to perform.
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“Forcing” the APT in place during cementing process can change balance of knee by damaging soft tissues.
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Failure mechanisms
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Bending of PE implant at periphery, as there is no underlying metal to support cantilever bend forces
•
PE implant bends, causing cement to crack. Implant then becomes loose.
What are the indications for a hinged prothesis?
This is absolute indication for hinge.
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Hyperextension conditions include
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Postpolio knee or spina bifida
•
Increased knee extension forces to lock and hold knee in extension during gait, eventually causing posterior capsule to stretch out
•
Erosion of posterior capsular attachments to bone as a result of
•
Advanced bony osteolysis
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Autoimmune disease states (particularly rheumatoid arthritis)
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Native knee removal
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Tumor
•
High-energy fracture with communication
•
Massive infection
TKA techniques to avoid patella maltracking
Reduction of excess valgus
□
Valgus deformity must be corrected to a neutral mechanical alignment—always.
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Severe valgus deformities that require radical ligamentous releases can be adequately managed with sophisticated revision-style prosthetic systems.
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Component positioning
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Patellar maltracking—causes
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Internal rotation of
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Femoral component
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Tibial component
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Medialization of
•
Femoral component
•
Tibial component
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Femoral component rotation
□
Femoral component should never be internally rotated.
Review the technique for internal rotation of the tibia component
why do you set 3-5 degrees of external rotation cut on the femur?
Review the 5 established techniques to address femoral rotation:
Review implant medialization
femoral implant should be lateralized
tibial implant should be lateralized
patella component may be medialized
what is the pre-op evaluation for a TKA revision?
Revision of a painful TKA without identification of a specific cause of pain is likely to have a poor outcome.
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First, pain must be determined as having either an intrinsic intraarticular source or an extrinsic source.
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Extrinsic sources of knee pain
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Referred pain from the hip
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Most common missed diagnosis
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Hip pain typically refers to anterior-medial knee region (distal branch of obturator nerve).
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Referred pain from the spine
•
Typically L3 nerve root
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Extraarticular at the knee
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Allodynia—chronic regional pain syndrome
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Local superficial neuroma
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Intrinsic sources of knee pain
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Mechanical loosening
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Osteolysis with PE debris synovitis
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Malposition and/or malalignment of implants
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Instability
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Infection
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Typically, presentation is constant global pain with abnormal infection biomarkers and positive aspiration findings.
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PJI is currently the number one reason for revision within the first 2 years of primary TKA.
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Hypersensitivity—rare
•
Typically, constant global pain with normal infection laboratory results and negative aspiration findings.
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Most common metal ion involved in knee hypersensitivity is nickel.
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Intraarticular aspiration
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WBC count greater than 3000 cells/μL raises suspicion for a chronic PJI.
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Intraarticular lidocaine challenge
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Administration of at least 15 mL of lidocaine or 50/50 mixture of lidocaine/bupivacaine
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Relief of more than 90% of pain constitutes a positive result.
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Indicates that pain emanates primarily from within the knee joint
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Radiographs
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Smooth radiolucent lines around cement mantle and metallic implants suggest aseptic loosening.
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Stem tilt to side of medullary canal with an outer cortical periosteal reaction suggests aseptic loosening.
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CT
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Evaluation for rotational malalignment of implants
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Posterior condylar axis of metallic femoral condyles should be compared with epicondylar axis.
•
Posterior condylar axis should be parallel or slightly externally rotated in relation to epicondylar axis.
•
Internal rotation of femoral implant is a known cause of flexion gap imbalance.
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Tibial implant axis should lie over medial third of tibial tubercle.
•
A tibial implant axis lying medial to tibial tubercle indicates malalignment (i.e., relative increase of tibial tubercle external rotation).
review the surgical technique for a revision TKA:
The prior incision should be used instead of a new incision made.
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Reason: a “skin bridge” can necrose from devascularization.
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If a second incision is required, the minimum distance between incisions is 7 cm.
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If two or more longitudinal incisions are present in the anterior knee, the most lateral incision should be chosen for revision.
•
Reason: blood supply for the anterior knee skin comes from medial side of distal thigh and knee.
▪
Difficult exposure sequence
□
Extended proximal arthrotomy
•
To most proximal end of quadriceps tendon
□
External rotation of tibial bone from soft tissue envelope
•
Subperiosteal dissection of soft tissues from medial tibial tubercle all the way around to posterior-medial corner of knee
•
Release of posterior-medial corner structures and, if needed, posterior tibial capsule
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Lateral knee débridement
•
Removal of scar from patella, tendon, and lateral gutter
•
Gradual eversion of patella as tibia is externally rotated
□
Lateral retinacular release—only if needed
□
Quadriceps tendon snip (usually 2 cm)
•
Transverse snip at most proximal region
•
Snip will not cause quadriceps dysfunction or lag
▪
Tibial tubercle osteotomy
□
Used as last resort
□
Best indication is for a stiff TKA (<90 degrees flexion) with patella baja deformity.
Implant System
▪
Comprehensive revision system must be available.
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Revision surgery is often unpredictable.
□
Constrained tibial insert option is a must.
□
Stems and metallic augmentations are required.
can you just change out the poly in a TKA revision?
Modular Bearing Change for Premature Excessive Wear
▪
Most current tibial PE bearings should last at least 13–15 years with an average patient wear scenario in a well-balanced knee.
□
Failure of a PE bearing within 5–7 years is premature and indicates a problem with knee balance and/or alignment.
▪
Failure rate of isolated modular bearing change for excessive premature PE failure is 30%–40%.
□
Reasons for premature failure
•
Unappreciated malalignment
•
Poor knee balancing in either coronal or sagittal plane
▪
Isolated modular bearing change in this scenario is not recommended.
review the workflow sequence of a revision TKA
Workflow sequence
□
First—implant removal
□
Second—joint line restoration with tibial implant
•
Joint line is generally 1.5 cm superior to top of fibular head.
•
Failure to restore joint line will result in diminished knee flexion.
□
Third—femur restoration
•
Extension gap restored first
•
Surgeon must ensure that patella is in appropriate position (i.e., no baja or alta).
•
Restoration of flexion gap
•
Gap balance fine tuned.
□
Fourth—adjustment of femoral and tibial implant rotation for best patellar tracking
□
Fifth—selection of constraint
•
The least constraint needed for stable knee function should be used.
□
Sixth—assessment of patellar tracking
•
Large lateral retinacular release should be avoided if possible. Lateral superior genicular artery should not be cut.
▪
Segmental defect femur or tibia
□
Metadiaphyseal trabecular cones are preferred solution to provide prosthetic support (Fig. 5.92).
□
Trabecular cones have predictable osteointegration to host bone.
Revision TKA—Patella
▪
Isolated patella component failure usually indicates subtle malalignment in patellar tracking.
□
Higher failure rate for isolated patellar revision
□
Full revision should be considered.
▪
A mechanically loose patellar component can cause significant patellar bone loss.
□
For revision to another patellar component, bone thickness must be at least 12 mm, and there must be enough bone to support PE pegs within bone.
□
If bone is inadequate for revision resurfacing
•
Débridement of patellar implant with bone retention is acceptable
•
Patellectomy is recommended for bony fragmentation.
Patella resurfacing vs non-resurfacing
Between the two techniques, neither method has been established as superior.
□
Overall, patellar resurfacing has a lower risk of reoperation than nonresurfacing.
□
Absolute indication for patellar resurfacing is autoimmune inflammatory arthritis.
▪
Problems with resurfaced patella
□
Patella component loosening
•
Due to maltracking
•
Due to osteolysis
□
Patella clunk and crunch
•
Clunk occurs when suprapatellar scar tissue gets entrapped within the posterior stabilized box as the knee comes from flexion into extension.
•
Clunk is unique to posterior stabilized design.
•
Patellar crunch occurs when scar accumulates around patellar component, creating a crunching noise as the knee comes from flexion to extension.
□
Patella fracture
•
Reason: bone cut too thin
•
Minimum thickness for patella is 13 mm.
□
Avascular necrosis of patella with fragmentation
•
Reason: peripatellar devascularization due to lateral retinacular release
•
Disruption of lateral superior geniculate artery
▪
Problems with nonresurfaced patella
□
Anterior knee pain
•
Incidence increases over time.
•
Articular cartilage wears away, and there is point loading upon patellar bone.
•
Results of secondary resurfacing are variable.
•
Pain relief not predictable
▪
Criteria for patellar nonresurfacing
□
Noninflammatory arthritis
□
Lower activity level
□
No dysplasia or maltracking
□
No baja
▪
Requirements for patellar nonresurfacing
□
Anatomic femoral component
•
V-shaped trochlea groove to match native patella
•
Deep trochlear groove to prevent overstuffing of patellar gap
□
Circumferential denervation of patella with electrocautery
review the operative techniques to address patella baja
Operative solutions to reduce baja in TKA
□
Superior placement of patellar component
•
Use of smaller patellar dome placed superiorly on patella
•
Trimming/tapering of inferior bone to reduce flexion impingement
•
Useful for mild baja deformity
□
Lower joint line—sophisticated technique (Figs. 5.103 and 5.104)
review the measures to avoid catastrophic poly wear
PE thickness at least 8 mm (for traditional PE)
▪
Congruent bearing design
□
High contact area
□
Low contact load
▪
Sliding wear on tibia minimized
□
PCL substitution or
□
PCL accepting prosthesis
•
PCL is used as a static stabilizer only (seen with anterior stabilized knee).
▪
Direct compression–molded PE bearing
□
No machining of articular surface
▪
Inert PE irradiation
□
γ-Irradiation sterilization in an oxygen-free environment
□
Quality packaging to minimize on-the-shelf oxidation
•
Packaging must keep oxygen from diffusing back into PE through it
▪
Oxygen scavengers embedded in PE material
□
Reduces effect of in vivo oxidation
□
Vitamin E is currently the most commonly added antioxidant.
review factors contributing to advanced poly wear:
“The Perfect Storm”
Metal-backed tibial baseplate with bone-conserving tibial bone cut
□
Thin PE, 5 mm
▪
Flat bearing design in coronal plane
□
Low contact area (a line)
□
High contact load
▪
PCL retention with flat PE insert
□
High sliding wear
▪
Ram bar PE with calcium stearate additive
□
Fusion defects in PE
▪
γ-Irradiation sterilization in air (i.e., oxygen)
□
Weakening of mechanical properties of PE
▪
Machined PE surface
□
Cutting-tool stretch effect upon PE
review factors associated with pre-mature poly wear
Etiology is macroscopic PE failure.
□
Problem is not a microscopic PE wear problem.
▪
Patient presents with a large knee effusion that may or may not be painful.
▪
Osteolysis is present but is a secondary problem.
▪
Multiple factors are involved to create the perfect storm of catastrophic wear.
Factors Involved in Catastrophic Wear
▪
The factors involved in catastrophic wear of a TKA implant:
□
PE thickness
□
Articular geometry
□
Knee kinematics
□
Surgical technique
□
PE processing
▪
Polyethylene thickness
□
Thin PE breaks.
□
To keep knee bearing contact stress below the yield strength of UHMWPE (12–20 mPA), the PE must be at least 8 mm.
□
This statement applies to “traditional” PE that is not highly cross-linked.
□
Many second-generation knee systems had PE knee inserts with a PE thickness of 4–5 mm in the thinnest region.
□
Current designs ensure that PE thickness in the thinnest areas of the insert is at least 8 mm.
▪
Articular geometry (Fig. 5.105)
□
Flat PE should be avoided.
•
Knee loads exceed yield strength of UHMWPE in flat design.
•
There is only a thin line of joint contact during loading in flat PE inserts.
•
A thin line of contact results in high contact loads to PE.
□
Goals of current tibial articular designs
•
Maximize contact area.
•
Minimize contact loads (i.e., force/area).
•
Best design is biplanar congruency (Fig. 5.106).
•
Congruent design in both coronal and sagittal planes
▪
Knee kinematics
□
Sliding wear is bad for PE.
□
Sliding wear occurs when the ACL is sacrificed.
•
When the ACL is removed and the PCL remains, the femur slides across the tibial PE during flexion and extension.
•
Sliding movements are most pronounced in a cruciate-retaining knee design with a flat PE insert.
•
Sliding movements are least pronounced in a posterior stabilized or anterior stabilized knee design with a congruent PE insert.
•
In laboratory testing sliding wear across the tibia created severe surface and subsurface cracking with high wear.
□
Current knee prosthetic systems are designed to minimize tibial sliding wear.
▪
Surgical technique
□
A tight flexion gap hastens sliding wear effect.
•
Stress is amplified with
•
Tight PCL
•
Anterior tibial slope (Fig. 5.107)
▪
Polyethylene processing
□
Fabrication
•
Ram bar extruded PE is not good.
•
Variation in PE quality within the bar
•
Calcium stearate additive is bad.
•
Causes fusion defects in PE
•
Best PE fabrication process: direct compression molding
•
PE powder is placed into a mold, heated, and compressed, creating an implant directly from the mold.
□
Sterilization
•
Irradiated PE in air is bad.
•
Oxidized PE chains
•
Reduced mechanical strength of PE
□
Machining (cutting-tool effect)
•
The cutting tool used to machine PE microscopically stretches PE chains (Fig. 5.108).
•
Amorphous areas are stretched.
•
The cutting-tool stretch effect is most pronounced 1–2 mm below the cut surface of the PE.
•
The stretched PE chains are more susceptible to radiation, resulting in greater oxidation in this region.
•
The clinical finding of the PE stretch/oxidation effect is the classic white band of oxidation in the subsurface of the PE (Fig. 5.109).
poly wear concerns
Etiology is macroscopic PE failure.
□
Problem is not a microscopic PE wear problem.
▪
Patient presents with a large knee effusion that may or may not be painful.
▪
Osteolysis is present but is a secondary problem.
▪
Multiple factors are involved to create the perfect storm of catastrophic wear.
Factors Involved in Catastrophic Wear
▪
The factors involved in catastrophic wear of a TKA implant:
□
PE thickness
□
Articular geometry
□
Knee kinematics
□
Surgical technique
□
PE processing
▪
Polyethylene thickness
□
Thin PE breaks.
□
To keep knee bearing contact stress below the yield strength of UHMWPE (12–20 mPA), the PE must be at least 8 mm.
□
This statement applies to “traditional” PE that is not highly cross-linked.
□
Many second-generation knee systems had PE knee inserts with a PE thickness of 4–5 mm in the thinnest region.
□
Current designs ensure that PE thickness in the thinnest areas of the insert is at least 8 mm.
▪
Articular geometry (Fig. 5.105)
□
Flat PE should be avoided.
•
Knee loads exceed yield strength of UHMWPE in flat design.
•
There is only a thin line of joint contact during loading in flat PE inserts.
•
A thin line of contact results in high contact loads to PE.
□
Goals of current tibial articular designs
•
Maximize contact area.
•
Minimize contact loads (i.e., force/area).
•
Best design is biplanar congruency (Fig. 5.106).
•
Congruent design in both coronal and sagittal planes
▪
Knee kinematics
□
Sliding wear is bad for PE.
□
Sliding wear occurs when the ACL is sacrificed.
•
When the ACL is removed and the PCL remains, the femur slides across the tibial PE during flexion and extension.
•
Sliding movements are most pronounced in a cruciate-retaining knee design with a flat PE insert.
•
Sliding movements are least pronounced in a posterior stabilized or anterior stabilized knee design with a congruent PE insert.
•
In laboratory testing sliding wear across the tibia created severe surface and subsurface cracking with high wear.
□
Current knee prosthetic systems are designed to minimize tibial sliding wear.
▪
Surgical technique
□
A tight flexion gap hastens sliding wear effect.
•
Stress is amplified with
•
Tight PCL
•
Anterior tibial slope (Fig. 5.107)
▪
Polyethylene processing
□
Fabrication
•
Ram bar extruded PE is not good.
•
Variation in PE quality within the bar
•
Calcium stearate additive is bad.
•
Causes fusion defects in PE
•
Best PE fabrication process: direct compression molding
•
PE powder is placed into a mold, heated, and compressed, creating an implant directly from the mold.
□
Sterilization
•
Irradiated PE in air is bad.
•
Oxidized PE chains
•
Reduced mechanical strength of PE
□
Machining (cutting-tool effect)
•
The cutting tool used to machine PE microscopically stretches PE chains (Fig. 5.108).
•
Amorphous areas are stretched.
•
The cutting-tool stretch effect is most pronounced 1–2 mm below the cut surface of the PE.
•
The stretched PE chains are more susceptible to radiation, resulting in greater oxidation in this region.
•
The clinical finding of the PE stretch/oxidation effect is the classic white band of oxidation in the subsurface of the PE (Fig. 5.109).
