Wear and special cases

Wear in Total Knee Replacement (TKR)

Catastrophic Wear vs. Long-Term Wear

  • Catastrophic Wear: Different from long-term submicron wear particles.
  • Influenced by various factors:
    1. Polyethylene (PE) thickness.
    2. PE manufacturing.
    3. PE sterilization.
    4. Articular geometry.
    5. Sagittal plane kinematics.

PE Thickness

  • Minimum acceptable thickness: 8mm.
  • Thinner polyethylene has insufficient yield strength.
  • Includes metal tray thickness in measurement.

Articular Geometry

  • Flatter Polyethylene:
    • Decreases congruency, increasing contact pressure.
    • Accelerates wear (e.g., edge loading, fatigue failure).
  • Better designs prioritize higher congruency and more surface area contact.

Sagittal Plane Kinematics

  • Without ACL rollback:
    • Includes sliding, which leads to subsurface wear and cracks in PE.
  • New designs:
    • Increased articular congruency reduces sliding.
    • Flexion achieved through increased slope, posterior center of rotation (COR), or posterior stabilized (PS) designs.

PE Sterilization

  • Oxidized PE is weaker.
  • Best sterilized with inert gas irradiation (prevents free radical oxidation).
  • Storage: In a vacuum with a shelf life of <2 years.
  • Avoid using highly cross-linked polyethylene in TKR due to its vulnerability to fatigue wear.

PE Manufacturing

  • Best Method: Direct compression molding.
  • Avoid RAM bar extruded PE due to inconsistent quality.
  • Subsurface white band indicates vulnerability to fatigue failure and delamination.

Total Knee Replacement in Specific Situations

Previous High Tibial Osteotomy (HTO)

Challenges:

  1. Multiple Scars:
    • Prefer MPP approach or lateral incisions for easier access.
  2. Joint Line:
    • Patella Baja common; use cuts that lower the joint line.
  3. Rotational Malalignment:
    • Do not rely on IM referencing.
  4. Ligament Balance:
    • More challenging post-HTO; use PCL substituting implants.

Previous Patellectomy

  • Challenges:
    • Reduced quadriceps strength (up to 50%).
    • Decreased flexion and no rollback of the femur relative to tibia.
    • Sagittal instability in flexion.
  • Recommendation: Use PCL substituting design to allow mechanical rollback.

Neuropathic Arthropathy (e.g., Charcot Joint Disease)

  • Challenges:
    • Worse outcomes; avoid TKR if possible.
    • Bone loss and wound problems due to neuropathy.
  • Management: Grafting likely necessary.

Inflammatory Arthritis

  • Approach: Resurface patella; use Posterior Stabilized (PS) Knee.
  • Be cautious of skin problems and peri-operative medication management.

Haemophiliac Arthropathy

  • Considerations:
    • Increased HIV risk.
    • Higher transfusion requirements.
    • Pre-operative angiogram recommended due to aneurysm risk.
    • Maintain factor 8 levels at 100% before surgery.

Periprosthetic Fractures Around Knee Replacements

Distal Femur Fractures

Risk Factors:

  • Osteoporosis, Rheumatoid arthritis, Immunosuppressants, Trauma.
  • Notching: Shown to weaken the distal femur but not clinically linked to increased fracture risk.

Classification:

  • Location of fracture.
  • Are components loose?
  • Adequate bone for fixation?

Management:

  1. Nonoperative:
    • For undisplaced fractures in medically unfit patients: Proximal tibial traction or retrograde nail.
  2. Supracondylar fractures:
    • Open box design, multiple blocking screws, ORIF with fixed angle plate.
  3. Revision TKR:
    • For loose components, very distal fractures, use stemmed revision knee or hinged knee replacement.
  4. Distal Femoral Replacement:
    • For elderly or comminuted fractures.

Tibial Fractures

Risk Factors:

  • Poor bone quality, Loose/infected implants, Osteolysis, Previous tibial tubercle osteotomy, Malalignment, Trauma.

Classification (Felix et al.):

  1. Type 1: Fracture of tibial plateau.
  2. Type 2: Fracture at the level of tibial stem.
  3. Type 3: Fracture distal to the stem.
  4. Type 4: Tibial tuberosity fracture.

Management:

  • Type 1: Non-operative or ORIF with screws/plates.
  • Type 2: Same as Type 1.
  • Type 3: Maintain tibial alignment, non-operative or ORIF, possible external fixation.
  • Type 4: ORIF with standard methods.

Patella Fractures

Risk Factors:

  1. Patient-Related:
    • Osteoporosis, Trauma, Rheumatoid arthritis, Obesity, High demand.
  2. Component-Related:
    • Patella resurfacing, single PEG patella, uncemented patella, metal-backed patella.
  3. Technical Factors:
    • Devascularization of patella, Maltracking, over-resection.

Classification:

  1. Type 1: Extensor mechanism intact, component stable.
  2. Type 2: Extensor mechanism absent, component stable or unstable.
  3. Type 3: Extensor mechanism intact, component unstable.

Management Principles:

  1. Priority: Preserve the extensor mechanism.
  2. Type 1: Non-operative treatment.
  3. Type 2: Surgical repair or reconstruction of the extensor mechanism, with possible allograft.
  4. Type 3: Remove loose components, partial or complete resection of the patella if necessary.
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