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.