Design Principles

Native Knee

  • Functions as a 4-bar linkage system with the axis of rotation (COR) through the cruciate ligaments.
  • COR moves posteriorly during flexion.
  • Femur rolls backward and rotates on the tibia:
    • Flexion: Tibia internally rotates due to the LFC being farther back.
    • Extension: Tibia externally rotates to “screw home” the knee.

Original TKR Design

  • Insall Total Contact Condylar Knee:
    • Highly conforming but did not allow femoral rollback.
    • Tibia initially all-polyethylene; later designs included metal-backed tibias.
    • Posterior femur impinged on poly during flexion, limiting flexion to <100°.
    • Resulted in flexion instability and loosening due to high conformity and impingement.
    • Despite limitations, the design achieved good outcomes.

Modern TKR Design

Focus: Achieving “normal” femoral rollback.

Two Methods

  1. Retain the PCL:
    • PCL tension during flexion prevents anterior femoral translation.
    • As the PCL angle changes, the femur rolls back.
  2. Substitute the PCL:
    • CAM and post mechanism replicate femoral rollback:
      • CAM engages the post, pushing the femur backward.
      • Post also prevents anterior instability.

PCL Retaining

Advantages

  • Less bone resection.
  • More “normal” rollback pattern (more sliding since ACL is resected).
  • Less constraint (flatter poly allows rollback), reducing the risk of loosening.

Disadvantages

  • Technically challenging to balance the knee:
    • PCL contracture can limit balancing in severe deformities.
  • Rollback is achieved through sliding, increasing poly wear.
  • Less constraint increases contact stress on the poly:
    • Newer designs use more conforming poly.
    • PCL acts more as a static stabilizer.
    • Flexion is achieved by restoring slope and posterior offsetting of the COR.

PCL Substituting

Advantages

  • Suitable for almost all knees.
  • More conformity reduces edge loading, sliding, and wear.
  • Post provides varus-valgus stability.

Disadvantages

  • Higher conformity may transfer more stress to the implant-bone interface:
    • Theoretical risk of loosening (not proven in practice).
  • More bone resection required.
  • Not ideal for hyperextending knees:
    • Risk of post fracture or wear.
  • CAM Jump: Can occur in high flexion or loose flexion gaps.
  • Patella Clunk Syndrome:
    • Soft tissue impingement in the notch, causing a clunk.

Indications for PCL Retaining vs. Substituting

PCL Retaining

  1. Knees with significant hyperextension.
  2. Patients requiring >130° flexion (e.g., certain cultural activities):
    • Avoids risk of CAM jumping.

PCL Substituting

  1. Post-patellectomy:
    • Weak quads increase the risk of anterior femoral translation.
  2. Severe deformities:
    • Non-functioning PCL.
  3. Inflammatory arthritis:
    • Potential PCL disease or incompetence.
  4. Past PCL injury.

Mobile Bearing/Rotating Platform Knees

  • Cruciate sacrificing design with more conforming poly.
  • The mobile poly adjusts during flexion and extension:
    • Accommodates minor component positioning errors.
    • Maintains better conformity throughout the ROM.
    • Reduces sliding, high contact stresses, and wear.

Unique Problem: Poly Spinout

  • Poly moves beyond the knee’s intended constraints and dislocates anteriorly:
    • Occurs due to a loose flexion gap.
    • Requires revision if it occurs.

Outcomes

  • Results are comparable to fixed-bearing designs but not significantly superior.
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