Factors that facilitate accurate patella tracking

1 - Design of the femur and patella

• Shape of the trochlea allows unconstrained movement in extension.  The patella is then captured by the groove as the knee flexes to 90°.
  
  
• The patellar dome has a 3mm medial offset.

2 - Position of the femur and patella

• Femoral component positioned in slight external rotation (maximum 4° to 6°) with respect to the posterior condylar line.
  
  
• Femoral component positioned slightly laterally (1-2mm) to the intercondylar mid line. (Whitesides’ Line)
  
  
• Anterior cut parallel to the distal quarter of the anterior femoral cortex to avoid elevating the trochlea.
  
  
• Patella measured prior to resection to ensure appropriate reconstruction of the original patella thickness.
  
  
• Patella cutting jig positioned so patella resection is symmetrical.
  
  
• Residual patella bone thickness of 12-15mm to avoid making it too thin so weakening the patella or too thick so leading to overstuffing. In smaller patients the patella thickness can be reduced to a minimum of 10 mm with the extra small button (to be introduced in the future).

3 - Soft Tissue Releases

• Release of the lateral patellofemoral  synovial fold close to the femur.
  
  
• Lateral retinaculum dissected off the lateral osteophyte of the patella to release the lateral retinacular contracture. (A sub-periosteal Peri-patellar release).
  
  
• At full extension, flip the replaced patella at 90° to the trochlea.  The retinaculum should be loose enough to allow the edge of the patella to reach medial to the mid line of the trochlea groove. (Flip test).
  
  

4 - Assessment of Patellar Tracking

• The medial patella facet should remain in contact with the medial trochlea and femoral condyle throughout the full range of movement.  The patella odd facet will bear against the medial femoral condyle in deep flexion (over 110°).  Ensure there is no impingement as the patella rotates internally at 120° of flexion.  
  
  
• If any tendency is observed for the medial facet to lift off from the femoral trochlea, then a further release of the lateral retinaculum from the border of the patella should be performed.  A mid lateral release is always avoided to prevent damage to the lateral retinacular vessels and causing a soft tissue haematoma.  (This considerably slows recovery).
  
  
• If tracking is not perfect, then a single stitch can be applied to the mid point of the retinaculum and the patella, and the tracking reassessed.  This simulates wound closure.  (Stitch test).
  
  
• If lateral mal-alignment persists then consider tibial tubercular osteotomy and medialisation or revert to a total knee replacement.