'Movement Patterns of the Knee Relevant to TKR' - Derek McMinn, London Knee Meeting, October 2014

In this talk, given in October 2014 at the London Knee Meeting, Professor Derek McMinn talks through the kinematics of the natural knee joint in flexion. Patella movement and maltracking can cause pain to patients and in this talk, Mr McMinn explains how a spiral condyle designed knee replacement, such as the Birmingham Knee Replacement (BKR), closely matches the natural kinematics of the knee to combat this problem.

Knee movement patterns have been a hotly debated topic for many years. John Goodfellow and John O’Connor have contributed massively to our understanding of the knee joint. Goodfellow described the position of the femur on the tibia as indeterminate, meaning ‘not exactly known, established or defined.’ Michael Freeman came to the opposite conclusion, precisely defining the movement of the femur on the tibia.

An MRI scan of the knee joint in full flexion shows how the lateral femoral condyle comes down the back of the tibia. At 140 degrees of flexion the medial side has also gone to the back of the knee, but not as great as the lateral side. This is a good example of a ‘J’ shaped femur with a small flexion radius and a larger extension radius. Movement on the medial side from 20 degrees to 110 degrees of flexion shows the femur does not go back or forwards. From 110 degrees to 160 degrees of flexion, the femur goes back 10mm. On the lateral side, there is a much more gradual movement back of the femur on the tibia with flexion.

Medial rotation with flexion shows that the medial femoral condyles stay still and the lateral condyle comes back in flexion and forwards in extension. Lateral rotation with flexion shows that the lateral condyle stays still and the medial side goes back in flexion and forwards in extension. Central rotation shows reciprocal front and back movement of each femoral condyle on the tibia. Rollback with a different loading regime shows that both femoral condyles move back in flexion and forwards in extension.

Both femoral condyles are spiral which induce a side–to-side translatory movement caused by a ‘screw in a nut’ effect, something which Goodfellow recognised could happen. In a paper published by the Rizzoli institute, it was found that during flexion, the femur translates about 5mm laterally on the tibia. This is important for patella positioning. If the femur and the patella groove do not translate laterally in flexion, the patella groove will not be under the laterally sited patella and maltracking will occur.

With standard knee replacements over the years, Mr McMinn found that on average there was 10mm of lateral patella maltracking. This was investigated at the Rizzoli institute on twelve cadaver legs with trackers on the tibia, patella and femur. With the normal cadaveric knee, the patella moves from its position in extension and with flexion it gradually moves laterally to its final position in full flexion. The same test carried out on the standard knee replacement showed the patella moved from its position in extension and with increased flexion, the patella moves gradually medially. There is approximately 10mm difference between where the patella started in flexion to where it ended up after a standard knee replacement. This puts tension on the lateral soft tissue as the patella is 10mm medial to where it was previously – causing pain to patients. Onto the femoral same cuts, a spiral condyle femoral design was placed and the patella tracking test re-done. The results showed that the patella ended up where the patella started in the normal knee.