Design of knee replacement- Can we approach normal knee function? Derek McMinn 2014
In this talk, presented May 2014 at the 6th Advanced Hip Resurfacing Course in Ghent, Mr Derek McMinn MD FRCS looks at knee replacement design and how that design influences joint function.
John Goodfellow and John O'Connor came to the conclusion that the position of the femur on the tibia is indeterminate whereas Michael Freeman found the opposite by precisely defining said positioning. Mr McMinn shows the medial, lateral and central rotation movement of the femur using fluoroscopy and a cadaveric knee. The natural femoral condyles of the knee are spiral, therefore inducing a side to side translatory movement during flexion and extension. A study shows that on twenty two cadaver knees during flexion, the femur translates laterally approximately 5mm on the tibia. This is vital for the patella as maltracking will occur if the femur and the patella groove do not translate laterally in flexion.
Mr McMinn looks at the flexion statistics of the PCL Retaining Total Knee Replacement. A study shows that in a group of 150 patients, seventy two percent of knees had impingement between the back of the femur and the back of the tibial polyethylene. The mean pre-operative range of flexion was 105° and post-operative was 105.9°. For every 2mm decrease in posterior condylar offset, the maximum flexion was reduced by 12.2°.
The major disadvantage of the Posterior Stabilised Total Knee Replacement is gross anterior to posterior mid-flexion instability. The Medial Rotation Total Knee Replacement is good in mid-flexion but not in high flexion where the femur slides forward on the tibia leading to impingement. In high flexion, the Birmingham Knee Replacement (BKR) brings the femur to the back of the tibia. The BKR has spiral femoral condyles, matching the natural kinematics of the knee. The combined static and dynamic effect is 10mm, laterally translating the femur in flexion and the opposite in extension.
Results for seventy nine BKRs (in seventy two patients) show the highest Oxford Knee Score of 12 for follow up – excluding ten patients whose inferior scores were due to other pathologies. Knee flexion results show a 21° post-operative improvement in range of flexion. Maximum walking speed is slower for patients with a standard knee replacement (6.5km/h) and the loading through the replaced side does not match the normal side. Comparatively, patients with a BKR have a faster maximum walking speed of 11km/h and the loading closely matches that of the normal knee.
National Joint Register PROMs data shows that nearly fifteen percent of Total Hip Replacement and nearly thirty percent of Total Knee Replacement patients are not much better since their operation. There is a lot of improvement needed in the design of knee replacements in order to achieve better function for knee replacement patients.