Computer Assisted Mechanical Axis and Kinematic TKA
Authors: McEwen Peter , Mahoharan Varaguna 
Published: March 2016

Introduction:

Total knee arthroplasty (TKA) has traditionally been and largely continues to be aligned mechanically, that being with a neutral coronal plane mechanical tibiofemoral axis and a joint line orientated at 900 to this axis. Femoral component rotation is set by gap balancing or by externally rotating 30 from any of a number femoral reference lines. This produces a rectangular flexion gap and relaxes patellar tracking. Kinematic alignment (KA) is an alternative technique that aims to restore premorbid alignment, joint orientation and ligament tension. The basic premise for this technique is based on evidence that the medial and lateral femoral condyles consistently equate to cylinders of equal or near equal size and that therefore with a fixed radius, cruciate retaining implant, matched distal femoral, posterior femoral and proximal tibial resections, accounting for bone and cartilage already lost will reproduce the premorbid joint line and restore native premorbid kinematics. Femoral rotation is therefore referenced off the prearthritic posterior condylar axis (PCA) that is on average internally rotated to the AP axis. Kinematic alignment therefore has the potential to challenge patellar tracking, increase patellar load and potentially increase patellar complications.

Method:

Case control study – level of evidence III-2. Between November 2012 and June 2013 the senior author completed 104 consecutive computer assisted (CAS) kinematically aligned total knee arthroplasties (TKA) with a cruciate retaining, fixed bearing, single radius implant. The results of these surgeries were compared with the results of 91 consecutive CAS mechanically aligned TKA done between November 2011 and October 2012 using the same navigation system and implant Implant sizing and positioning as well as gap measurement and ligament balance was done with computer assistance in all cases. Data was collected prospectively and analysed retrospectively.

Results:

The Oxford Knee Score improved from 22.2 preoperatively to 42.3 postoperatively in the kinematic group and from 21.5 to 42.1 in the mechanical group. Improvements in the KOOS domains were similar between the two groups except for the sport domain that favoured the kinematic group. Postoperative range of motion was slightly better in the mechanical group. On average there was a difference of femoral component rotation of 4.50. In the kinematic group femoral position was 2.30 internally rotated relative to the AP axis (range 10.50 IR to-6.50 ER) whereas in the mechanical group femoral position was 1.20 externally rotated (range 50 IR to 90 ER. There were six patellofemoral complications in five patients in the kinematic group requiring four additional procedures. Complications included patellar stress fracture / AVN in four patients, one acute patellofemoral dislocation and one chronic patellofemoral dislocation. There were no patellofemoral complications in the mechanical group.

Conclusions:

The overall results of CAS kinematic and mechanical TKA are similarly excellent but patellofemoral complications occur at an unacceptably high rate with strict kinematic positioning. Patellar resurfacing should be undertaken with caution in kinematic TKA. In the context of kinematic conflict between the posterior condylar and AP axes the femoral component should be externally rotated to reduce load on the patella.