Introduction.
The function of the differential nature of rollback that generates a medial pivot in the human knee is unclear and therefore its importance in total knee arthroplasty remains undefined. The affirmative hypothesis of this study was that the medial pivot functions to negate the Trochlear Groove offset (TT-TG) at 90° of knee flexion, when the patella is under maximum compressive load against the trochlea.
Method.
Thirty-three primary cruciate – retaining total knee arthroplasties were completed using an optical navigation system. Digital measurements of TT-TG distance were taken from the most distal point of the Trochlea to the centre of the Tibial Tuberosity with the knee in multiple positions, in both the native and post implantation states. Rotation of the tibia relative to the femur in the axial plane was recorded through the full arc of motion. After testing for normality, continuous variables were analysed for central tendency with a student’s t-test and linear correlations with the Pearson Correlation Coefficient.
Results.
Strong correlations were present between the TT-TG and medial pivot ranges in the native and post-implantation states (r=0.693, p<0.0001 and r= 0.748, p<0.0001 respectively). A very strong correlation was present between native and post—implantation TT-TG ranges (r=0.849, p<0.0001) and a strong correlation was present between native and post-implantation pivot ranges (r=0.766, p<0.0001).
The mean TT-TG at 90° of flexion was 0 mm in both native and post-implantation states (native -0.1mm and post – implantation -0.2mm, p=0.966 (paired students t-test)).
Discussion.
A medial pivot would appear to be necessary for favourable patellofemoral mechanics when the patella is under maximum compressive load. Poor pivot mechanics may be a cause of early patellar component failure or persisting anterior knee pain.
Conclusions.
The medial pivot of the native and replaced knee negates TT-TG offset so that the patella is optimally aligned with the distal trochlea at 90° of knee flexion.