INTRODUCTION: Patellofemoral dysfunction following total knee replacement
(TKR) is significant. Due to the intimate relationship of the femoral component
and patella, aspects of knee arthroplasty such as femoral component malrotation
and patellofemoral overstuff have been sited as predisposing factors to such
complications. The principal structures of the extensor retinaculum act as a
checkrein for the patella as it tracks over the femur. Little biomechanical
information is available on the behaviour of these structures, or the effects of
TKR on them.
AIM: The aim of this thesis was to measure retinacular behaviour in the normal
knee and following TKR. We hypothesise that TKR will cause significant
elongation of the retinacula and this would be further deranged with the addition
of femoral component malrotation and patellofemoral overstuff. METHODS:
Retinacular length changes were measured by threading fine sutures along the
retinacula and attaching these to displacement transducers. The intact knee was
flexed-extended on a custom built rig, while the quadriceps were tensed.
Measurements were repeated post-TKR (Genesis II CR, Smith & Nephew Co.),
following internal/external rotation of the femoral component 5° and finally
altering the resurfaced patellar thickness by 2mm increments. RESULTS: The
medial patellofemoral ligament (MPFL) was close to isometric, whereas the
lateral retinaculum slackened significantly with knee extension. TKR did not
cause statistically significant elongation of the retinacula. Internal rotation of the femoral component resulted in the MPFL slackening whereas external rotation
resulted in the MPFL tightening as the knee extended. The lateral retinaculum
showed no significant differences. Overstuffing the patellofemoral joint caused
significant stretching of the MPFL at all angles of knee flexion, but very little
change in the lateral retinaculum.
CONCLUSION: This work has shown a correctly positioned TKR does not cause
significant retinacular length changes sufficient to affect knee function. It has
shown that small changes in femoral component rotation and patellofemoral
overstuff of 4mm cause significant changes, particularly in the medial structures
and not the lateral structures, contrary to current understanding. This work has
described for the first time how the lateral retinaculum’s mobile attachments allow
its principal fibres to move anteriorly and posteriorly with the patella, taking up
any slack/tension produced by abnormal patellar shift/tilt. This work provides
important insight into the contribution of the retinacula to patellofemoral
biomechanics after knee replacements and may help in developing a more soft
tissue friendly knee prosthesis
