This work has shown a novel way by which computer navigation can be used to analyse soft tissue behaviour during TKA beyond the coronal plane and throughout range of motion. Despite subjective stress testing, our results show reproducible patterns of soft tissue behaviour-in particular a wide range of mid-flexion excursion. It also quantifies the limits within which a cruciate-retaining TKR can maintain knee stability. This functionality may guide the surgeon in identifying and/or preventing soft tissue imbalances intra-operatively, improving functional results.

It was concluded that femoral rotation alone could not restore all aspects of both patellar and tibial kinematics to normal with this specific implant. The clinical relevance of this is that it appears to be inadvisable to reposition the femoral component, in an attempt to improve patellar tracking, if that repositioning may then cause abnormal tibiofemoral kinematics. Further, the pattern of patellar tracking, with the type of TKA used in this study, could not be adjusted to normal by femoral component rotation.

Patellofemoral dysfunction following total knee replacement (TKR) is a significant clinical problem, but little information exists on the mechanics of the patellofemoral retinacula or the effects of TKR on these structures. We hypothesized that TKR would cause significant elongation of the retinacula. Retinacular length changes were measured by threading sutures along the retinacula, fixing the sutures to the patella and the iliotibial band (ITB), and attaching the femoral ends to displacement transducers. The intact knee was flexed-extended while the quadriceps and ITB were tensed and the retinacular length change patterns were recorded. The measurements were repeated post-TKR. The medial patellofemoral ligament (MPFL) was close to isometric, stretching 2 mm in terminal knee extension, whereas the lateral retinaculum slackened 8 mm from 1108 to 08 flexion. TKR did not cause significant elongation of either of the retinacula, the largest change being 3 mm elongation of the MPFL around 408, which stretched the MPFL by 1.4 mm above its maximum natural length. Thus, this work did not support the hypothesis that TKR causes significant elongation of the retinacula sufficient to affect knee function. Keywords: knee; patellofemoral; retinaculum; TKR; length change patterns Demand is increasing for improved knee function after surgery. Despite continuing evolution in prosthesis design and surgical methods, stiffness after total knee replacement (TKR) remains a disabling complication. The prevalence appears to be relatively low, but has not been clearly defined, primarily due to ambiguity in the literature concerning its definition. The prevalence of knee stiffness was recently reported as 1.3%-3.7%, 1,2 though higher levels (8%-12%) were reported in older literature. 3Stiffness after TKR may be caused by limited preoperative motion, biological predisposition, intraoperative technical problems, poor patient motivation, and inadequate postoperative rehabilitation.1,4--6 During knee motion, tensions in the ligaments, capsule, and extensor envelope may restrict flexion and affect implant wear.6,7 Awareness of the role of the extensor retinaculum in patellofemoral kinematics is increasing.8 While studies to date have focused on its role in patellar instability, [8][9][10][11] the influence of TKR is unknown. The positioning and thickness of TKR components may cause the surrounding soft tissues to be stretched. Abnormal soft tissue tensions post-arthroplasty may alter patellofemoral kinematics and stability; the literature shows that these mechanical factors can lead to clinical problems such as pain, stiffness, and excessive wear. Our aims were: to measure the length change patterns of the medial and lateral retinacula during knee extension; and to test the hypothesis that insertion of a TKR would cause significant elongations of the retinacula. MATERIALS AND METHODS Specimen PreparationEight right knees aged 69 years (range, 57-87 years) and including 20 cm of femur and 15 cm of tibia were obtained from a tissue b...