Background: Previous studies of the patellofemoral joint have been
limited by the use of invasive techniques, measurements under
non-weight-bearing conditions, cadaveric specimens, or computerized models. It
has been shown that soft tissue and bone can be accurately quantified with
magnetic resonance imaging. The present study was designed to define the
relationship between the patellofemoral contact area and patellofemoral
kinematics in vivo.
Methods: Ten subjects with clinically normal knee joints were
scanned with high-resolution magnetic resonance imaging while they pushed a
constant weight (133 N) on the foot-plate of a custom-designed load-bearing
apparatus. Images were obtained at five positions of flexion between
—10° and 60°. Three-dimensional reconstructions were used to
measure the patellofemoral cartilage contact area, patellar centroid, patellar
medial and inferior translation, patellar medial and inferior tilt, and
patellar varus-valgus rotation. All translation and area measurements were
normalized on the basis of the interepicondylar distance. Random-effects
models of quadratic regressions were used to evaluate the data.
Results: The mean contact area ranged from 126 mm2 in
extension to 560 mm2 at 60° of flexion. The patella translated
inferiorly to a maximum distance of 34 mm at 60° of flexion and translated
medially to a maximum distance of 3.2 mm at 30° of flexion before
returning to nearly 0 mm at 60° of flexion. The patella tilted inferiorly
to a mean of nearly 35° at 60° of flexion and medially to a maximum of
4.2° at 30° of flexion. By 60° of flexion, the centroid of the
contact area had shifted to an inferior and posterior maximum of 20 and 10 mm,
Conclusions: We found that lateral patellar subluxation and tilt
occurred in these normal knees at full extension and the patella was reduced
into the trochlear groove at 30° of flexion. Therefore, we believe that
lateral patellar tilt and subluxation observed during arthroscopy of the
extended knee may not represent a pathological condition.
Clinical Relevance: This study may be useful for refining
arthroplasty design and surgical technique.