Background: Little information is available on the kinematics of the normal knee in deep flexion. The purpose of this study was to use magnetic resonance imaging to analyze the patellofemoral articulation in deep flexion.
Methods: Axial scans were made of the patellofemoral joint of twenty healthy Japanese volunteers with the knee in approximately 90° of flexion, in maximum active flexion (mean [and standard deviation], 140° ± 10°), and in maximum passive flexion (mean, 156° ± 5°). A fat-suppressed, three-dimensional, fast low-angle shot sequence was used to visualize the articular cartilage. The patellofemoral contact area was determined on sequential images and was reconstructed three-dimensionally.
Results: At 90° of flexion, the contact area on the patella was continuous over the medial and lateral facets in fourteen knees and was located in the proximal half of the articular surface. At maximum active and passive flexion, the odd facet engaged in fifteen and eighteen knees, respectively. At maximum passive flexion, the contact area of the lateral facet moved distally and decreased significantly (p = 0.0002). From 90° of flexion to maximum active flexion, the mean total contact area remained constant (3.43 ± 0.70 and 3.62 ± 0.72 cm
2 , respectively); it then decreased significantly in maximum passive flexion (2.96 ± 0.78 cm
2 , p = 0.04).
Conclusions: The contact area on the patella was divided into two parts (the odd and lateral facets) and moved distally in deep knee flexion. The size of the contact area on the lateral facet significantly decreased in maximum passive flexion.
Clinical Relevance: This study provides information that may be useful for the design of a knee prosthesis that permits a greater range of motion. Design modifications that include a smoother shape of the distal intercondylar notch to increase the contact area may be beneficial.