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Magnetic Resonance Imaging of Patellofemoral Kinematics with Weight-Bearing

¹ Department of Orthopaedic Surgery, University of California, 400 Parnassus, ACC-3, San Francisco, CA 94117
² Magnetic Resonance Science Center, University of California, San Francisco, 1 Irving Street, AC109, San Francisco, CA 94143. E-mail address for S. Majumdar: sharmila.majumdar{at}radiology.ucsf.edu
³ Biostatistics Core, UCSF Comprehensive Cancer Center, University of California, 400 Parnassus, MU 423 West, San Francisco, CA 94117

Investigation performed at the University of California, San Francisco, San Francisco, California

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from the National Institutes of Health, the Orthopaedic Research and Education Foundation, and the American Association of Hip and Knee Surgeons. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

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, respectively.

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.

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