Scientific Articles   |    
Impact of Patellofemoral Design on Patellofemoral Forces and Polyethylene Stresses
Darryl D. D'Lima, MD; Peter C. Chen, PhD; Mark A. Kester, PhD; Clifford W. ColwellJr., MD
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In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from Stryker Howmedica Osteonics. In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Stryker Howmedica Osteonics). One or more commercial entities paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2003 Nov 01;85(suppl 4):85-93
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Background: The patellofemoral joint is a complex articulation because it relies mainly on soft-tissue constraints for stable tracking. The presence of a functioning posterior cruciate ligament and the design parameters of a total knee arthroplasty, such as trochlear groove alignment and cam-post configuration, can have a major effect on patellar biomechanics.

Methods: A finite element model of a knee implanted with femoral, tibial, and patellar components was generated. The model was validated by experimental testing of three cadaver knees implanted with a total knee prosthesis and a patellar force transducer. Two femoral component designs with different valgus angles of the trochlear groove (0° and 7°) were studied. The effects of femoral rollback, rotation of the femoral component, medialization of the patellar implant, and alignment of the trochlear groove on patellar forces were then analyzed.

Results: A consistent reduction of up to 7% in patellofemoral forces was seen with progressive magnitudes of femoral rollback. The 0°-aligned trochlear groove generated some lateral patellofemoral shear force throughout knee flexion. The 7°-aligned trochlear groove generated medial shear force at flexion angles of <20° and lateral shear force at flexion angles of >20°. A more medial patellar component position reduced peak lateral shear forces by up to 10 to 15 N. However, a corresponding increase in medial shear forces was seen.

Conclusions: This model predicted substantial reduction in patellofemoral lateral shear forces with a medialized patellar component or with external rotation of the femur. The model supported the hypothesis that femoral rollback reduces patellofemoral forces by improving the efficiency of the extensor mechanism.

Clinical Relevance: Patellofemoral complications after total knee arthroplasty include anterior knee pain, patellar subluxation and dislocation, abnormal polyethylene wear and damage, and loosening. There is a wide variation in the design features of current total knee prostheses, such as the sagittal radius, depth, and orientation of the trochlear groove of the femur and the geometry of the patellar component surface. The finite element model used in the present study can provide insight into the effects of design parameters on patellofemoral forces and on local contact stresses.

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