Effect of femoral component designs on the contact and tracking characteristics of the unresurfaced patella in total knee arthroplasty.

OBJECTIVES To determine the effect of 5 different femoral components used in total knee arthroplasty (TKA) on the contact area and tracking characteristics of the nonresurfaced patella and to identify any design features that might adversely affect these characteristics. DESIGN An in-vitro study. SETTING The biomechanics laboratory, Department of Mechanical Engineering, McGill University, Montreal. SPECIMENS Six fresh-frozen cadaveric knee-joint specimens. INTERVENTIONS An unconstrained quadriceps simulator was used to apply the conditions of static lifting to the specimens first in their normal state and then sequentially implanted with femoral and tibial components of various designs (Miller/Galante II, Anatomic Modular Knee [AMK] System, Whiteside Ortholoc Modular, press-fit condylar and Insall-Burstein II). OUTCOME MEASURES Patellar 3-dimensional tracking characteristics, determined by using a 6 degrees-of-freedom electromechanical goniometer attached directly to the patella, and patellar contact pressure measurements, obtained using low-range Fuji Prescale film. RESULTS Articulation of the normal patella on a prosthetic femoral component resulted in alterations in the normal patellofemoral contact and tracking characteristics. The exact departure depended on the design of the prosthetic trochlea. Although all of the selected prostheses demonstrated satisfactory contact characteristics near extension, marked alterations occurred at higher flexion angles. With 90 degrees or more of flexion, there was incompatibility between the geometries of the prosthetic notch of 2 femoral designs (AMK and PFC) and the normal knee. CONCLUSION The design of the prosthetic femoral component must be taken into account when determining whether or not to resurface the patella at the time of TKA.