Effects of patellar thickness on compression and shear forces in total knee arthroplasty

Ten unembalmed cadaveric knees were dissected to the capsule preserving the ligamentous and tendinous structures. A posterior cruciate ligament sparing total condylar knee arthroplasty was implanted routinely. A force transducer that measured compression force and shear was implanted into each patella. For each knee, 3 thicknesses of the patellar composite (osteotomized patella, transducer, polyethylene component) were evaluated: (1) precut patellar thickness, (2) precut plus 2 mm, and (3) precut plus 4 mm. The knees were tested in an Oxford Knee Testing Rig, which allowed dynamic testing with 6 degrees of freedom. Values of patellar forces were obtained throughout a range of motion of 0 degrees to 95 degrees flexion. At higher flexion angles (45 degrees and above), the total patellofemoral shear forces for the precut plus 2 mm and the precut plus 4 mm composites were altered significantly from the precut thickness. Increasing the patellar thickness results in a significant increase in shear forces, potentially leading to early loosening of the component or increased wear or both. Therefore, bone conserving resections that increase the patellar composite thickness above the precut thickness should be avoided.