In vivo comparison of femorotibial contact positions for Press-Fit posterior stabilized and posterior cruciate–retaining total knee arthroplasties

The objective of this study was to determine the in vivo medial and lateral femorotibial condyle contact positions for 20 subjects having either a posterior cruciate–retaining (PCR) or posterior-stabilized (PS) total knee arthroplasty (TKA) while sitting and kneeling. The two-dimensional radiographic images were converted into three-dimensional images using an iterative computer model-fitting technique. Anteroposterior contact positions, axial rotation, and condylar lift-off were assessed for each subject. In a seated position, the femorotibial contact points were, on average, posterior for both TKA groups (PCR: medial = −2.4 mm, lateral = −3.4 mm; PS: medial = −5.1 mm, lateral = −8.9 mm; medial, P=.21; lateral, P=.08). In a kneeling position, the contact position shifted anteriorly for the PCR TKA group (medial = 0.9 mm, lateral = −0.8 mm), whereas the contact positions in the PS TKA group remained posterior (medial = −5.6 mm, lateral = −8.3 mm; medial, P=.002; lateral, P=.0004). It is hypothesized that while in a kneeling position, the posterior cruciate ligament has less resistance to the anterior thrust of the femur relative to the tibia than in a PS TKA, in which this force is absorbed in the cam-and-post mechanism.