In vivo determination of condylar lift-off and screw-home in a mobile-bearing total knee arthroplasty.

Twenty subjects implanted with the low-contact stress (LCS) cruciate-sacrificing, mobile-bearing total knee arthroplasty underwent dynamic videofluoroscopy during in vivo weight-bearing conditions using a 3-dimensional computer-aided design (CAD) interactive modeling method. Ninety percent of the subjects demonstrated significant lift-off during stance phase of gait. Condylar lift-off was present at both the medial and the lateral condyles. The maximal medial lift-off was 2.12 mm, whereas the greatest lateral lift-off was 3.53 mm. The maximal positive screw-home was 9.6 degrees, whereas the maximal negative or reverse screw-home was 6.2 degrees. The average screw-home rotation was positive 0.5 degrees. In 50% of patients, medial condylar translation was unexpectedly greater than lateral condylar motion. Condylar lift-off and screw-home motion are significant kinematic functions in this rotationally unconstrained total condylar knee arthroplasty.