The TRAC PS mobile-bearing prosthesis: design rationale and in vivo 3-dimensional laxity.

We present a posterior stabilized mobile-bearing prosthesis, TRAC PS, which has congruent contact from full extension to full flexion, allows for freedom of internal-external rotation, and has an automatic posterior shift in tibiofemoral contact on the tibia to maximize the quadriceps lever arm in flexion. TRAC PS has 2 radii of curvature in the sagittal plane, 1 for the distal femoral condyles and 1 for the posterior femoral condyles, as does the normal knee. The distal and posterior femoral condyles articulate congruently in the inner tracks or the outer tracks of the polyethylene bearing, respectively. Anterior or posterior sliding of the femoral condyles on the bearing or of the bearing on the tibial tray cannot occur, providing inherent anterior and posterior stability. Three-dimensional knee laxity testing was performed on 17 patients from 12 months to 25 months after total knee arthroplasty with the TRAC PS and on 18 healthy control subjects of similar ages. Normal ligament balancing and normal internal-external rotational laxity were achieved with the TRAC PS prosthesis. Anterior and posterior laxity in the patients with TRAC PS was significantly reduced compared with that of the control subjects.