SUMMARY Malrotation of the tibial component can lead to complications after total knee arthroplasty (TKA). Despite reports of internal rotation being associated with more severe pain or stiffness than external rotation, the biomechanical reasons remain largely unknown. We used a computer simulation model and evaluated traction forces in the lateral collateral ligament (LCL) and medial collateral ligament (MCL) with a malrotated tibial component during squatting. We also examined tibiofemoral and patellofemoral contact forces and stresses under similar conditions. A dynamic musculoskeletal knee model was simulated in three different constrained tibial geometries with a prototype component. The testing conditions were changed between 15° external and 15° internal rotation of the tibial component. With internal rotation of the tibial component, the MCL force increased progressively; the LCL force also increased, but only up to less than half of the MCL force values. A higher degree of constraint of the tibial component was associated with greater femoral rotational movement and higher MCL forces. The tibiofemoral and patellofemoral contact forces were not influenced by malrotation of the tibial component, but the contact stresses increased because of decreased contact area. This altered loading condition could cause patient complaints and polyethylene problems after TKA. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1658–1666, 2014.