Medial tilting of the joint line in posterior stabilized total knee arthroplasty increases contact force and stress.

BACKGROUND: Kinematically aligned total knee arthroplasty is based on the concept to represent the premorbid joint alignment with cruciate-retaining implants, characterized by medial tilt and internal rotation. However, kinematic and kinetic effects of kinematically aligned total knee arthroplasty with posterior-stabilized implants is unknown. The purpose of this study was to examine the effect of medial tilting of the joint line with posterior-stabilized implants.

METHODS: A mechanical alignment model, and medial tilt 3° and 5° models were constructed. Knee kinematics and contact forces were simulated using a musculoskeletal computer simulation model. Contact stresses on the tibiofemoral joint and the post area were then calculated using finite element analysis.

FINDINGS: From 0° to 120° of knee flexion, greater external rotation of the femoral component was observed in medial tilt models (-0.6°, 1.8° and 4.2° in mechanical alignment, medial tilt 3° and medial tilt 5° models, respectively). The peak contact stresses on the tibiofemoral joint and the post area at 120° of knee flexion were higher in medial tilt models. The peak contact stresses on the post area in medial tilt 3° and 5° models were 2.2 and 3.8 times greater than that in mechanical alignment model, respectively.

INTERPRETATION: Medial tilting of the joint line causes greater axial rotation even with posterior-stabilized implants, which can represent near-normal kinematics. However, medial tilting of the joint line in total knee arthroplasty with posterior-stabilized implants may have a higher risk for polyethylene wear at the tibiofemoral joint and post area, leading to subsequent component loosening.