Total knee arthroplasty reduces knee extension torque in-vitro and patellofemoral arthroplasty does not

Patients often have difficulty recovering knee extension strength post total knee arthroplasty (TKA), and that may reflect alteration of the mechanics including geometry and rollback kinematics, so the purpose of this work was to explore this by comparing the knee extension torque (KET) of the native knee, TKA and patellofemoral arthroplasty (PFA) in response to quadriceps tension. Eight fresh-frozen knees were mounted in a knee extension rig with quadriceps loading and tibial extension torque measurement. Each knee was subject to four conditions: native knee, PFA, cruciate-retaining (CR) and posterior-stabilized (PS) TKA. The KET was measured from 120° to 0° knee flexion. Data were analyzed using one-way ANOVA and post-hoc paired t-tests. The native KET was lowest in terminal extension and 70-100° flexion, and maximal at 20-30° flexion. PFA produced the greatest KET (p < 0.008) compared with native, CR- and PS-TKA, at 30-40° flexion. CR- and PS-TKA had lower KET across 0-50° flexion (p < 0.001 across 0-30°), falling to 25% of the native knee KET or the PFA at full extension. PFA had the highest KET in early flexion possibly due to increased trochlear offset and/or preservation of the cruciate mechanism, so PFA may be more beneficial during the functional range of motion. The claimed benefits of PS- over CR-TKA in deep flexion were not detected. Both CR- and PS-TKAs led to lower KET than the native and PFA knee states across 0-50° flexion. This mechanical effect may help to explain clinical findings of knee extension weakness post-TKA.