In vitro investigation of the influence of tibial slope on quadriceps extension force after total knee arthroplasty

The purpose of this study was to investigate the influence of tibial base plate angulation on knee kinematics and kinetics during knee arthroplasty. The amount of quadriceps force required to extend the knee and the anteroposterior displacement of a mobile bearing insert as well as tibiofemoral position were measured during an in vitro simulation of an isokinetic knee extension cycle. Human knee specimens (n = 7, mean age 62, range 52–75 years, all male) were tested in a kinematic knee simulating machine after total knee arthroplasty (TKA) with a mobile bearing insert prosthesis (Interax^®, Stryker/Howmedica). During simulation, a hydraulic cylinder applied sufficient force to the quadriceps tendon to produce an extension moment of 31 N m about the knee. The quadriceps load was measured using a load cell attached to the quadriceps tendon, the anteroposterior displacement of the mobile bearing insert as well as the relative tibiofemoral position was measured using an ultrasound base motion analysis system (CMS 100^®, Zebris). Quadriceps load, insert and tibial displacement were first investigated with the tibial base plate implanted with a neutral tibial base plate orientation, and subsequently after 10° posterior angulation. The quadriceps forces needed to produce a 31 N m knee extension moment after TKA with neutral slope reached levels as high as 1,391 N (SD 82 N). After applying a posterior slope of 10°, maximum quadriceps force was measured to be up to 1,303 N (SD 34 N, P = 0.04). The mobile bearing insert was observed to move up to 0.1 mm (SD 4.2 mm) anteriorly relative to the tibial base plate with neutral tibial slope, and up to 1.0 mm (SD 4.5 mm, P = 0.47) with tibial slope. Femoral position relative to the tibia moved from a posterior position of 13.1 mm (SD 4.0 mm) anteriorly up to 0.5 mm (SD 6.3 mm), and from 16.0 mm (SD 6.4 mm, P = 0.67) to 9.5 mm (SD 9.9 mm, P = 0.33) with a 10° tibial slope. Posterior slope of the tibial base plate resulted in a more physiologic insert movement with a more posterior position of the femur and reduced quadriceps force especially in knee flexion angles above 60° compared to TKA with a neutral slope of the tibial base plate. Thus, the data suggest that the quadriceps lever arm was improved, which might have positive effect in mobilization of patients after TKA.