Initial stability of a new cementless fixation method of a tibial component with polyaxial locking screws: a biomechanical in vitro examination.

INTRODUCTION: Cementless fixation of the tibial component is critical as reduced initial stability leads to implant failure. In this experimental in vitro study, a new fixation method of the tibial component using polyaxial locking screws is evaluated using Roentgen stereophotogrammetric analysis (RSA). MATERIALS AND METHODS: A special prototype of a tibial component with four polyaxial locking screws was tested on 10 fresh-frozen human tibia specimens. The components were tested with an axial load of 2000 N for 10,000 cycles. Radiographs in two views were performed before loading, after 1000 and after 10,000 cycles, respectively. Besides rotation and translation along the x-, y-, and z-axes, endpoints for RSA were maximum subsidence (MaxSub), maximum lift off (MaxLiftOff) and maximum total point motion (MTPM). RESULTS: MaxSub increased from -0.5 mm (SD = 0.2) after 1000 cycles to -0.9 mm (SD = 1.1). MaxLiftOff was 0.1 mm after 1000 cycles and did not increase after 10,000 cycles. The MTPM was 0.7 mm (SD = 0.3) after 1000 cycles and 1.1 mm (SD = 1.1) after 10,000 cycles. Two out of nine implants showed an MTPM ≥ 1.0 mm after 10,000 cycles. CONCLUSIONS: Polyaxial locking screws can potentially improve the initial stability of tibial components. The results of this study indicate that the use of such screws in total knee arthroplasty may be of interest in the future. Further experimental and clinical investigation is needed.