Abstract
Introduction
Initial stability of the tibial component influences the success of uncemented total knee arthroplasty. In uncemented components, osseointegration provides long-term fixation which is particularly important for the tibial component. Osseointegration is facilitated by minimising bone-implant interface micromotion to within acceptable limits. To investigate initial stability, this study compares the micromotion and initial seating of two uncemented hydroxyapatite-coated tibial components, the Genesis II and Profix. This is the first stability comparison of two hydroxyapatite-coated tibial components.
Methods
Six components of each type were implanted into synthetic tibias by a single orthopaedic surgeon. Good coverage was achieved. No screws or articular inserts were used. Initial seating was measured using ImageJ software at five areas on each tibia. Tibias were transected and their proximal section implanted into a molten alloy parallel to horizontal. Dynamic mechanical testing was performed using a hydraulic 858-Bionix machine. Prostheses underwent unilateral axial point-loading of 700N cyclically applied four times. The load was applied to three locations approximating femoral loading points. The loading cycle was repeated six times at each point, allowing micromotion to be recorded at three contralateral locations. Micromotion was measured by optical lasers. After dynamic testing, two tibial components of each type were removed with claw pliers while measuring the force required on the 858-Bionix machine. Implant under-surfaces were photographed for wear.
Results
The micromotion readings allowed a directional (subsidence or lift-off) movement profile to be constructed. The absolute micromotion recordings demonstrated areas experiencing the most micromotion. Micromotion was not significantly different between components (P>0.05). Absolute micromotion during posterolateral loading was significantly different (P<0.05). Loading points producing the most absolute micromotion were antero- and centrolateral in Genesis II prostheses and anteromedial and posterolateral in Profix prostheses. The areas which showed the greatest absolute micromotion were anteromedial in Genesis II prostheses and posteromedial and posterolateral in Profix prostheses. Average absolute micromotion did not exceed 75μm. Initial gap ranged from 535–633 μm in Genesis II prostheses and 631–799 μm in Profix prostheses. Initial gap did not significantly correlate with either prosthesis. Pullout force was significantly different (P<0.0001), requiring less than 75N for Profix prostheses and greater than 150N for Genesis II prostheses. Wear was seen anteromedially in all Profix components. In Profix prostheses the only loading point to consistently produce liftoff was anteromedially.
Conclusions
Average micromotion is not significantly different in Genesis II and Profix trays during point loading central condylar areas in synthetic tibias. With posterolateral loading the Genesis II was significantly more stable. Unilateral loading demonstrated a pivot type micromotion pattern about the tibial stem in both designs. Seating was not a significant factor influencing micromotion, presumably while the initial gap is small (<800μm). The deficit of an anteromedial peg in the Profix prostheses predisposes to liftoff when this point is loaded. Using a force approximating that of walking, distributed through typical femoral loading points, results in micromotion in both designs at a level not expected to prevent osseointegration.
