Influence of stem length on component flexion and posterior condylar offset in revision total knee arthroplasty.

BACKGROUND: Hyperextension of the femoral component and excessive slope of the tibial component may delay the cam-post engagement in semi-constrained revision total knee arthroplasty (TKA). Further, it may compromise the posterior condylar offset (PCO). No prior study has determined whether a short 50-mm stem, or longer stems (100mm and 150mm) lead to less hyperextension of the femoral component or excessive slope and its influence on the posterior condylar offset.

METHODS: Flexion/extension of the femoral component with respect to the sagittal femoral anatomic axis of the distal diaphysis (SFAA) and the tibial slope were measured from rotationally controlled lateral X-rays of 126 consecutive patients with a one- or two-stage revision TKA. Stems of 50mm, 100mm and 150mm were analyzed. Further, reconstruction of the PCO with respect to stem length and component flexion was evaluated.

RESULTS: The average flexion (+) or extension (-) of the femoral component in the sagittal distal anatomical axis was -4.1°±2.8° (mean ± std) for components, which have been implanted with 50-mm stems and 1.1°±1.1° for components with 100- or 150-mm stems, respectively (P=0.0001). The average slope of the tibial was 2.6°±1.5° for tibial trays with 50-mm stems, which was more than the 1.3°±1° for trays with 100-mm stems (P=0.0001). The average PCO ratio was 0.8±1.7 for femoral components with 50-mm stems, which was less than the 1.0±1.7 for trays with 100-mm stems (P=0.0001).

CONCLUSIONS: Because 50-mm stems resulted in about three degree hyperextension of the femoral component with respect to the SFAA compared to 100-mm or 150-mm stems, the longer stems do not alter the natural femoral flexion and a delay of the cam-post engagement may be avoided. Further, a better reconstruction of the PCO may be archived with the use of longer stems.