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 (100 mm and 150 mm) lead to less hyperextension of the femoral component or excessive slope and its influence on the posterior condylar offset.
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 50 mm, 100 mm and 150 mm were analyzed. Further, reconstruction of the PCO with respect to stem length and component flexion was evaluated.
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).
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.