Is there a true match in the femoral medullary cavity between neutral and flexion alignment of the stem on a female (Asian) population?

BACKGROUND: In total hip arthroplasty, stem alignment has been proposed to occur in the neutral and flexed positions. Nonetheless, there is a paucity of research on the relationship between the shape of the medullary cavity cross-section and its compatibility with the stem shape. METHODS: A total of 216 female (Asian) patients with no history of fracture or surgery on the contralateral side with complete computed tomography (CT) data were included in the study. The plane of the medullary cavity in the neutral (N group) and flexion (F group) alignments was defined. The ratio of the plane of medullary cavity widths at 80 mm and 130 mm distal from the center of the femoral head in these two medullary cavity planes was defined as the modified canal flare index (mCFI). The mCFI was used to assess the compatibility between the configuration of the medullary canal plane and that of the stem. RESULTS: All patients in the N group formed a plane, whereas 58 patients in the F group (26.9%) lacked a clear calcar femorale. Fifteen patients in the F group (9.4%) were excluded because of the inability to insert the stem, leaving 143 patients in the F group. The mean mCFI was significantly lower in the F group (0.692 ± 0.128) than in the N group (0.807 ± 0.073). The mCFI values of frequently used stems were all close to the mCFI of flexion alignment. CONCLUSIONS: The medullary cavity shape differed in the neutral and flexion alignments. Preoperative three-dimensional planning is helpful because the flexion alignment may not allow for the insertion of an optimally sized stem.