How does the stem design affect the degree of freedom of the stem version and position? A computer simulation study

Controlling stem anteversion is crucial in total hip arthroplasty to prevent prosthetic impingement. The degree of freedom in stem version varies depending on the stem design. However, few studies have quantified the version freedom across different stem designs. This study aimed to quantify the differences in the version freedom and position according to stem version changes between tapered-wedge and fit-and-fill stems.

Fifty-one hips were examined using preoperative CT. Three-dimensional templating was performed using tapered-wedge and fit-and-fill stems. The difference between the maximum and minimum stem anteversions was defined as the version freedom. Stem alignment changes in the coronal and sagittal planes and stem depth changes according to the changes in stem version were examined. Changes in the version with respect to the native femoral version were also compared.

The mean version freedom for the tapered-wedge stem (21.7°) was significantly greater than that for the fit-and-fill stem (9.8°, P < 0.0001). The stem alignment changes of the tapered-wedge stem in the coronal and sagittal planes were significantly greater than those of the fit-and-fill stem, with no significant difference in the stem depth change. The version freedom significantly correlated with stem alignment changes. Both stem designs more easily increased stem anteversion but were more limited in decreasing it relative to the native femoral anteversion.

Stem design affects the version freedom. Increased adjustability of the tapered-wedge stem may improve implant positioning. Meticulous surgical planning and precise intraoperative control remain crucial to achieving target anteversion and ensuring optimal long-term outcomes.