Association Between Stem Anteversion and Femoral Rotation in Endoprosthetic Proximal Femoral Replacement: Insights from Two Different Prosthetic Designs

<b>Background/Objective</b>: Endoprosthetic proximal femoral replacement is a reconstructive procedure for preserving ambulatory function following tumor resection. Different prosthetic systems for endoprosthetic proximal femoral replacement may result in different stem placement techniques, especially regarding the anteversion angle of the stem. The aim of this study was to evaluate femoral rotation and stem anteversion following endoprosthetic proximal femoral replacement using two different prosthetic systems, and to investigate their influence on postoperative quality of life. <b>Methods</b>: We retrospectively reviewed 30 patients who underwent endoprosthetic proximal femoral replacement at our institution between 2008 and 2022. The evaluated parameters included patient demographics, anatomical and functional stem anteversion, femoral rotation, femoral resection length, implant type, and Musculoskeletal Tumor Society score. <b>Results</b>: The cohort comprised 16 males and 14 females with a mean age of 65.2 ± 13.5 years. Twenty patients received the Global Modular Replacement System implants and 10 received the Kyocera Modular Limb Salvage System implants. The mean anatomical stem anteversion was 17.0 ± 17.7°, and the mean femoral rotation was 14.4 ± 22.6°. The Global Modular Replacement System implants demonstrated less variability in anatomical stem anteversion (11.7 ± 15.2°) compared to the Kyocera Modular Limb Salvage System (27.6 ± 18.4°, <i>p</i> = 0.02). A significant negative correlation was found between anatomical stem anteversion and femoral rotation (r = -0.78, <i>p</i> < 0.01), and a positive correlation between femoral rotation and functional stem anteversion (r = 0.62, <i>p</i> < 0.01). Musculoskeletal Tumor Society scores were available in 14 patients and correlated significantly with functional stem anteversion (r = -0.62, <i>p</i> = 0.02) and femoral resection length (r = -0.61, <i>p</i> = 0.02), but not with anatomical stem anteversion or femoral rotation alone. <b>Conclusions</b>: This study demonstrated that stem placement angles differ between prosthetic systems. These differences are attributable to variations in surgical implantation techniques and prosthesis design philosophies. In particular, the Global Modular Replacement System incorporates built-in anteversion, and when using such prostheses, referencing the linea aspera enables more stable restoration of the anatomical stem anteversion. Excessive reduction in anatomical stem anteversion is not recommended to avoid excessive external femoral rotation.