Relationship between implant fit-and-fill and the mechanical stability of a custom-made osseointegrated transfemoral prosthesis

Osseointegrated prostheses are a promising alternative for transfemoral amputees, offering a more physiological load transfer than socket-type prostheses. Their use is limited by complications and anatomical eligibility, as most commercial intramedullary stems are produced in a restricted set of standard circular sizes that do not reflect the variability of femoral canal morphology. A custom-made stem (OsteoCustom) was therefore developed to improve fit-and-fill. This thesis investigated how post-operative morphometric conditions of the residual femur and stem design influence primary stability in transfemoral osseointegration. An imaging-based analysis quantified cortical bone volume removal, sector-wise cortical thickness reduction and distal stem-bone contact area, and related these CT-derived indicators to primary stability metrics (inducible micromotions) from a previous in vitro study. Human cadaveric femurs were implanted with OsteoCustom; contralateral specimens, when anatomically eligible, received a commercial stem (OTN). Pre- and post-implant CT scans were processed to extract parameters. OsteoCustom tended to require less distal cortical volume removal than the commercial stem, while preserving comparable proximal bone volume. Sector-wise analysis showed better preservation of thickness, especially in lateral and antero-lateral sectors. Manyfemurs that would have fallen below the commonly adopted 2 mm cortical thickness threshold with a commercial stem could be safely implanted with OsteoCustom, expanding the range of treatable anatomies. Distal stem–bone contact, particularly at the osteotomy level, was higher for OsteoCustom and emerged as a stronger predictor of smaller micromotions than cortical volume removal. Overall, this work suggests that a custom-made stem can improve fit-and-fill by preserving a safer cortical shell and increasing effective stem–bone contact, with potential benefits for primary mechanical stability and patient eligibility.