Small interprosthetic gaps do not increase femoral peri-prosthetic fracture risk. An in vitro biomechanical analysis.

It has been hypothesized that the interprosthetic gap between ipsilateral hip and knee replacements acts as a stress riser affecting bone fracture behaviour. The aim of this study was to quantify femoral strength and fracture morphology for a wide range of interprosthetic gaps. Seven interprosthetic gaps (0-20cm) were created in artificial femora (N = 6-9/group). All specimens were loaded to failure following a compressive loading protocol. Fracture load and fracture morphology were recorded. Outcomes were compared to femora with a hip implant only (N = 6; reference group). Fracture load was highest for 0 cm gaps. All other interprosthetic gaps had fracture loads similar to that of the reference group. Fracture occurred most frequently with a medial butterfly fragment located at the tip of the hip stem.We conclude that small gaps do not act as stress risers. The specific fracture morphology may benefit from different treatment than peri-prosthetic hip fractures.