Migration and cyclic motion of a new short-stemmed hip prosthesis--a biomechanical in vitro study.

BACKGROUND: Uncemented, short-stemmed hip prostheses have been developed to reduce the risk of stress shielding and to preserve femural bone stock. The long-term success of these implants is yet uncertain. Prerequisite for osseointegration is sufficient primary stability. In this study the cyclic motion and migration patterns of a new short-stemmed hip implant were compared with those for two clinically successful shaft prostheses. METHODS: The prostheses were implanted in paired fresh human femura and loaded dynamically (gait cycle) with increasing load (max 2,100 N) up to 15,000 cycles. Relative displacements between prosthesis and bone were recorded using a 3D-video analysis system. FINDINGS: The short stem displayed a biphasic migration pattern with stabilisation at maximum load. Initial migration was predominantly into varus and was greater than that for the shaft prostheses. Failure occurred in cases of poor bone quality and malpositioning. Cyclic motion of the short prosthesis was less than that for the shaft prostheses. Surface finish showed no effect. System stiffness for the new stem was lower than for the shaft prostheses. INTERPRETATION: The new stem tended to migrate initially more than the shaft prostheses, but stabilised when cortical contact was achieved or the cancellous bone was compacted sufficiently. Bone quality and correct positioning were important factors for the short stem. The lower cyclic motion of the new stem should be favourable for bony ingrowth. The lower system bending stiffness with the new implant indicated a more physiological loading of the bone and should thereby reduce the effects of stress shielding.

Biomechanics of a new short-stemmed uncemented hip prosthesis: An in-vitro study in human bone.

The migration pattern, cyclic motion, system stiffness and failure load of a new short-stemmed hip prosthesis were compared to a clinically successful shaft prosthesis during the initial loading phase. The influence of implant-sizing on mechanical stability was also investigated for the new stem, in particular with relation to the bone quality. Prostheses were implanted in paired human femora and loaded cyclically up to 3515 cycles. Relative displacements between prosthesis and bone were measured using a 3D-camera and reflective marker system. Migration of the new stem was predominantly into varus and was higher than for the shaft prosthesis. The test set-up was proposed to simulate a worst-case loading scenario since muscle forces, which tend to reduce bone deformation, were not simulated. It could therefore be expected that clinical migration of the implants would be less pronounced than that observed in this study. Cyclic motion for the new stem was similar to that for the clinically successful shaft prosthesis, suggesting that bone ingrowth could be expected for the new stem. No significant difference in fracture load was observed between the implants. The system stiffness with the new stem was lower than for the shaft prosthesis, indicating more physiological load transfer. Smaller implant sizing with 'cancellous' fixation seems favourable, as it led to similar migration and smaller cyclic motion values than with 'cortical' fixation. A trend for higher cyclic motion and migration was observed in femora with poor bone quality. Hence, sufficiently good bone stock is necessary, when implanting the new short-stemmed prosthesis.