A biomechanical study of the Birmingham mid head resection arthroplasty: effect of stem size on femoral neck fracture.

The Birmingham mid head resection (BMHR) arthroplasty can be used as an alternative to conventional stemmed total hip arthroplasty in young patients unsuitable for hip resurfacing. This study investigated the effect of stem size on femoral neck fracture in the BMHR. Sawbones composite femurs were randomly allocated to one of the following groups: (1) unprepared femur with no prosthesis, (2) femur prepared with a Birmingham hip resurfacing (BHR) prosthesis, (3) femur prepared with a BMHR stem size 1 (BMHR-1) and (4) femur prepared with a BMHR stem size 3 (BMHR-3). Each femur was subjected to a compressive force using a materials testing machine until fracture of the femoral neck occurred. The highest force at fracture was in the unprepared femurs with a mean (±standard deviation) force at failure of 5.9 ± 0.2 kN. The mean force at failure for the femurs fitted with a prosthesis was 2.6 ± 0.4, 3.0 ± 0.4 and 3.5 ± 0.5 kN for the BHR, BMHR-1 and BMHR-3, respectively. Statistical analysis showed that the failure force for the unprepared femur was significantly (p<0.05) greater than that of the BHR, BMHR-1 and BMHR-3. There was a significant difference (p<0.05) between the force at failure for the BMHR-1 and BMHR-3, indicating that these two stem sizes have an effect on fracture force.