Inference of hammering force during non-cemented total hip arthroplasty

Abstract

In cementless stem fixation, the strong hammering of the stem required to ensure sufficient fixation force is occasionally complicated by the occurrence of bone fracture. Because small bone fractures that are not visible to the unaided eye or through X-ray radiographs are not detected, however, the reported incidence of small bone fractures is currently underestimated. We determined the amount of stress applied on the femur during cementless stem press-fit fixation using stress measurements and finite element analyses. Employing the same method as that used in total hip arthroplasty, we hammered the stem into artificial femurs which were mechanically equivalent to living femurs. The hammer was attached to a load sensor which allowed us to compare the amount of stress applied on the osseous tissue. Using stress measurements and finite element analysis, we then evaluated the conditions under which bone fractures are likely to occur. Under these conditions, we found that the hammering force caused fracture of the artificial femur. Our measurements showed that the hammering force was more than 9.25 kN and von Mises stress generated by the first and second hammer strikes was 31 and 68 MPa, respectively. The first two hammer strikes caused large displacement, and were therefore unlikely to generate intraoperative fractures. Two strikes are sufficient to emplace stems requiring a large displacement.