Prediction of Intraoperative Fracture by Hammering Sound Frequency Analysis and Stress Estimation during Total Hip Arthroplasty

When a stem is inserted into the femur during total hip arthroplasty, sufficient fixation depends on the surgeon’s experience. An objective method of evaluating whether the stem has been correctly fixed may aid clinicians in their decision. We examined the relationship between the sound frequency caused by hammering the stem and the internal stress in artificial femurs, and evaluated the utility of sound frequency analysis to prevent intraoperative fracture. Surgeons inserted one of two types of cementless stems (SL-PLUS and modified CLS) using routine operational procedures into 13 artificial femurs. These are the standard Zweymullers used in Europe. The difference is the lateral shape; SL-PLUS has holes for removal and the modified CLS has fins to prevent rotation. We estimated stress in the femur via finite element analysis, measured the hammering force, and recorded the sound of hammering for frequency analysis. Finite element analysis revealed that the hammering sound frequency decreased as the maximum stress increased. A decrease in frequency suggested that fixation was sufficient and that continued hammering would increase the risk of fracture. Thus, evaluation of the change in sound frequency during stem insertion may indicate when the hammering force should be reduced, thereby preventing intraoperative periprosthetic fractures. Further frequency change may also predict fractures prior to visual confirmation. We concluded that sound frequency analysis has potential as an objective evaluation method to help prevent intraoperative periprosthetic fractures during stem insertion.