The results presented in this study provide a basic understanding of the range of forces occuring during the insertion of femoral stems (type Sportono, model CBC Evolution, Mathys Ltd.) and forces leading to an IPFF. Two cadavers could be treated bilaterally. A third screened cadaver had proximal femur fractures treated on both sides with osteosynthesis material still in place. Therefore, biomechanical testing could not be carried out in this case. Due to the small sample size, the observed values are only indicative, as there are many influences such as age, gender, secondary diseases, implant- and surgeon-specific differences and especially the bone mineral density (BMD) [17–20]. As can be seen from the results of the failure test, the fracture did not always occur at the maximum peak force. Only in implantation 4, the maximum peak force was equal to the failure force. This suggests that the fracture is caused by the interaction between the number of blows and the force level. Therefore, in our opinion, the most important aspect is the presentation and classification of the force ranges of the two different testing methods.
Only a few studies described a similar project or provided comparable data. Both Tijou et al. and Dubory et al. conducted similar biomechanical studies [21, 22]. However, they focused their evaluation on the correlation between the number of strokes recommended by a surgeon and by a support system presented by them. Therefore, they did not provide force data [21, 22]. Sakai et al. also investigated the forces required for the insertion of a femoral stem. However, they carried out the investigations on artificial composite femurs and had a much stiffer bearing of the bone than is the case in situ. No information was given on the weight of the hammer. It can be assumed that the stiffer bearing led to the higher observed forces of 9.25 kN ± 1.71 kN . In comparison, an average of 3.06 kN ± 0.63 kN and a maximum of 4.35 kN, i.e. less than half, were determined in our experiments when the stem was inserted. Sakai’s submitted forces are even higher than those observed for the failure tests in our study. Therefore, the data were barely comparable.
It is crucial for the success of inserting femoral stems not to exceed the transition point from higher initial stability to fracture. The data given in this study showed a significant difference between the peak forces required for stem insertion and those led to a fracture. Furthermore, between 6 and 54 blows were required for a perceptible fracture after adequate stem stability was established. Therefore, the risk seems to be rather low. But, the fractures were only detected visually and acoustically by the surgeon. However, in many cases, intraoperative fractures due to THA can only be detected via postoperative radiography . Therefore, it is preferable, on the one hand, to provide the surgeon with support systems that recognise a sufficiently high initial stability and, on the other hand, to protect the patient from intraoperative femoral fractures. There are already various concepts for this. Sakai et al. described a close correlation between changes in the sound frequency and the initial stability of stems and the occurrence of fissures in artificial femurs . The finite element analyses described by Sakai et al. were able to show that a decrease in the tone frequency at the hammer is related to an increase in the estimated maximum stress in the femoral shaft. When a decrease in frequency was observed, adequate initial stability of the stem occurred and there was a risk of fracture when hammering was continued. Based on the relationship between stress and frequency, evaluation of frequency shifts can be useful to prevent intraoperative fractures . Another method was described by Tijou et al. who used a hammer equipped with a force sensor to monitor the insertion of femoral stems . The insertion depth of the stems were recorded using video motion tracking. Furthermore, the number of strokes and forces at the hammer were recorded. By calculating and evaluating two parameters, their approach allowed them to determine the number of strokes at which the surgeon should stop insertion to obtain optimal initial stability. This study can also be a basic element in the development of a decision support system to assist the surgeon while performing THA .
This is a small case series resulting from the limited resources of suitable cadavers, costs for the examinations and time-limited availability until cremation. Therefore, only two cadavers could be treated. As a further consequence of time-limited availability, the BMD of the cadavers, as a factor influencing failure behaviour, could not be determined.
Furthermore, the surgeon considered the striking pad of the hammer used to be too small. As a result, occasionally the impactor tool was only partially hit, which caused a too small force measured by the sensor. However, this influence was largely eliminated, as the measurement only started when a force of 500 N was exceeded due to the software.