Intertrochanteric fracture of the proximal femur was a common osteoporotic fracture in the elderly[17, 18]. The optimal treatment and implant device selection for the intertrochanteric fracture remained controversial[19, 20]. DHS was a common implant used to treat stable-type intertrochanteric hip fractures. However, a few studies reported that the DHS should be avoided in the intertrochanteric fracture with a broken LFW for the high incidence of reoperation[5, 4]. Furthermore, Hsu.et al reported that the lateral wall thickness was a reliable predictor of post-operative lateral wall fracture and concluded that intertrochanteric fractures with a lateral wall thickness < 20.5 mm should not be treated with DHS alone. Another study also showed that the lateral wall thickness was a reliable predictor of intraoperative lateral wall fracture during DHS fixation. We thought that the thinner LFW was too weak to provide sufficient lateral buttress for the proximal fragment, and the large-diameter hole drilled into the LFW would be another factor contributing to weaken the strength of the LFW. Nevertheless, to the author’s knowledge, there were no studies relating to simulation and comparison of biomechanical performance of DHS fixation with different LFW thickness.
Finite element analysis was a common tool used in biomechanical validation studies of orthopedic or dental implants. In present study, we constructed 3D finite element models with different LFW thickness of the intertrochanteric fractures treated with DHS to compare the differences in biomechanical properties. Our results showed that the stress of the proximal femur, LFW and implant in the 10mm model were higher than that in 20.5mm model and 30mm model. Furthermore, the maximum displacement in the 30mm model was much smaller than that in the10mm model and 20mm model, and the maximum displacement in the 10mm model and 20mm model was similar.
The LFW fracture was an important predictor of a reoperation in the intertrochanteric fracture treated with a sliding compression hip-screw device. Palm et al. investigated 214 consecutive patients with an intertrochanteric fracture treated with a 135° sliding compression hip screw and the results showed that patients with a fractured LFW underwent a significant higher reoperation rates than those with an intact LFW postoperatively within six months (22% versus 3%, p < 0.001). Moreover, 74% (thirty-four) of the forty-six fractures of the LFW occurred during the operative procedure itself. Another study revealed that 19.5% (thirty-four) of the 135 patients underwent LFW fractures during surgery. Both of these two studies revealed that a compromised lateral femoral wall was a reliable predictor of intraoperative or post-operative LFW fracture in the intertrochanteric fracture. In present study, we found that the stress concentration in the LFW located at the area around the blade entry point in all of the three models. Furthermore, we discovered that the thinner of the LFW thickness, the greater the stress on the cortical bone of the lateral femoral wall. The peek von Mises stress of the 10mm model and 20.5mm model increased by 89.26% and 66.39% when compared with the 30mm model, respectively. One reason might be that the high-quality LFW could prevent excessive sliding of the proximal fragment and reduce the stress concentration significantly when compared with the comprised LFW in the intertrochanteric fracture after DSH fixation. Therefore, we thought that the intertrochanteric fractures with a comprised LFW would have a higher risk of intraoperative or postoperative LFW fracture after DHS fixation and should not be treated by DHS alone.
The loss of integrity of the medial or lateral wall of an intertrochanteric region had been suggested as the most important cause of instability in intertrochanteric fractures[5, 21, 22]. The medial wall consisted of the medial cortex and medial calcar located in its deep side. The loss of the medial support had been proven to cause coxa varus postoperatively due to the hinge. When a lateral wall fracture occurred, the medial support played a important role in maintaining the stability of the intertrochanteric fracture. Furthermore, we postulated that the thinner LFW couldn’t provide sufficient support for the DHS and further stress would concentrated on the medial wall. Then, coxa varus could occur. In this study, we found that the stress concentration area was located at the medial-inferior part of the proximal femur and the intertrochanteric fracture with a thicker LFW showed the smaller stress concentration compared with thinner LFW. The 10mm model had the largest peek von Mises stress and the 20.5mm model had the slightly smaller peek von Mises stress compared with 30mm model. Therefore, we thought that the thinner of the LFW thickness, the higher risk of varus could occur in the intertrochanteric fractures after DHS fixation.
A decrease in the peak von Mises stress of the implant could decrease the risk of implant failure after daily loading. In this study, the stress distribution in the DHS was also evaluated. The stress in the DHS was concentrated near the junction of the barrel and side plate of each model. The maximum stress on the DHS increased by 44.25% and 47.52% when comparing 20.5mm model and 10mm model with 30mm model, respectively. Therefore, the DHS of the 20.5mm model and 10mm model had higher risk of fatigue failure than the 30mm model. Furthermore, the yield strength of Ti-6Al-7NB alloy was 921 MPa, while the maximus stress in the DHS of the 10mm model, 20.5mm model and 30mm model was 368.18MPa, 360.03MPa and 249.58MPa, respectively. The maximum von Mises stress of the DHS was lower than the yield stress among all models. This mechanical property might reduce the breakage risk of the implant.
The total displacement of the fractured femur stabilized by a DHS device was used to evaluate the stability. In present study, the total displacement of the 10mm model and 20.5mm model increased by 20.59% and 16.19% when compared with 30mm model, respectively. This indicated that a thicker LFW could increase the stability of the intertrochanteric fracture and the intertrochanteric fracture with a thinner LFW tended to increase the risk of failure after DHS fixation. Therefore, the intramedullary nail or an addition of trochanteric stabilization plate (TSP) was recommended with the intertrochanteric fractures with a thinner LFW.
To the best of the authors’ knowledge, this was the first study to investigate the biomechanical performance of intertrochanteric fractures with different LFW thickness treated with DHS device. Nevertheless, there were several limitations in present study. First, the femur and implants were anisotropic materials. However, in this study, in order to reduce complexity of analysis, they were simplified into homogenous, isotropic and elastic materials. Second, only static analysis was conducted in this model, while dynamic analysis was ignored. More motion forms would be considered in the future research. Finally, this study did not conduct experimental validation, which definitely was a common limitation of similar simulation research. However, the purpose of this study was to compare relative values, and intertrochanteric fractures were difficult to achieve in vivo. Therefore, the lack of experimental validation was acceptable. This study analyzed the biomechanical performance of intertrochanteric fractures with different LFW thickness treated with DHS device only, and further research was needed to analyze more available fixation options (e.g., intramedullary nail).