Modelling and analysis of a dynamic hip screw: biomechanical analysis of a dynamic hip screw under different load conditions

Internal fixation is a choice of treatment of hip fractures employing implants like dynamic hip screw to stabilize the fracture.The choice of proper implant has a great effect on the healing process and early mobility of patients. Since the majority of the hip fracture patients are elderly, surgeons are reluctant to perform implant removal operation. A better understanding of biomechanical behaviour of dynamic hip screw during daily activities and at impact from a low energy fall will aid the patients with the implanted femur. This study carries out a finite element analysis to establish the effect of load conditions and implant material properties on biomechanical behaviour of the intact and implanted femur after internal fixation. A three-dimensional finite element model of an intact femur and a threedimensional dynamic hip screw within the same femur were developed. By means of literature data, four load configurations were chosen; simulating stance phase of normal gait, stair climbing, stair descending and impact from a fall from standing height, on the lateral aspect of the hip. To the author’s knowledge there has been little research done considering load conditions such as stair climbing and stair descending, or falling on the implanted femur containing a dynamic hip screw. In the case of stair climbing, two different load regimes are chosen, one with simple stair climbing loads, and the second including active muscle forces, resulting in 15 different cases which were investigated. In addition, the implanted femur was investigated with two different materials for the implant,involving Stainless steel and a Titanium alloy. The implanted femurs were compared for deformation, and von Mises stresses under all load conditions relative to those in the intact femur. The implants were also assessed and compared for stress in relation to their different material properties. The results show that the presence of the dynamic hip screw significantly increases stress within the implanted femur. However, peak stresses are below the yield stress of both the bone and implant during daily activities. The most critical activity is stair climbing according to the stress results. Investigating two different load regimes for stair climbing case revealed that inclusion of active muscles increases the stress on the implanted femur, which is close to real physiological loadings. Walking and stair descending induce less stress and cause less deformation on the implanted femur. During fall, high stress concentrations are experienced within the dynamic hip screw rather within the femur itself when compared to the intact femur under the same load condition. Titanium alloy as the material for dynamic hip screw decreases the stress levels within the implanted femur by average of 40% compared with the stainless steel, and may be considered as the preferred implant material.