Computational modelling of tissue engineering scaffolds with a fracture fixation assembly under physiological loads for bone tissue regeneration in osteoporotic fractures

Introduction: Globally, there are an estimated 200 million people effected by osteoporosis [1] and 1 million total hip replacements are implanted each year [2]. The occurrence of periprosthetic femoral fractures (PFF) has increased in people with osteoporosis due to decreased bone density and bone strength, as well as stress shielding from the prosthetic implant. The treatment for PFF in the elderly is a genuine concern for orthopaedic surgeons as no effective solution is currently available. Therefore, this study aimed to design and develop a novel scaffold device to work in parallel with fracture fixation solutions in the femoral bone. This scaffold will drive the fracture healing, thanks to the release of bone-regulating biomolecules, while the standard fixation solutions sustain the physiological loads and avoid mechanical failure during the bone regenerative process. Materials and Method: A computed tomography scan (516×516 pixels, pixels size of 0.815 mm, and slice thickness of 1 mm) was used to develop the 3D geometry of the femur. CAD models for the fixation plate, bone screws, cables, cable holders, femoral stem, and device were designed in SolidWorks. A parametric computer design was performed to determine the internal topology of the scaffold. In particular, channels were used to connect the central pores to generate a porous network, create access points to allow the scaffold to be filled with a hydrogel containing bone-regulating biomolecules, and direct diffusion towards the fracture site (Figure 1). The scaffold device was fabricated from a polymeric blend of poly-L-lactic acid, polycaprolactone, and poly 3-hydroxybutyrate-co-3-hydroxyvalerate, whose mass ratios were optimised in terms of mechanical properties and degradability. A Young’s modulus of 2.52 GPa was measured experimentally for the polymeric blend and implemented in the model. The material properties for all the other components were taken from the literature [3] and it was assumed that all device components were bonded with the bone (complete osseointegration). Physiological loading simulating the peak loads through standing, walking, and stair climbing were investigated [3]. Results and Discussions: The peak stresses in the scaffold device were 4.20 MPa, 9.22 MPa, and 25.77 MPa for standing, walking, and stair climbing loading conditions. The fracture fixation assembly showed the highest peak stress value for the three physiological loading conditions investigated. Within the plate fixation assembly, bone screw 3 demonstrated the highest stress levels, followed by bone screws 1 and 2 (Figure 1). Conclusions: The following conclusions from this study are: · Polymeric scaffold device is non-load bearing as the majority of the load was carried by fracture fixation assembly. · Mechanical design was validated as the stresses within the scaffold device were 8%, 17%, and 49% of the maximum yield strength (53 MPa) of the polymeric blend at standing, walking, and stair climbing peak loading conditions.