Hip fractures : A biomechanical analysis of fracture strength prediction, prevention, and repair

Abstract: Due to the aging population, hip fracture incidence has been increasing over the past decades. Measurements of bone mineral density with dual energy X-ray absorptiometry are the gold standard for hip fracture risk assessment, where patients with a low bone density have a high risk of fracture. However, many people that are not diagnosed to be at risk, still fracture their hip. Calculations of bone strength using subject-specific finite element (FE) models, can improve fracture risk prediction, but further improvement is required.Patients with a high fracture risk are often prescribed pharmaceutical treatment in order to increase bone density systemically. As systemic response to treatment is limited, other options to prevent fractures by improving the bone strength are investigated. One of those options is the injection of biomaterials in the femoral neck. In case of a hip fracture due to a low-energy fall, total hip replacement is generally preferred over joint-preserving methods like fixation using a dynamic hip screw. Screw fixation comes with a risk of screw instability, especially in low-density bone. Bone cements can be used to improve fixation of orthopaedic implants and fracture fixation devices. Calcium sulphate/hydroxyapatite (CaS/HA) is an injectable biomaterial that has been used, for example, to reinforce collapsed vertebrae and to stabilize wrist fractures. The work presented in the thesis aims to improve fracture risk prediction, and fracture prevention and repair methods with use of CaS/HA. This is achieved through a combination of experimental mechanical tests at organ and tissue scale, and development and thorough validation of FE models of the proximal femur.In the first part of this thesis, 12 cadaveric femora were used in an experiment where the bones were loaded until fracture in a configuration developed to replicate a fall to the side. During loading, high-speed cameras were used to image both the medial and lateral side of the femoral neck allowing for full-field strain measurements using digital image correlation. The femora were imaged with clinical CT before and micro-CT before and after mechanical testing. Using the acquired CT images, FE models were developed at two different resolutions to determine their ability to capture the fracture force, fracture location and surface strains. The FE models based on the clinical CT images were able to accurately capture the fracture force and identify regions where the bone would fracture. These models could also capture the strains with high accuracy. However, the strains were not predicted as accurately in regions with high surface irregularity. The models based on the micro-CT images could show with higher accuracy how the strains were distributed around local porosity (e.g., due to vascularization) in the femoral neck and how these influenced the fracture pattern.The thesis continues with an investigation of fracture prevention and repair methods through the use of CaS/HA. The ability of CaS/HA to increase the fracture strength of the proximal femur for fracture prevention and its ability to stabilize a dynamic hip screw used for fracture repair was investigated. The increase in fracture strength was investigated using FE models. These models showed that CaS/HA can increase the fracture strength of the femur approximately 20% when injected close to the cortex in the lateral neck. Pullout tests using a dynamic hip screw were performed on synthetic bone blocks and femoral heads from hip fracture patients. In the synthetic blocks, CaS/HA significantly increased the pullout strength. However, in the human bone the stability of the screw was not improved, because the cement could not easily spread into the threads of the screws. The mechanical behaviour of CaS/HA and bone was further investigated using high-resolution synchrotron X-ray tomography. Cylindrical trabecular bone specimens with and without CaS/HA were imaged with tomography during in-situ loading of the samples. The images revealed that CaS/HA reinforced the bone, and that CaS/HA is a brittle material that will crack before the bone.To conclude, in this thesis FE models are presented showing accurate prediction of fracture strength, which can be used for improved fracture risk assessments. Furthermore, the work provides insight in how CaS/HA behaves mechanically and how it can be used to increase the fracture strength and to stabilize fixation devices in the femur, improving fracture prevention and fracture repair methods.