Finite element analysis of an internally fixated pelvis during the process of bone recovery from a ‘Denis I’ sacral fracture

The pelvis is a key part of the human musculoskeletal system and serious fracture can greatly incapacitate patients. A greater understanding of the structural mechanics of the pelvis during bone recovery is therefore required in order to help contribute to this issue. This research aimed to address the subject of prolonged patient immobilisation in patients with unstable pelvic fractures through the development of our current understanding of the structural mechanics of a fixated pelvis during the process of bone recovery. This was achieved through the assessment of the structural mechanics of the pelvis during bone recovery. In order to understand the structural mechanics of the pelvis, FEA was employed for the assessment of pelvic stresses and strains. An FE model was thus developed based on methods and parameters commonly employed in relevant literature. The FE model was based on an accurate saw bone model of the pelvis, which was also employed for validation of the FE model. The FE model was developed to be subject specific to the physical model through analysis and implementation of data obtained from a laser topography model and CT images of the saw bone specimen. Using the aforementioned data, a model was created with a variable thickness cortical shell surrounding the inner 3D cancellous bone core of the pelvis. Validation studies were conducted on the FE model through the comparison of FE results with results obtained through the experimental testing (static loading of the pelvis to a single body weight) of the saw bone specimen using an Instron machine. This specimen was subsequently assessed after fixation using a multi-axial spinal fixation, imposition of a Denis I fracture and inducing of stiffness recovery at the fracture site (experimentally using a curing adhesive). Post validation, FE studies were conducted to account for a number of factors which play a key role in the facilitation and assessment of the potential for early mobilisation. To ensure that these findings were similarly applicable in situations representative of the actual mobilisation process, a subsequent study focused on the assessment of non-static real world mobilisation activities, particularly gait. Gait loading patterns were applied to the validated FE model of the pelvis and a similar assessment conducted on peak pelvic and fixation stresses. Further studies targeted inclusion of key ligaments and cartilage in the FE model of the pelvis, with assessment of the model focused on the impact of ligament avulsion as often occurs during pelvic fractures. The final study attempted to analyse the effectiveness of a number of different fixation systems in facilitating early mobilisation on a Denis I fractured pelvis. Results from the validation study showed good agreement between experimental tests and FE analyses. Assessment of pelvic and fixation stresses during gait indicated significant potential for early mobilisation, with pelvic stresses converging towards levels in the undamaged pelvis once “recovering bone” at the site of fracture had restored to a fraction of its original property. Further assessment of the peak strains in the recovering bone at the site of fracture also indicated that strains were below the required threshold for healing to continue under such loading, thus showing further potential for the early mobilisation of patients at early stages of healing. Assessment of the FE model with modelled ligaments and cartilage highlighted that these results were similarly applicable when taking into account the worst case scenario where major pelvic ligaments are severed. Finally, analysis of a number of different fixation systems highlighted significant differences in the ability of fixations to facilitate mobilisation. Consideration of bone-fixation interface strains indicated fixation loosening to be a key criteria of concern during early mobilisation, with iliosacral screw type fixations showing high risk of fixation loosening. In comparison, the posterior tension band fixation and the multi-axial spinal fixation were found to potentially allow for the early mobilisation of patients with Denis I fractures to the pelvis.