Abstract
Background: Sacroiliac joint dysfunction affects 13-32% of all patients suffering from chronic lower back pain, however, the joint still seems underappreciated as a source of mechanical lower back pain. Literature is scarce on sacroiliac joint dysfunction because its relevance is often overshadowed by better-known causes of back pain which include structural abnormalities, infection, inflammatory metabolic disorders and joint degeneration. There is little literature focussing on the anatomy of the joint and the kinematics of structures causing mechanical pain in the sacroiliac joint.
Aims and objectives: The main aim of this thesis is to contribute to the clinical literature on the biomechanics and anatomy of the sacroiliac joint, specifically aiding in the understanding of the mechanisms involved in sacroiliac joint dysfunction. This will be achieved via the following three objectives: (1) to investigate and describe the normal anatomy and biomechanics of the sacroiliac joint, (2) to quantify the presence of fat in the posterior sacroiliac joint and to establish a method applicable to clinical imaging, (3) to describe the mineral bone density distribution of the subchondral bone layer of both articular surfaces of sacroiliac joints in the ‘healthy, pain and pathology-free’ conditions and compare the results to a cohort of patients with sacroiliac joint dysfunction pre- and post-sacroiliac joint arthrodesis.
Methods: For the first objective, a systematic literature review was performed on the normal anatomy of the sacroiliac joint, as well as the biomechanics of the joint in healthy individuals and those with sacroiliac joint dysfunction. For the second objective, 78 hemipelves were sliced and photographed. The posterior sacroiliac region and visible fat within the region were segmented manually using a computational software and the overall fat volume was estimated using Cavalieri’s method. MATLAB was used to quantify fat volume in CT scans, this time using fat Hounsfield unit values (-150 to -50). For the last objective, 20 hemipelves were CT-scanned and indented to establish the relationship between the mechanical bone strength and bone mineral density visualised using densitograms derived from computed tomography osteoabsorptiometry. Densitograms of 39 healthy patients, and 27 sacroiliac joint dysfunction (pre- and post- sacroiliac joint arthrodesis) patients were compared to analyse differences in bone mineralisation between the groups.
Main results: Results from objective one found two main areas lacking in research following the literature review. These were: fat presence within the posterior sacroiliac joint region and the subchondral bone density of the sacrum and iliac auricular surfaces. In suit of these findings, main results for objective two were that fat is present in the posterior sacroiliac joint region and females have a higher fat volume and percentage. A novel semi-automated approach was developed in this thesis. It was found to be investigator-independent and time-efficient and can be applied to computed tomography scans for clinical use. Further research is needed in regards to the presence of fat in the posterior joint space of the SIJ. For the third objective, biomechanical indentation revealed that bone density derived from CT-OAM reflects the biomechanical properties of the sacroiliac auricular subchondral bone plate. From this, evidence has been found that the iliac subchondral lamella is mechanically denser than the sacral side and that bone density and bone strength of articulating sides do not correlate, indicating biomechanical non-conformity in healthy individuals. Anterior and inferior region differences in mineralisation were found between healthy patients and dysfunction-sufferers. Furthermore, surgically fused joints reflect altered morpho-mechanical density patterns when compared to healthy joints. Changes in mineralisation may relate to the surgical approach and implant-placement and provide time-related information on the overall appearance of the joint and its state of dysfunction.