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Total hip arthroplasty (THA) is the treatment of choice to relieve joint pain and loss of mobility as a result of advanced stage osteoarthritis or other hip pathologies. Despite their general success, THAs do fail, with revision rates estimated near 5% per year. Instability, defined as the complete subluxation (dislocation) of the femoral head from the acetabular socket - which usually occurs due to implant impingement - has recently supplanted wear-induced osteolytic aseptic loosening as the leading cause of failure in THA. Soft tissue integrity has long been recognized as influencing joint stability, and therefore there has been great interest recently in improving soft tissue restoration following THA. However, there is little quantitative information related to the degree of soft tissue repair necessary to restore joint stability. Additionally, impingement events, besides their role in prelude to frank dislocation, hold potential to damage new-generation hard-on-hard bearings, due to the relatively unforgiving nature of the materials and designs. Despite the largely biomechanical nature of these impingement-related complications, they remain under-investigated relative to their burden of morbidity. In addition to impingement, failure modalities unique to hard-on-hard bearings merit careful biomechanical scrutiny. This includes investigation of catastrophic fracture in ceramic-on-ceramic bearings, as well as analysis of patient, implant and surgical variables associated with increased wear and adverse soft tissue engagement potential for metal-on-metal implants. Toward the goal of improving current biomechanical understanding of failure modalities in THA and to provide an objective basis for orthopaedic surgeons to choose the most favorable implants and to identify optimal intraoperative parameters which minimize failure propensity, a novel, anatomically-grounded finite element model was developed, and used to perform multiple parametric finite element investigations of these failure modes.

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Total hip arthroplasty commonly involves a hard metallic/ceramic femoral ball component articulating against an acetabular ultra-high molecular weight polyethylene (UHMWPE) counter-bearing. A novel six-station, wear tribometer, featuring programmable load and motion, was designed to further the investigation into wear, sub-surface plasticity and debris generation of UHMWPE. This thesis describes the pin-on-plate device, its validation and subsequent use to assess unirradiated (-PE) and gamma-irradiated highly crosslinked (+PE) UHMWPE wear behaviour. With the emphasis on dynamic loading and a closer gait matched open wear path, the tribometer improves on the clinical relevance of pin-on-plate testing. There is a requirement for this type of machine in order to investigate the directional dependence of wear and debris generation of UHMWPE more accurately, where “simplified” tribometers (adequate for constant load/constant velocity and constant load/sinusoidal velocity work) are not capable or suitable. For the first time in orthopaedic pin-on-plate studies, tests were conducted using an advanced dynamic load synchronised to a more physiologically accurate elliptical motion path. To validate the machine, three orthopaedic polymers of clinical relevance; Polytetrafluroethylene, Polyacetal, and UHMWPE were subjected to linear-reciprocating (LR) and novel elliptical motion paths under a Paul-type load profile. All three polymers showed higher wear factors under elliptical motion, by up to 2 orders of magnitude, agreeing well with explanted values. The UHMWPE elliptical wear factor was comparable to that reported for clinical, where kelliptical = 1.56 x 10-6 mm3/Nm. In the crosslinked study, the mean steady state wear of -PE and +PE groups under linear reciprocating motion was not significantly different. However, under elliptical motion, crosslinking reduced UHMWPE wear by up to 92% when compared to the unirradiated group. In –PE pins worn under LR motion and in +PE pins subjected to both motion paths a sub-surface damage zone with reduced crystallinity and increased strain was measured using Raman spectroscopy. This was attributed to large strain accumulation in the slower wearing surfaces providing a mechanism for de-crystallisation. The discovery of such a near-surface layer is in good agreement with critical strain wear models. In disagreement, however, we found the sliding induced layer to extend to greater depths than previous reported.

The aim of this thesis was to determine if the size of the femoral head can influ-ence corrosion at the head-neck taper interface of total hip arthroplasty (THA) prosthe-ses. A hypothesis was developed that large head sizes could result in a greater toggling torque at the head-neck taper interface by increasing the distance between the centre of the femoral head to the centre of the neck taper. This could result in increased micromotion and deteriorate the passive oxide film along the head-neck taper interface; thus, making the taper interface vulnerable to corrosion. A retrieval analysis of 74 THA prostheses studied the corrosion damage at the head-neck taper interface. This study revealed that prostheses featuring 36 mm femoral heads had significantly greater head taper corrosion than prostheses with a 28 mm head. Finite element analysis was performed afterwards to identify if the use of large femoral heads can increase the micromotion at the head-neck taper interface due to a greater toggling torque. This experiment demonstrated that with a larger head size the micromotion at the head-neck taper interface increases. An in vitro corrosion fatigue study was performed afterwards following ASTM F1875-98. When applying an off-axis fatigue load, prostheses featuring a 36 mm femoral head displayed significantly more corrosion damage at the head-neck taper interface than those with a 28 mm femoral head. Axial fatigue loading was also applied; negligible corrosion damage at the head-neck taper interface was discovered in comparison to the prostheses that received an out of axis load. This verifies that the use of large femoral heads can result in increased head-neck taper corrosion due to a greater toggling torque.

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Background: Highly porous Trabecular Metal™ acetabular components are increasingly being used in revision hip arthroplasty as they facilitate ingrowth, provide a useful mechanism to deal with bone loss and may decrease the risk of infection. The purpose of this audit was to describe: 1) the short-term radiological outcomes of revision hip arthroplasty with Trabecular Metal™ components and augments. 2) the total number of hip arthroplasty surgeries over five years, and indications for revision. Methods: A retrospective folder and radiograph review of all patients who had revision total hip arthroplasty (THA) at a tertiary level hospital from February 2012 to February 2017 was done. Results: There were 979 THAs performed over the period – 863 (87%) primary THAs, and 116 (12%) hip revision cases performed in 107 patients. Of the 116 (107 patients) hip revisions, there were seven (6%) re-revisions in five patients. Trabecular Metal™ was used for revision in 16 hips (14 patients), which is 13.7% of the total 116 revisions. There were ten females and four males with an average age of 61 years. The average duration of follow-up in this group was 18.5 months (1.5– 39.2). In these 16 Trabecular Metal™ hips, there were three (18.7%) early failures of fixation due to technical errors. The indications for revision were aseptic loosening 67 (58.6%), septic loosening 11 (9.5%), liner wear 18 (15.5%), periprosthetic fracture five (4.3%), other 15 (13%). Conclusion: In our institution, Trabecular Metal™ revisions had a 18.7% early failure rate due to technical error. 12% of the arthroplasty is revision surgery. The indications for revision are similar to published literature.

Surgical interventions on the hip joint have greatly increased over the past decade, with the cumulative cost total hip arthroplasties (THA) alone exceeding $400B/year by 2020. Although positive patient-reported outcomes and satisfaction after THA and hip preservation for cam femoroacetabular impingement (FAI) are among the highest in orthopaedics, a limited number of research has investigated the biomechanics of dynamic activities following-up the surgery. This doctoral thesis examined the kinematics, muscle force component, and hip contact loading in pre- and postoperative patients during the deep squat motion. Specifically, this research: 1) examined muscle strength and pelvic kinematics in asymptomatic FAI, 2) examined lower-limb kinematics and muscle activity in postoperative patients who underwent either THA or FAI correction during a deep squat task, and 3) examined muscle force contributions and hip contact forces (HCF) during dynamic motion in postoperative FAI patients. First, clinical and medical imaging evaluations classified the participants into three groups: symptomatic FAI, asymptomatic FAI (FAD – participants had the cam deformity, but no pain), and healthy controls. The FAD participants had significantly greater hip extensor strength compared to the FAI and CTRL groups, which allowed them to achieve greater pelvic mobility and squat as deep as the CTRL group. Second, at the follow-up for the FAI surgery the patients showed increased pelvic ROM during the squat, and weakness associated with hip flexion and hip flexion-with-abduction were associated with postoperative alterations. For the THA follow-up analyses, the patients using a dual- mobility (DM) prosthesis reached an anterior pelvic tilt similarly to the CTRL during the dynamic parts of the squat; however, without returning its neutral tilt at the bottom of the squat, while the single- bearing (SB) prosthesis was associated with excessive hip abduction during the squat. Third, a generic full-body musculoskeletal model (MSKM) was optimized to allow for the analysis of tasks with a high range of motion (ROM; e.g. deep squat task), which controlled muscle moment arms during the high joint flexions to avoid the model’s motor tendon units (MTU) to penetrate the bony structures and respect the anatomical via points. Simulation performed during gait demonstrated that FAI patients enhance medial-lateral hip stability postoperatively, allowing reduced dynamic forces of the muscles associated with the sagittal aspect of the gait due to a less compensatory strategy to stabilize the hip joint. Furthermore, simulations performed during deep squat showed a higher anterior pelvic tilt in postoperative FAI patients as a ‘restore to native’ mechanism once the cam-deformity was no longer present. Increased semimembranosus force was linked to higher vertical HCF and total magnitude. The outcomes of this research include findings for gait and squat analyses that provide a better understanding of the pelvic mobility and hip muscle forces in hip diseases. In silico models can improve biomechanical assessment of postoperative patients in order to quantify surgical effectiveness and support clinicians in making subject-specific case decisions. The contributions also lay on the assertion of helping us to formulate future research directions in biomechanics applied to the orthopaedics field.

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