Degenerative conditions of the spine are a major public health problem, leading to severe back pain, reduced quality of life and chronic disablement in a proportion of sufferers. For some of these patients, spinal fusion surgery is a treatment that can alleviate back pain and restore normal function. However, limitations in the availability of graft material mean that alternative grafts are needed and tissue-engineering approaches have been employed. Using a novel self-organising collagen scaffold combined with nano-hydroxyapatite and chondroitin sulphate and by employing the latest materials techniques, I have studied the osteogenic capability of a biomimetic graft for use in spinal fusion surgery. The mineralised collagen scaffold has compressive strength comparable to human cancellous bone and can support the proliferation of viable human mesenchymal stem cells. This porous scaffold can be combined with human mesenchymal stem cells to further promote bone growth, as evidenced by an upregulation in the levels of bone-forming genes and mineralisation of the scaffold. This scaffold can act as a carrier system for BMP-2, with wider application for other growth factors or drugs, providing sustained release when fabricated as a layer-by-layer scaffold. An alternative bone substitute for use in spinal surgery has been designed and characterised, with exciting potential for use in vivo.

Although the tensile mechanical properties of tendon have been well characterised, the viscoelastic and anisotropic properties remain uncertain. This thesis addresses the anisotropic and viscoelastic material properties of tendon. A method to characterise the three-dimensional shape of tendon is reported and experiments to characterise the fibre-aligned and fibre-transverse viscoelastic properties of tendon are presented. The cross-sectional profiles of bovine digital extensor tendons were determined by a laser-slice method. Linear dimensions were measured within 0.15 mm and cross-sectional areas within 1.7 mm². Tendons were compressed between two glass plates in creep loading at multiple loads. Compression was then modelled in a finite element environment. Tendon was found to be nearly incompressible and reproduction of its isochronal load-displacement curve was achieved with a neo-Hookean material model (E ≃ 0.3 MPa). The fibre-aligned tensile mechanical properties were described using a Quasi-Linear Viscoelastic model. The model was effective at reproducing cyclic loading; however, it was ineffective at predicting stress relaxation outside the scope of data used to fit the model. When all experimental results are considered together, two significant conclusions are made: (1) tendon is much stiffer in fibre-aligned tension than in fibre-transverse compression and (2) the fibre-aligned tensile response is strain dependant, while the transverse response is not.

Cam-type Femoro-Acetabular Impingement (FAI) is a common cause of hip osteoarthritis (OA). In this condition a bony abnormality at the head-neck junction of the femoral head, called the “cam”, abuts against the acetabulum causing labral damage and articular cartilage delamination, which in turn may lead to progressive degeneration and OA. The understanding of the damage mechanism is currently at a conceptual level. The aim of the thesis is to develop a more detailed understanding of the underlying mechanism so as to improve methods of detection and treatment of cam-type FAI and thus to help prevent hip OA. A geometric-kinematic model combining hip joint motion and hip joint geometry was cre- ated to determine what motions, activities or cam shapes give rise to cam-type impingement, which was quantified by the proximity of the acetabular and femoral bony surfaces. Five normal subjects and five symptomatic cam-type FAI patients were modelled. The FAI patients experienced early impingement during the impingement test but did not have impingement during common functional activities. The early impingement was possibly due to the larger coverage and protrusion of their cams and the smaller overall proximity in their hip joints. A 2D finite element (FE) model was created to simulate cam-type FAI. As idealised 2D rectangular and circular geometries did not reproduce the damage seen clinically, subject- specific geometry, loads, and motions were introduced. Under some circumstances, as the cam entered the hip joint, large shear strains developed near the cartilage-bone interface of the acetabulum which would result in cartilage delamination. In vitro experiments were undertaken to validate the FE model and verify the damage mech- anism by which cam-type FAI leads to cartilage delamination. Porcine cartilage-bone samples were loaded under conditions similar to those generated by a cam (shear and compression). A validation FE model was created that used the same material and contact representations and analysis framework as the impingement FE model but mimicked the experimental setup. The cartilage shear strains assessed with a video-based method were similar to predicted FE results. In vitro damage experiments demonstrated that delamination can be caused by repetitive shear and compressive loading that lead to large shear strains near the cartilage-bone interface. The impingement FE model was used to further explore the effect of cam anatomy. In hips with low clearance, cams with large protrusions (75% hip joint clearance) would not enter into the hip joint, but caused high shear strains in the labrum, which would result in labral tears. A narrower cam caused damage to the labral tip, whereas a wider cam caused damage to the labral-bone junction. In contrast, cams with small protrusion (25% hip joint clearance) were able to enter the joint and caused damage at the articular cartilage-bone interface, which would result in cartilage delamination. The wider the cam, the further into the hip joint the damage was initiated. The FE model was used to explore the effect of different labral anatomy and of reshaping surgery. A labrum connected to the articular cartilage resulted in shear strains of up to five times greater in the articular cartilage and labrum compared to an unconnected labrum and was more likely to cause articular cartilage delamination. For a cam that damages the articular cartilage, surgical removal of the cam reduced shear strains. For a cam that abuts the labrum, surgical removal of the cam eliminated labral abutment and increased the range of motion of the hip, but resulted in greater shear strains in the articular cartilage. It is not known whether these shear strains are normal or could possibly be damaging. Also, reshaping the head to be spherical resulted in slightly reduced shear strains in the articular cartilage compared to the current surgical practice of cutting deeper into the femoral head when removing the cam. This study has, for the first time, using a validated FE model demonstrated the mechanism by which a cam can cause articular cartilage delamination and labral tearing. Further analysis using the geometric and FE model should help identify cam deformities that would be likely to cause OA and the best way to treat them surgically so as to prevent OA.