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Degenerative disc disease (DDD) is a natural degenerative process that affects the intervertebral discs of the spine and is a major cause of lower back pain. After failure of conservative measures DDD may be treated surgically by fusion or Total Disc Replacement (TOR). Fusion results in loss of motion at the operated level, causing increased motion at adjacent levels. This is thought to accelerate further degenerative changes. The intention of TOR is to replace the disc and maintain normal motion in the spine. CAdisc-L is a novel TOR implant; it preserves motion by deforming under load rather than articulating like existing devices. The objective of the study was to ascertain the effect of implantation of CAdisc-L on sagittal biomechanics. Spine specimens were tested before and after implantation to asses implant stiffness, range of motion and stability. Additionally the position and migration of the Instantaneous Axis of Rotation (IAR) of the specimens was recorded. Finally, the ultimate failure load of implanted specimens in compression was evaluated. Monosegmental cadaveric spine specimens were used for the study. Loads and moments were applied by a uni-axial test machine and movement of the specimen was captured using reflective markers and a motion tracking camera. Implantation resulted in a reduction in compressive stiffness, but an increase in overall disc height which maintained the intervertebral space in compression up to 4.4kN. A reduction in flexural stiffness was seen after implantation as well as a reduction in neutral zone. Tracking of the IAR of the specimen indicated that it migrated by a similar range to intact specimens both horizontally and vertically. The position of the IAR was displaced posteriorly my 5.2mm at 6° of flexion compared to intact specimens. Ultimate failure loads were not found to be affected significantly.

Subsidence is a major complication associated with implantable devices currently used for total disc replacement and interbody fusion. These innovative implants are placed within the intervertebral spaces of the spinal column, an area that holds major weight-bearing responsibility. The subsidence event arises through a combination of poor structural support offered by the underlying bone and inadequate device design, resulting in the implant sinking through the vertebral endplates. This ultimately leads to a decrease in distance between the two vertebral bodies, thereby, reducing the intervertebral foramen, causing both nerve entrapment and radicular pain. Consequently, patients suffering from bone weakening diseases, such as osteoporosis, are currently contraindicated for these life improving procedures. This project aimed to provide the underpinning basic science on which to widen surgical indications for the elderly population, a growing demographic of patients that would most benefit from spinal interbody surgery. The first phase developed an in vitro subsidence model validated within a retrospective total disc replacement radiograph study, which ultimately recreated clinically relevant failure patterns. Prophylactic vertebral body cement augmentation is a potential method for inhibiting implant subsidence and was shown to significantly increase subsidence resistance when statically loaded with a total disc replacement shaped indenter. A 20% polymethylmethacrylate cement fill produced the greatest mechanical resistance but required an evenly distributed fill within the vertebral body to maintain the resistance across the end plate. Cyclic loading of total disc replacement implanted spinal motion segments only showed significant reduction in subsidence when the vertebrae above and below the implant were augmented with cement. Implant fixation between spiked and keel designs did not affect subsidence in cement augmented vertebrae. Finally, cement augmentation demonstrated significantly reduced subsidence in interbody fusion devices with both polymethylmethacrylate and calcium phosphate cements demonstrating similar levels of subsidence resistance. The culmination of this project provides guidelines for the effective use of prophylactic cement augmentation as a method for reducing interbody device subsidence in osteoporotic patients.

An ideal engineered scaffold should support the regeneration of natural extracellular matrix; for bone this is principally Type I collagen and hydroxyapatite. The present work investigated the fabrication and characterisation of scaffolds comprised of (a) PCL or (b) Type I collagen +/- PDLLA produced via electrospinning and studied their influence on osteogenic regeneration. A parallel study examined the cellular response of human bone marrow stromal cells to nano-crystalline hydroxyapatite particles in a foamed PDLLA scaffold. Both the influence of particle size and chemical substitution were considered. Characterisation of materials involved scanning electron microscopy, Fourier transform infrared spectroscopy, circular dichroism, atomic force microscopy and histology. Electrospun PCL nanofibrous scaffolds potentially supported osteogenic regeneration. However, the scaffolds had to be treated post- spinning with NaOH and fetal calf serum to make them more hydrophilic to support cell attachment. With a view to producing a biomimetic material and knowing that hydrophobicity is not an issue for Type I collagen, rat-tail derived Type I collagen was selected for further studies. A number of solvents were explored to successfully electrospin the collagen. Although the wet stability of resultant scaffolds was poor, crosslinking improved scaffold stability, particularly the use of glutaraldehyde vapour. The observed denaturation. of collagen was determined to be a consequence of conformational changes rather than scission of collagen polypeptides, indicating that the electrospun collagen was not a simple analogous to gelatin. An alternative approach to stabilisation involved electrospinning collagen with a co-polymer, PDLLA, which successfully stabilised the fibrous scaffolds in the ratio ranges of 40- 60 wt% PDDLA: 60-40 wt% collagen. Future work will combine these materials with the optimised apatite filler to produce a composite material since the parallel study on cell culture of minerallPDLLA composites suggested that incorporation of mineral components did indeed enhance the alkaline phosphatase activity of human bone marrow stromal cells. Strontium substituted hydroxyapatite introduced potential benefits on alkaline phosphatase activity of bone derived cells in basal media.

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Since the introduction of medical implants into human bodies, corrosion and wear have been regarded as key issues for their long-term durability. There has been a recent renewed interest in the use of large diameter metal-on-metal (MoM) hips, primarily because of the reduced volumetric wear compared with the wellestablished polyethylene-on-metal joints. Long term durability of MoM joints relies on control of both their corrosion resistance (relating to ion release) and wear behaviour (relating to creation of nanometre-scale wear debris). Concerns about the potential risk of released metal ions to the biological environment (patient) are of great importance. In this respect tribocorrosion is a serious consideration in joint performance. An integrated electrochemical cell on a reciprocating tribo-meter was employed to evaluate the corrosion and tribocorrosion behaviour in a protein rich solution (Bovine Serum), a cell culture solution (DMEM) and a saline solution (NaCI) with the attempt to isolate the organic species effects. Three commonly used orthopaedic materials were involved in this study. A High Carbon Cobalt-Chromium-Molybdenum alloy, a Low Carbon Cobalt-Chromium-Molybdenum and UNS S31603 Stainless Steel (316L). A range of electrochemical methods were used in the assessment of materials under biotribocorrosion systems and results were supported by surface analysis and bulk solution analysis techniques. The material degradation rate is strongly dependent upon the charge transfer (corrosion), the mechanical damage (tribology) and also their interactions (tribocorrosion) in these simulated biological environments.

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