Mechanical and biological aspects of impaction bone grafting in revision hip surgery and the use of a new synthetic bone graft

AIMS: This thesis aims to examine the biological and mechanical factors which influence the strength of impacted morsellised bone graft. The use of synthetic materials as bone graft enhancers was also analysed (Controlled Release Glass - Corglaes®, Giltech Ltd, Ayr, Scotland and Tricalcium Phosphate Hydroxyapatite - TCP/HA, Stryker Howmedica Osteonics, Berkshire, UK). METHODS: Phase I - the mechanical strength of different fresh frozen human bone graft preparations were analysed in the laboratory, looking specifically at the effect of particle size, washing the graft and using synthetic additives. Phase II - mixtures of bone graft and bone graft/Corglaes®, which had been identified as being mechanically stronger than standard bone graft, were analysed for their biological response in an in-vivo ovine defect model, compared to controls. Bone densitometry and histological analysis was performed. Phase III - a mixture of bone graft and Corglaes® was compared to the current 'gold standard' of allograft bone alone, in a simulated revision in-vivo ovine femoral hip replacement model. Outcome measures of subsidence, micromotion on cyclical loading and histomorphometry were performed. RESULTS: Phase 1 - Fresh frozen human bone-graft behaves in a similar fashion to theoretical predictions based on Engineering principles. These principles follow Soil Mechanics theory and are commonly used by engineers when designing stable foundations for roads or buildings. In general, when the spread of different particle sizes is uniform over a given range, the material is stronger (more resistant to shear) than if the particles are all the same size. This allowed determination of which bone mills produce the strongest graft. These results were dependent on the degree of fluid release on graft milling, with more fluid release when the average particle size is reduced. Shear strength was improved for all mills after washing the morsellised graft or by the addition of synthetic additives (Corglaes® and TCP/HA). Phase 2 - The defect model allowed analysis of remodelling of the impacted pellets, highlighting rapid dissolution of the Corglaes®, without a significant inflammatory response. The model may be closer to a fracture model when the histological results of phase III are considered. Phase 3 - No statistically significant difference in subsidence over the implantation period (12 weeks) or micromotion of the retrieved implant / femur composites could be elicited between the two groups. Histological analysis revealed the distal impacted graft to be in an isolated environment, both from biological ingress and solution exchange. Bone graft and Corglaes® that was remodelled or resorbed after time in the Phase II defect experiments, was little changed with time in the distal femur. The proximal femur histologically behaved in a similar fashion to the defect experiments. This suggests that a defect model alone is not ideal to analyse materials for impaction grafting. CONCLUSIONS: • Graft strength is variable depending on the bone mill that produces it - washing bone graft improves the strength from all bone mills tested. • Tight compaction with smaller particles does not inhibit neovascularisation. • Novel biomaterials by themselves were inferior mechanically and biologically. • 50/50 mixes of allograft and Corglaes® are stronger mechanically and do not appear to have an adverse effect on biological incorporation. • In this sheep hemiarthroplasty model, subsidence, micromotion and histomorphometry results better replicate the equivalent reported human results than previous models, especially unloaded defect models in lower vertebrates