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MECHANICAL ANALYSIS OF BONESAVE DURING REMODELLING

Biomedical Engineering, Institute of Orthopaedics, UCL, RNOH, Stanmore, Middlesex, HA7 4LP

Introduction: Impaction allografting allows an initial stable function of revision hip replacements and a method of reconstituting the bone stock. A previous in-vivo ovine study has found that the density of impacted morsellised allograft reduces after six weeks but recovers by twelve weeks. This reduction in density during remodelling may also correspond with a reduced mechanical strength. A probable cause of the low density is osteoclastic bone resorption prior to vascular in growth and the formation of new bone by osteoblasts. BoneSave is a 4–6mm porous granules of hydroxyapatite and tricalcium phosphate, and is designed as be used as a 50:50 mix with morsellised allograft. Bonesave takes a long time to be resorbed and replaced with bone compared with allograft. We hypothesised that the inclusion of BoneSave could slow resorption down and hence maintain the mechanical strength of the graft during remodelling. This study investigated the mechanical strength of BoneSave mixed with allograft at six and twelve weeks after insertion into a defect, with pure allograft as a control.

Methods: Twelve yews were used in this study, half were terminated at six weeks the remainder at twelve. The test site was a 15mm diameter hole, approximately 10mm deep, in the medial femoral condyle. Both femurs were operated on consecutively, with allograft on one side and a BoneSave/allograft mix on the other. After euthanasia the distal femurs were removed and CT scans performed to evaluate density. Sixteen millimetre discs were cut from each femur, exposing the test site 4 mm from its proximal end. These graft site was then subjected to non-destructive compression tests in Zwick loading machine. Bone remodelling in the graft was determined using histology.

Results: Wilcoxon paired test were used to compare densities of the allograft group with the BoneSave group at 6 and 12 weeks, at both time points there was a significant difference between the groups (p<0.05). There was no statistical difference in the density of the allograft groups between 6 and 12 weeks, or the Bonesave groups between 6 and 12 weeks using the Mann-Whitney U test (p>0.05). There was no significant difference between the stiffness of the two groups at both time points using the Mann Whitney U test (p>0.05).

Discussion and Conclusion: This result was unexpected in the allograft group because in a previous study looking at different sized allograft chips there was a significant difference between the density at 6 and 12 weeks. The most likely cause for this is that lower forces were used to impact the graft in this experiment compared with the graft size study. This would have resulted in lower density at time zero, so perhaps this lower density didn’t invoke such a large resorption response. Bone-Save is able to maintain mechanical strength during remodelling when used as a bone graft extender.

Correspondence should be addressed to Mr Carlos Wigderowitz, Honorary Secretary BORS, University Dept of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School, Dundee DD1 9SY.