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MECHANICAL TESTING OF APAPORE AS A BONE GRAFT EXTENDER

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

Introduction: ApaPore is a synthetic bone graft extender, made from Hydroxyapatite. It is designed to be used as a 50:50 combination by volume with morcelised bone graft. A typical use for such a product may be in impaction allografting during revision hip surgery. The aim of this study was to compare the mechanical stability of stems inserted using impaction allografting where the graft was composed of a 50:50 mixture of ApaPore and allograft with only allograft.

Methods: Twelve large left sawbones were used in this study, the femoral head was cut off each one and the equivalent cancellous bone was cored out to resemble a femur at revision. Impaction allograft was performed on six of the sawbones, with rinsed human morsellised graft. This procedure was repeated on the remaining six sawbones, with graft produced as a 50:50 mixture by volume of rinsed allograft and blood soaked Apapore. The mechanical test was performed in the red rocket, a six station hydraulic loading machine. The sawbones were mounted in the anatomical position of 7 degrees valgus and 9 degrees posteriorly and the test was run in batches of three. Sinusoidal loading at 2Hz was applied under the following loads: 600N, 1kN, 1.4kN, 1.8kN and 2.2kN, each loading step lasting for 5000 cycles. Migration of the stem during loading was measured using LVDT. Vertical displacement of the prosthesis head was measured using digital height callipers at the beginning and end of each loading step. X-Rays were taken before and after mechanical testing.

Results: There is a significant difference between the groups in the overall displacement of the prosthesis head, measured with the digital height calipers, (Mann-Whitney U Test p=0.01). Total average head movement allograft group: 3.5mm and ApaPore group: 1.8mm. The total average subsidence measured with the LVDT’s was 0.295 mm in the allograft group and 0.119mm in the ApaPore/Allograft group. A sideways displacement of the prosthesis head was observed on the x-rays, which is a direct relationship of head rotation, measurements showed a significantly less rotation in the ApaPore/Allograft group (Mann-Whitney U Test p=0.002).

Conclusions: Rotation of the stem in all planes during loading resulted in a greater observed displacement of the prosthesis head than that measured by the LVDT’s. These results show that ApaPore, when used as a bone graft extender is able to reduce initial rotation and subsidence of the stem. Further research needs to be undertaken to investigate the long-term feasibility of using ApaPore.

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