Abstract
Problems associated with allograft are well known. The addition of hydroxyapatite (HA) to allograft has various mechanical advantages, especially within revision arthroplasty. The mixing of bone and HA results in mechanical properties different from the individual parts. However, at present the changes in material properties the mix have not been fully investigated and the optimum mixing ratio not characterized. A compressive uniaxial chamber was used to investigate the change in mechanical properties occurring with the addition of HA in varying proportions to morcellised bone graft (MBG).
Materials and methods
MBG was prepared using femoral heads donated from patients undergoing total hip replacement surgery using a bone mill in a standard manner. Non porous HA (npHA) was prepared using a precipitation method of Calcium Carbonate and Orthophosphoric acid. The porous HA, (pHA) is a 60% macroporosity HA commercially prepared.
Chamber
The uniaxial compression chamber was a 30mm diameter, steel chamber. Holes were drilled to allow fluid drainage. Loads were applied using a 10 kN load cell. Specimens were prepared in the volumetric proportions pure HA, pure MBG, 2:1, 1:1, 1:2 ratio of MBG to HA. The samples were subjected to compressive forces of incrementally increasing loads of up to 2 KN for 60 cycles. The sample was then allowed to creep under a stress of 2 kN. MBG was also tested up to forces of 7 kN. The mechanical parameters that were examined were the stiffness of the sample at the 60th cycle, (Ec60), and creep
Statistical analysis
The different bone: HA mixes were compared by Mann Whitney U tests. The parameters analysed were the Ec60 and creep for the different mixes. All samples were also compared to MBG compacted with higher forces.
Results
Apparent stiffness. The Ec60 of the MBG increased with increasing axial stress. The Ec60 of the different proportions of MBG and both npHA and pHA lay between the values of the MBG and HA individually and were significantly different from one another (p<0.05). MBG:npHA were significantly stiffer than the comparative mixes of MBG:pHA. (p<0.05)
Creep rate
With increasing axial stress the creep rate for pure MBG decreased up to a precycling stress of 4 MPa. Subsequent increases in stress had no significant effect. Pure npHA and all of the mixes of MBG:npHA demonstrated significantly less creep than the comparable mixes of MBG:pHA (p=0.01).
Applicability of results
The increase in apparent stiffness associated with an increase in the volume fraction of HA is accompanied by a concomitant decrease in the creep rate. Comparison of the mechanical response of the MBG at the high precycling stresses (4 - 7 MPa) to the mechanical response of the MBG:HA mixes at 3 MPa demonstrated an overlap in the mechanical response of the pure MBG at high stress and both the MBG:npHA and MBG: pHA mixes.
Conclusions
Greater proportions of HA in the mix led to improvement in the mechanical response without the need for increased force of impaction. These improvements can be quantified by using a uniaxial compression test. The results raise the interesting possibility of tailoring bone graft mixtures to specific patient requirements dependent upon their weight and the quality of their bone stock.
