Spinal fusion induced by porous hydroxyapatite blocks (HA). Experimental comparative study with HA, demineralized bone matrix and autogenous bone marrow.

Numerous materials have been studied and recently used in clinical practice as synthetic bone biosubstitutes to integrate or even replace autogenous bone grafts. This study consisted of an experimental vertebral fusion model in rabbits using porous hydroxyapatite (HA), both alone and in various combinations with demineralized bone matrix (DBM), as a vehicle for the osteoinductive agent (Bone Morphogenetic Protein-BMP), and with autogenous bone marrow (BM), which supplies BMP-sensitive reticular cells. The animals were divided into two main groups on the basis of the physical form of the biomaterial implanted: HA blocks and HA granules, which in turn included several subgroups based on the composition of the implants (HA, HA+DBM, HA+BM, HA+DBM+BM). There were also control groups with no implant or with bone autografts (ACB). Two months after the operation, analysis of the results (radiograph, fusion stability test on dynamic radiographs, histological evaluation) demonstrated the following: clinical and radiographical results of the group implanted with HA blocks were superimposable on those with autogenous bone grafts, especially in the subgroup treated with DBM and BM, while in the group implanted with granules the results were poor, thus indicating the fundamental influence of the physical state of the biomaterial on bone ingrowth. This was then further confirmed by the histological study in the HA block group, whereas the HA granule group showed a certain fibrous reaction around the granules with poor porous bone ingrowth.