Evidence of osteoinduction by Grafton demineralized bone matrix in nonhuman primate spinal fusion

A nonhuman primate posterolateral lumbar intertransverse process arthrodesis model was used to evaluate osteoinductive bone graft materials.

To test two new formulations of Grafton demineralized bone matrix (Flex and Matrix) for evidence of osteoinduction and their potential efficacy as an extender or enhancer for autogenous bone in a previously validated nonhuman primate posterolateral lumbar fusion model.

Whereas several demineralized bone matrix formulations have been shown to be variably osteoinductive in rodent ectopic bone assays and rabbit spine applications, few have demonstrated efficacy in higher species and in more challenging applications such as posterolateral spine fusion. The authors are not aware of any published studies describing any demineralized bone matrix that has been tested in a nonhuman primate posterolateral spine fusion model.

After approval by the institutional animal care and use committee, eight skeletally mature rhesus macaques underwent single-level posterolateral arthrodesis. In four animals, autograft (4 g/side) was implanted with a piece of human Grafton Flex demineralized bone matrix. In the other four animals, rhesus Grafton Matrix demineralized bone matrix, a new and more porous formulation of Flex, was implanted with autograft (4 g) on one side of the spine, and Matrix with half the amount of autograft (2 g) was implanted on the opposite side. Radiographs were taken at intervals until the animals were killed at 24 weeks. Spinal fusion was evaluated by manual palpation (status was fused or not fused), and computed tomography was done to visualize the amount of bone formation.

Fusion was ascertained by palpation in two of four monkeys receiving Flex with autograft and in three of four monkeys receiving Matrix with autograft. Evidence of osteoinduction was seen in all four monkeys on the Matrix with 4 g autograft side, which had larger fusion masses than in the other treatments. Histologic examination showed that the bone formed was normal.

The rhesus Matrix formulation performed better than the human Flex. Evidence of osteoinduction was seen in all four monkeys that received Matrix, which improved the fusion success of autograft. This alone suggested that it might play a role as a graft enhancer, not merely as a graft extender. Human studies are warranted.