Purpose: To test a CaSO4/CaPO4-TCP composite bone graft substitute in a crtically sized bone defect.

Method: Twenty dogs had a contained medullary defect created in the proximal humerus. In ten dogs, the defect was treated with CaSO4/CaPO4-TCP composite graft (PRO-DENSE^™, Wright Medical) and studied for 13 weeks (N=5) and 26 weeks (N=5). In the other ten dogs, the defect was treated with autograft and followed for 13 weeks. An additional ten unoperated humeri were used to establish the properties of normal canine bone. The area fraction, ultimate compressive stress and modulus of elasticity of bone in the experimental and normal humeri were quantified using histomorphometric and mechanical methods and analyzed using the Mann-Whitney test.

Results: At 13 weeks, the area fraction, compressive stress and modulus of elasticity of new bone in the defects was several-fold greater (p ≤ 0.005) using CaSO4/CaPO4-TCP composite graft compared to defects treated with autograft. The area fraction and compressive stress of new bone using CaSO4/CaPO4-TCP composite graft were also several fold greater (p≤.009) compared to normal bone, but there was no difference in the modulus of elasticity. Although the compressive stress was still greater (p=0.047) at 26 weeks for defects treated with the composite graft compared to normal bone, the regenerated bone had remodeled to a normal cancellous architecture, incorporating minute fragments of residual graft.

Conclusion: CaSO4/CaPO4-TCP composite graft produced a several-fold greater amount and strength of bone than autogenous graft bone at 13 weeks. There was no modulus mismatch between the regenerated and native cancellous bone. The composite graft holds promise for non-load bearing applications where dense, strong bone formation at earlier time points would be advantageous, potentially resulting in quicker return to activity.