Background:

Calcium phosphate materials have been employed clinically as bone void fillers for several decades. These materials are most often provided in the form of small, porous granules that can be packed to fill the wide variety of size and shape of bony defects encountered. ReBOSSIS-85 (RB-85) is a synthetic bioresorbable bone void filler for the repair of bone defects with handling characteristics of glass wool-like (or cotton ball-like). The objective of this study is to evaluate the in vivo performance of RB-85 (test material), compared to a commercially available bone void filler, Mastergraft Putty (predicate material), when combined with bone marrow aspirate and iliac crest autograft, in an established posterolateral spine fusion rabbit model.

Methods:

One hundred fifty skeletally mature rabbits had a single level posterolateral fusion performed. Rabbits were implanted with iliac crest bone graft (ICBG), Mastergraft Putty™ plus ICBG, or one of 4 masses of ReBOSSIS-85 (0.2, 0.3, 0.45, or 0.6 g) plus ICBG. Plain films were taken weekly until euthanasia. Following euthanasia at 4, 8, and 12 weeks, the lumbar spine were tested by manual palpation. Spinal columns in the 12 week group were also subjected to non-destructive flexibility testing. MicroCT and histology were performed on a subset of each implant group at each euthanasia period.

Results:

Radiographic scoring of the fusion sites indicated a normal healing response in all test groups. Bilateral radiographic fusion rates for all test groups were 0% at 4 weeks; ICBG 43%, Mastergraft Putty 50%, RB-85-0.2g 0%, RB-85-0.3g 13%, RB-85-0.45g 38%, and RB-85-0.6g 63% at 8 weeks; and ICBG 50%, Mastergraft Putty 50%, RB-85-0.2g 0%, RB-85-0.3g 25%, RB-85-0.45g 36%, and RB-85-0.6g 50% at 12 weeks.Spine fusion was assessed by manual palpation of the treated motion segments. At 12 weeks, ICBG, MGP, and RB-85-0.6g were fused mechanically in at least 50% of the rabbits. All groups demonstrated significantly less range of motion in both flexion/extension, lateral bending, and axial rotation compared to normal unfused controls.Histopathology analysis of the fusion masses, in all test groups, indicated an expected normal response of mild inflammation with macrophage and multinucleated giant cell response to the graft material at 4 weeks and resolving by 12 weeks. Regardless of test article, new bone formation and graft resorption increased from 4 to 12 weeks post-op.

Conclusions:

This animal study has demonstrated the biocompatibility and normal healing features associated with the ReBOSSIS-85 bone graft (test material) when combined with autograft as an extender. ReBOSSIS-85 was more effective when a larger mass of test article was used in this study.

Clinical relevance:

ReBOSSIS-85 can be used as an extender negating the need for large amounts of local or iliac crest bone in posterolateral fusions.