Comparison of Two Synthetic Bone Graft Products in a Rabbit Posterolateral Fusion Model

Abstract

Background: The drawbacks of iliac crest autograft as graft material for spine fusion are well reported. Despite continued modifications to improve bone healing capacity, the efficacy of synthetic graft materials as stand-alone replacements remains uncertain. The rabbit posterolateral fusion model is an established environment for testing of fusion concepts. It offers the opportunity to obtain radiographic, biomechanical and histological data on novel fusion materials. The objective of this study was to compare the spine fusion capability of two synthetic bone graft products in an established rabbit posterolateral spine fusion (PLF) model: Signafuse® Bioactive Bone Graft Putty and Actifuse® ABX.

Methods: Bilateral intertransverse spine fusion was performed at the L5-L6 transverse processes (TPs) of New Zealand White rabbits using either Signafuse or Actifuse ABX as the bone graft material. Bone remodeling and spine fusion were assessed at 6 and 12 weeks using radiographic, biomechanical and histological endpoints.

Results: Fusion rate by manual palpation at 6 weeks was greater for Signafuse (33%) compared to Actifuse ABX (0%), and equivalent in both groups at 12 weeks (50%). Biomechanical fusion rate based on flexion-extension data was 80% in Signafuse group and 44% for Actifuse ABX. Histology revealed a normal healing response in both groups. MicroCT and histomorphometric data at 6 weeks showed greater new bone formation in the Signafuse group compared to Actifuse ABX (p <0.05), with no differences detected at 12 weeks. Histological fusion scores were greater in the Signafuse group at 6 and 12 weeks, indicated by higher degree structural remodeling and tendency towards complete bridging of the fusion bed compared to the Actifuse ABX group.

Conclusion: Confirmed by several metrics, Signafuse outperformed Actifuse ABX as a standalone synthetic bone graft in an established PLF model, demonstrating greater rates of bone remodeling and spine fusion. The combination of 45S5 bioactive glass and biphasic HA/βTCP granules of Signafuse appear to provide greater bone healing capability in comparison to the 0.8% silicate-substituted hydroxyapatite material of Actifuse ABX.

Figure 1.

Time 0 histology showing the granulate and polymer carrier components of the Signafuse (A) and Actifuse ABX (B) graft materials.

Figure 2.

Representative bilateral microCT sagittal images of the fusion defects at 6 and 12 weeks for Signafuse (5017, 5001) and Actifuse ABX (5027, 5022).

Figure 3.

Representative bilateral non-decalcified histology of the fusion defects at 6 and 12 weeks for Signafuse (5017, 5001) and Actifuse ABX (5027, 5022).

Figure 4.

Representative hi-magnification non-decalcified histology at 6 and 12 weeks showing the remodeling progression of the graft materials for Signafuse (A) and Actifuse ABX (B).