Biologic Fusion Is More Than Filling Space

The Biology Behind a Durable Fusion

Successful spinal fusion depends on more than instrumentation alone. Long-term stability ultimately relies on whether the graft can integrate, remodel, and form living bone across the fusion bed.

Autograft has long remained the gold standard because it provides osteogenic, osteoinductive, and osteoconductive properties together. However, donor-site morbidity, limited graft availability, and increased operative burden continue to drive the search for biologically active alternatives.

Not all graft materials behave the same way once implanted. Some primarily function as space occupiers, while others actively participate in the healing environment through angiogenesis, cellular signaling, and progressive bone remodeling. Clinically, that difference may determine whether durable bone bridging develops.

Evaluating Biologic Graft Behavior

This preclinical study evaluated whether combining demineralized bone matrix (DBM) with bioactive glass could create a more effective fusion environment than bioactive glass alone.

Investigators compared NanoFUSE® DBM — a combination of DBM and bioactive glass with standalone bioactive glass putty (NovaBone Putty®), autograft, and combined graft constructs in a rabbit posterolateral fusion model involving 64 rabbits evaluated over 24 weeks. Outcomes included radiographic fusion, manual palpation, histology, and biomechanical testing. 

Remodeling Versus Residual Graft Persistence

What mattered most was how the graft behaved over time.

NanoFUSE® DBM demonstrated progressive new bone formation, bridging fusion, and active remodeling throughout the study period. By 24 weeks, histologic analysis showed only 1–25% residual implant material remaining, suggesting that much of the graft had been resorbed and replaced by newly formed bone. 

In contrast, the standalone bioactive glass group demonstrated persistent residual graft material with limited incorporation into native bone. No successful fusion was observed in that cohort through either manual palpation or radiographic assessment. 

When combined with autograft, NanoFUSE® DBM achieved fusion behavior approaching autograft controls, with 24-week fusion rates of 56% compared with 67% for autograft alone by manual palpation assessment. 

Biomechanical testing further supported these findings. NanoFUSE® DBM combined with autograft demonstrated comparable load tolerance and stiffness to autograft alone, while also demonstrating the highest energy absorption values among all treatment groups. 

Image 1: Representative 24-week histologic sections demonstrating progressive graft remodeling and incorporation within the posterolateral fusion environment.

Image source: https://sciencepublishinggroup.com/article/10.11648/j.ijbmr.20150303.11 

Why Graft Behavior Matters

One of the more important takeaways from this study is that radiographic graft presence alone may not reflect successful fusion biology.

A graft that simply remains visible on imaging may contribute less effectively to long-term stability than one that actively remodels and incorporates into native bone. Graft activity including osteoinduction, angiogenesis, and progressive remodeling may ultimately matter more than graft volume itself.

These findings also reinforce the growing role of active graft extenders designed to reduce autograft burden while still supporting robust fusion performance.

The Bigger Shift in Fusion Strategy

As spine surgery continues moving toward tissue preservation and biologic optimization, graft technologies that combine osteoconductive and osteoinductive mechanisms may become increasingly relevant.

This study suggests that combining DBM with bioactive glass may create a more favorable healing environment than bioactive glass alone not simply by filling space, but by actively participating in remodeling and bone formation over time. 

Key Clinical Insight

Fusion success depends not only on graft placement, but on graft integration and remodeling within the healing environment. In this preclinical model, NanoFUSE® DBM demonstrated more favorable incorporation, bridging bone formation, and biomechanical performance compared with standalone bioactive glass, supporting continued interest in biologically active graft technologies for spinal fusion.

Interested in exploring the full study? https://sciencepublishinggroup.com/article/10.11648/j.ijbmr.20150303.11