In vitro assessment of expansion, mechanical stability, and cohesion across a panel of bone graft scaffolds

Abstract Background. Over 400 000 spinal fusions are performed annually in the United States, and nonunion has been reported in 5%–45% of cases, with graft selection contributing to this variability. Scaffold behavior in the hydrated intervertebral environment, particularly volume retention, structural cohesion, and resistance to dissociation, is a less commonly characterized determinant of early graft performance than handling or porosity, despite its mechanistic relevance to space maintenance and cellular ingrowth. Objective. To compare three in vitro performance indicators, axial force generation, volumetric expansion, and material cohesion, across a panel of commercially available bone graft substitutes representing the principal categories used in spinal fusion, including demineralized bone matrix, synthetic ceramic, and peptide-enhanced grafts, together with an amphiphilic fiber-network synthetic scaffold, OsteoFlo® HydroFiber™ by SurGenTec. Testing was performed under standardized hydrated conditions to mimic in vivo environments within an FDA-cleared interbody device. While comparative in vitro evaluations of individual graft categories have been reported, concurrent measurement of these three endpoints across categories within the same testing bed has not, to our knowledge, been described. Methods. Materials were packed into FDA-cleared ALIF PEEK interbody implants and assessed using three standardized tests: axial force generation measured with calibrated load sensors, volumetric expansion determined by vertical displacement, and material cohesion evaluated by stability and dissociation following immersion. The implants were sealed for unidirectional expansion and submerged in deionized water for 48 h. Ten commercially available comparators from nine manufacturers and three graft categories were evaluated alongside the amphiphilic fiber scaffold, with one specimen per product. Comparators were pooled as a reference group; group-level contrasts were assessed using independent t-tests, with results interpreted as descriptive given the single-replicate design. Results. The amphiphilic fiber scaffold exhibited mean vertical expansion of 4.7 mm above the implant rim compared with 0.2 mm for the pooled comparators. Mean maximum axial force was 1.29 N for the amphiphilic fiber scaffold versus 0.33 N for the pooled comparators, corresponding to a 3.8-fold ratio at low absolute force levels. All ten comparators exhibited dissociation on inversion within 48 h, whereas the amphiphilic fiber scaffold retained cohesion at both 24 and 48 h. Conclusion. Under the static hydrated conditions tested, the amphiphilic fiber scaffold demonstrated greater volume retention and cohesion than the comparator materials. The three endpoints evaluated, sustained volumetric expansion, structural cohesion under fluid exposure, and consistent axial contact within the interbody device model, correspond to biomaterial properties widely recognized in the bone healing and spinal fusion literature as mechanistically relevant to bone interface contact, cellular ingrowth, vascularization, and arthrodesis. These in vitro findings are intended as biomechanical performance indicators warranting translational investigation; given the single-replicate design, replicated in vitro testing and subsequent in vivo evaluation are required before inferences can be drawn about clinical fusion performance.