Minimizing the Severity of rhBMP-2-Induced Inflammation and Heterotopic Ossification With a Polyelectrolyte Carrier Incorporating Heparin on Microbead Templates

Study Design. A rodent model of posterior spinal fusion. Objective. The aim of this study was to evaluate the efficacy of low-dose recombinant human bone morphogenetic protein-2 (rhBMP-2) delivered with a heparin based polylectrolyte complex (PEC) carrier in facilitating posterior spinal fusion while concurrently minimizing seroma and heterotopic ossification. Summary of Background Data. rhBMP-2 is being used to augment spinal fusion. However, complications such as heterotopic ossification and local soft tissue swellings have been reported. These are attributed to supraphysiological amount of rhBMP-2 and the poor modulation capacity of absorbable collagen sponge. Methods. Forty rats were randomized into 6 groups as follows. Group I: absorbable collagen sponge without rhBMP-2 (n = 4); group II: positive control, absorbable collagen sponge + 10 mu g rhBMP-2 (n = 4); group III: alginate-(poly-l-lysine)-heparin (PEC) without rhBMP-2 (n = 8); group IV: PEC + 4.5 mu g rhBMP-2 (n = 8); group V: PEC + 1.5 mu g rhBMP-2 (n = 8); group VI: PEC + 0.5 mu g rhBMP-2 (n = 8). Results. Between postoperative days 5 and 7, seroma was observed in all rhBMP-2 implanted groups irrespective of carrier and dose. However, the rate and size of seroma differed considerably. Although all animals (100%) in positive control group showed seroma, only one animal (12.5%) in group VI developed seroma at the implant site. The size of seroma in group VI was significantly smaller than that in positive control group. Micro-computed tomography evaluation revealed comparable mean fusion scores in all rhBMP-2 implanted groups. More importantly, although new bone was well contained within the cage in group VI, heterotopic ossification beyond the cage was observed in positive control group. Conclusion. A new carrier has demonstrated capacity to minimize seroma formation as well as heterotopic ossification associated with rhBMP-2 by reducing the efficacious dose needed for consistent fusion. The results of this study indicate that PEC alginate microbeads may represent a new opportunity to define an efficient rhBMP-2 carrier.