#467 Reducing the Severity of Rhbmp-2 Induced Peri-implant Tissue Swellings: An in vivo Evaluation of Alginate Microbeads with Heparin in a Rodent Model of Posterior Spinal Fusion

General Session: What's New in Biologics and Biomechanics

Presented by: M. Wang


M. Wang (1), (2)
S.A. Abbah (1)
J. Goh (3)
H.K. Wong (1)

(1) National University of Singapore, Department of Orthopaedic Surgery, Singapore, Singapore
(2) Duke-NUS Graduate Medical School, Singapore, Singapore
(3) National University of Singapore, Division of Bioengineering, Singapore, Singapore


Introduction: Recombinant human bone morphogenetic protein 2 (rhBMP-2) delivered on absorbable collagen sponge (ACS) has been used to augment spinal fusion. However, a number of complications including ectopic bone formation and inflammation response (seroma formation) have been reported [1]. These complications are attributed to “supraphysiological” amount of rhBMP-2 applied at the fusion site and the poor modulation capacity of ACS, which leads to protein burst release and washout from implant site. In the present study, a new carrier material, based on alginate microbead surface modified with heparin, was evaluated on its efficacy to deliver minimized amount of bioactive rhBMP-2.

Methods: Thirty eight Sprague Dawley (SD) rats underwent posterior spinal fusion in six groups, namely: (1) 0ug rhBMP-2 on ACS (n = 2 )(negative control); (2) 10ug rhBMP-2 on ACS (n = 4 ) (positive control); (3) 0ug rhBMP-2 with functionalized microbeads (n = 8 ); (4) 4.5ug rhBMP-2 with functionalized microbeads (n = 8 ); (5) 1.5ug rhBMP-2 with functionalized microbeads (n = 8 ); (6) 0.5ug rhBMP-2 with functionalized microbeads (n = 8 ). All carriers were implanted with poly (ε-caprolactone) -20% tricalcium phosphate (PCL-TCP) cage. Rats were monitored for post-operative complications and sacrificed six weeks post implant for Micro CT (mCT) imaging and histology analysis.

Results: On post operative day 5 to 7, gross inspection revealed that seroma formation was observed in all treatment groups (implanted with rhBMP-2) irrespective of carrier material and dose of rhBMP-2; however the rate of seroma formation and size differed considerably. While all animals (100%) in positive control group showed formation of seroma, only 12.5% of the animals in group 6 developed seroma at implant site. Besides, the size of seroma in group 6 was significantly smaller than that in positive control group. Evaluation of fusion using mCT imaging revealed comparable fusion rates in all treatment groups. More importantly, whereas new bone was well contained within the cage in group 6 (0.5ug rhBMP-2 with microbeads), heterotopic bone formation beyond the cage was observed in positive control group (10ug rhBMP-2 on ACS). Similarly, histomorphometric analysis demonstrated that the new carrier with lower dosage of rhBMP-2 maintained bone volume fraction (BV/TV) (0.5ug rhBMP-2, 20.9%) comparable with positive control group (10ug rhBMP-2, 20.6%).

Conclusion: A new carrier material has demonstrated the capacity to minimize the complications associated with rhBMP-2 by reducing the dose with no significant effect on fusion rate. This carrier material utilizes heparin affinity for rhBMP-2 and has been shown to considerably lower protein burst release and washout [2]. The results of the present study indicate that heparin functionalized alginate microbead may represent a new opportunity to define an efficient rhBMP-2 carrier for comparable if not superior clinical outcome with reduced complications and cost.


1. Mroz, T.E., et al., Complications related to osteobiologics use in spine surgery: a systematic review.Spine (Phila Pa 1976), 2010.35(9 Suppl): p. S86-104.

2. Abbah, S.A., et al., Enhanced control of in vivo bone formation with surface functionalized alginate microbeads incorporating heparin and rhBMP-2.Tissue Eng Part A, 2012.

Seroma formation