Can rhBMP-2 Overcome the Inhibitory Effect of Alendronate Sodium on Spinal Fusion?

Purpose: To determine if rhBMP-2 can overcome the inhibitory effect of alendronate sodium on spinal fusion.

Materials and Methods: Sixty New Zealand white rabbits underwent a L4-L5 posterolateral /inter-transverse process fusion (n=15/group): Group I- iliac crest autograft control; Group II - autograft + Fosamax; Group III - rhBMP-2 control; Group IV- rhBMP-2 + Fosamax. Based on allometric calculations, rabbits were administered either 1.4mg/animal/dose of alendronate sodium (Grps II+IV) or saline via oral gavage.

Results: Fifty-four rabbits completed the study, Grp I- 13, Grp II - 14, Grp III - 12 and Grp IV- 15 rabbits. By palpation, 72% were considered fused (54%, 43%, 100% and 100% respectively; p 6.5 ROM in flexion-extension as a criteria for pseudarthrosis, inter-group fusion was present in 54%, 36%, 100% and 100% respectively. Both rhBMP-2 groups exhibited significantly lower ROM in each testing mode (flexion-extension, lateral bending, rotation), even when compared to "fused" specimens in Grp I and II (p0.05 for all comparisons of Grp I vs. Grp II, or Grp III vs. Grp IV). Histologically, successful bony union was observed in 54%, 43%, 100% and 100% of cases respectively.

Conclusions: The rhBMP-2 control group showed a 100% fusion rate; however, surprisingly the rhBMP-2 + Fosamax group also showed a 100% fusion rate, with an average Emory score, radiographically evaluated bridging bone, and biomechanical testing which was slightly better than the Fosamax control group. Thus, rhBMP-2 can overcome the inhibitory effect of Fosamax in a rabbit PL fusion model, as well as have a synergistic effect on spinal fusion. This implies that Fosamax may NOT need to be discontinued in clinical practice when a spine fusion is contemplated.

Hibbs Award Nominee for Best Basic Science Paper