Combined effects of recombinant human BMP-2 and Nell-1 on bone regeneration in rapid distraction osteogenesis of rabbit tibia

Distraction osteogenesis (DO) has been accepted as an effective technique for bone lengthening. However, the long treatment period and possible fibrous union or nonunion hampers its further clinical application. Bone regeneration in DO involves multiple stages of repair and coordinated action of multiple cell types. Consequently, it may be possible to enhance bone regeneration through treatment strategies that target more than one repair process or cell types. The goal of this study was to determine the combined effects of recombinant human bone morephogenetic protein 2 (rhBMP-2) and NEL-like molecule-1 (NELL-1) on bone formation in DO. Unilateral tibiae in 48 rabbits were lengthened for 7 days at a rate of 2 mm/day after 3-day lag. At the end of distraction, the animals were randomly divided into four groups (n = 12) and received phosphate-buffered saline, 50 μg rhNell-1 or 50 μg rhBMP-2, or both 25 μg rhBMP-2 and 25 μg rhNell-1 at the lengthened segment, respectively. After 4-week consolidation bony healing was assessed using histology, radiography, dual energy X-ray absorptiometry, micro-CT, and three-point bend testing. Treatment with rhNell-1 and/or rhBMP-2 resulted in better bone formation and higher BMD and BMC than the saline group, whilst excellent bone formation and the highest BMD and BMC was observed in the combined treatment group. Both rhNell-1 and rhBMP-2 groups presented more mature characteristics in the micro-architecture than the saline group, whereas the combined treatment group presented the highest BV/TV, Tb.Th and Tb.N as well as the lowest Tb.Sp. The peak load of the lengthened tibia increased by 71% in the combined treatment group, 54% in the rhBMP-2 group, and 25% in the rhNell-1 group compared to the control group, respectively. This work suggests that BMP-2 and Nell-1 enhance each other's ability and dual delivery of two agents can significantly improve bony healing in tibial DO.