Maximizing bone formation in posterior spine fusion using rhBMP‐2 and zoledronic acid in wild type and NF1 deficient mice

Abstract

Spinal pseudarthrosis is a well described complication of spine fusion surgery in NF1 patients. Reduced bone formation and excessive resorption have been described in NF1 and anti-resorptive agents may be advantageous in these individuals. In this study, 16 wild type and 16 Nf1 þ/À mice were subjected to posterolateral fusion using collagen sponges containing 5 mg rhBMP-2 introduced bilaterally. Mice were dosed twice weekly with 0.02 mg/kg zoledronic acid (ZA) or sterile saline. The fusion mass was assessed for bone volume (BV) and bone mineral density (BMD) by microCT. Co-treatment using rhBMP-2 and ZA produced a significant increase (p < 0.01) in BV of the fusion mass compared to rhBMP-2 alone in both wild type mice (þ229%) and Nf1 þ/À mice (þ174%). Co-treatment also produced a significantly higher total BMD of the fusion mass compared to rhBMP-2 alone in both groups (p < 0.01). Despite these gains with anti-resorptive treatment, Nf1 þ/À deficient mice still generated less bone than wild type controls. TRAP staining on histological sections indicated an increased osteoclast surface/bone surface (Oc.S/BS) in Nf1 þ/À mice relative to wild type mice, and this was reduced with ZA treatment. Keywords: neurofibromatosis; NF1; scoliosis; spine fusion Neurofibromatosis type 1 (NF1) is a genetic disease with an incidence of 1 in 3,000 that results from mutations in the NF1 gene. 1 NF1 is expressed in many tissues and its deficiency is associated with a range of characteristic features that present with variable penetrance. Orthopedic manifestations are one of the standard diagnostic criteria for NF1. As many as 25% of individuals will go on to develop some degree of scoliosis. 2,3 Spine fusion surgery to correct the curvature and prevent future deformity is not uncommon, however with NF1 this can have an increased complication rate. 4 Pseudarthrosis at the fusion site may develop in as many as 25% of NF1 cases; in these incidents the adjacent vertebrae are united by fibrous tissue rather than with a solid bone bridge. 5 Solid fusion is seen in as few as 7% of cases. 6 These poor surgical outcomes are likely due to primary defects in NF1 bone metabolism. Up to 30% of adolescents and adults with NF1 show reduced bone mineral density (BMD), 7,8 likely related to reduced bone formation and increased bone resorption. 9 Moreover, poor bone healing is observed in murine models of NF1-deficient and NF1-null fracture repair, and these feature delayed union and pseudarthrosis, respectively. 10,11 Orthopedic models have likewise shown a bone anabolic deficiency as well as excess resorption from abundant osteoclasts. 9 Consequently, combination therapies using recombinant human bone morphogenetic proteins (rhBMPs) together with bisphosphonates have been proposed for the treatment of tibial pseudarthrosis. This strategy is supported by both genetic mouse models of NF1 12,13 and a published case series of NF1 tibial pseudarthrosis patients. 14 Currently therapeutic development for NF1 spine fusion is hampered by a lack of ...

Unilateral Creeping Destruction of Deformed Mandibular Ramus and Angle Associated with Extensive Facial Plexiform Neurofibroma in Neurofibromatosis Type 1: A Case Report with Analysis of the Literature for Diagnosing Osteolytic Events of the Mandible in Tumor-suppressor Gene Syndrome

“…Recently, in vitro and in vivo attempts to influence osteoblasts of patients with NF1 using bisphosphonates have yielded contradictory results for the suitability of this group of compounds for the treatment of NF1-associated osteoporosis (59)(60)(61)(62). The patient presented here had not been medicated with bisphosphonates, therefore the effect of this group of substances has only theoretical significance with respect to the pathogenesis of bone disintegration in NF1 in the presented case.…”

Development of Murine Anterior Interbody and Posterolateral Spinal Fusion Techniques

“…Our posterolateral model is distinguished from previous models in its use of a fusion bed encompassing both the facets and the transverse processes, which increases the total surface area available for fusion, thereby improving the fusion rate. Although a prior mouse model using only intertransverse fusion had a fusion rate of 62.5%, the combination of decortication with autograft was not reported; likewise, most models in the literature have reported decortication alone or adjunct supplementation [11][12][13]15,[18][19][20][21] . The fusion rate in the present study is also greater than the 67% fusion rate observed in a rabbit posterolateral model, indicating that our mouse model is optimized to study fusion biology whereas the rabbit model is better optimized to study differences between fusion adjuncts 22,23 .…”

The Skeletal Phenotype in Neurofibromatosis Type 1 - Structural Defects, Molecular Mechanisms and Therapeutic Approaches

“…However, this approach did not diminish accumulation of fibrotic tissue within the callus. Another approach tested in a posterolateral fusion model of lumbar spine the combination of BMP-2 and zoledronic acid [48]. This Citation: Seifert W, Kolanczyk M, Kühnisch J (2016) The Skeletal Phenotype in Neurofibromatosis Type 1 -Structural Defects, Molecular Mechanisms and Therapeutic Approaches.…”