Recombinant Human Bone Morphogenetic Protein-2 Improves Spine Fusion in a Vitamin D-Deficient Rat Model.

BACKGROUND: The effects of vitamin D deficiency on spinal fusion are not well studied, nor are approaches to overcoming deficiency-related detrimental effects. The purpose of this study was to (1) evaluate the effects of vitamin D deficiency on spine fusion in a rat model, and (2) determine whether recombinant human bone morphogenetic protein-2 (rhBMP-2) can improve outcomes in deficient rats. METHODS: Sprague-Dawley rats were assigned to a vitamin D group: vitamin D sufficient (14), vitamin D deficient (16), vitamin D postoperative rescue (15). Posterolateral fusion was performed at L3-4 and L5-6, with one level receiving rhBMP-2 and the other allograft. Following 6 weeks, the spines were harvested for micro-computed tomography (micro-CT) and histological analyses. Fusion was assessed via manual palpation and micro-CT assessment. Micro-CT images were analyzed for bone microarchitecture in intact L5 vertebral bodies and within fused bone masses treated with rhBMP-2. RESULTS: There were no significant effects of vitamin D status on fusion assessments. However, the microarchitecture of native bone in the intact L5 vertebral bodies of vitamin D-sufficient rats showed significantly greater trabecular thickness (P < .001) and bone volume fraction (P < .001), with decreased trabecular spacing (P < .001), than that of vitamin D-deficient rats. Fusion masses of rhBMP-2 levels also showed significant effects of vitamin D supplementation on both bone volume fraction and trabecular thickness. Histological analysis confirmed that robust bone formation was observed in rhBMP-2-treated fusions, but not in fusion levels treated with allograft. CONCLUSIONS: Overall, vitamin D deficiency decreased trabecular bone microarchitecture, and treatment with rhBMP-2 improved outcomes across all vitamin D groups. CLINICAL RELEVANCE: Given the prevalence of vitamin D deficiency in spine surgery patients, vitamin D supplementation may be a cost-effective method for reducing the risk of pseudoarthrosis.

Nanomechanical mapping of the osteochondral interface with contact resonance force microscopy and nanoindentation.

The bone-cartilage, or osteochondral, interface resists remarkably high shear stresses and rarely fails, yet its mechanical characteristics are largely unknown. A complete understanding of this hierarchical system requires mechanical-property information at the length scales of both the interface and the connecting tissues. Here, we combined nanoindentation and atomic force microscopy (AFM) methods to investigate the multiscale mechanical properties across the osteochondral region. The nanoindentation modulus M ranged from that of the subchondral bone (M=22.8±1.8GPa) to that of hyaline articular cartilage embedded in PMMA (M=5.7±1.0GPa) across a narrow transition region <5µm wide. Contact resonance force microscopy (CR-FM), which measures the frequency and quality factor of the AFM cantilever's vibrational resonance in contact mode, was used to determine the relative storage modulus and loss tangent of the osteochondral interface. With better spatial resolution than nanoindentation, CR-FM measurements indicated an even narrower interface width of 2.3±1.2µm. Furthermore, CR-FM revealed a 24% increase in the viscoelastic loss tangent from the articular calcified cartilage into the PMMA-embedded hyaline articular cartilage. Quantitative backscattered electron imaging provided complementary measurement of mineral content. Our results provide insight into the multiscale functionality of the osteochondral interface that will advance understanding of disease states such as osteoarthritis and aid in the development of biomimetic interfaces.

Administration of high-dose macrophage colony-stimulating factor increases bone turnover and trabecular volume fraction.

Macrophage colony-stimulating factor (M-CSF) is a hematopoietic growth factor that plays a critical role in early osteoclastogenesis. To characterize the skeletal effects of M-CSF, we administered soluble M-CSF to mice. It was hypothesized that M-CSF would stimulate bone formation through coupled activity of osteoclasts and osteoblasts. Twenty-four male C57BL/6 J mice (n = 12/group, aged 7 weeks) received subcutaneous injections of human M-CSF [5 mg/(kg day)] or inert vehicle (VEH) for 21 days. M-CSF increased serum bone turnover markers (+57% TRAP-5b and +44% osteocalcin). Microcomputed tomography revealed an anabolic effect on tibial trabecular bone, with higher bone volume fraction (+35%), connectivity density (+79%), and number (+18%), as well as lower trabecular separation (-18%). M-CSF had no significant effect on cortical bone mineral content, geometry, or strength. There was no change in quantitative histomorphometry parameters of femoral cortical bone. These results reveal the complex, site-specific effects of M-CSF. In particular, we have demonstrated an anabolic effect of M-CSF on trabecular bone achieved through coupled activation of osteoblasts. However, in contrast to previous studies, M-CSF was found to have no effect on cortical bone. M-CSF was demonstrated to significantly influence both bone modeling and remodeling in relatively young animals.