Ovariectomy-Induced Osteoporosis Does Not Impact Fusion Rates in a Recombinant Human Bone Morphogenetic Protein-2-Dependent Rat Posterolateral Arthrodesis Model

Abstract

Study Design Randomized, controlled animal study. Objective Recombinant human bone morphogenetic protein-2 (rhBMP-2) is frequently utilized as a bone graft substitute in spinal fusions to overcome the difficult healing environment in patients with osteoporosis. However, the effects of estrogen deficiency and poor bone quality on rhBMP-2 efficacy are unknown. This study sought to determine whether rhBMP-2-induced healing is affected by estrogen deficiency and poor bone quality in a stringent osteoporotic posterolateral spinal fusion model. Methods Aged female Sprague-Dawley rats underwent an ovariectomy (OVX group) or a sham procedure, and the OVX animals were fed a low-calcium, low-phytoestrogen diet. After 12 weeks, the animals underwent a posterolateral spinal fusion with 1 μg rhBMP-2 on an absorbable collagen sponge. Representative animals were sacrificed at 1 week postoperative for alkaline phosphatase (ALP) and osteocalcin serum analyses. The remaining animals underwent radiographs 2 and 4 weeks after surgery and were subsequently euthanized for fusion analysis by manual palpation, micro-computed tomography (CT) imaging, and histologic analysis. Results The ALP and osteocalcin levels were similar between the control and OVX groups. Manual palpation revealed no significant differences in the fusion scores between the control (1.42 ± 0.50) and OVX groups (1.83 ± 0.36; p = 0.07). Fusion rates were 100% in both groups. Micro-CT imaging revealed no significant difference in the quantity of new bone formation, and histologic analysis demonstrated bridging bone across the transverse processes in fused animals from both groups. Conclusions This study demonstrates that estrogen deficiency and compromised bone quality do not negatively influence spinal fusion when utilizing rhBMP-2, and the osteoinductive capacity of the growth factor is not functionally reduced under osteoporotic conditions in the rat. Although osteoporosis is a risk factor for pseudarthrosis/nonunion, rhBMP-2-induced healing was not inhibited in osteoporotic rats.

Keywords: osteoporosis; pseudarthrosis; rats; rhBMP-2; spinal fusion.

Fig. 1

Study design. Animals were divided into two groups of 12 animals. Animals in both groups were aged to 24 weeks and underwent either a sham procedure (control group) or ovariectomy (OVX group). Prior to undergoing the spine fusion procedure, the control group received a regular diet and the OVX group received a low-calcium, low-phytoestrogen diet for 12 weeks. One week after the spine fusion procedure, four animals from the OVX group and three animals from the control group underwent serum cytokine analysis. The other eight animals in each group were euthanized at 4 weeks postoperatively for further evaluation with wet uterine weights, radiographs, manual palpation, micro–computed tomography (microCT), and histology.

Fig. 2

Average uterine wet weights for the control and ovariectomy (OVX) groups after harvest at 4 weeks postoperatively.

Fig. 3

Representative radiographs of rodent spine from the (a) control and (b) ovariectomy (OVX) groups 2 weeks postoperatively. Qualitative analysis demonstrated that more osteopenia in the OVX group was supported by mean gray value calculations of the L3 vertebral body in radiographs at (c) 2 weeks and (d) 4 weeks.

Fig. 4

(a) Fusion scores and (b) fusion rates as determined by manual palpation of the control and ovariectomy (OVX) groups 4 weeks postoperatively. Spines with palpation scores greater than or equal to 1.0 were considered fused. Neither fusion scores nor rates were significantly different between the two groups.

Fig. 5

Three-dimensional reconstruction micro–computed tomography (micro-CT) images of bone formation in representative (a) control and (b) ovariectomy (OVX) groups. (c) Average new bone formation by micro-CT analysis at 4 weeks postoperatively in the control and OVX groups were not significantly different (p = 0.39).

Fig. 6

Serum (a) alkaline phosphatase (ALP) and (b) osteocalcin levels in the control and ovariectomy (OVX) groups measured 1 week postoperatively.

Fig. 7

Hematoxylin and eosin stain of sagittal cross sections of the transverse processes at L4 and L5 of animals from (a) the control and (b) ovariectomy groups. Qualitative analysis demonstrates solid fusion masses between L4 and L5 transverse processes in both groups with no substantial differences.