105. Sex-based differential response in rhBMP-2-mediated spinal fusion in vivo

Abstract

BACKGROUND CONTEXT Sex-based differences in clinical outcomes for spinal fusion have been investigated with conflicting results. Estrogen is well known to have a major influence on bone and interacts with the recombinant human bone morphogenetic protein-2 (rhBMP-2) pathway, yet literature investigating the potential but unsubstantiated sex-dependent differential response to rhBMP-2 spine fusion is scarce. Clarifying whethera sex-based differential bone regenerative response could improve and personalize patient care in the setting of spine and other orthopedic procedures requiring bone healing. Herein, we investigated whether there is a sex-dependent bone regenerative response to rhBMP-2 in a rat spinal fusion model. PURPOSE The purpose of this study was to assess and validate the presence and extent of the sex differences in bone response to rhBMP-2 in vivo. STUDY DESIGN/SETTING Pre-clinical. PATIENT SAMPLE Male and female Sprague-Dawley rats, ages 12-16 weeks. OUTCOME MEASURES Radiography, fusion scoring, microCT. METHODS Twenty-eight Sprague-Dawley male and female rats underwent L4-L5 posterolateral fusion with bilateral placement of an absorbable collagen sponge loaded with 10 μg of rhBMP-2/animal (INFUSE™; 5 μg/implant; N=14/group). Bone regenerative response and fusion were assessed eight weeks postoperatively via radiography and blinded manual palpation, where each specimen was assigned a score of 0 (unfused, no bridging bone), 1 (unilaterally fused, L4-L5 bridging), or 2 (bilaterally fused). MicroCT imaging (N=3/group) was used for microarchitectural analysis to evaluate new bone, the bone volume fraction, and trabecular thickness. RESULTS As expected at this dose, radiography and manual palpation showed 100% fusion rates for both sexes. Fusion scores, though, were significantly different with females having a lower fusion score (female: 1.26±0.31 vs males: 1.55±0.33, *p=0.001). Radiography showed noticeably smaller fusion mass volumes in females vs males, but the fusion mass was found to be less compressible on manual palpation in females relative to males. MicroCT evaluation showed significant differences in females vs males, with females having significantly lower new bone volumes (female: 26.591±3.871 mm3 vs male: 37.595±11.174 mm3, *p=0.046), but a significantly higher bone volume fraction (female: 0.266±0.116 vs male: 0.126±0.064, *p=0.028) and significantly greater trabecular thickness (female: 0.198±0.016 mm vs male: 0.178±0.014 mm, *p=0.041). CONCLUSIONS This study shows that there may indeed be a sex-based difference in bone regeneration induced by rhBMP-2. We showed that female rats have higher quality, but less volume of new bone formation compared to males. Future work will evaluate the extent of this distinction, especially at sub-therapeutic doses of the growth factor, and will investigate the interplay between the BMP-2 pathway and sex hormones in bone regenerative outcomes. FDA DEVICE/DRUG STATUS Unavailable from authors at time of publication.