Blockade of Osteoclast-Mediated Bone Resorption With a RANKL-Inhibitor Enhances Bone Formation in a Rat Spinal Fusion Model

Study Design. Rat spine fusion model. Objective. The present study aimed to determine whether administration of osteoprotegerin (OPG) in a rat model of spinal fusion increases bone volume, bone density, and decreases osteoclasts in the fusion mass. Summary of Background Data. OPG is a soluble RANK-ligand inhibitor that blocks osteoclast differentiation and activation. This makes it a potential agent to control the remodeling process and enhance bone mass during spinal fusion. Materials and Methods. Forty-eight male Sprague-Dawley rats received a one-level spinal fusion of L4–L5 with bone allograft. Rats were then divided into four groups according to initiation of treatment: (1) saline on day 0 (saline), (2) OPG on day 0 (OPG D0), (3) OPG on day 10 (OPG D10), and (4) OPG on day 21 (OPG D21) postsurgery. After their initial injection, rats received weekly subcutaneous injections of OPG (10 mg/kg) and were euthanized six weeks postsurgery. MicroCT analysis of the fusion site and histological analysis of bone surface for quantification of osteoclast lining was performed. Results. Increased bone volume in the fusion site and around the spinous process was seen in OPG D0 and OPG D10 when compared with saline. Mean trabecular thickness was greater in all groups receiving OPG compared with saline, with OPG D0 and OPG D10 having significantly greater mean trabecular thickness than OPG D21. All OPG groups had less bone surface lined with osteoclasts when compared with Saline, with OPG D0 and OPG D10 having fewer than OPG D21. Conclusions. This study indicates that OPG inhibited osteoclast bone resorption, which led to greater bone at the fusion site. Future studies investigating OPG on its own or in combination with an osteogenic factor to improve spinal fusion outcomes are warranted to further elucidate its potential therapeutic effect.