Strontium releasing composite resin cements

Abstract

While vertebroplasty has proven effective in relieving pain and stabilizing fractures in osteoporotic patients, it currently fails to address the underlying pathology. Strontium releasing composite resin cements, based on borate glass reinforced hydrophilic resin, may present a method of incorporating anti-osteoporotic therapy into a vertebroplasty material, however, a thorough understanding of the release of strontium is needed in order to tailor ion release to achieve and maintain therapeutic thresholds. To achieve this goal, strontium-sodium-borate glasses were investigated to assess the effects of composition on glass properties and ion release. While strontium release was found to be diffusion controlled, high sodium glasses resulted in borate precipitation, possibly presenting a mechanism of mitigating cytotoxicity without limiting strontium release. When incorporated into a Bis-GMA, TEG-DMA , HEMA resin, this material was found to maintain sufficient handling properties for this intended use. Lower HEMA content cements decreased in strength from 44 to 28 MPa over 60 days, while strength was maintained for high HEMA contents (16 MPa), an unusual finding for composite resins. Increased HEMA resulted in increases in borate release effciency (up to 15%) as expected; strontium release was limited (<1.8%) by the formation of strontium phosphate precipitates on the resin surface, demonstrating strong mineralization potential. Strontium release greatly increases in calcium rich SBF, demonstrating the importance of the ionic environment. Ion release under mechanical loading (an implantation variable frequently overlooked in the evaluation of ion release from orthopedic materials) was investigated to assess the effect of compression on release kinetics. While both cyclic and static compression resulted in decreased water sorption, the release of both strontium and boron was increased. This significant increase in ion release demonstrated an important limitation to the unloaded ion release experiments commonly performed. In a pilot implantation model, strontium-releasing cement demonstrated signi cant implant swelling, and direct tissue integration. New bone formation, however, was greater surrounding a non strontium releasing control resin, demonstrating that the beneficial effects were likely imparted by the highly hydrophilic resin, and not strontium release. While materials demonstrated significant release of strontium ions under in vitro testing conditions, no in vivo therapeutic effect was observed.