Biotechnologically modified calcium phosphate cement for the stabilization of osteoporotic vertebral compression fractures
Osteoporosis is characterized by reduced bone mass and disruption of bone architecture, resulting in an increased risk of fragility fractures, most of which are located in the lumbar spine. Bioresorbable calcium phosphate cement (CPC), mechanically stabilized by the addition of poly(l-lactide-co-glycolide) acid (PLGA) fibers, may be suitable for the vertebroplasty/kyphoplasty of osteoporotic vertebral fractures. However, additional targeted delivery of osteoinductive bone morphogenetic proteins (BMPs) in the CPC may be required to counteract the augmented local bone catabolism and support complete bone regeneration. The present work thus aimed at: 1) analyzing the release kinetics of three BMPs (GDF5, BB-1 and BMP-2) from an injectable brushite-forming PLGA fiber-reinforced CPC and their bioactivity in vitro; 2) generating and characterizating a minimally invasive large animal sheep model for lumbar ventrolateral vertebroplasty; and 3) testing the PLGA-fiber reinforced CPC loaded with low-dose BMP-2 in the newly established model. The quantification of the release by ELISA within 30/31 days showed a maximum of 34% and 17% of the applied dosages (2, 10, 200, and 1000 g/ml) respectively, for GDF5 and BB-1 (BMP-2: 25.7% release of 400 g/ml within 14 days).The addition of 5% and 10% PLGA fibers augmented the BMP release within 14 days. Notably, in several cell lines the BMPs released within 3 days demonstrated bioactivity, in selected cases augmented by the addition of 10% fibers. In order to develop a large animal model suitable for preclinical studies, lumbar defects were created by a ventrolateral percutaneous approach in aged, osteopenic, female sheep (L1: untouched; L2: empty defect; and L3: CPC). The minimally invasive model resulted in short operation time and postoperative recovery, very limited local trauma, and high experimental reproducibility. The model was then used for the vertebral injection of CPC containing different dosages of BMP-2 (1, 5, 100, and 500 g). Three and 9 months after operation, BMP-2 improved all parameters of bone formation, bone resorption, and bone structure, as well as the compressive strength. The BMP-2 effects on bone formation were dose-dependent, with 5 to 100 g as the optimal dosage. Overall the results confirmed the suitability of the drug-loaded CPC as a carrier for a one-time delivery of osteogenic proteins such as BMP-2 in minimally invasive surgery. Thus, the new CPC may represent an alternative to the bioinert, supraphysiologically stiff polymethylmethacrylate cement currently used to treat osteoporotic vertebral fractures by vertebroplasty/kyphoplasty.