Abstract
Summary Statement
The present study demonstrates the beneficial effects of strontium (Sr) modified calcium phosphate cement to improve new bone formation in a metaphyseal osteoporotic fracture defects in rats compared to calcium phosphate cement and empty defects. Keywords: strontium, fracture, calcium phosphate, bone formation
Introduction
Impaired fracture healing with subsequent implant failure is a dramatic problem in osteoporotic fractures. Biomaterials are of interest to stimulate fracture healing in osteoporotic defects and the objective of the current study is to investigate the effects of Strontium modified calcium phosphate cement (SrCPC) in a critical-size metaphyseal fracture defect of osteoporotic rats compared to calcium phosphate (CPC) and empty defect control group.
Methods
45 female Sprague-Dawley rats were randomized into 3 groups: SrCPC, CPC and empty defect (n=15 for each). A combinatorial approach of multi-deficiency diet for 3 months after bilateral ovariectomy was used for induction of osteoporosis. Left femur of all animals underwent a 4mm wedge-shaped metaphyseal osteotomy that was internally fixed with a T-shaped plate. The defect was then either filled with CPC or SrCPC and internally stabilised with a T shaped mini-plate. Empty defect served as a control. After 6 weeks femora were harvested followed by histological, histomorphometrical, immunohistochemical (bone-morphogenic protein 2, osteocalcin and osteoprotegerin), and molecular biology analysis (alkaline phosphatase, collagen10a1 and osteocalcin) to demonstrate the effects of the biomaterials on new bone formation. Time of flight secondary ion mass spectrometry (TOF-SIMS) technology was used to assess the distribution of released strontium ions and calcium appearance of newly formed bone.
Results
Histomorphometric analysis showed a statistically significant increase in the bone formation at the tissue-implant interface in the SrCPC group (p<0.001). A statistically significantly more cartilage and unmineralised bone formation was also seen in the SrCPC group in comparision to the CPC group alone (p<0.05) and also to the empty defect (p<0.05) in the former fracture defect zone. These data were confirmed by the immunohistochemistry results which revealed an increase in bone-morphogenic protein 2, osteocalcin and osteoprotegerin and an increase in expression of genes responsible for bone formation viz. alkaline phosphatase, collagen10a1 and osteocalcin. TOF-SIMs analysis showed a higher release of Sr from the SrCPC into the interface region and related to a higher calcium content in this area compared to CPC.
Discussion/Conclusion
SrCPC treatment showed enhanced new bone formation in a metaphyseal osteoporotic fracture defect of rats after 6 weeks compared to CPC-filled and empty defects in histomorphometry, immunochemistry and gene expression analysis. Strontium ranelate is a well-known anti-osteoporotic drug increasing bone formation and reducing bone resorption. As revealed by TOF-SIMS release of Sr out of the the SrCPC cement is most likely attributable for new bone formation. Therefore, Sr seems to be a good candidate not only for systemic treatment in osteoporosis but also in Sr-modification of biomaterials for local stimulation of new bone formation in osteoporotic fracture defects.
