236. Ectopic bone formation by submicron structured calcium phosphates: role of the innate immune system

Abstract

BACKGROUND CONTEXT Ectopic implantation of calcium phosphates (CaP) with an advanced submicron surface topography can lead to de novo bone formation. Since implantation initiates a wound healing response, the innate immune system may play a role in this process. PURPOSE Tis study is to investigate the role of immune cells (ie, macrophages) and osteoclasts in material-driven bone formation. STUDY DESIGN/SETTING TCP-B with micron-scaled surface (1-2mm, 1cc/implant) and BCP with submicron-scaled surface (MagnetOs, 1-2mm, 1cc/implant) were implanted in the dorsal muscles of nine beagles for 3 days, 1, 3, 6 and 12 weeks. Histology, immune-histochemistry and bioassays were performed to detect alkaline phosphatase (ALP), tartrate-resistant acid phosphatase (TRAP), Cathepsin K (CTSK), Tumor necrosis factor-alpha (TNF-alpha), Interleukin-10 (IL-10) and bone formation. PATIENT SAMPLE N/A OUTCOME MEASURES Bone formation was identified by histology and ALP expression. M1/M2 macrophages were characterized by IL-10 level (M2) and TNF-alpha level (M1) and the presence of osteoclasts was determined by TRAP level, TRAP staining and CTSK staining. Histomorphometry was also performed to quantify bone formation. One-way ANOVA with Tukey pos-hoc test was applied to distinguish differences between groups (p METHODS At harvesting, samples (n=4 per condition) were cut into halves for bioassays, histology and immune-histochemistry (IHC). Bioassays (Elisa for IL-10 and TNF-alpha; pNPP method for ALP and TRAP) were performed with lysates and the data were normalized to the mass of the samples. Histology (HE, Masson-Golder, ALP and TRAP) and IHC stain (CTSK) were applied on undecalcified sections. HE-stained sections were used to quantify bone (area percentage of bone in available space). RESULTS Bone was absent in TCP-B implants at all time-point, while bone was formed in BCP at week 6 (3/4, CONCLUSIONS Higher IL-10 level in BCP than TCP-B up to week 3, indicates that BCP favors pro-healing M2 macrophage polarization. The higher TNF-alpha level in BCP than TCP-B up to week 1, showed that initially more pro-inflammatory M1 macrophages are present in BCP compared to TCP-B. TRAP/CTSK-positive multinucleated cells were present at week 3, before bone formation (at week 6), which suggests that osteoclastogenesis occurs prior to osteogenesis. These data imply that calcium phosphates with a submicron topography facilitate macrophage recruitment/proliferation, enhance pro-healing M2 macrophage polarization and osteoclast-like cell formation, ultimately leading to ectopic bone formation. FDA DEVICE/DRUG STATUS MagnetOs Granules (Not approved for this indication)