Introduction: A previous study reported on the effect of Na doping of calcium polyphosphate (CPP) powders on sintering behaviour to form porous Na-CPP implants as biodegradable bone substitutes or for osteochondral defect repair and regeneration. The present study investigates processing parameters to form such implants with the goal of process optimization to get higher strength, faster degradation and maintain biocompatibility and suitability for in vitro articular cartilage formation. Methods: The formation of 0.01 Na2O/CaO mole ratio CPP was achieved by appropriate blending of sodium carbonate into a precursor powder which after calcining, melting and further processing using a 2-Step sinter/anneal, resulted in sodium-doped CPP (Na-CPP) porous structures. Samples for mechanical and in vitro degradation studies were made using three different Step-2 crystallization anneals (720,835 or 950 C) and characterized by XRD, SS NMR and SEM examination. For mechanical testing (as-made and after in vitro degradation in PBS solution - pH=7.4), 4mm я × 6mm cylindrical compressive test and 4mm я× 2mm DCT samples (for tensile strength determination) were evaluated as-made and after up to 30 days degradation. Weight loss was estimated from Ca2+ concentrations using ICP-AES. Additional 4mm Φ disc samples of optimized-processed' samples were prepared for in vitro cartilage formation as reported using primary bovine chondrocytes. After 3 weeks in culture, the resulting biphasic samples were assessed by histology and biochemical analysis. Non-doped CPP 'controls' were also included for comparison. Results: SEM examination of prepared Na-CPP and CPP 'control' samples indicated larger sinter necks for the Na-CPP samples (sinter neck ratio ≈ 0.6±0.1 cf 0.4±0.1). Fig 1 shows the mechanical test results for the as-made samples prepared using the three different Step-2 annealing temperatures while Fig 2 shows the estimated sample weight loss after 30 days degradation. From these results, the 'optimized-processed' samples were selected as the 835°C Step-2 samples. These displayed strengths and degradation rates approximately double that of pure CPP. Histology and biochemical analysis of in vitro-formed cartilage was the same as for pure CPP 'control' samples. Fig 1 Mechanical strengths of Na-CPP under different Step-2 sintering temperatures. Fig 2 Weight losses of Na-CPP using different Step-2 sintering temperatures after 30-days degradation Discussion: With the goal of higher strength and moderately faster degradation rates compared with the 'control' porous CPP samples, the results suggest optimum processing of Na-CPP using an 835°C, Step-2 anneal which resulted in increased strength due to larger sinter necks and possibly strengthening due to second phase precipitates (sodium polyphosphate below XRD detection limit but suggested by NMR results) and faster degradation due to the presence of this second phase. These effects did not appear to affect biocompatibility as indicated by the observed successful in vitro cartilage formation. Conclusions: The results suggest an advantage of porous Na-CPP over pure CPP when formed using appropriate processing.
展开▼
