Hollow calcium phosphate microcarriers for bone regeneration: In vitro osteoproduction and ex vivo mechanical assessment

Abstract Synthetic grafting materials, such as calcium phosphates (hydroxyapatite, HA; tricalcium phosphate, TCP), polymers, or composites thereof, can be used as osteoconductive scaffolds and delivery vehicles for osteoinductive growth factors. Carrier materials must be engineered to deliver these factors in a controlled fashion at a rate and dose consistent with the biological need and responsiveness of the system to optimize bone formation and ingrowth. They should also simultaneously provide mechanical support and slowly resorb as new bone is formed. This investigation assessed the elution characteristics of BMP-7 (OP-1) from hollow calcium phosphate spheres of varying chemical composition (HA/beta-TCP) and porosity (dense/porous). The pharmacokinetics indicated a bimodal trend of protein release with protein elution peaking between fifteen and thirty minutes in solution (bolus release) and continuing through the eight-week time point (sustained release). Eluted OP-1 bioactivity was characterized over a three-week period using mesenchymal stem cell (MSC) cultures and included assessment of the protein's differential, proliferative, and calcified nodule forming abilities. Alkaline phosphatase enzyme (ALP) activity in MSCs peaked between 12 and 16 days post-OP-1 exposure. Elutant from the HA dense treatment group induced the highest degree of ALP expression while elutant from the beta-TCP treatment groups induced the formation of significantly higher numbers of calcified nodules in culture. The aggregate modulus of a clinically relevant 2 cc dose of carriers was quantified using custom designed testing fixtures to investigate the effects of carrier size, porosity, chemical composition, and the presence of a central hole on mechanical integrity. Significant increases in moduli were noted for carrier size and chemical composition (HA>beta-TCP). These preliminary in vitro and ex vivo results indicate the clinical potential of the hollow calcium phosphate carriers as successful load-bearing delivery vehicles for OP-1.