Polymer-calcium phosphate composites for use as an injectable bone substitute

Calcium phosphate (CP) materials have been used extensively for bone replacement and augmentation due to their similarity to the mineral component of bone. In addition to being non-toxic, they are biocompatible, not recognized as foreign in the body, and most importantly, exhibit bioactive behavior, being integrated into the tissue by the same processes active in remodeling healthy bone. This leads to an intimate physicochemical bond between the CP implants and bone, termed osseointegration. The main limitation, however, of bulk CP materials is their brittle nature and poor mechanical properties. As a result, these materials have been used clinically only in non-load-bearing indications, primarily as granules and blocks. The inability to sculpt the bulk materials to conform to irregular defects and the possibility of the granules migrating from the implant site has resulted in the formulation of self-setting calcium phosphate cements (CPCs). These materials set by a precipitation reaction similar to that of gypsum, and can be molded to desired shapes or injected into defects in minimally invasive procedures. Nevertheless many of these cements still suffer from poor mechanical properties. In the present study, the possibility of strengthening a commercially available CPC, ±-BSMTM, by incorporating polymers into the cement paste was investigated. A number of polyelectrolytes, poly(ethylene oxide), and the protein Polymer-calcium phosphate composites for use as an injectable bone substitute file:///E|/pjflory users/Rafal/Thesis/Thesis-4.htm (2 of 42) [7/19/2001 4:36:20 PM] bovine serum albumin (BSA) were blended with the cement paste to create calcium phosphate-polymer composites. Composites formulated with the polycations poly(ethyleneimine) and poly(allylamine hydrochloride) exhibited compressive strengths up to six times greater than the pure ±–BSMTM material, with a maximum value reached at intermediate polymer content, and for the highest molecular weight studied. Composites containing BSA developed compressive strengths twice that of the original cement at concentrations between 10-25% by weight. In each case, XRD studies correlate the improvement in compressive strength with reduced crystallite dimensions as evidenced by a broadening of the (0,0,2) reflection. This suggests that polycation or BSA adsorption inhibits crystal growth, and possibly leads to a larger crystal aspect ratio. SEM results indicate a denser, more cohesive, and interdigitated microstructure. The increased strength was also attributed to the capacity of the polymeric matrix to absorb energy through plastic flow, and the ability of the high molecular weight polymers to both bridge between crystals and form entanglements with other chains, resulting in an interconnected mesh. Thesis Supervisor: Anne M. Mayes Title: Associate Professor of Polymer Physics