(55c) Resorbable Mineralized Bone Particle/Polyurethane Composites for Bone Tissue Engineering

Introduction. Bone healing is a complex process that involves the restoration of both anatomy and physiology, there is a recognized need for innovative biomaterials that are able to improve the quality of treatment of patients endure orthopedic defects such as fractures.  We are developing a family of weight-bearing mineralized bone particle (MBP)/polyurethane (PUR) composites to address this clinical need. These MBP/PUR composites are the results of a reactive mixture which can be compression molded or used as a cement paste.  Polymers synthesized from lysine triisocyanate (LTI) and polyester polyols have been reported to support cell attachment and biodegrade to non-cytotoxic porducts^1,2.  We have synthesized composites incorporating 79 wt% MBP and polyester polyol molecular weights. 

Figure 1: Stress-strain plot PUR/MBP composites synthesized from 300 and 600 MW polyols showing a significant increase in modulus and strength using the 300 MW polyol.   

Materials and Methods. Six
New Zealand white rabbits were used in this femoral plug study was designed to study four MBP/PUR treatment groups.  MBP/PUR composites plugs were synthesized from LTI, a polyester polyol, and rabbit mineralized bone particles (MBP ~105 mm). Co-polymer triols (60% e-caprolactone, 30% glycolide, 10% DL-lactide, [6C3G1L]) of molecular weights of 300 and 600 g/mol were synthesized using known published methods. Untreated MBP as well as surface demineralized BP (SDBP) was investigated. The four treatment groups comprised of a combination of MBP and SDMBP and the used of both 6C3G1L300 and 6C3G1L600. Compression testing was conducted on a MTS with a 13 kN load cell.  A 6 mm x 11 mm defect was prepared in the metaphysis of the distal femur of each rabbit, and four treatment groups were randomly divided among the rabbits. The rabbits were sacrificed at 6 weeks and Faxitron x-ray images of the extracted femurs were obtained.  The femurs were embedded in Technovit 7200, cut on an band saw, and ground to approximately 75 microns in thickness.  The prepared slides were stained with Sanderson's rapid bone stain.   

Results. The stress-strain plot of the composites, figure 1, illustrates the effect of polyol molecular weight on the MBP/PUR composite modulus and strength.  Synthesizing composites from 6C3G1L300 achieved a modulus of  5.52 GPa, while composite synthesized with 6C3G1L600 achieved a modulus of 3.1 GPa.  Strength values followed the same trend with values of 165 and 112 MPa for 6C3G1L300 and 6C3G1L600, respectively.  Histology micrographs from the distal femur rabbit study indicate some remodeling at 6 weeks, as shown in Figure 2.  There were no signs of any adverse reactions due to the implant, and the normal healing process proceeded uninterrupted.  Fig. 2A shows the implant surrounded by newly formed bone suggesting a creeping flow mechanism of remodeling.  Higher magnification, fig. 2B, shows resorption of a bone particle as new bone is being concurrently form on other regions of the surface. 

Figure 2: Histology micrographs at a 6 week time point of MBP/PUR composite plug, incorporating 6C3G1L600 polyol and SDBP, implanted in the distal femur of
New Zealand white rabbit. Bone is stained tan/pink, polymer is stained dark blue, and soft tissue is stained light blue.  Newly formed bone is stained dark pink with osteocytes stain blue within the newly mineralized matrix.   (A: 1.25X, B:20 X)

Discussion. The data from these studies suggest that MBP/PUR composites are promising biomaterials for bone tissue engineering.  By varying the molecular weight of the polyol, the modulus of the MBP/PUR composites can be tuned to targets suitable for clinical applications as well as the degradation rate.  The preliminary rabbit study suggests that PUR/MBP composites are biocompatible and biodegradable The radiograph and histological data suggest that the composites remodel when implanted in a femoral plug model in NZW rabbits. 

References. 1. Guelcher SA, Patel V, Gallagher K, Connolly S, Didier JE, Doctor J, Hollinger JO. Tissue Eng 2006;12(5):1247-1259. 2. Guelcher SA, Didier JE, Srinivasan A, Hollinger JO. Synthesis, mechanical properties, biocompatibility, and biodegradation of cast poly(ester urethane)s from lysine-based polyisocyanates and polyester triols. Manuscript in preparation.

Acknowledgements. This work was funded by the Center for Military Biomaterials Research (DOD-2110-PO5-694362) and
Vanderbilt University.