Poly(phosphoesters) as Bioabsorbable Osteosynthetic Materials
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Poly(phosphoesters) as Bioabsorbable Osteosynthetic Materials
S. Kadiyala, M. Richards*°, B. Dahiyat, J.D. MichelsonA and K.W. Leong* Department of Biomedical Engineering The Johns Hopkins University Baltimore, MD 21218 *4"The Biomechanics, Trauma and Sports Medicine Laboratory, University of Michigan, Ann Arbor, Michigan 48105 ADepartment of Orthopaedic Surgery *To whom correspondence should be addressed
Introduction Polymeric prosthetic devices enjoy the advantage of tailor-made properties. In particular bioabsorbable polymers possess distinct advantages. In orthopedic applications, most osteosynthesis has been achieved using metallic devices to stabilize fixation of bony fragments. The different elastic moduli of the metallic implants versus that of bone often causes cortical bone to atrophy . The theoretical advantage of gradual load transfer from the fixation device to the bone and the elimination of surgical removal after the healing of a fracture make an absorbable osteosynthetic material extremely useful. A resorbable porous coating material would also have the potential of improving fixation by promoting bony ingrowth and reducing cartilage destruction. The structural support function of a polymeric device can also be integrated with the drug-carrier function to take advantage of the recently discovered growth factors such as bone morphogenic proteins in enhancing bone union. In evaluating the potential of poly(phosphoesters) as degradable biomaterials (1), we have been encouraged by the initial results and have started to examine the interactions of bone with these polymers. The advantages of poly(phosphoesters) lie in their versatility. The physicochemical properties of the polymers can be manipulated by altering either the backbone or the side-chain structure. They are biodegradable because the phosphoester bond in the backbone is cleavable under physiological conditions. The phosphate structure is of special interest to us as phosphate is known to complex with calcium ions, which may lead to heightened interaction with bone. We have optimized the synthesis of these poly(phosphoesters) (2), evaluated their in vitro and in vivo degradation behavior, determined their mechanical properties (1), and examined their drug-carrier characteristics (3). We have also investigated the material response of these polymers to bone through push-out and histologic studies. Mat. Res. Soc. Symp. Proc. Vol. 252.'c 1992 Materials Research Society
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Experimental Synthesis of the poly(phosphoesters) with a bisphenol-A backbone was achieved by an interfacial polycondensation as described previously. Molecular weight distributions of the polymers were determined by gel permeation chromatography (GPC). The chemical structure was confirmed by FT-IR and FT-NMR. The thermal properties were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Contact angles for water in air were measured by goniometry. Swelling studies were conducted according to ASTM Method #D570. The mechanical properti
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