Small changes in polymer chemistry have a large effect on the bone-implantinterface: evaluation of a series of degradable tyrosine-derived polycarbonates in bone defects

In a series of homologous, tyrosine-based polycarbonates, small changes in the chemical structure of the polymer pendent chain were found to affect th e bone response in a long-term (1280 d) implantation study. Identically siz ed pins, prepared from poly(DTE carbonate), poly(DTB carbonate), poly(DTH c arbonate), and poly(DTO carbonate) were implanted transcortically in the pr oximal tibia and the distal femur of skeletally mature New Zealand White Ra bbits. The tissue response at the bone-implant interface was characterized in terms of the absence of a fibrous capsule (direct bone apposition, indic ative of a bone bonding response) or the presence of a fibrous capsule (ref erred to as the encapsulation response). The relative frequency of direct b one apposition versus encapsulation was recorded for each polymer throughou t the entire period of the study. While all four polymers were tissue compa tible, there was a correlation between the chemical structure of the penden t chain and the type of bone response observed, with poly(DTE carbonate) ha ving the highest tendency to elicit direct bone apposition. Based on in viv o degradation data and the ability of moder polymers with carboxylate group s at their surface to chelate calcium ions, it is proposed that the ability of poly(DTE carbonate) to bond to bone is caused by the facile hydrolysis of the pendent ethyl eater groups which creates calcium ion chelation sites on the polymer surface. The incorporation of calcium chelation sites into the chemical structure of an implant material appears to be a key requireme nt if direct bone apposition/bone bonding is desired. This study demonstrat es that very subtle changes in the chemical composition of an implant mater ial can have significant effects on the long-term tissue response in a clin ically relevant model. (C) 1999 Elsevier Science Ltd. All rights reserved.