An Enhanced Strength Retention Poly(Glycolic Acid)-Poly(l-Lactic Acid) Copolymer for Internal Fixation

In general, most bioabsorbable polymers used for internal fixation are composed of copolymers of glycolic acid and lactic acid (l and/or dl). Within this family of polymers resides the latitude to design materials with specific degradation properties to meet a variety of clinical needs. One material with an extensive clinical history is LactoSorb copolymer, which is composed of a molar ratio of 82:18 poly(l-lactic acid)-poly(glycolic acid). Fixation devices derived from this copolymer have been used in orthopedic and craniofacial surgeries. With a strength loss profile of approximately 12 weeks, it mirrors the profile of normal healing; however, in cases in which delayed healing is anticipated, a copolymer with a longer strength retention profile may be desirable. This study compared the hydrolytic degradation, in vitro, of the 82:18 copolymer with a related 85:15 poly(l-lactic acid)-poly(glycolic acid) copolymer. In addition to a difference in the copolymer ratio, the latter had a larger inherent viscosity (molecular weight) and a smaller polydispersity (molecular weight distribution). This copolymer had an equivalent initial tensile modulus as the 82:18 copolymer (0.8 GPa) but a strength retention profile that spanned 44 weeks. The activation energy of hydrolysis for the 85:15 copolymer was 38.8 kcal/mol, which was greater than the 23.8 kcal/mol historical value for the 82:18 copolymer. This suggests that hydrolysis of the 85:15 copolymer is more temperature-sensitive than that of the 82:18 copolymer. These results were interpreted in the context of the differences in the compositions of these 2 copolymers.