Time-dependent failure in load-bearing polymers: a potential hazard in structural applications of polylactides

With their excellent biocompatibility and relatively high mechanical strength, polylactides are attractive candidates for application in load-bearing, resorbable implants. Pre-clinical studies provided a proof of principle for polylactide cages as temporary constructs to facilitate spinal fusion, and several cages already made it to the market. However, also failures have been reported: clinical studies reported considerable amounts of subsidence with lumbar spinal fusion cages, and in an in vivo goat study, polylactide spinal cages failed after only three months of implantation, although mechanical testing had predicted sufficient strength for at least eight months. The failures appear to be related to the long-term performance of polylactides under static loading conditions, a phenomenon which is common to all glassy polymers and finds its origin in stress-activated molecular mobility leading to plastic flow. This paper reviews the mechanical properties and deformation kinetics of amorphous polylactides. Compression tests were performed with various strain rates, and static stress experiments were done to determine time-to failure. Pure PLLA appeared to have a higher yield strength than its co-polymers with d-lactide, but the kinetic behaviour of the polymers was the same: an excellent short-term strength at higher loading rates, but lifetime under static stress is rather poor. As spinal implants need to maintain mechanical integrity for a period of at least six months, this has serious implications for the clinical application of amorphous polylactides in load bearing situations. It is recommended that standards for mechanical testing of implants made of polymers be revised in order to consider this typical time-dependent behaviour

Poly(L-lactide) implants for repair of human orbital floor defects:Clinical and magnetic resonance imaging evaluation of long-term results

Purpose: The purpose of this study was to evaluate the long-term outcome of repair of orbital floor defects in patients with resorbable as-polymerized poly(L-lactide) (PLLA) implants and to determine whether these patients showed symptoms that could be indicative of the presence of a late tissue response. Patients and Methods: Six patients (four women, two men; mean age, 39 years; range, 18 to 67 years) treated with PLLA implants for orbital floor fractures were recalled for follow-up examination after a period ranging from 31/2 to 61/2 years. The examination consisted of an interview and a physical examination, including an ophthalmologic and orthoptic consultation, For evaluation of the orbital tissues, coronal spin echo T1- and T2-weighted magnetic resonance images (MRls) were made through both orbits. Results: None of the patients reported any problems in the years preceding the follow-up examination that might have indicated complications, Clinical examination of the operative sites revealed no abnormalities. At ophthalmologic and orthoptic consultation, normal eye function, without diplopia or restriction of motility, was found in all patients. The MRls showed no indication of an abnormal or increased soft tissue reaction in the orbital region. Conclusions: Based on the results of this study, it can be concluded that PLLA orbital floor implants have the potential for successful use in repair of human orbital floor defects

The influence of morphology on the (hydrolytic degradation of as-polymerized and hot-drawn poly (L-lactide))

The influence of morphology on the hydrolytic degradation behavior of poly(L-lactide) has been studied. High molecular weight and highly crystalline as-polymerized poly(L-lactide) was obtained in high yields through melt polymerization. Poly(L-lactide) fiber with a draw ratio of 5.6 was obtained by hot-drawing solution-spun fiber. During the bulk degradation of as-polymerized poly(L-lactide), a rapid decrease of molecular weight and tensile properties was observed. This could be explained by the morphology of the material and the presence of thermal stresses and subsequent generation of microcracks. The lamellar crystallites in as-polymerized poly(L-lactide) appeared to be very stable towards hydrolysis. The resorption time of high molecular weight as-polymerized poly(L-lactide) in vivo was estimated at 40-50 yr by extrapolation of molecular weight data. Hot-drawn poly(L-lactide) fiber showed exceptional hydrolytic stability under a static load and substantially retained its mechanical properties over a period of more than 5 yr. The high perfection of the crystalline fiber and the elimination of micro-voids obtained by hot-drawing prevented penetration of water and induced surface erosion of the fiber