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

Abstract

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.

Characterization of load dependent creep behavior in medically relevant absorbable polymers

“…Early creep investigations into scaffolds manufactured through solventcasting and particulate leaching from PLLA and P(D,L)GA suggested that the behavior may be material specific, at least at very low loads (Mikos et al, 1993). More recently, studies of absorbable craniofacial plates and spinal cages suggest that the creep rates are load dependent and that rupture can proceed well before the estimated degradation time Pietrzak, 2012;Smit et al, 2010Smit et al, , 2008. While these studies provide useful creep information, the results are most applicable for the specific form of medical device studied and do not characterize creep from a material perspective or investigate its ability to recover from creep accumulation.…”

The Performance Envelope of Spinal Implants Utilizing Thermoplastic Materials

“…The authors conducted long-term, static failure experiments at various loads and found that PDLLA cages loaded to approximately 50% of their static compressive strength failed within one day. The authors attribute this time dependent behavior of the polymer to its structure, stating that the material maintains to some extent its behavior as a fluid, resulting in plastic flow under sustained loads (Smit et al 2008;Smit et al 2010). These conclusions correlate well to some of the clinical failure experience in PDLLA cages described in the literature.…”

Competitiveness of chitosan-based implants

“…Furthermore, other studies showed an increased incidence of nonunion and post-surgical cage migration in patients undergoing lumbar interbody fusion with PLA-based biodegradable cages versus carbon-fiber implants [71] and PEEK-based implants [72]. As spinal interbody implants need to maintain mechanical integrity for a period of at least six months [73], this has serious implications for the clinical application of absorbable polymer-based implants in load bearing situations. Moreover, some authors declare that the disintegration of absorbable polymerbased implants into particles with a very slow hydrolytic degradation rate, as recommended for spinal fusion applications, can induce and maintain a clinically detectable swelling, with the occurrence of foreign body reactions, allowing skepticism regarding the value of these bioabsorbable implants [74,75].…”