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

Smit TH, Engels TA, Söntjens SH, Govaert LE - J Mater Sci Mater Med (2009)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Orthopaedic Surgery, VU University Medical Centre, P.O. Box 7057, 1007MB, Amsterdam, The Netherlands. th.smit@vumc.nl

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.

Show MeSH

Related in: MedlinePlus

Plastic deformation of e-beam sterilised 70/30 PLDLLA cages (top right) with an original height of 10 mm under various low-loading regimes. After three months of compressive loading at 500 N, the cage was severely deformed (lower right). When sterilized by EtO, the cages showed much less deformation (open circles)
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2837159&req=5

Fig7: Plastic deformation of e-beam sterilised 70/30 PLDLLA cages (top right) with an original height of 10 mm under various low-loading regimes. After three months of compressive loading at 500 N, the cage was severely deformed (lower right). When sterilized by EtO, the cages showed much less deformation (open circles)

Mentions: In order to place previous findings into clinical perspective, we also compressed e-beam sterilised 70/30 PLDLLA cages under real-time degradation conditions (Fig. 7). Cages with an original height of 10 mm had a yield strength of 7.1 kN at a strain rate of 1.3 mm/min (0.2 mm/s) [33]. When placed under a static load of only 500 N, plastic deformation was more than 1 mm after only three weeks of loading, and at 3 months, the cage had failed entirely (Fig. 7, right). It should be emphasised, that the instantaneous strength (i.e. the yield strength at a loading rate of 1.3 mm/min) after 6 months of real-time degradation at 37°C without loading was still 5.8 kN [33], an order of magnitude higher than the load applied in this experiment. Thus, plastic deformation of the cage is due to viscous flow of PLDLLA, not to degradation.Fig. 7


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

Smit TH, Engels TA, Söntjens SH, Govaert LE - J Mater Sci Mater Med (2009)

Plastic deformation of e-beam sterilised 70/30 PLDLLA cages (top right) with an original height of 10 mm under various low-loading regimes. After three months of compressive loading at 500 N, the cage was severely deformed (lower right). When sterilized by EtO, the cages showed much less deformation (open circles)
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2837159&req=5

Fig7: Plastic deformation of e-beam sterilised 70/30 PLDLLA cages (top right) with an original height of 10 mm under various low-loading regimes. After three months of compressive loading at 500 N, the cage was severely deformed (lower right). When sterilized by EtO, the cages showed much less deformation (open circles)
Mentions: In order to place previous findings into clinical perspective, we also compressed e-beam sterilised 70/30 PLDLLA cages under real-time degradation conditions (Fig. 7). Cages with an original height of 10 mm had a yield strength of 7.1 kN at a strain rate of 1.3 mm/min (0.2 mm/s) [33]. When placed under a static load of only 500 N, plastic deformation was more than 1 mm after only three weeks of loading, and at 3 months, the cage had failed entirely (Fig. 7, right). It should be emphasised, that the instantaneous strength (i.e. the yield strength at a loading rate of 1.3 mm/min) after 6 months of real-time degradation at 37°C without loading was still 5.8 kN [33], an order of magnitude higher than the load applied in this experiment. Thus, plastic deformation of the cage is due to viscous flow of PLDLLA, not to degradation.Fig. 7

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Orthopaedic Surgery, VU University Medical Centre, P.O. Box 7057, 1007MB, Amsterdam, The Netherlands. th.smit@vumc.nl

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.

Show MeSH
Related in: MedlinePlus