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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.

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Left: True strain versus loading time for increasing stresses. Right: Stress dependence of the time-to-failure (−α). Dots are single measurements
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Fig4: Left: True strain versus loading time for increasing stresses. Right: Stress dependence of the time-to-failure (−α). Dots are single measurements

Mentions: The time-dependent failure of glassy polymers is illustrated by the behaviour of PLLA in compression under a variety of strain rates and stresses (Figs. 3 and 4). Figure 3 (left) shows the intrinsic behaviour of PLLA as measured under compression at a constant true strain rate, resulting in homogeneous deformation over large strains. Initially the material behaves linear-elastic, eventually reaching a maximum: the yield stress (here at 4% strain and a stress of app. 94 MPa). Subsequently, two characteristic phenomena are observed [30]: (1) Strain softening, the initial decrease of true stress with strain, which implies that less energy is required for further deformation of the specimen, so that failure continues. (2) Strain hardening, the subsequent upswing of the true stress–strain curve, which implies that further deformation requires more energy, thereby inhibiting further failure. The interplay between strain softening and strain hardening for a large extend determines the toughness of a material: materials with strong softening and weak hardening behave brittle, and materials with weak softening and strong hardening tough [31]. Polylactides thus fail brittle, at least under higher loading rates, and this was also observed in the in vivo goat study [28] (Fig. 2b). For the designer of polylactide implants, this implies that stress concentrations, such as holes or sharp teeth on the rims of cages, should be avoided.Fig. 3


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)

Left: True strain versus loading time for increasing stresses. Right: Stress dependence of the time-to-failure (−α). Dots are single measurements
© Copyright Policy
Related In: Results  -  Collection

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

Fig4: Left: True strain versus loading time for increasing stresses. Right: Stress dependence of the time-to-failure (−α). Dots are single measurements
Mentions: The time-dependent failure of glassy polymers is illustrated by the behaviour of PLLA in compression under a variety of strain rates and stresses (Figs. 3 and 4). Figure 3 (left) shows the intrinsic behaviour of PLLA as measured under compression at a constant true strain rate, resulting in homogeneous deformation over large strains. Initially the material behaves linear-elastic, eventually reaching a maximum: the yield stress (here at 4% strain and a stress of app. 94 MPa). Subsequently, two characteristic phenomena are observed [30]: (1) Strain softening, the initial decrease of true stress with strain, which implies that less energy is required for further deformation of the specimen, so that failure continues. (2) Strain hardening, the subsequent upswing of the true stress–strain curve, which implies that further deformation requires more energy, thereby inhibiting further failure. The interplay between strain softening and strain hardening for a large extend determines the toughness of a material: materials with strong softening and weak hardening behave brittle, and materials with weak softening and strong hardening tough [31]. Polylactides thus fail brittle, at least under higher loading rates, and this was also observed in the in vivo goat study [28] (Fig. 2b). For the designer of polylactide implants, this implies that stress concentrations, such as holes or sharp teeth on the rims of cages, should be avoided.Fig. 3

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