Time-Dependent Mechanical Strength of 70/30 Poly(l,dl-lactide): Shedding Light on the Premature Failure of Degradable Spinal Cages

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Abstract

Study Design.

In vitro studies on the mechanical strength of 70/30 poly(l,dl-lactic acid) (70/30 PLDLLA) cages.

Objective.

To evaluate the effect of loading rate, humidity, temperature, and continuous static loading on the strength of 70/30 PLDLLA, to elucidate the mechanism of premature failure of degradable spinal cages observed in earlier studies.

Summary of Background Data.

Degradable 70/30 PLDLLA cages have been designed to withstand mechanical loads in a goat lumbar spine for at least 6 months. Yet mechanical failure was observed after only 3 months in vivo. We hypothesize that this observation can be related to the time-dependent nature of the polymer.

Methods.

Degradable 70/30 PLDLLA cages were loaded to failure at loading rates between 10−3 and 10−1 mm/s under standard loading conditions (in air at room temperature: ±23°C). The experiments were also done at body temperature (37°C) and under wet conditions. Furthermore, we determined the time-to-failure for 70/30 PLDLLA cages subjected to loads well below their instantaneous mechanical strength.

Results.

The mechanical strength of 70/30 PLDLLA cages was lower for lower loading rates, higher temperature, and higher humidity. The cages already failed within less than 5 minutes when statically loaded at 75% of their strength, and within 1 day when loaded at about 50% of their strength. Extrapolation predicts cage failure at 3 months when loaded at 25% of their strength.

Conclusion.

Premature failure of 70/30 PLDLLA cages, as observed in vivo in earlier studies, is owing to mechanical loading and the time-dependent mechanical properties of the material. The standards for mechanical testing of implants made of strongly time-dependent materials like polylactide should be reconsidered.

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