Time-dependent mechanical strength of 70/30 Poly(L, DL-lactide): shedding light on the premature failure of degradable spinal cages.
METHODS
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.


BACKGROUND
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 and 10 mm/s under standard loading conditions (in air at room temperature: +/-23 degrees C). The experiments were also done at body temperature (37 degrees 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.


CONCLUSIONS
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.
Smit
Theo H
TH
VU University Medical Centre, Department of Physics and Medical Technology, Amsterdam, The Netherlands. th.smit@vumc.nl
Engels
Tom A P
TA
Wuisman
Paul I J M
PI
Govaert
Leon E
LE
D020341:Absorbable Implants; D019245:Compressive Strength; D019544:Equipment Failure Analysis; D066298:In Vitro Techniques; D011091:Polyesters; D011475:Prosthesis Failure; D013314:Stress, Mechanical; D013997:Time Factors; D016474:Weight-Bearing
D011091:Polyesters; C033616:poly(lactide)