Evaluation and Modeling of Biodegradable Metallic Implants for Load-based Fixation.

摘要

Biodegradable metal implant research is increasing in importance as a possible means to reduce complications associated with multiple surgeries in cases where load based fixation is necessary, but long term physical constraint of bone tissue is problematic. Recent advances in metallurgical studies of magnesium alloys have shown promise for their use as biodegradable implants, but the results of in vitro studies vary widely and do not compare well with in vivo studies.;This research focuses on quantifying the strength performance of a magnesium alloy over time in a balanced salt solution and modeling the performance to optimize implant performance in vivo. Two studies were undertaken. In the first study, three different magnesium plate configurations were evaluated: solid, small holes simulating a porous structure, and large-hole samples simulating a bone plate. Samples were subject to varying lengths of submersion from 0 to 20 weeks in Hanks Balanced Salt Solution at the common human body temperature of 37 degrees Celsius and tested using a four-point bend test. The results indicate slow degradation of the proof load (0.2% offset) over time with a corresponding decrease in average bending stiffness. Corrosion product accumulation caused a gain in mass over time as expected, with the large-hole configuration accumulating the most and the no-hole samples accumulating the least. Small-hole configured samples were similar in mass gain to the large-hole samples. The small-hole and large-hole samples lost approximately 25% of their strength over the 20 week test period and the no-hole samples lost approximately 10%.;In the second study, bone constructs consisting of simulated bone (polyacetal DelrinRTM), AZ31 alloy bone plates, and AZ31 bone screws were subjected to corrosion fatigue testing while immersed in Hanks Balanced Salt Solution for varying lengths of time. The test system imparted three different load mechanisms in 4 point bending orientation to 5 samples each over test durations of 0, 2, 4, 6, 8, 10, 14, 16, and 20 weeks. One set of samples was subjected to static loading, another set to dynamic or cyclic loading and the third set to noload condition. After the corrosion process, corrosion products were removed via chromic acid etching and samples weighed and tested in a 4-point bend test. Bending strength, bending stiffness, and mass loss per week were recorded and compared for each sample.;A mathematical model of the corrosion of AZ31 magnesium alloy was developed to estimate the service life of a resorbable implant. The bending stiffness of previously tested constructs was used to derive a change in the cross sectional area of inertia. The change in cross sectional area of inertia is assumed to be the result of corrosion, and simplifying assumptions of rectangular cross section and pitting corrosion approximating general corrosion allow the derivation of a corrosion penetration model. By applying an estimate of corrosion penetration, the estimate of bending strength can be calculated. The derived model is tested against 2 sets of data, one with corrosion media exposure to 3 sides of a construct, and one with 4 sides exposed. The model agrees within 30% of testing results, and is applicable to any corrosion medium and any alloy as long as corrosion rate estimates are known. The corrosion model may be applied to situations where the corrosion process is not consistent, as is the case with magnesium plates in contact with soft tissue or cortical bone.