MECHANICAL-PROPERTIES OF BIORESORBABLE MA TERIALS IN LACTIC-ACID POLYMERE

Purpose of the study The potential applications of biodegradable osteo synthesis implants present many advantages over conventional metallic devices. Polyesters of the ply and hydroxy-acid type were recognized e arly as serious candidates. These polymers have demonstrated a very go od biocompatibility and are biodegradable in vivo. After biological an d chemical testing poly L. lactic acid 98 (PLA 98) was selected as a c andidate. We used a static and dynamic investigation in vitro to asses s firstly the material properties of PLA 98 and secondly how its chara cteristics could be modified within a physiological environment.Materi al Michel Vert and colleagues have shown that polymers of lactic acid have a similar time to resorption providing they contain 98 per cent o f the ''L'' form of the polymer. In vitro studies were assessed on bar s made in PLA 98. Methods In a first time in vitro studies in traction and flexion on bars allowed an assessment of mechanical properties of PLA 98. in a second time stresses were applied on bars using a physio logical environment (Haemacel -37 degrees C). In a third time we asses sed the mechanical properties at the temperature of 37 degrees C with dynamic tests on bars in traction and flexion. Results The stress-stra in curves on bars showed that the material is fragile. Sterilisation w ith ethylene-oxide did not affect the mechanical properties. When bars were placed in a thermostatically controlled (37 degrees C) physiolog ical environment, the stess-strain curve showed that the material beca me ductile. With a temperature of 37 degrees C and with a frequency be tter that one hertz, the dynamic tests on bars showed that the materia l endurance is good up to 20000 cycles. At 37 degrees C and at the end of one month, the Young modulus and the maximal strain before breakin g lose 50 per cent of their initial value. Discussion All things consi dered and as the digital value showed, the PLA 98 appear to be ten tim es less strong than steel. In a physiological environment the mechanic al properties improved due to hydratation of the polymer. The material become quickly ductile or malleable. This allowed transient loading w ithout causing breakage. Conclusion The mechanical properties of biore sorbable materials are very different from those of stainless steel an d there is a learning curve in their utilisation. The PLA 98 polymer h as demonstrated a very good biocompatibility and is totally biodegrada ble in vivo. With these results we think that PLA 98 can be used in cl inical practice. Indications and clinical use should remain limited to bones regions with low applied stresses.