Effect of radio frequency glow discharge treatment on resorption of poly-L-lactide to be used as intra-bony space maintainer
Puranik, Chaitanya Prakash
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Early loss of deciduous teeth in children can lead to problems in mastication and eruption of succeeding permanent teeth, as a result of closure of space for eruption of permanent teeth. Similarly, loss of bony spaces in adult jaws, required to accommodate dental implants, after extraction and healing, is an undesirable outcome. This investigation anticipates the custom fabrication of implantable bony space maintainers for such situations, employing the biocompatible, resorbable polymer poly-L-lactide (PLLA) in devices beneficially surface-modified by short-time (minutes) low-temperature Radio Frequency Glow Discharge Treatment (RFGDT) for sterilization and favorable bio-adhesion qualities. PLLA is a polymer that generally loses its mechanical strength in about 6 months in body implant sites as a result of degradation due to simple hydrolysis, but the rates and consequences of such biodegradation are unreliably associated with variances in material fabrication, cleaning, and conventional sterilization protocols. In this study, RFGDT of compression-molded PLLA specimens for 2.5 minutes in residual air at a weak vacuum of 0.1-0.2 torr was found to significantly increase the specimens' initial shear and compressive strengths, simultaneously increasing their Critical Surface Tensions (CST) and making their hydrolytic dissolution in physiologic saline more predictable than in the as-manufactured state. Weekly and monthly specimen and saline extracts examination by Multiple Attenuated Internal Reflection Infrared Spectroscopy (MAIR-IR), Differential Interference Contrast (DIC) light microscopy, and pH measurements after 37°C incubation in physiologic saline for 180 and 90 days, respectively, was followed by shear strength and compression strength testing of samples that uniformly maintained their original exterior dimensions. RFGDT specimens had faster strength loss with a tighter mass balance profile and quicker, more uniform breakdown product generation, as judged by pH suppression to the region of 4.0 and more rapid appearance of carboxyl absorption bands in the MAIR-IR spectra of saline extracts. Microscopic inspection, confirming earlier surface texture measurements, revealed that RFGDT diminished larger scale surface blemishes while imparting a fine micro-porosity to the surface zone, consistent with micro-ablation of a stressed superficial layer at the previously reported rate of 100-200A 0 /minute for the equipment utilized. Surface erosion during saline immersion generally preceded bulk strength loss, with a first layer loss at 4-5 weeks post-immersion and then an RFGDT-accelerated bulk hydrolysis obvious by 7 weeks, maintaining material surface chemistry essentially constant throughout as monitored by MAIR-IR spectroscopy. At the 8-week immersion time, generalized surface dissolution contributed to a matte appearance of the original specular reflection for all PLLA specimens, with an accompanying increased CST, but a persistent advantage of the RFGDT specimens in rate of dissolution and strength loss. These significant, measured decrements in bulk strength of RFGDT PLLA while retaining exterior specimen dimensions support the concept that space-maintenance PLLA implants might be custom fabricated for intra-oral implantation and expected to predictably yield to secondary tooth emergence forces over periods of at least 6 months. RFGDT of as-manufactured specimens, by annealing of their superficial layers, provides them with increased and more narrowly distributed starting shear and compressive strength values. Dynamic testing of representative specimens in tissue-wrapped pouches, to simulate intra-oral implant placement, in an "artificial chewing" device for 1,000,000 cycles showed persistence of physical properties in spite of dynamic loading conditions. Fabrication and surface analysis of root-form PLLA implants showed additional benefits of RFGDT as a means to eliminate waxy contaminants from the implant casting process, while providing standardization of the resorbable polymer surface state for more reliable dissolution rate performance.