Influence of Laser Irradiation on the Constant Composition Kinetics of Enamel Dissolution

Objectives: As the clinical use of lasers for caries removal and prevention becomes a reality, it is important to establish optimal treatment conditions.  Methods: We compared the influence of CO₂ laser irradiation (l = 9.6mm; with different parameters such as fluence, pulse duration and pulses per spot) on enamel dissolution kinetics using a modified constant composition (CC) method.  The reactions were made in undersaturated calcium phosphate solutions (pH = 4.5, relative undersaturation with respect to hydroxyapatite s = -0.72), enabling extents of dissolution of less than 10 nanomoles of enamel to be quantified, and rate measurements as a function of depth on a micron scale.  Results: Fluences of 0.5 and 1.0 J/cm² yielded dissolution rates (0.98  ± 0.08 and 0.82 ± 0.21 nmol/min, respectively) significantly below those for non-irradiated enamel surfaces (1.82 ± 0.11 nmol/min).  In contrast, higher fluences (1.5 and 3.0 J/cm²) resulted in rates for the first few microns in depth (2.53 ± 0.33 and 2.55 ± 0.45 nmol/min, respectively) appreciably greater than the non-irradiated samples, suggesting surface melting, phase transformation and/or ablation.  This was followed by a rapid decrease with depth, until the final dissolution rates of samples irradiated at 1.5 and 3.0 J/cm² (0.85 ± 0.16 and 0.80 ± 0.06 nmol/min, respectively) were comparable to rates at 1.0 J/cm².  It was also demonstrated that surface damage could be limited to the first 1-2 microns by lowering the number of pulses per spot during the irradiation procedure from 25 to 10.  Conclusion: The constant composition technique allows for precise dissolution kinetics studies of small enamel surfaces (16 mm²).  Using this method, an optimal technique for ablative removal of carious tissue was established, while simultaneously decreasing the solubility of the underlying enamel.

Supported by NIH/NIDCR DE03223 and NIH/NIDCR DE09958.