Fried Daniel, Featherstone John D B, Le Charles Q, Fan Kenneth
Department of Preventive and Restorative Dental Sciences, University of California, San Francisco, California 94143, USA.
Lasers Surg Med. 2006 Oct;38(9):837-45. doi: 10.1002/lsm.20385.
Previous studies have demonstrated that lasers can be used to modify the chemical composition of dental enamel to render the mineral phase more resistant to acid dissolution with minimal peripheral thermal damage. Transverse excited atmospheric (TEA) CO(2) lasers tuned to the strong mineral absorption of hydroxyapatite (HAP) near lambda = 9 microm are well-suited for the efficient ablation of dental hard tissues if the laser-pulse is stretched to greater than 5-10 microseconds to avoid plasma shielding phenomena. Moreover, TEA CO(2) lasers can be operated at very high repetition rates and are inherently less expensive and more versatile than Er:YAG and Er:YSGG solid-state lasers. In this study a lambda = 9.3-microm TEA CO(2) with a pulse duration of 8 microseconds and a repetition rate of 300 Hz was used to uniformly treat bovine enamel surfaces at ablative irradiation intensities. We hypothesized that a uniform surface layer of modified enamel of improved crystallinity and CaP phase composition would be formed with an enhanced resistance to acid-dissolution in the ablated areas at higher scanning rates used with the water spray. Such a modified layer of enamel formed at the base and walls of a cavity preparation under the irradiation conditions employed in this study have the potential to inhibit secondary caries under sealants and restorations.
STUDY DESIGN/MATERIALS AND METHODS: The surfaces of bovine enamel blocks (3 x 3 mm(2)) were rapidly scanned across the laser beam at rates of 2, 3, and 6 mm/second with and without a water-spray at an incident fluence of 30 J/cm(2). The resistance to acid dissolution was evaluated using controlled surface dissolution experiments on laser-irradiated and control samples.
The groups irradiated at a fluence of 30 J/cm(2) with a repetition rate of 300 Hz and a high scan rate of 6 mm/second with and without water-cooling significantly reduced the overall surface dissolution rates (P < 0.001). At low scan rates (2-3 mm/second) excessive heat deposition resulted in the formation of an outer layer of asperities containing non-apatitic CaP phases that were more susceptible to acid-dissolution. At a scanning rate of 6 mm/second even without the water spray a layer of purer phase HAP was formed without thermal damage, indicating that a high scanning rate can be used to avoid excessive thermal damage during ablation. The best results (80% inhibition) were attained for the higher scanning speed 6-mm/second combined with a water spray.
This study demonstrates that an enamel surface with enhanced resistance to acid dissolution is produced after ablation with lambda = 9.3-microm TEA CO(2) laser pulses delivered at high-repetition rates if sufficiently high scanning rates are used with or without a water-spray.
先前的研究表明，激光可用于改变牙釉质的化学成分，使矿物相更能抵抗酸溶解，同时将周边热损伤降至最低。调谐至波长λ = 9微米附近羟基磷灰石（HAP）强矿物吸收峰的横向激励大气压（TEA）CO₂ 激光，若将激光脉冲拉伸至大于5 - 10微秒以避免等离子体屏蔽现象，则非常适合高效消融牙齿硬组织。此外，TEA CO₂ 激光可在非常高的重复频率下运行，与Er:YAG和Er:YSGG固态激光相比，其本质上成本更低且用途更广。在本研究中，使用波长λ = 9.3微米、脉冲持续时间为8微秒、重复频率为300 Hz的TEA CO₂ 激光，以消融辐照强度均匀处理牛牙釉质表面。我们假设，在使用水喷雾且扫描速率较高的情况下，在消融区域会形成一层结晶度和CaP相组成得到改善的改性牙釉质均匀表面层，其对酸溶解的抗性增强。在本研究采用的辐照条件下，在窝洞制备的底部和壁部形成的这种改性牙釉质层有潜力抑制封闭剂和修复体下方的继发龋。
研究设计/材料与方法：以30 J/cm² 的入射能量密度，在有和没有水喷雾的情况下，以2、3和6毫米/秒的速率使牛牙釉质块（3×3 mm²）表面快速扫过激光束。通过对激光辐照样品和对照样品进行可控表面溶解实验，评估其对酸溶解的抗性。
在30 J/cm² 的能量密度、300 Hz的重复频率以及6毫米/秒的高扫描速率下，无论有无水冷进行辐照的组，其整体表面溶解速率均显著降低（P < 0.001）。在低扫描速率（2 - 3毫米/秒）下，过多的热量沉积导致形成一层含有非磷灰石CaP相的粗糙外层，该外层更易被酸溶解。在6毫米/秒的扫描速率下，即使没有水喷雾也形成了一层更纯相的HAP，且无热损伤，这表明高扫描速率可用于避免消融过程中的过度热损伤。对于6毫米/秒的较高扫描速度并结合水喷雾，获得了最佳结果（抑制率达80%）。
本研究表明，若使用足够高的扫描速率，无论有无水喷雾，在以高重复频率发射波长λ = 9.3微米的TEA CO₂ 激光脉冲进行消融后，可产生对酸溶解抗性增强的牙釉质表面。
