Hard Tissue Ablation and Modification with IR Lasers

Infrared lasers are ideally suited for the selective and precise removal of carious dental hard tissue while minimizing the healthy tissue loss. Since the initial investigations of Stern[1] over 30 years ago, several unique laser applications have evolved for dentistry, namely laser ablation of dental hard tissue, caries inhibition treatments by localized surface heating, and surface conditioning for bonding[2]. During high intensity laser irradiation, marked chemical and physical changes may be induced in the irradiated dental enamel. These changes can have profound effects on the laser ablation/drilling process and may lead to a reduction in the ablation rate and efficiency, increase peripheral thermal damage and even lead to stalling without further removal of tissue with subsequent laser pulses. Moreover, thermal decomposition of the mineral can lead to changes in the susceptibility of the modified mineral to organic acids in the oral environment. Morphological changes may result in the formation of loosely attached layers of modified enamel that can delaminate leading to failure during the bonding to restorative materials [3, 4]. Therefore, it is important to thoroughly characterize the laser (thermal) induced chemical and crystalline changes after laser irradiation. The mineral, hydroxyapatite, found in bone and teeth contains carbonate inclusions that render it highly susceptible to acid dissolution by organic acids generated from bacteria in dental plaque. Upon heating to temperatures in excess of 400 °C, the mineral decomposes to form a new mineral phase that has increased resistance to acid dissolution[5]. Recent studies suggest that as a side effect of laser ablation, the walls around the periphery of a cavity preparation will be transformed through laser heating into a more acid resistant phase with an enhanced resistance to future decay[6]. However, poorly crystalline nonapatite phases of calcium phosphate may have an opposite effect on plaque acid resistance[7] and may increase the quantity of poorly attached grains associated with delamination failures.IR spectroscopy has been used for half a century to study the structure of bony tissue[8, 9]. Fowler and Kuroda [7, 10] used IR transmission spectroscopy to show the chemical changes induced in laser irradiated dental enamel. Recently, we demonstrated that FTIR in specular reflectance mode could be used effectively to nondestructively measure the laser-induced chemical changes in enamel and that the carbonate loss could be determined as a function of incident laser intensity[11]. However, this technique requires smooth surfaces of large area in order to acquire suitable spectra. The high brightness of the ALS enables spectra to be acquired on specific areas across ablation craters with a resolution of 10 μm.