Current Concepts of Laser-Oral Tissue Interaction

Abstract

Abstract: Fundamental to the adjunctive use of laser photonic energy for delivering therapy and tissue management, is the ability of the incident energy to be absorbed by target tissues. The aim of this review is to examine the differential performance of the separate components of oral hard and soft tissues when exposed to laser photonic irradiance of variable wavelengths and power values. Through an examination of peer-reviewed published data and materials, the interaction of laser photonic energy and target tissues are explored in detail. Varying laser wavelength emissions relative to anatomical structures explores the ability to optimise laser-tissue interactions, and also identifies possible risk scenarios as they apply to adjacent non-target structures. The concepts and practical aspects of laser photonic energy interactions with target oral tissues are clearly demonstrated. Emphasis was placed on optimising the minimum level of laser power delivery in order to achieve a desired tissue effect, whilst minimising the risk or outcome of collateral tissue damage.

Keywords: dentistry; laser; laser–tissue interaction; optical properties of tissues; photobiomodulation; photothermolysis.

Figure 1

An overview of the manipulation of incident photonic energy, such as laser light as an adjunct to screening, diagnostic and therapeutic clinical activity. Key: LED—light emitting diode, UV—ultra-violet, NIR—near infra-red, with multi-wavelength representation as vertical incident arrows. OCT—optical coherence tomography, aPDT—antimicrobial photodynamic therapy. Re-emission, as direct beam or scattered irradiance is represented by complete and dotted arrows, either straight or non-linear.

Figure 2

Absorption spectral profiles of major dental/oral structural elements and chromophores associated with soft tissue management. Absorbance is shown relative to wavelength of irradiation. The depth of penetration is shown as 1/absorbance. Original data and graphics: Parker S. Data source: Parker, S; et al. Laser Essentials for the Dental Practitioner: Foundation Knowledge—Construction, Modes of Operation and Safety. EC Dental Science 2019, 18.9, 2020–2027 [2].

Figure 3

Absorption spectral profiles curves of major tissue elements associated with bone and dental hard tissue management. Absorbance is shown relative to wavelength of irradiation. Key: water (blue), protein/collagen (green), hydroxyapatite—HA and carbonated HA (white). Original data and graphics: Parker S. Data source: Parker, S; et al. Laser Essentials for the Dental Practitioner: Foundation Knowledge—Construction, Modes of Operation and Safety. EC Dental Science 2019, 18.9, 2020–2027 [2].

Figure 4

Relationship between incident photonic power density and exposure time (seconds). Changes in the two components of laser–tissue interactions—power density (irradiance) and exposure time, may affect the interaction level. Original graphics: Parker S. Data source: Boulnois, J-L. Laser Med. Sci. 1986, (1), 47–66 [5].

Figure 5

Laser wavelengths in common use in clinical dentistry, arranged according to wavelength in nanometers (10⁻⁹ m), from blue-visible to the far infra-red regions.

Figure 6

Comparative light micrographs of laser interaction with porcine oral soft tissue. Top: Nd:YAG 1064 nm (an example of the shorter wavelengths employed), causes a wider, crater-shaped area of ablation, with some areas of thermal conduction. Right: Longer wavelengths such as FRP Er:YAG 2940 nm create a sharper “V” shaped incision, whereas Bottom: CO₂ 10,600 nm, being a gated CW emission, results in some features of the other two—surface configuration ascribable to absorption in water, but some thermal spread arising from a comparative lack of thermal relaxation.

Figure 7

Soft tissue surgery. A mucocele excision, lower lip. The laser used was a diode 980 nm 1.25W CW Fluence 12.2 J/cm². Time taken: two minutes with pauses. A—pre-op, B—excision with haemostasis, C—immediate post-op, D—healing at one month.

Figure 8

Scanning Electron Microscope (SEM) micrographs of laser-mediated enamel and dentine ablation. Top left and right: the resultant enamel surface is rugged, fragmented and capable of accepting a resin-based composite restoration once any unstable fragments have been removed. Bottom left and right: dentine is rendered smear layer-free, with an intact and stable cut surface.

Figure 9

SEM micrograph of laser mediated ablation of bone. Top left, right and bottom left: the use of an erbium laser system enables an accurate and clean ablation without evidence of charring or thermal cracking. Bottom right: this is in contrast to the use of a Nd:YAG system (bottom right), which can result in extensive heat-damage, and melting of the parent hydroxyapatite structure.

Figure 10

Use of the er YAG laser in tooth cavity preparation. Note the co-axial water spray to aid dispersal of ablation debris and cool the target hard tissue site.

Figure 11

Algorithm depicting the multi-level interaction of varying incident photonic energy as applied to target tissues of quantifiable optical properties. Of prime concern is the thermal containment of laser–tissue interactions. Excessive application of incident power may cause overheating and carbonisation, leading to aberrant interactive effects.