Soft tissue laser dental surgery basics

Peter Vitruk, PhD, MInstP, CPhys, sheds some light on the many applications of lasers

Laser light absorption by the biological tissues — ablative and nonablative lasers

The key to the success of lasers in soft tissue surgery is their ability to (a) cut and (b) coagulate the soft tissue at the same time. Not all laser wavelengths are suitable to comply with both requirements.

To understand how laser light cuts and coagulates the soft tissue, one must consider how the laser light is absorbed and scattered by the soft tissue.^1-3 Most commercially available dental lasers fit into three wavelength categories:

• Near-infrared circa 1,000 nm (810-1,064 nm diodes and 1,064 nm Nd-laser)
• Mid-infrared circa 3,000 nm (2,790 and 2,940 nm Er-lasers)
• Infrared circa 10,000 nm (9,300 and 10,600 nm CO₂ lasers)

The absorption spectra of oral soft tissue are summarized in Figure 1 along with Ablation Thresholds and Coagulation Depths for popular soft tissue dental lasers.

Because near-infrared (Near-IR) photons are weakly absorbed (1,000-10,000 times weaker than CO₂ and Erbium), dental diodes are not used to cut soft tissue with photons. Instead, dental diodes cut soft tissue thermomechanically with hot charred glass tips, whereas the tip-tissue contact time defines the depths of its thermal impact. These diode laser wavelengths, however, are scattered much stronger than they are absorbed, resulting in a strong and widespread Near-IR halo of light underneath the irradiated surface of the soft tissue.³ These Near-IR diode laser wavelengths are referred to as nonablative, as opposed to ablative Erbium and CO₂ laser wavelengths, which are weakly scattered but strongly absorbed by the soft tissue.

Mid-IR Erbium laser wavelengths are highly energy-efficient and spatially accurate photothermal ablation tools, but with an insufficient depth of coagulation (significantly less than blood capillary diameters).

IR CO₂ laser wavelengths are highly efficient and spatially accurate photothermal ablation tools with good coagulation efficiency due to a close match between coagulation depth and oral soft tissue blood capillary diameters.

SuperPulse and Thermal Relaxation Time

Thermal Relaxation Time (TRT) is a measure of how fast the irradiated tissue diffuses the heat into the adjacent tissues.^3,4 The most efficient heating of the irradiated tissue takes place when the laser pulse duration is shorter than the TRT. The most efficient cooling of the tissue adjacent to the ablated zone takes place if the time duration between laser pulses is greater than the TRT. Such laser pulse design in CO₂ lasers is referred to as SuperPulse,³ depicted in Figure 2. SuperPulse minimizes the amount of heat that is diffusing away from the cutting/ablation zone to the adjacent healthy tissue.

SuperPulse CO₂ laser removal of biofilms from dental implant surfaces

Efficient decontamination of the implant surfaces can be achieved by the SuperPulse CO₂ laser beam^5,6 without heating or damaging the implant^7,8 for the following reasons:

• This 10,600 nm wavelength is highly efficiently absorbed by water-rich bacterial biofilms on the implant surface (Figure 1).
• The short pulse duration of the SuperPulse allows for highly efficient confinement of the laser-
  generated heat inside the biofilm without thermal conduction of heat away into the implant (Figure 2).
• SuperPulse’s high peak power allows for fast vaporization (ablation) of biofilms since the laser-generated heat inside the biofilm is sufficient for vaporizing it.

The SuperPulse 10.600 nm CO₂ laser treatment with an average fluence in excess of 40 J/cm² provides a 100% removal of an in vitro biofilms of approximately 10 µm thickness grown on titanium and titanium-oxide implant specimens with a moderately rough surface topography.⁶

Such CO₂ laser irradiation of the titanium implant surface during open flap procedures allows for high success outcomes⁹ when treating peri-implantitis as illustrated in Figure 3.

Implant-Safe Lasers — Implant-Safe SuperPulse CO₂ Laser settings

Safe decontamination of implant surfaces with laser light can be achieved if the laser energy is efficiently reflected off the implant surface after the diseased tissue and biofilms are vaporized (ablated) from the implant surface.^6-8

This condition is easily met for titanium implants.^6-8 As illustrated in Figure 4, the 10.6 micrometer CO₂ laser wavelength is highly reflected (>90%) from titanium.¹⁰ Diode, Nd-laser, and Er-laser wavelengths are not as efficiently reflected and produce 3 to 4 times greater rate of the implant heating than the CO₂ laser for a comparable dose of laser energy. This property makes the CO₂ laser wavelength the safest available wavelength for peri-implantitis treatment.

Titanium implants (Biomet 3i NanoTite™ NIITP4310 and Biomet 3i Osseotite™ IFNT510 — all are trade names and trademarks of Biomet) have been demonstrated⁸ to safely handle a SuperPulse CO₂ laser’s average fluence in 320 J/cm² –360 J/cm² range, which are much greater fluence values than those proven efficient for biofilm ablation (> 40 J/cm²).⁶ It has been also demonstrated in the same study⁸ that SuperPulse CO₂ laser irradiation of these implants produces significantly less heat when compared to the diode laser wavelengths.

Soft tissue surgery and incision/ablation depth with laser beams

For the ablative CO₂ and Erbium lasers, the power density of the focused laser beam is equivalent to the mechanical pressure that is applied to a cold steel blade.¹¹ Greater laser fluence (i.e., greater power density and slower hand speed) results in a greater depth and rate of soft tissue vaporization. More specifically, as illustrated in Figure 5, the depth of incision is proportional to laser beam average power and inversely proportional to laser beam spot size and the surgeon’s hand speed.

Understanding how the depth of the incision is controlled through power, spot size, and hand speed, a skilled dental surgeon can easily adjust the technique for a very wide range of applications. When decontaminating the dental implant surfaces described earlier, the depth of the biofilms removed is under 20 micrometers. During the closed-flap sulcular debridement with the CO₂ laser, the diseased epithelium can be removed¹² with similar micrometer-range precision and accuracy as illustrated in Figure 6. In certain popular soft tissue applications — e.g., lingual frenectomy¹³ — the skilled laser user can carefully debride the fibrous tissue even from around blood vessels if need be to fulfill the complete release of restrictions without puncturing the exposed blood vessels as illustrated in Figures 7 and 8.

Depth of coagulation/hemostasis with laser beams

Coagulation occurs as a denaturation of soft tissue proteins that takes place in the 60°C to 100°C temperature range³ leading to a significant reduction in bleeding (and oozing of lymphatic liquids) on the margins of ablated tissue. Photothermal coagulation is also accompanied by hemostasis due to shrinkage of the walls of blood and lymphatic vessels due to collagen shrinkage at increased temperatures. Since blood is contained within and transported through the blood vessels, the diameter of blood vessels B (20-40 µm from measurements in human cadaver gingival tissue¹⁶) is a highly important spatial parameter in considering the efficiency of photothermal coagulation.

For diode laser wavelengths, optical absorption (Near-IR attenuation) and coagulation depths are significantly greater than blood vessel diameters; coagulation takes place over extended volumes, extending far beyond ablation margins where no coagulation is required.^3,11,17

For pulsed lasers (e.g., SuperPulse CO₂ and free-running pulse Erbium lasers) the coagulation depth is proportional to the absorption depth of light in the soft tissue. The depth of coagulation/hemostasis can be extended by longer and lower power laser pulses, which allow the heat to diffuse away from the irradiated superficial depths into the deeper regions of the target tissue. ^3,11

For Erbium laser wavelengths, the optical absorption depth and the coagulation depth are significantly smaller than blood vessel diameters (Figure 1); coagulation takes place on a relatively small spatial scale and cannot prevent bleeding from the blood vessels severed during efficient tissue ablation.^3,11

For CO₂ laser wavelengths (Figure 1), sub-100 µm^5,7 coagulation extends just deep enough into a severed blood vessel to stop the bleeding; the coagulation and hemostasis are more efficient than for Erbium laser wavelengths.^3,11

Figures 6-10 illustrate great hemostasis and coagulation on surgical margins during and immediately after the CO₂ laser surgery.

Reduced scarring and reduced postoperative pain following CO₂ laser surgery¹⁸

Both the reduced scarring and postoperative pain can be partially explained by the optimal coagulation/hemostasis depth on CO₂ laser surgical margins. Several studies have found the reduced presence of contractile myofibroblasts — cells accountable for postoperative scarring — in CO₂ laser surgical wounds when compared to scalpel surgery, which explains the lesser postoperative scarring following CO₂ laser surgery. Also, decreased extravasation of blood and lymphatic fluids into the CO₂ laser wound space impedes the release of inflammatory mediators. This results in less edema around the wound then following conventional surgery and delayed minimal inflammatory response. It may also account for the reduced immediate postoperative pain after CO₂ laser surgery. Despite the abundant published case studies and research as well as anecdotal reports regarding reduced pain following CO₂ laser surgery, the exact mechanism behind it remains to be explained.

Understanding how the depth of the incision is controlled through power, spot size, and hand speed, a skilled dental surgeon can easily adjust the technique for a very wide range of applications.

Nonablative lasers and near-infrared diode hot glass tip

As previously explained and further illustrated in Figure 11, the near-IR laser wavelengths of diode lasers are weakly absorbed by the soft tissue and cannot radiantly ablate/vaporize the soft tissue^3,11,17-21 (except for high melanin content epithelium).

Since diode Near-IR laser wavelengths are not suitable for the oral soft-tissue cutting, there is a different technique of non-optical (non-radiant) tissue cutting with the so-called “hot tip,”^19-21 as illustrated in Figure 12.

A critical component of such hot tip-tissue interaction is the creation of the optically dark char deposit on the very end of the glass tip. The diode laser optical energy is then used to heat the charred glass fiber’s tip up to 1,000ºC to 1,500ºC.^19-21 The hot glass tip heats soft tissue through heat conduction (i.e., heat diffusion) from (and through) hot glass tip to (and through) the soft tissue. Charred glass tip diode lasers perform as non-laser wavelength-independent ablation thermal devices with coagulation depths in the range 250-1,000 µm²² (similarly to electrocautery where soft tissue is cut by the hot metal tip). The cutting speed of the charred hot glass tip is limited by its disintegration at elevated temperatures (up to 1,500ºC), thus raising concerns about biocompatibility¹⁹ of the burnt tip’s cladding chemicals and thermally fractured glass.

Degradation of the char on glass tip surface creates the “optically leaky” tip with a reduced tip temperature, and increased risk of Near-IR induced sub-surface thermal necrosis²⁰ as well as mechanical tearing of the tissue by the cold glass’ sharp edges, which may result in both deep thermal trauma and excessive superficial bleeding as illustrated in Figure 13.

ANSI Z136.3 Standard for Safe Use of Lasers in Health Care

Laser Institute of America (LIA) has originally developed and now regularly updates the Laser Safety regulations (ANSI Z136.3 Standard for Safe Use of Lasers in Health Care – 2018 Edition²³), which are mandatory for implementation by all laser users in mitigating laser beam and non-beam hazards.

The most important top-level Laser Hazard mitigation measures are typically well understood and properly implemented by the majority of the surgical laser users — i.e., Laser Safety Officer (LSO) appointment, Laser In Use Sign, and Laser Safety Goggles Administration. Additionally, the ANSI Z136.3 Standard defines laser plume as one of the non-beam laser hazards since it contains viral, bacterial, and other cellular and aerosolized particulates, gaseous toxic compounds, etc.

ANSI Z136.3 Standard specifies safety measures to mitigate the laser plume hazards, i.e., the mandatory use of Local Exhaust Ventilation (LEV) device — for example, wall suction devices in dental offices (aka high-volume evacuation devices and mobile/portable smoke evacuators in physicians’ and veterinarians’ offices) — equipped with a proper filter (with ANSI Z136.3 Standard’s defined filtration at 0.12 µm at 99.999% efficiency) and with wide aperture suction nozzle held as close as possible to the surgical site (no further than 2 inches per the CDC²⁴ — Figure 14) to safely remove laser plume.

According to ANSI Z136.3(2018), the use of N95 is preferred over surgical masks, but only in addition to laser plume evacuation.

It is important to emphasize that in surgical laser uses that do not utilize water sprays, laser plume can be safely removed according to and in compliance with ANSI Z136.3 Standard. However, users of dental lasers that utilize water sprays should be aware that ANSI Z136.3 Standard does not address the safe removal of the laser plume dispersed by the water spray.²⁵