11.1 Introduction

The use of laser technology in dentistry has gained popularity since its first introduction half century ago. The indications and applications for laser therapy have also expanded, including stand-alone monotherapy or as an adjunctive aid. There are various laser devices with specific wavelengths and advantages that target different tissue types. Practitioners must learn the basic principles and characteristics of laser therapy in order to select the ideal laser device and settings for their practice or treatment procedures. For instance, peri-implant diseases are emerging serious complications and a major concern in modern dentistry that leads to tissue destruction, inflammation with pocketing, and disintegration of dental implants. Given that the traditional mechanical debridement did not provide predictable results, the utilization of laser therapy for the treatment of peri-implant diseases has therefore become the focus of recent clinical applications and researches. Although several in vitro and in vivo studies have shown favorable results with the use of laser therapy, preliminary clinical studies showed inconclusive results. Despite further well-controlled clinical studies are warranted to fully evaluate the efficacy of laser therapy, the benefits of hemostasis, enhanced healing, patient comfort, and possible positive outcomes have already been recognized by the clinicians. This chapter reviews the characteristics of laser therapy, especially for the treatment of peri-implant diseases. Existing nonsurgical and surgical treatment protocols are also compared together with their expected outcome for clinical implications.

11.2 Physics of Laser

Laser, which stands for light amplification by stimulated emission of radiation, is a medical device that delivers energy to a target. There are different media inside the laser that exert the energy, which can be gas, solid, or semiconductor. There are different mechanisms to excite the active medium to go through optical resonator for amplification, but it is the medium that determines the type of laser, which includes:

Gas: Carbon dioxide (CO₂) and argon

Semiconductor: Diodes

Generally, laser wavelengths are medium specific and cannot be changed. The most common range of wavelengths used in periodontics and implantology spans from 400 to 10,600 nm. This range includes both the invisible and visible electromagnetic spectra. The wavelength of a specific laser determines its unique characteristic and application. Studies have used different lasers for the treatment of peri-implant diseases, and thus the results should not be compared directly and data interpreted with caution.

11.2.1 Characteristics of Laser Therapy

Different substances or tissues have their efficiency in terms of resorption for various lasers (absorption of coefficient), and therefore therapeutic laser usually would target certain tissue; these include epithelium, water, bacteria, blood, pigmentation, bone, dentin, and enamel. The performance of a laser is determined by the degree of absorption from these chromophores. In these living tissues, the main components that influence the absorption are the presence of free water, proteins, pigments, and inorganic components (such as apatite). Simplified absorption spectra for each laser can be categorized into two groups: CO₂ and erbium (mainly water and hydroxyapatite) and Nd:YAG and CO₂ (pigmentation, hemoglobin, and melanin) (for full absorption coefficient curve, see [1]).

The penetration depth is an important characteristic for laser. It indicates the laser energy scattering deep into the tissue after penetration. Interestingly, the depth of the absorption is strongly associated with the absorption coefficient in water [2, 3]. The lower the absorption coefficient rate in water, the laser exerts deeper penetration. Therefore, the deep penetrating types of lasers are diode and Nd:YAG lasers, whereas the superficially absorbed types of lasers are CO₂ and erbium laser (high absorption in water). The absorption of energy by the water may also minimize collateral thermal effect to the adjacent tissues during irradiation.

As for hard tissue ablation, it is thought to occur as a result of thermal-mechanical effects following photothermal interactions [5, 6]. The erbium laser seems to be the only laser that can effectively apply to hard tissue. During the hard tissue ablation process, water molecules within the hard tissue are vaporized after absorbing the energy and increasing pressure that provokes “micro-explosions.” These water micro-explosions can cause mechanical breakdown of the hard tissues leaving a microstructured appearance with minimal thermal alteration [7–9]. The water micro-explosion is believed to play a major role in dental implant surface decontamination, especially over exposed implant surface within infrabony defects where undercut surface is expected under the threads. However, it is also important to preserve the microstructure of the dental implant after surface decontamination to allow re-osseointegration. Both Nd:YAG and Er:YAG lasers have shown promising results in vitro for implant surface decontamination [10, 11].

The most intriguing part of the laser–tissue interaction is the biostimulation or the photobiomodulation of the adjacent tissues. When high-level energy scatters into low-level energy, it may stimulate photochemical reactions within the cell and tissue without irreversible changes. It is also a concurrent and desired effect [12]. Although the detailed mechanism is still not clear, several clinical benefits are believed to be a result of biostimulation, including promoting wound healing [13–15], reduction of inflammation [16], and pain relief [13, 17]. Using laser to debride defect around the periodontal or peri-implant defect may also exert positive effect on regenerative therapy, which will be discussed in later sections.

Taken together, the effects of laser energy are diverse and synergized if used correctly. Laser therapy shows promising aid in wound healing, tissue ablation, bacterial reduction, epithelial ablation, connective tissue remodeling, bone metabolism, hemorrhage, and pain control.

11.2.2 Definition and Prevalence of Peri-implant Diseases

Peri-implant diseases constitute both peri-mucositis and peri-implantitis. The term peri-implantitis was first introduced almost 30 years ago [18] and then had the first definition in the 1990s [19] to describe an inflammatory progress that results in progressive bone loss around a dental implant. Contrary to peri-implantitis, peri-mucositis is a reversible inflammatory disease that does not cause bone loss beyond initial bone remodeling around the implant. It is similar to the definition of gingivitis and periodontitis around natural dentition. While the definition of peri-mucositis maintains the same, the definition of peri-implantitis kept evolving. Most recent consensus report from the American Academy of Periodontology [20] emphasizes that bone loss should be beyond the biological bone remodeling. However, the biological bone remodeling has yet to be determined and most likely varies among individuals, especially when peri-implant soft tissue thickness may play an important role in the initial bone remodeling process to determine the biological width around the dental implant [21]. Despite how much bone loss defines peri-implantitis, its rapid progressive bone loss associated with inflammation is well recognized as a serious concern following the popularity of dental implant therapy.

Peri-implantitis is an emerging new disease with a prevalence ranging from 7.1% up to 58% [22, 23] averaging 22% in a recent systematic review [24]. Although early stage mucositis may be controlled nonsurgically, established peri-implantitis lesions are still a challenge to manage due to the unpredictable results [25]. Before discussing about the treatment for peri-implant diseases and the role of laser therapy, it is prudent to understand the etiology of peri-implant diseases and recognize each contributing factor that needs to be addressed all together.

11.2.3 Main Etiologic Factors Associated with Peri-implant Diseases

Therefore, before using the laser, the clinician must recognize if there are other factors that need to be addressed, including active periodontal disease, smoking, diabetes, or any systemic conditions that compromise the immune system [36, 37]. A prognosis system was developed considering whether all the other factors can be addressed or not. If all the factors can be addressed, the prognosis of the implant can be considered favorable; should there be some uncertainty, it will give questionable or unfavorable prognosis [38]. For the laser therapy, it can potentially both target bacteria decontamination over implant surface and biomodulate the host inflammatory response around the defect. The following paragraph will discuss the rationale and benefits of using laser as an adjunctive tool for treating the peri-implant diseases.

11.2.4 Why Can Laser Assist in the Treatment of Peri-implant Diseases

11.2.4.1 Implant Surface Detoxification with Laser Therapy

It was hypothesized that the main challenges in treating peri-implant diseases are from detoxification of the exposed contaminated dental implant surface. The conventional mechanical debridement with hand instrument may not be able to thoroughly scale the microstructure of the surface, whereas using rotary instruments may result in damaging the surface texture and compromising the re-osseointegration. Studies have evaluated the effectiveness of different methods for implant surface decontamination [39]. Among the available approaches for decontamination, laser therapy has been proven to reduce bacterial load [40–42] without damaging the implant surface (Nd:YAG and Er:YAG laser) [10, 11]. The notion that Nd:YAG can target pigmented pathogen (e.g., Porphyromonas gingivalis) may warrant further investigation as it may still present with relative abundance in peri-implantitis lesions [43]; however, its potential tendency in damaging implant surface even with lower energy power may require precaution [44]. Laser therapy not only can kill or devitalize the bacteria but also ablate or inactivate toxic substances, such as endotoxins [45, 46]. Therefore, together with the abovementioned micro-explosions, those characteristics of laser therapy make it a promising and effective way to detoxify implant surface.

However, given the limited clinical studies so far, it was difficult to assess if the goal of “surface decontamination” is achieved or not clinically and if it has an impact over the clinical outcome [47]. In animal models, there are several studies showing that with the Er:YAG laser treatment in preparation for bone grafting procedure, there is an increased regenerated bone-to-implant contact for a better regenerative outcome [48, 49]. Therefore, it may be more reasonable to assess the clinical attachment gain or radiographic bone fill over the specific defect site to indirectly assess the benefit of implant surface decontamination.

It is also important to note that proper laser therapy protocol is required to reach the bactericidal energy threshold for surface decontamination [42]. As most studies did not specify the setting of the laser and the detailed application time and methods, it may not have a strict protocol. Before more studies develop evidence-based application, it is prudent to recognize that a thorough implant surface decontamination takes time and should not be rushed.

11.2.4.2 Soft and Hard Tissue Wound Healing Following Laser Therapy

In addition to bactericidal effects, laser therapy has a distinct wound healing pattern following the treatment. The biostimulation effect from low-level energy laser is also an emerging area of interest, which may be an important characteristic of laser therapy to promote healing and tissue regeneration. Many earlier in vitro, in vivo, and clinical studies also have shown the benefits of laser to promote wound healing [13–15, 50] reduction of inflammation [16, 51, 52], as well as pain relief [13, 17, 53], which are all desirable biostimulatory effects [54].

At the cellular level, in vitro studies showed that low-level laser irradiation could attenuate the production of pro-inflammatory mediators (IL-1β and PGE₂) from both stimulated human gingival fibroblast and PDL cells [51, 52] as well as macrophages [55]. Diode laser at low dose can also activate human fibroblast transforming factor β1 signaling pathway and induce the expression human β-defensin 2 (HBD-2), both of which are potent factors to promote wound healing [56]. Additionally, low-level laser stimulates osteoblast proliferation, differentiation, release of several growth factors [57–59], and the formation of bone nodules in vitro [60]. Therefore, these cellular responses following laser biostimulation can attenuate inflammation and enhance both soft and hard tissue healing.

In the animal models, in vivo studies demonstrated that low-level laser therapy could induce dose-dependent reduction of tumor necrosis factor-alpha in acute inflammation [61]. Yamazaki et al. showed that low-level CO₂ laser induces expression of heat shock proteins in the collateral tissue exhibiting partial coagulation necrosis, but it promotes the repair process and tissue remodeling [62, 63]. Low-level laser therapy enhances fractured long bone healing in the animal model [64]. LED light irradiation promotes osteogenesis in periodontal intrabony defect with open flap debridement [65].

There are several interesting studies on laser enhancing implant osseointegration and healing of peri-implant tissues. Low-level laser therapy can stimulate faster osseointegration in the animal model [66] or even irradiate the recipient alveolar bone before implant placement to enhance the vitality of the osteocyte and promote faster bone formation around the dental implant for quick osseointegration [67, 68]. Omasa et al. utilized low-level laser to increase the stability of the mini implants placed in rat tibiae and found higher expression of the bone morphogenetic protein 2 from the surrounding cells as well [69]. Direct application of Er:YAG laser over contaminated implant surface can also induce more bone-to-implant contact with regenerative therapy [49], although such finding is not known to be a direct result of surface decontamination or biostimulation.

In a human split-mouth design study, using low-level laser therapy after gingivectomy incisions promotes better healing [70]; another human split-mouth study used diode laser to modulate modified Widman flap and found less edema of pain postoperatively [71]. More recently, Er:YAG laser has been used to conduct second-stage procedure to expose dental implant that resulted in significantly less postoperative pain score [53]. Another obvious clinical benefit for tissue management is hemostasis that is well recognized by the clinicians. However, strong hemostatic effect may compromise the wound healing, especially when hemostasis was generally achieved with carbonization, coagulation, and thermal denaturation of the adjacent tissue that clots the small blood vessels. Therefore, incision of the soft tissue with laser may not be the right application if severe bleeding is not expected. The carbonization of the incisional tissue would impair primary closure and delay wound healing [72–74]. Erbium lasers may not be the first choice in terms of hemostasis because it creates minimal thermal degeneration of the collateral tissue owing to its superficially penetrating characteristic [75]. However, given such characteristic, wound healing following erbium laser therapy may be more favorable. Sawabe et al. demonstrated faster and better gingival wound healing with Er:YAG laser compared with electrosurgery in rat animal models [74].

In summary, the effect of implant therapy is not only limited to implant surface decontamination but also enhances the soft and hard tissue healing around the inflamed peri-implant defect (Fig. 11.1).

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