Erbium laser‐assisted ceramic debonding: a scoping review

Abstract Purpose Removal of ceramic restorations and appliances can be time consuming, invasive, and inconvenient. Erbium lasers offer an alternative noninvasive method for debonding of ceramic appliances. This paper aims to provide a comprehensive review of current literature on the effectiveness of erbium lasers for removal of ceramic restorations and appliances from natural teeth and dental implants. Methods A comprehensive search of 7 databases, including Medline (Ovid), Embase, Dentistry and Oral Sciences Source (DOSS), Web of Science, Cochrane Library, and ProQuest Dissertations and Theses was performed. The inclusion and exclusion criteria were agreed prior to the literature search. Two reviewers independently screened the title and abstract. A third reviewer then broke the tie, if any. The selected articles then underwent full text review and the data was extracted. Results The search identified 4117 unique articles published through June 10, 2021. Studies were assessed and categorized based on the type of restoration/appliance, type of abutment, type of laser, laser settings, efficacy of debonding, and pulpal temperature rise. Thirty‐eight full‐text articles were reviewed for inclusion. Time for ceramic debonding varies depending on the type of restorations and materials. Removal of zirconia crowns from teeth and implant abutments requires a longer period of time compared to lithium disilicate crowns. Temperature increases were reported as 5.5 degrees or less. Laser setting and laser type affect the debonding time and the increase in temperature. Examinations of debonded ceramics demonstrated no known structural damages resulting from laser applications. Conclusions Erbium lasers are effective noninvasive tools to remove all ceramic restorations/appliances from natural teeth and implant abutments without causing harm to abutments. Laser‐assisted debonding should be considered as a viable alternative to rotary instrumentation for ceramic crowns; however, clinical studies of erbium‐assisted ceramic retrieval are needed.

loosening or breaking. The conventional removal procedure of ceramic restorations is often performed by sectioning with rotary instruments using diamond or tungsten carbide burs. This traditional removal method is inconvenient and damages the integrity of the restoration. 5 The removal procedure can be lengthy and result in damage to the underlying natural tooth or implant abutment. [6][7][8] Removing all-ceramic restorations or orthodontic brackets from natural teeth can also be challenging due to the similarity between the color of the cement and underlying tooth structure. 9,10 The use of erbium lasers has been explored as an alternative method for debonding of ceramic appliances from natural teeth and implants, 2,[11][12][13] including removal of translucent restorative materials such as composite restorations, 14,15 fiber reinforced composite posts, 16 veneers, [11][12][13] and orthodontic brackets. 10,[17][18][19] The light emitted by erbium lasers, such as Erbium, Chromium-doped Yttrium Scandium Gallium Garnet (Er,Cr:YSGG) and Erbium-doped Yttrium Aluminum Garnet (Er:YAG) lasers carrying wavelengths of 2780 nm and 2940 nm, respectively, can be transmitted through the translucent ceramic materials and selectively absorbed by water molecules and residual monomers in luting cements, thus resulting in the vaporization of these molecules and debonding of the cement. 5,16,20 This method offers many advantages, however its efficiency can be affected by several clinical operating factors including chemical composition, shade and thickness of the cement, type, shade, opacity, and thickness of the ceramic restoration, and laser parameters such as power, pulse duration, and frequency. 6,10,16,21 Recent studies have demonstrated predictability in debonding of ceramic restorations with both erbium lasers. 20 Several concerns with the laser-assisted crown debonding remain around heat generation and thermal injury to the adjacent tissues. An increase in pulpal temperature by 5.5 • C can cause irreversible damage to the pulp tissue. 22 An increased osseous temperature by 10 • C can cause bone damage. 23 Temperature increases to 6 • C can damage periodontal ligament. 24 The objectives of this study are therefore (1) to provide a summary of the current comprehensive literature on erbium family laser-assisted ceramic restoration and orthodontic appliance debonding from teeth and implant abutments; (2) to provide insight into the erbium laser settings, clinical outcomes, and possible complications; and (3) identify knowledge gaps, scope a body of literature, and clarify current concepts of Erbium lasers in ceramic debonding

Identification of relevant literature
A systematic search was conducted in the following databases on July 31, 2020 and updated on June 10, 2021: Medline (Ovid), Embase, Dentistry and Oral Sciences Source (DOSS), Web of Science, Cochrane Library, and ProQuest Dissertations and Theses. Keywords and controlled vocabulary were used to search for the concepts of dental debonding and erbium-doped yttrium, aluminum, and garnet (Er:YAG) and erbium, chromium-doped yttrium, scandium, gallium and garnet (Er,Cr:YSGG) lasers. No date restrictions were used, however, results were limited to those published in English. The detailed search strategy used in Medline (Ovid) is provided in Table 1. The PRISMA checklist is provided as  Supplementary Table 1.

Study selection and data collection
Study selection was conducted in two phases. First, 3 abstract reviewers (J.G.D., J.M., K.G.L.) independently reviewed titles and abstracts of citations obtained from the systematic literature search. Articles marked for inclusion by at least 2 reviewers were retained for the second phase of full-text review. Three reviewers examined abstracts for inclusion. Two reviewers examined all full-text articles independently (J.M., K.G.L.), and a third reviewer (J.G.D.) was consulted to resolve any disagreements.
Inclusion criteria included in vitro and in vivo studies of Er:YAG/Er,Cr:YSGG laser debonding (removal, retrieval) of veneers, laminates, lithium-disilicate, zirconium-oxide crowns, prosthetic crowns, fixed bridges, implant crowns, brackets. Exclusion criteria included non-English papers, review, clinical reports, opinions, editorial papers; nonerbium laser or no laser; only adhesive removal; only resin composite debonding, or debonding of fiber posts, glass fibers; and laser surface preparation before bonding or debonding procedures.
Two authors (J.M. and K.G.L) reviewed the final included full text articles and collected the following information: sample size, study group composition, sample type (e.g. type of abutment, type of restoration/appliance etc.), laser parameters, radiation exposure/retrieval time, and temperature changes. A third author (J.G.D.) reviewed and compiled the data from full text articles into summary. The fourth author (S.B.) verified the validity of the charted data and finalized the results.

Study selection and characteristics of the included studies
The electronic literature search yielded a total of 7114 articles, of which 4117 were unique citations. The eligible manuscripts were published from 2011 to 2021. Review of titles and abstracts resulted in the exclusion of 4064 records. Fifty-three full-text articles were reviewed for inclusion, and 38 were retained for this review. Figure 1 displays the database searches, number of articles retained during each phase of review, and reasons for exclusion. Table 1 provides the detailed search strategies used to search each database.
Comparisons included: ceramic crowns on natural teeth, ceramic crowns on various implant abutments and veneers. All studies were assessed for the type of erbium family laser. In all studies, including repeated experiments, a total of 80 ceramic crowns were debonded from implant abutments and 160 ceramic crowns were debonded with erbium lasers from teeth. Of the veneer samples, 355 debonded, while 183 ceramic disc samples debonded.

Relationship between restorative or luting materials and debonding time
Based on reported data, type of cement plays a role in the debonding time and has been associated with structural changes seen on microscopic and electron microscopic analyses. 7,8,10,13,20,21,26,29,30,32,38,43,48,50,53 While Er,Cr:YSGG laser was deemed efficient for veneer ( removal without causing deleterious effects to the enamel, presence of residual cement and changes in enamel prism structure were observed in association with certain resin cements (RelyX Veneer, 3M-ESPE and Variolink Veneer, Ivoclar Vivadent). 37 Resin cements in general are greatly affected by the erbium laser irradiation, however the ablation thresholds and ablation volume loss are dependent on the chemical composition of a particular resin cement. Some cements, such as was determined in G-Cem LinkAce, GC America, and Multilink N, Ivoclar Vivadent cements, exhibit higher volume loss compared to other such as Rely X Unicem U100, 3M-ESPE, Variolink II Ivoclar, and Panavia F-20, Kuraray Dental, which appear to have less volume loss. 51 Debonding of lithium disilicate crowns from titanium implant abutments requires a shorter time when crowns are cemented with a resin-modified glass-ionomer cement (97.5 seconds) versus resin cement (196.5 seconds). 8 Thicker veneer restorations show more resistance to debonding. 42 Translucency of the same type of ceramic appears to have little influence on debonding. No significant difference in debonding veneers from bovine incisors was reported among feldspathic ceramics with different translucency. 44 While the translucency of the ceramic plays a nonsignificant role in the laser transmission and debonding, the thickness of ceramic appears to be critical to the laser transmission and thus the debonding efficiency. Transmission ratio through the ceramic material decreased with increasing thickness of the ceramic sample. The highest transmission ratio was determined for lithium disilicate-reinforced ceramic with 0.5 mm thickness (88%), and the lowest was determined for feldspathic ceramic with 1 mm thickness (44%). 54 The details on debonding time are discussed below. Time to debond a crown has been positively correlated with surface area and volume of the tooth and crown, and the volume of the luting cement. 49 The larger abutment surface area was positively correlated to the longer retrieval time. 48 Laminate veneers with an average thickness at the midfacial of 0.75 mm can be removed in 14.16 ±0.60 seconds using Er,Cr:YSGG laser. 38 As the thickness of a veneer increases, more time is necessary to remove it. 38 Time for debonding varies for all-ceramic crowns depending on the material. Studies reported that it takes longer to debond a zirconia crown (226-312 seconds) compared to a lithium disilicate crown (190 seconds) from a human molar. 6,48 This has also been reported for debonding of all-ceramic crowns from implant abutments. Removal of zirconia crowns from zirconia abutments took on average up to 355 seconds, 20 while debonding of lithium disilicate crowns can be accomplished in shorter time from zirconia abutments (192-181 seconds), 8 and titanium abutments (97.5-196.5 seconds). 7

Pulpal temperature changes
Several manuscripts measured pulpal temperature changes during laser irradiation. 27,36,38,49 No study reported a significant increase in pulpal temperature beyond the physiological limit of 5.5 • C during irradiation with any erbium laser. Irrigation used during the debonding process may sometimes even lower the pulpal temperature. 27 Pulpal temperature changes ranged between 0.71 • C to 4.28 • C. For bracket removal, the temperatures increased 0.83 • C, while for crown removal temperature increased up to 4.5 • C. The temperature increases may also be influenced by the proximity of the pulp chamber to the irradiated surface and the thickness of the remaining tooth structure. The intrapulpal temperature increase in central incisors was significantly higher than that of the premolar teeth when removing brackets with Er:YAG laser at 1.2 W, 600 mJ, 2 Hz. 36 A similar trend was observed when debonding prefabricated zirconia crowns from primary teeth with larger pulps and thinner layer of dentin and thus closer proximity to the irritated surface. 49 Increasing thickness of veneer restoration does not necessarily result in an increase in pulpal temperature despite requiring longer time of irradiation for debonding. 38

Type of the erbium laser and settings
Five studies compared the efficiency of debonding between the Er,Cr:YSGG and Er:YAG lasers. Time for removal was not always statistically different. However, the debonding times in general tend to be shorter for the Er:YAG laser compared to the Er,Cr:YSGG laser. 20,27,29,31,50 Both erbium lasers feature comparable times to debond orthodontic brackets (Table 2), 10,29,31 and do not result in the temperature increase inside the tooth. 27 While both erbium lasers are efficient in removing zirconia crowns from teeth or implant abutments, Er:YAG laser accomplishes the task faster than Er,Cr:YSGG. 20,50 While power of 3 Watt, 10 seconds in the scanning mode with energy density 22-28 J/cm2, pulse duration of 100 µs seems to be adequate for orthodontic bracket removal, 31 crown removal requires laser settings to be in the range of at least 3.5 and 4 Watt of power with 25 Hz pulse rate. 47 Most studies used settings between 4 and 6 Watt. Power of 5 Watt; 100-250 mJ, 20-50 Hz has been used for veneer debonding 41 and orthodontic bracket removal, 30 while 4.5 Watt, 300 mJ, 15 Hz was reported to be efficient and safe for removal of lithium disilicate and zirconia crowns from implant abutments 7,8,20 and teeth. 48

Examinations of irradiated ceramics and sample surface analyses
Most studies reported successful removal of restorations or appliances with no detectable physical damage. All of the all-ceramic crowns were successfully debonded from teeth or implant abutments with the erbium lasers. No crowns were fractured and no damage to the underlying abutment was detected. The bonding/luting cement was usually deteriorated due to the erbium laser irradiation. Veneer debonding with the Er:YAG laser at 100 mJ and 30 Hz resulted in intact underlying tooth structure. 13 F I G U R E 2 Clinical examples of laser debonding using Er:YAG laser; (top) Debonding of an all-ceramic lithium disilicate crown; (middle) Debonding of a monolithic zirconia fixed partial denture; (bottom) Debonding of orthodontic brackets.
Fifteen out of 38 studies (Tables 2, 3, 4, and 5) examined the debonded ceramics. Scanning electron microscopy (SEM) was the most common method of examination. 7,8,10,13,20,21,26,29,37,48,50,53 SEM was sometimes applied with energy dispersive spectroscopy (EDS), 20,37 secondary electron imaging (SBI), or backscattered electrons (BSE). 10 EDS provides chemical compositions of materials, while SBI and BSE provide further details on ceramic cracks and surface structural damages. It is particularly useful to see that the composition of the debonded ceramic materials, either ceramic crowns or implant abutments, appeared to have no chemical or structural damages from erbium laser application for debonding. Multiple repeated laser debonding appeared to have little to no effect on the ceramic surface structure. 20 Results from all studies showed that erbium lasers are effective in debonding all ceramic restorations with no damage to abutment teeth/implant and with none or minimal alterations to the ceramic restorative/orthodontic appliance surfaces.

Discussion
This study is one of the first to comprehensively review contemporary research on ceramic debonding using erbium lasers. Results from this scoping review revealed that erbium lasers have been reported as an option to remove ceramic restorations from natural teeth and dental implant abutments. An orthodontic bracket appliance or a ceramic crown can be removed without any damage to the natural tooth or implant abutments in a few minutes. The light energy emitted by erbium lasers transmits through the translucent ceramic materials and is selectively absorbed by water molecules and residual monomers in cements resulting in cement ablation and hydrodynamic ejection. 5,7 The bond between the ceramic restoration and the tooth or implant abutment is disrupted mainly within the cement layer and at the ceramic/cement interface (Fig 2).
A majority of the included studies were performed on extracted natural teeth, thus data obtained in clinical in vivo trials is lacking. In this scoping review, there was no in vivo or in human studies included because only case reports or case series were currently present in the literature search. The most robust evidence is available for debonding of orthodontic brackets which is based on human and bovine extracted teeth. The erbium laser irradiation capacity to remove and degrade composite resin materials is applied for the removal of bonded ceramic materials to tooth structure. 6 The improvement of ceramic-natural tooth bonding technology presents a challenge to clinicians. In addition, removal of a wellfitted implant supported cement-retained fixed prostheses from customized implant abutments can also be a clinically challenging task. Only a few ex vivo studies have examined the application of erbium lasers for this implant prosthesis removal purpose. 7,8,20 The optimal laser power settings for a majority of ceramic restorations/appliances have been reported in the range of 4.0 to 5.5 Watt. 8,12,20,41,47,48 It appears that the time required to debond ceramic restorations correlates to the volume of the luting cement, 49 the size of the abutment surface area, 48 thickness and characteristics of the restorative materials, 6,38,42,48 and chemical composition of the luting cement. 37 In general, it takes longer to debond zirconia than lithium disilicate crowns from either natural teeth or implant abutments. 7,8,20 The zirconia material which is mainly crystalline structure is possibly less penetrable compared to lithium disilicate material which has more matrix in the structure. 20 Resin and glass-ionomer cements display different ablation thresholds and ablation volume loss based on their chemical composition, 51 resulting in different debonding efficiency of the restoration. 8 This is likely due to various amounts of water molecules and unpolymerized monomers left in the cements. Translucencies of ceramics seem to have no significant effect on time required to debond veneers. 44 The safety concern of raising pulpal temperature while using the erbium lasers to facilitate restorative debonding has been studied. The increase in pulpal temperature did not exceed 5.5 • C, which could cause thermal injury to the pulpal tissue. 22 The temperature raised in all studies pointed to the safety of erbium lasers to the pulpal tissue, 22 periodontium, 24 and bone. 23 None of the studies reported significant problems with intrapulpal temperature increases during laser irradiation using the proposed settings. Furthermore, many studies examined the debonded ceramics using microscopy, scanning electron microscopy (SEM), and energy dispersive x-ray analysis (EDX) and reported no detectable damage to the abutment and ceramic surface. 7,8,20 It can thus be suggested that laser-assisted ceramic restoration removal does not present a thermal risk for the tooth or implant nor does it damage the ceramic restorative material.
While both erbium lasers are efficient in debonding restorations, the Er,Cr:YSGG laser may take longer due to its lower absorption coefficients compared to the Er:YAG laser. 20,27,29,31,50 The Er,Cr:YSGG laser wavelength penetrates deeper into the tissue and requires more time to heat up to the evaporation temperature, while the substance heated by the Er:YAG laser will reach ablation temperatures faster. 55 More importantly the absorption peak to the water molecule of the Er:YAG laser is slightly closer than the peak of Er,Cr:YSGG laser, therefore the Er:YAG laser appears to be more proficient than Er,Cr:YSGG laser in ceramic debonding. 20 However, the two erbium pulsed lasers may be used interchangeably for debonding of ceramic restorations and brackets despite having different wavelengths, as both of them target water as their chromophore.
It is important to note the limitations of this scoping review. First, all studies included in this review were ex vivo or simulated. Extracted human and bovine teeth, as well as implant fixtures, were used in typodont simulated or other models. This may not be reflective of clinical situations where the patient's oral structures, tongue and cheek, as well as optimal access to the laser intraorally, can present challenges in manipulating the laser handpiece and application. Debonding time in clinical application may likely take longer than the simulated debonding time. More importantly, there is a real need for clinical studies to demonstrate the in vivo-based debonding time as well as to assess patient's compliance and acceptance of the laser assisted ceramic debonding. In addition to the fact that all studies in this scoping review are in vitro, the in vitro procedures were varied and not standardized. For example, some studies utilized manikins to simulate a clinical setting, others did not or did not report the use of a typodont or manikin. It is likely that in the clinical setting the time of retrieval would be longer. Therefore, a meta-analysis was not performed which would have been appropriate if more clinical studies were available. Future prospective controlled clinical trials are recommended. Second, there is no standardized protocol for laser settings in restorative/orthodontic appliance removal. While this scoping review has shown the consistency in the similar applications of laser settings, there is a need for a consensus for this procedure for both Er:YAG and Er,Cr:YSGG lasers. Finally, this scoping review only included English-language peer-reviewed literature. Attempts were made to review theses/dissertations, conference papers, as well as other sources of gray literature. However, no study was found relevant to this scoping review. It is possible that there are some unpublished works that may have been missed.

Conclusion
Erbium lasers present a noninvasive protocol for retrieval of ceramic restorations/appliances from teeth and implant abutments. Laser-assisted ceramic restoration and orthodontic bracket removal provides efficient restoration retrievability without damage to the material or the abutment surface. While the majority of the current literature did not compare the laser method to conventional rotary instruments, the laser method is a safe, predictable, and time efficient alternative to rotary-assisted crown removal. Clinicians need to possess adequate knowledge regarding laser safety and parameters with reference to different tissues and ceramics. Further well-designed controlled clinical trials and longitudinal prospective studies are needed to determine the safest and most effective laser parameters for irradiation of ceramic restorations with varying thicknesses and luted with cements of various chemical compositions.