## 1 Introduction  
Recent advancement of digital technologies, including intraoral scanning, computer-aided design and computer-aided manufacturing (CAD/CAM), as well as biomaterials, allows dentists to fabricate fixed prostheses such as a crown in a rapid streamline process [1; 2; 3; 4]. These crowns, most frequently made of ceramic materials such as lithium disilicate or zirconia, are esthetically pleasing, with superior biocompatibility and physical properties as well as survival rates [5; 6; 7]. The crown material, especially lithium disilicate, can be bonded to the natural tooth abutment using acid etching and composite resin cement [8; 9; 10], while zirconia materials used to make these crowns are perhaps the strongest ceramic materials available in contemporary dentistry [11; 12]. These physical strengths and bonding capabilities allow for long lasting restorations in the oral cavity. These clinically desirable properties however become a major obstacle when the crown needs to be removed [13; 14]. When the crown retrieval is indicated due to recurrent dental caries, misfits, esthetic reasons, material damage or a need for endodontic therapy, a rotary handpiece with a cutting bur is a common conventional method to remove the crown. This process leads to reversible damage of the crown and possibly the tooth structure, resulting in a need for a new restoration, which can be a time consuming and costly process for both dentist and patient. Ideally, the dentist should be able to retrieve the crown from the abutment tooth without trauma and quickly without damaging the crown or tooth structure. The crown would then be re-cemented back onto the tooth. Such a process would reduce chair time, discomfort, time, frustration, and cost to both the provider and the patient. The use of lasers for removal of restorative materials from natural teeth or dental implants is not new. High power lasers such as erbium-doped yttrium aluminum garnet laser (Er:YAG) have been used to retrieve all-ceramic restorations [15; 16]. The energy wavelength of 2940 nm is passed through the translucent ceramic material with little absorption and targets the water and monomer molecules in the cement layer. The energized water and monomer molecules create an ablation cascade of cement and therefore the all-ceramic restorations can be retrieved without any structural damage to the crown or to the natural abutment tooth. Er:YAG laser has been used to remove composite resin restorations [17], veneers [18; 19], orthodontic brackets [20; 21] and crowns [22]. Recent studies used Er:YAG laser to retrieve lithium disilicate crowns from zirconia or titanium implant abutments [23; 24; 25] and zirconia crowns from zirconia implant abutments [25] in a few minutes without any damage to the crown or abutment materials. These studies also demonstrate the ability of the Er:YAG laser to retrieve and reuse all-ceramic (lithium disilicate or zirconia) crowns multiple times without any damages to the dental implant crown or abutment materials [23; 24; 25]. When applying Er:YAG laser at 2940 nanometer (nm) wavelength to remove lithium disilicate crowns from dental implant abutments, the removal time ranged from \(\sim\)3 to 5 min (min) [23; 24], and the temperature of the crowns appeared to increase by 4-6 \({}^{\circ}\)C [23; 24]. When similar set up and laser was used to remove zirconia crowns from implant abutments, the average removal time increased to almost 6 min, while the temperature increased by 3-7 \({}^{\circ}\)C [25]. The Er:YAG laser at 2940 nm wavelength does not seem to have any adverse effects toward lithium disilicate or zirconia crown or implant abutment materials [23; 24; 25]. However, repeated Er:YAG laser assisted crown retrieval seems to increase the amount of remaining cement in the implant crown [24; 25] and increase the surface roughness through roughening of feldspathic glaze porcelain on the zirconia implant crown surface [25].