## 4 Discussion  
While some clinical reports and research on a non-invasive retrieval of all-ceramic crowns such as lithium disilicate and zirconia using Er:YAG laser exist in literature, there is little information on the different responses to laser-assisted crown retrieval from various ceramic materials. The results of this study highlighted the fact that crowns made from different ceramic materials behave differently when Er:YAG laser is used for their retrieval. As expected, lithium disilicate crowns are easier to retrieve than zirconia crowns. The time required to retrieve a lithium disilicate crown on average ranges from 3 min  
Figure 4: SEM analyses of zirconia crowns. SEM of the cameo surface of lithium disilicate at the 1 st (Z1), 2 nd (Z2), and 3 rd (Z3). Figure 3: SEM analyses of lithium disilicate crowns. SEM of the cameo surface of lithium disilicate at the 1 st (LD1), 2 nd (LD2), 3 rd (LD3), and a control of a higher laser setting based on a previous proposed protocol [22]. 40 s to 4 min 37 s. The time required to retrieve a zirconia crown on average ranges from 5 min 55 s to 6 min 59 s. Zirconia crowns therefore take 2-3 min longer to retrieve. These retrieval times align with findings from previous studies with the same laser settings [23; 24]. The retrieval times for these natural teeth are slightly higher than retrieving a zirconia crown from zirconia implant abutment, which are approximately 5 min 15 s to 5 min 20 s for the first time cementation and the repeated cementation [25]. This perhaps is a result of denser crystalized material of zirconia compared to less crystalized lithium disilicate. Furthermore, based on the study by [PERSON] et al. [26] lithium disilicate material shows higher translucency values than crystalline-based ceramics (zirconia), which can explain the results of this study. In theory, the laser can therefore penetrate through lithium disilicate better than zirconia [26]. This is reflected in longer retrieval time as well as 1 \({}^{\circ}\)C to 2 \({}^{\circ}\)C lower pulpal temperatures in the zirconia compared to the lithium disilicate group. It is interesting to note here that even the highest recorded temperature of \(\sim\)28 \({}^{\circ}\)C in the pulpal chamber is still considered safe. The increased pulpal temperatures are also similar to increased temperature of zirconia implant abutment [24; 25]. The increase in pulpal temperature over 5.5 \({}^{\circ}\)C could cause irreversible damage to the pulp tissue [27]; however, temperature lower than 30 \({}^{\circ}\)C is considered safe for the vitality of the dental pulp [27; 28; 29; 30]. A rise in temperature to 50 \({}^{\circ}\)C for 1 min on the root surfaces can cause irreversible damage to the periodontal ligament and bone that can lead to bone resorption and tooth ankylosis [31; 32]. For both materials examined in this study, the temperature rises during crown retrieval using Er:YAG laser irradiation appear not harmful to the dental pulp or adjacent tissues. This study demonstrated that both lithium disilicate and zirconia crowns can be reused without compromised structural properties following re-retrieval with Er:YAG laser. These findings are consistent with previous observations on implant crowns [23; 24]. The Er:YAG laser appears to have little or no effect on lithium disilicate material [33; 34]. The laser settings used here appear to be too low to alter the structure of lithium disilicate or zirconia [23; 24; 35; 36; 37]. The goal of our study was also to achieve minimal retrieval time at the lowest possible settings to avoid potentially harmful temperature increases and irreversible damage to the tooth substance. Various authors have proposed a wide range of laser settings for debonding of restorations. While trying to determine optimal efficiency with safe parameters, the pilot experimented with a higher laser setting from a published protocol for retrieval of all-ceramic crowns (508 mJ, 10 Hz) [15; 22], resulted in macroscopically noticeable significant damage to the abutment tooth.