## 4 Discussion
They exhibited differences in the MD of implant superstructures ranging from 13.21 to 75.26 \(\upmu\)m for the CAD/CAM groups and 64.89 to 115.27 \(\upmu\)m for the group that was conventionally cast [27]. In addition, the use of Ti-base abutments may affect the MD values and can successfully support single zirconia crowns. According to a retrospective study, 118 patients who received CAD/CAM-milled bilayer and monolithic zirconia crowns using implants and Ti-base abutments with up to 5 years of follow-up were not diagnosed with periimplantitis [28]. Another study revealed that Ti-base abutments did not have a negativeeffect on the peri-implant soft and hard tissue parameters [29]. Therefore, Ti bases have a similar stability and mechanical behavior as Ti abutments, which is still considered the \"gold standard\" for implant-supported single reconstructions [30]. Although several studies showed no superiority when using Ti bases in implant-supported prostheses, one study showed that the peri-implant tissues responded to Ti-base abutments in a similar manner to one-piece abutments over a 5 year period [31]. Another study showed no significant differences in human marginal bone levels between the Ti base and one-piece abutments at any of the follow-ups [32]. No definitive conclusion can be drawn based on these studies due to the low power and short follow-up periods. A few studies that examined MD values did not support the results found in the present study. They reported that cementation did not cause a significant increase in MD values. One study revealed no significant differences in MD values before and after cementation for the analyzed groups with MD values below 120 \(\upmu\)m [33]. This may be because they compared the MD values of fixed dental prostheses (FDP) under a stereomicroscope at \(\times\)15 magnification. Whereas in this study, the MD values between restoration and implant abutment were examined. A two-dimensional SEM was used to measure the MD values under \(\times\)500 magnification by one clinician who performed the scanning, determined the finish line, and the zirconia restoration. The CAD/CAM system was used because it is an advanced three-dimensional technology which reduces manual intervention and leads to more accurate manufacturing variations and fitting restorations in implants, which may lead to increased implant survival [34]. Additionally, the system has the potential to conserve materials and has an advantage in fabricating complex geometries, although limited evidence is available regarding applying CAD/CAM technologies in implant restorations [35]. As a result, three points of three aspects of each sample were marked in this study to ensure the same surfaces were scanned before and after cementation to provide more accurate results and to evaluate the differences between the three aspects. Due to the accuracy of the CAD/CAM system, no differences were found between the three aspects. However, the mean MD values before cementation were not close to zero because a certain degree of inaccuracy is inevitable even with CAD/CAM, which also depends on the clinician's expertise and on the equipment used [15]. In the current study, zirconia was chosen as the material for CAD/CAM restoration since its use has become increasingly widespread because of its excellent unique combination of high flexural strength and fracture toughness to resist deformation under contraction forces [36]. Nevertheless, several studies reported similar mechanical stabilities between zirconia meso-abutments bonded to Ti in comparison to customized Ti abutments. In addition, various types of ceramic crown material, including zirconia, showed good stability when used in the monolithic state, without significant differences [30]. Ti base and crown surfaces should be treated mechanically and/or chemically to increase stability. Mechanical treatment of the Ti surface increased the stability and retention forces of the bonding interface between the Ti-base abutments and the crown.