Implant-supported single zirconia crowns for posterior teeth using completely digital work-flow – a case report

Introduction Planning fixed prosthodontic reconstruction can be challenging task in everyday practice. When the last tooth in dental arch is missing, a single implant-supported crown is recommended. With the evolution of digital technology, it became possible that these restorations can be made using completely digital approach. The aim of this case report was to present complete clinical procedure of making implant supported single zirconia crowns for posterior teeth using completely digital approach. Case report A 53-year-old patient presented to the dental office with missing both first molars in the lower dental arch. The decision was made to make two implant-supported single screw-retained crowns, using digital approach. Intraoral scanning of the soft tissues and the implants’ position was done using intraoral scanner Medit i500. The laboratory steps followed: computer-assisted design (CAD) and computer-assisted manufacturing (CAM) of zirconia crowns. Conclusion Implant-supported single crowns for posterior teeth are an excellent solution for patients when the last tooth in dental arch is missing. Digital approach -from initial intraoral scanning (IOS), to designing the restoration in software and further processing of monolithic CAD/CAM generated crowns out of zirconia gives predictable, highly esthetic and functional results for implant-supported single crowns.


INTRODUCTION
Planning fixed prosthodontic reconstruction can be challenging task in everyday practice, considering all biological and technical risks during the procedure. Prosthodontic restorations can be tooth or implant-supported. In the posterior region, the decision about gap reconstruction is made based on the condition of adjacent teeth. If they are intact or with minimal restoration, then bridge would be poor choice as it would damage abutment teeth. In that case, implant-supported single crown is the first choice, presenting the most tissue-preserving option [1]. Also, when the last tooth in the dental arch is missing, a single implant-supported crown is a recommended.
When making an implant-supported single crown, there are two options: screw-retained and cemented restorations. The choice is usually made according to clinician preferences since there is no evidence that any method is better [2].
The evolution of intraoral scanning technology (IOS) completely changed dental practice. The digital workflow begins with intraoral scanning of the soft tissues and implants' position. Afterward, the laboratory steps follow: computer-assisted design (CAD) and computer-assisted manufacturing (CAM). According to design, final monolithic restoration is manufactured from zirconia, lithium disilicate, or hybrid ceramic materials [3,4].
The aim of this case report is to present complete clinical procedure of making an implant supported single zirconia crown for posterior teeth using completely digital approach.

CASE REPORT
A 53-year-old patient presented to the dental office with missing both first molars in the lower dental arch. First molars were his last teeth in the lower dental arch. The decision was made to restore them with implant-supported single crowns, because other fixed prosthodontic restorations were not possible. Implants Blue Sky (Bredent Medical Gmbh&Co.KG, Germany) were placed in the region of the teeth 36 and 46. After the healing period of 6 months, gingival formers were placed and peri-implant mucosa was allowed to heal for 14 days. At the very beginning of the procedure, shade determination was done to avoid dehydration and change of teeth color later on ( Figure 1). Intraoral photographs were taken with camera (Canon R, Canon 100mm 2.8 L, Yongnuo YN-24EX TTL Macro Flash) and sent to the laboratory to achieve better color effects and matching with adjacent teeth.
The scanning was done using Medit i500 intraoral scanner (Medit corp., Seoul, Korea). Firstly, the whole lower dental arch was scanned, with gingival formers on implants ( Figure 2). Then, using a tool in MEDIT software part of the digital impression with gingival formers was deleted ( Figure 3). To represent the position and orientation of dental implants in intraoral scanning procedure scan body SKYUSCAI (Bredent Medical Gmbh&Co.KG, Germany) was used ( Figure 4). One scan body was used for both sides, scanning one implant at a time, using HD (high definition) option of the scanner ( Figure 5).
The scan was saved as STL file that was sent to the dental laboratory ( Figure 6). Digital models were printed in dental laboratory using printer DWS xfab2500pd and resin DWS Precisa RD097 and restorations were designed       Figure 9). It was easier to make all shape changes in this phase instead of on a full-ceramic crown. Since only minor changes were made, they were sent back to the laboratory. The occlusal surfaces of the crowns were designed to avoid premature contacts during mastication and movements. According to the new situation, final full-ceramic restorations were made of zirconia and sent back for the final step ( Figure 10). Before positioning crowns in patient's mouth, the crowns were left in chlorhexidine antiseptic for 2 minutes. Final crowns were screwed on the top of the implants in patients' mouths using a torque wrench calibrated at 25 Ncm (Bredent Medical Gmbh&Co.KG, Germany) ( Figure 11). The screw access holes on the occlusal surfaces of the restorations were closed with teflon tape plug ( Figure 12) and composite resin (GC gradia direct, GC Corporation, Tokyo, Japan) without using bond ( Figure 13).

DISCUSSION
Implant-supported single crowns in the posterior region are the first choice when the adjacent teeth are intact or with minimal restorations. They show high survival rates after an observation period of 5 years (94.5% and 96.3%)      [5,6]. Another study showed that biological and technical complications are frequent (33.6%) [7].
Biological complications of tooth-supported reconstructions include dental caries, loss of pulp vitality, and periodontal disease progression. In implant reconstructions, technical complications are more common, and they include mechanical damage of implants, implant components and/or suprastructures [1]. According to meta-analysis, the incidence of screw or abutment loosening is 12.7% and 0.35% for screw or abutment fracture after 5 years, and for supra-structure-related complications, the incidence of ceramic or veneer fractures is 4.5% [5].
The crowns made in this report were screw-retained. Screw-retained crowns have many advantages, and the most important one is retrievability. Since the cumulative incidence of screw or abutment loosening was 12.7% in a 5-year period according to Jung et al. [5], it is important that they can be easily removed and retightened if needed, which is not the case with cemented ceramic crowns. Screw retained crowns also eliminate the risk of excess cement that can compromise soft tissues surrounding the implant [8].
The use of digital impressions is becoming more and more popular due to its numerous advantages-they are more comfortable for the patients, less time-consuming, and easier to store and share than conventional models. However, the conventional impressions are still considered the gold standard in fixed prosthodontics. The accuracy of digital models and comparison with conventional models is a topic of numerous investigations [9][10][11][12]. According to recent studies, intraoral scanning accuracy is high for single crowns, and the deviations in virtual implant positions are clinically acceptable [10,13,14]. In our case, the results showed great precision of digital impression, and just minor changes regarding occlusion were made on plastic crowns.
Generally, patients report less inconvenience and they prefer digital impression procedure than the conventional procedure [15]. Moreover, one of the advantages of digital impression is that if there is an error in the scan, it is not necessary to retake complete impression; only the critical spot can be scanned again, which significantly saves time. Also, the communication with the dental laboratory is faster and safer, since digital impression is sent digitally and it is not possible to damage it during transport.

CONCLUSION
Implant-supported single crowns for posterior teeth are an excellent solution for patients when the last tooth in dental arch is missing. Digital approach -from initial intraoral scanning (IOS) to designing the restoration in the software and further processing of monolithic CAD/CAMgenerated crowns out of zirconia gives predictable, highly esthetic and functional results for implant-supported single crowns. Zubotehnička laboratorija "Dental Lab Bulić", Beograd, Srbija