The Complete Digital Workflow in Fixed Prosthodontics Updated: A Systematic Review

Digital applications have changed therapy in prosthodontics. In 2017, a systematic review reported on complete digital workflows for treatment with tooth-borne or implant-supported fixed dental prostheses (FDPs). Here, we aim to update this work and summarize the recent scientific literature reporting complete digital workflows and to deduce clinical recommendations. A systematic search of PubMed/Embase using PICO criteria was performed. English-language literature consistent with the original review published between 16 September 2016 and 31 October 2022 was considered. Of the 394 titles retrieved by the search, 42 abstracts were identified, and subsequently, 16 studies were included for data extraction. A total of 440 patients with 658 restorations were analyzed. Almost two-thirds of the studies focused on implant therapy. Time efficiency was the most often defined outcome (n = 12/75%), followed by precision (n = 11/69%) and patient satisfaction (n = 5/31%). Though the amount of clinical research on digital workflows has increased within recent years, the absolute number of published trials remains low, particularly for multi-unit restorations. Current clinical evidence supports the use of complete digital workflows in implant therapy with monolithic crowns in posterior sites. Digitally fabricated implant-supported crowns can be considered at least comparable to conventional and hybrid workflows in terms of time efficiency, production costs, precision, and patient satisfaction.


Background
The global trend towards digitization dominates all fields of dentistry today. Particularly in fixed prosthodontics, as a technique-oriented discipline, computerized dentistry has enabled new clinical protocols and production processes [1]. While the continuous development of computer-aided design and computer-aided manufacturing (CAD/CAM) techniques is the driving force in dental technology, the adoption of intraoral scanners (IOS) has significantly changed clinical procedures in recent years [2]. Together, these technologies now enable complete digital workflows for single-visit treatments for tooth-borne (tooth-supported) and implant-supported monolithic fixed dental prostheses (FDPs) [2].
Complete digital protocols consist of three main work steps: (i) the 3D acquisition of the individual patient situation directly in the mouth with IOS; (ii) digital design with dental software applications (CAD) for rapid prototyping such as milling or 3D printing (CAM) in a fully virtual environment without any physical dental models (plaster casts); and (iii) clinical delivery of the dental restoration [3]. Crucial steps are the generation, transfer, and further processing of the created IOS data (in Standard Tessellation Language [STL] format) [4]. Overall, the digital workflow is associated with mechanically high-quality monolithic restorations and reproducible fabrication in a simplified process with a reduced need for manual human interaction [5].

Focused question (PICO)
Is a complete digital workflow with intraoral optical scanning (IOS) plus virtual design plus monolithic restoration for patients receiving prosthodontic treatments with (A) tooth-borne or (B) implant-supported fixed restorations comparable to conventional or mixed analog-digital workflows with conventional impression and/or lost-wax-technique and/or framework and veneering in terms of feasibility in general or survival/success-analysis including complication assessment with a minimum follow-up of one year or economics or esthetics or patient-centered factors?

Inclusion Criteria
This systematic review focused on RCTs as the highest level of clinical evidence, particularly those describing complete digital workflows in fixed prosthodontics that analyzed at least one of the following parameters: economics in terms of time and/or cost analyses, esthetics, patient-centered outcomes with or without follow-up, as well as survival and success rate analyses including assessments of complications of at least 1 year under function. The following inclusion criteria were defined [6]: • Clinical trials, limited to RCTs with at least 10 patients; • Treatment concepts with FDPs, either tooth-borne or implant-supported for single-or multi-unit restorations; • Processing of a complete digital workflow (without physical models); and • Reporting of information on the used clinical work steps and technical production.

Selection of Studies
Title and abstract screening were performed by two independent researchers (S.A.B. and T.J.), who considered the defined inclusion criteria. If the provided information was not sufficient, full texts were retrieved and evaluated by both reviewers. Several publications reported on the same patient population; these publications that summarized different outcomes were merged. Selected articles were subjected to further analyses. Throughout this complete process, disagreements were resolved through discussion.

Data Extraction
The following information were extracted from the included publications: author, year of publication, description of the specific study design, number of patients treated and examined, type of fixed restorations (including the number of abutment teeth and/or dental implants), clinical treatment concept, methodological approach for laboratory processing, description of the material properties, as well as defined primary (and secondary) outcomes.
Finally, all included studies were subdivided into four groups based on the type of prosthetic abutments and the number of units: A1. tooth-borne single crowns; A2. toothborne multi-unit FDPs; B1. implant-supported single crowns; and B2. implant-supported multi-unit FDPs. The information extracted from the articles was tabulated, and if possible, a meta-analysis was to be conducted.

Included Studies
The systematic search was completed on October 31, 2022, and results are current as of this date. Of the 394 titles retrieved by the electronic search, 42 potentially relevant abstracts were identified; however, 28 of these were excluded from the final analysis. In addition, two studies were found through manual search, resulting in a total of 16 studies for data extraction (Figure 1).
year of publication, description of the specific study design, number of patients treated and examined, type of fixed restorations (including the number of abutment teeth and/or dental implants), clinical treatment concept, methodological approach for laboratory processing, description of the material properties, as well as defined primary (and secondary) outcomes.
Finally, all included studies were subdivided into four groups based on the type of prosthetic abutments and the number of units: A1. tooth-borne single crowns; A2. toothborne multi-unit FDPs; B1. implant-supported single crowns; and B2. implant-supported multi-unit FDPs. The information extracted from the articles was tabulated, and if possible, a meta-analysis was to be conducted.

Included Studies
The systematic search was completed on October 31, 2022, and results are current as of this date. Of the 394 titles retrieved by the electronic search, 42 potentially relevant abstracts were identified; however, 28 of these were excluded from the final analysis. In addition, two studies were found through manual search, resulting in a total of 16 studies for data extraction (Figure 1).
The reasons for exclusion were (n = 28):  No data on complete digital workflows (n = 5)  Not an RCT (n = 17)  Workflow did not investigate final prosthetic restorations (n = 6)  The reasons for exclusion were (n = 28): • No data on complete digital workflows (n = 5) • Not an RCT (n = 17) • Workflow did not investigate final prosthetic restorations (n = 6)

Descriptive Analysis
The 16 identified RCTs reported on a total of 440 patients, with 236 tooth-borne restorations and 422 implant-supported restorations. Only one of the 16 RCTs included follow-up examinations. General data for study design, type of fixed restoration, number of subjects, and defined outcomes are summarized in Table 2. Based on the prosthetic design, included studies were divided into four groups: (A1) six publications for tooth-borne single-unit restorations [11][12][13][14][15][16][17]; (A2) no publication for tooth-borne multi-unit restorations; (B1) eight publications for implant-supported single-unit restorations [18][19][20][21][22][23][24][25]; and (B2) two publications for implant-supported multi-unit restorations [26][27][28][29].    RCT − Two-armed design B1. Implant-supported crowns − n = 9: CAD/CAM provisional crowns prepared prior to surgery and immediate restoration. − n = 9: Stratified provisional crowns based on a conventional impression 10 days after surgery. Due to the heterogeneity of the included RCTs with different study designs and outcomes, a direct comparison among the identified publications was not feasible, and consequently, a meta-analysis could not be performed. Thus, the review of the full texts followed a descriptive analysis. Detailed information of each study, categorized in A1-B1-B2, is shown in Tables 3-5. Figure 2 displays the risk of bias for the included studies. No additional analyses were performed. Due to the heterogeneity of the included RCTs with different study designs and outcomes, a direct comparison among the identified publications was not feasible, and consequently, a meta-analysis could not be performed. Thus, the review of the full texts followed a descriptive analysis. Detailed information of each study, categorized in A1-B1-B2, is shown in Tables 3-5. Figure 2 displays the risk of bias for the included studies. No additional analyses were performed.        Esthetic: − White esthetic score was comparable in both groups (p = 0.45), trend to higher score for the conventional workflow. − Tendency for higher pink esthetic score for the test group (p = 0.057). Precision: − No significant differences regarding precision needed (occlusion: p = 0.70, interproximal contact: p = 0.69), but in about half of the cases in both groups adjustments were necessary. Patient satisfaction: − IOS impressions were significantly more comfortable compared to conventional impressions (p = 0.014). 8.
Precision: − At try-in and delivery, efficacy of prosthetic manufacturing was similarly high in both workflows. Time efficiency: − Mean total impression time was shorter for digital impressions (9.5 ± 3.5 min) compared to conventional impressions (15.1± 4.6 min) (p < 0.0001). − Mean total working time of the dental technician for centralized complete digital workflow (131 ± 31 min) and hybrid workflow (218 ± 31 min) (p < 0.0001). − Mean total waiting time for centralized complete digital workflow (8593 ± 4407 min) compared to the hybrid workflow (764 ± 65 min) (p < 0.0001) CAD/CAM, computer-aided design/computer-aided manufacturing; FDP, fixed dental prostheses; IOS, intraoral scanning; LS2, lithium disilicate; PICN, polymer infiltrated ceramic network; PMMA, polymethyl methacrylate acrylic; STL, stereolithography files; vs., versus; ZrO 2 , zirconium dioxide.  (Table 3) Six studies compared digital to conventional workflows for tooth-borne single crowns, with a total of 236 prosthetic units. Regarding the precision of marginal fit of the FDPs, three studies found no statistically significant differences of the fabricated crowns between workflows [14,16,17]. One RCT documented a trend towards better marginal fit for the conventional workflow [13]. Another RCT found better marginal and internal adaption for crowns fabricated with digital workflows, but the clinical evaluation showed similar marginal adaptation [12]. Occlusal contacts were found to be better for digitally produced crowns, while no differences were found for marginal fit, proximal contact, and crown morphology [11].

Group A1-Tooth-Borne Single-Unit Restorations
Four studies also investigated time efficiency, with two studies reporting no statistically significant differences in total clinical treatment times [13,15], and one study showing a shorter impression time for IOS compared with a conventional workflow [14]. The other RCT investigated a complete digital workflow, different hybrid workflows with a physical cast, and a conventional workflow as a control [15]. Laboratory fabrication time was significantly shorter for the conventional cast compared to all CAD/CAM casts because the digital workflow included delivery of the CAD/CAM cast from the manufacturer to the dental laboratory. Delivery of the crowns was significantly faster for the fully digital workflow, followed by the conventional workflow. (Table 4) This group comprised eight studies, including 302 patients. A total of 342 implantsupported single crowns were examined. The most frequently considered topic was time efficiency (n = 6), followed by precision (n = 5), patient satisfaction (n = 4), esthetics (n = 4), marginal-bone loss (n = 2), and cost efficiency (n = 1). For economic analyses, all studies that examined time efficiency found significantly higher time savings for digital workflows (n = 6/6 studies), and costs were also significantly lower for the complete digital approach (n = 1/1 study). Patient satisfaction was rated significantly better for digital solutions in most publications (n = 3/4 studies). For the three other parameters (precision, esthetic outcome, and marginal bone loss), no significant differences between the workflows were reported. (Table 5) Two studies investigated implant-supported multi-unit restorations with a total of 30 patients. Eighty implant-supported three-unit FDPs were fabricated either with digital or conventional workflows. Both studies examined time efficiency. Hashemi et al. stated significant less mean laboratory time and a shorter total fabrication time for the digital workflow [28]. Joda et al. [29] investigated time efficiency of two different digital and one conventional workflow. Significant differences in time efficiency for pairwise comparisons of the total work time were observed. The proprietary digital workflow 3Shape (IOS: TRIOS 3 Pod) was shown to be more time-efficient than the conventional workflow, while the proprietary digital workflow Dental Wings (IOS: Virtuo Vivo) required more time. The cost analysis was favored the digital workflow, with significantly lower production costs for completely digital fabricated FDPs [29]. Based on the same study population, patient-centered outcomes and clinical performance were also investigated [27]. Patient satisfaction with the final monolithic ZrO2 FDPs, as assessed in a double-blind testing, revealed no significant difference between the different workflows, but significantly lower overall ratings were reported by the dental professional than by patients [27]. Finally, the mean total chairside adjustment time, as the sum of interproximal, pontic, and occlusal corrections, was not significantly different among all three workflows [26].

Discussion
This systematic review aimed to summarize recent data from RCTs on conventional versus complete digital workflows for fabrication of FDPs. Overall, data from 658 FDPs from 16 RCTs were summarized. The review found ambiguous results for clinical parameters in tooth-borne single-unit restorations, while for implant-supported single-and multi-unit restorations, significantly shorter fabrication time at lower costs was demonstrated for digital compared to conventional workflows.
The systematic search strategy and inclusion criteria used in the present review were identical to those used in the previous review [6], only the time frame was adjusted. In general, RCTs provide the best clinical evidence for generating a systematic review, and only this study type was included. Consequently, the number of included publications was smaller than if all study types had been considered. Compared with the previous review that covered publications up to September 2016 and identified three RCTs, the present systematic review covering the last 6 years identified five times as many RCTs, including two studies of multi-unit restorations. Nevertheless, this still represents relatively few studies compared with the hundreds of publications that report FDP treatment using purely conventional protocols. This suggests that the long-predicted hype for digitization in the MedTech industry has yet to be realized.
Interestingly, the number of studies in the subgroups A1-B1-B2 showed a heterogeneous distribution, and no RCT was identified that investigated a complete digital workflow for tooth-borne multi-unit restorations. Most included RCTs (10/16; 63%) investigated implant-supported restorations. Possible explanations for this include a general trend towards more implant-driven treatment concepts, the fact that complete digital workflows are simply predestined for monolithic restorations on implants, or that the funding of clinical trials by industrial sponsors favors implant concepts.
The correct application of a workflow (digital or conventional) to an appropriate indication is crucial for the success of the overall prosthetic therapy and for a satisfied patient [31]. For digital processing, a teamwork approach is particularly important-this equally includes the clinician, the dental assistants, and the technician [32]. The complete digital workflow has the potential to become a game-changer in (fixed) prosthodontics [33].
Nevertheless, the conventional workflow remains the current gold standard. In recent years, individual components in the workflow have increasingly become digitized for both tooth-borne and implant-supported restorations. This digital change began in dental technology with the introduction of CAD/CAM technology. As a consequence, the technical-dental protocol has been transformed to a hybrid analog-digital workflow. For the indication of single crowns, especially on implants in the posterior region, there seems to be a strong trend in favor of complete digital workflows with monolithic restorations and pre-fabricated titanium base abutments [34]. Subsequently, IOS ideally has completed the clinical gap [35]. The continuous development of digital scanning techniques has enabled quick, safe, and patient-friendly 3D capturing of the clinical situation [36,37]. Use of IOS is particularly beneficial in implant therapy because it is not necessary to optically record an individual preparation margin on the tooth, but only a standardized supra-mucosal localized scanbody. For single-unit restorations, the digital bite registration is much easier and more reproducible than for multi-units [38]. Finally, economic factors offering highlevel quality restoration with reduced treatment time and lower production costs are the biggest driver [39,40].

Conclusions
Based on the findings of this systematic review, it can be concluded that the amount of qualitative clinical research investigating complete digital workflows has increased within the last 6 years. However, the absolute number of RCTs, in particular those investigating treatment with multi-unit restorations, is still low. Good quality clinical evidence exists supporting the use of complete digital workflows in implant therapy with monolithic crowns in posterior sites. Digitally fabricated implant-supported single units can be considered at least comparable to conventional and hybrid workflows in terms of time efficiency, production costs, precision, and patient satisfaction.
Future clinical research based on RCTs is imperative to gain clarity on the clinical performance of digital workflows. The difficulty is the rapid digital evolution, so that the devices and tools from the clinical trials are already "obsolete" after 1 to 2 years (when the data are published). It is therefore particularly important that the versions of hardware and software used are always specified.