Computer-Engineered Complete Dentures: Where Are We Now? A Review

AUTHORS

Sarah Bukhari, BDS, MS, is an assistant professor in the advanced specialty education program in prosthodontics at the Loma Linda University School of Dentistry. She is a fellow of the American Board of Prosthodontics. Conflict of Interest Disclosure: None reported.

Abdulaziz A. Alzaid, BDS, MS, is a teaching assistant in the prosthetic dental science department in the College of Dentistry at King Saud Bin Abdulaziz University for Health Sciences and King Abdullah International Medical Research Center in Riyadh, Saudi Arabia. He is a fellow of prosthodontics and digital technology at the Loma Linda University School of Dentistry. Conflict of Interest Disclosure: None reported.

Raneem Alduraiby, BDS, is a graduate student in the advanced specialty education program in prosthodontics at the Loma Linda University School of Dentistry. Conflict of Interest Disclosure: None reported.

Periklis Proussaefs, DDS, MS, is an associate professor in the advanced specialty education program in prosthodontics and implant dentistry at the Loma Linda University School of Dentistry. Conflict of Interest Disclosure: None reported.

Mathew T. Kattadiyil, BDS, MDS, MS, is a professor and the director of the advanced specialty education program in prosthodontics at the Loma Linda University School of Dentistry. Conflict of Interest Disclosure: None reported.

ABSTRACT

Background: Computer-engineered complete dentures (CECDs) or digital dentures have become a popular option for treating completely edentulous patients. CECDs are fabricated using additive or subtractive methods of manufacturing after the digitizing and designing process. The advantages and disadvantages of CECDs have been reported in the literature. The purpose of this review article was to evaluate the current status of CECDs by conducting a search of related literature.

Types of studies reviewed: An electronic search of the English language literature was conducted using PubMed/MEDLINE database from January 1990 to July 2020. Fifty-six articles were reviewed in-depth and variables related to CECDs studied such as accuracy, adaptation, retention, properties, patient satisfaction, number of post insertion adjustments, surface treatment and denture teeth.

Results: CECDs continue to be studied vigorously as evidenced by reports in the scientific literature. Milled CECDs are mostly reported to have superior adaptation, flexural strength, modulus of elasticity, fracture toughness, color stability and require less clinical chairtime for fabrication.

Practical implications: Despite numerous advantages associated with CECDs, there is no clear scientific evidence yet that they offer a significant improvement in patient satisfaction and quality of life when compared to conventional CDs.

Key words: Computer-engineered complete denture, CECD, milled dentures, printed dentures, digital dentures

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Computer-engineered complete dentures (CECDs) or digital dentures have become a popular option for treating completely edentulous patients. Introduced as a concept in the early 1990s, [1] the technology used to fabricate complete dentures using a digital process has expanded significantly in the last 10 years. CECDs are fabricated using additive or subtractive methods of manufacturing after the digitizing and designing process. [2] The advantages and disadvantages of CECDs have been reported in the literature. The purpose of this review article is to evaluate the current status of CECDs by conducting a search of related literature.

An electronic search of the English language literature was conducted using the PubMed/MEDLINE database. Search terms included were CAD-CAM AND complete dentures, digital AND complete dentures, computer-engineered AND complete denture, milled AND dentures, 3D printed, rapid prototyping AND dentures. The search period was from January 1990 to July 2020. The inclusion criteria for the search were: The article be in English and include the specified search terms in the title or the abstract. Articles that did not include search terms described in the inclusion criteria or repetitive data from another article were excluded. The exclusion criteria for the search were technique articles. Systematic reviews were reviewed to look for missed articles, if any. Articles that did not meet descriptions of CAD/ CAM as defined by the Glossary of Prosthodontic Terms [3] and not related to the topic of interest were also excluded.

The search strategy included three stages: title search, abstract review and full text article selection and review. In stage one, titles obtained from PubMed/ MEDLINE search terms were analyzed according to the inclusion criteria. Before excluding an article, any disputes in selection were resolved by discussion among the authors (SB, AA, RA). Disputed articles (those that did not meet unanimous consensus among the authors) were still included in the abstract stage for further review. In stage two, abstracts of all selected titles were reviewed, and selected articles and disputed articles were included for full-text analysis. In stage three, the full text of selected articles was reviewed in detail. Any articles that didn’t meet inclusion criteria were excluded and the final list of articles was reviewed in-depth to satisfy the objectives of the systematic review.

The PubMed electronic search resulted in 1,726 titles of which 161 titles were considered for further review of abstracts. This resulted in the elimination of 105 articles. The remaining 56 articles were reviewed in-depth for variables related to CECDs studied such as accuracy, adaptation, retention, properties, patient satisfaction, number of post insertion adjustments, surface treatment and denture teeth. The references of these 56 articles were reviewed to ensure no relevant articles were omitted. This best evidence consensus conclusions are based on the review of the selected articles and the parameters studied related to CECDs. A detailed TABLE is available at cda.org/BukhariTable.

Accuracy of Fit and Adaptation

In an in vitro study by Lee et al., [4] a surface comparing software was used to study the accuracy of denture bases fabricated by injection molding, milling and 3D printing. Milled and 3D printed denture bases displayed higher accuracy than the denture bases fabricated by injection molding. Reproducibility was higher for the injection molded denture bases.

In an in vitro study, Yoon et al. [5] evaluated the trueness of 3D printed denture bases and compared the tissue surface adaptation of 3D printed denture bases with milled and compression molded denture bases. Milled denture bases performed better than the 3D printed ones for trueness (P < .001). 3D printed denture bases had the lowest value for fit discrepancies followed by milled and injection molded denture bases. Significant differences were not seen with respect to tissue surface adaptation of the denture bases, regardless of the fabrication technique (P > .05).

Srinivasan et al [6] evaluated the trueness of compression molded, injection molded and milled dentures bases. Milled and injection molded denture bases had significantly lower trueness of the intaglio surface compared to conventional denture bases. The authors reported that the trueness of the intaglio surface of all three techniques remained within the clinically acceptable range.

McLaughlin et al. [7] in an in vitro study compared the denture base shrinkage fabricated by milling, compression molding and injection molding of denture base resins. Milled and injection molded dentures had significantly better adaptation than compression-molded dentures for shallow palates. Most of this difference in space was in the shallow palate groups, which were noted as more susceptible to shrinkage.

Hsu et al. [8] evaluated the denture base adaptation of milled, 3D printed and compression molding techniques (n = 10). Silicone thickness between the denture base and edentulous model under a 49-N load was measured at eight sites. Adaptation was evaluated by superimposing the scanning data from denture bases and models. Milled dentures had the highest level of adaptation and the 3D printed dentures bases had the lowest value for trueness of the intaglio surface of the denture base.

Steinmassl et al. [9] compared milled dentures provided by four different manufacturers were generated from 10 different master casts. The authors reported superior adaptation with milled denture bases compared to compression molded ones.

Yoon et al. [10] in a clinical study investigated the tissue surface adaptation of compression molded, milled and 3D printed denture bases for 12 edentulous arches. There was no significant difference in the tissue surface adaptation between milled, 3D printed and compression molded denture bases. These findings are in disagreement with the findings of Steinmassl et al.

The authors reported superior adaptation with milled denture bases compared to compression molded ones.

Goodacre et al. [11] compared the adaptation of denture bases fabricated conventionally (compression molding, pour, injection molding) and milled denture bases using a surface matching software. Milled denture bases exhibited superior accuracy and reproducibility compared to the conventional techniques of fabrication.

Kalberer et al. [12] evaluated the trueness of 3D printed and milled CECDs in an in vitro study. They reported significantly superior trueness for the entire milled CECD intaglio surface including the posterior crest, palatal vault, posterior palatal seal, tuberosity, anterior ridge, vestibular flange, midpalatal raphe areas and the entire intaglio surface.

Retention

Tasaka et al. [13] evaluated the accuracy and retention of 3D printed and compression molded denture bases. 3D printed denture bases exhibited higher retention and more accuracy.

Alhelal et al. [14] in a clinical study compared the retention of compression molded and milled denture bases in the maxillary arch of 20 edentulous subjects. A custom-designed electronic testing device was used to measure denture retention. Significantly higher retention with milled denture bases was reported.

Alrumaih et al. [15] using the same methodology evaluated the effectiveness of denture adhesive on the retention of milled and compression molded denture bases. Denture adhesive application interestingly decreased the overall retention of milled denture bases compared to the retention when no adhesive was used. The authors reported that the use of denture adhesive did not significantly improve retention values between the milled and the compression molded bases. The retention of the conventional bases was not significantly increased by using adhesive.

Physical Properties

Aguirre et al. [16] performed an in vitro study comparing the flexural strength flexural modulus of milled, injection molded and compression molded denture base material (n = 10 per group). The authors used a three-point bend test with a universal testing machine until failure. The authors reported that milled and compression molded denture base material exhibited more brittle behavior than injection molded denture bases. However, milled denture base material showed higher flexural strength than the other two groups.

Steinmassl et al. [17] in an in vitro study comparing physical properties of six milled, one injection molded and one autopolymerized denture base materials (n = 10) used a three-point bend test along with electron scanning microscopy (ESM) to evaluate breaking load, fracture toughness, modules of elasticity and fracture surface roughness. The authors found that all milled denture base materials had higher elastic moduli than the conventional materials, but only two of the milled denture base materials had a significantly higher fracture toughness than the conventional materials. The authors reported that milled denture base materials do not generally have a higher fracture tolerance or a higher resistance against crack propagation than conventionally processed denture base materials and that there are large variations in fracture resistance between different milled materials. Furthermore, surface analyses revealed that the resins’ microstructure might be accountable for their mechanical properties, rather than the polymer chain length.

Srinivasan et al. [18] conducted an in vitro study comparing biocompatibility and mechanical properties of milled and compression molded denture base materials. The authors concluded that milled denture base materials have comparable biocompatibility and presented with more improved mechanical properties than the conventional denture base materials.

Ayman [19] in an in vitro study evaluated flexural strength and flexural modulus (n = 10), surface hardness (n = 10) and residual monomer content (n = 15) of milled and compression molded denture base materials. The author reported that compression molded denture base material displayed significantly higher flexural strength and lower flexural modulus compared to milled denture base material. Also, milled denture base material had a significantly higher surface hardness and lower monomer release than compression molded denture base material. The author concluded that milled denture base material is a clinically suitable resin with superior physical properties for denture bases.

Einarsdottir et al. [20] conducted an in vitro study evaluating dimensional stability of milled, injection molded and compression molded denture base materials (n = 15). The authors adapted a double processing method in their protocol. The authors found that the second processing had almost no effect on dimensional stability and most errors were incorporated during the first processing, among which the milled denture base group had significantly less errors. The other two groups had no statistically significant difference in dimensional stability.

Ayman concluded that milled denture base material is a clinically suitable resin with superior physical properties for denture bases.

Prpic ́ et al. [21] in an in vitro study indicated that the printed denture base had the least flexural strength while the milled and the polyamide material had the highest. Also, surface hardness was not favorable for the 3D printed material as well as for the polyamide. The authors concluded that milled denture base material, even though not all of them performed similarly, had better mechanical properties than the other materials and polymerization type cannot be used as an indicator of the material properties. Similarly, Al-Dwairi et al. [22] in another in vitro study concluded that milled denture base material showed significant improvement in flexural strength, impact strength and flexural modulus when compared to the conventional material. In their study, there was no significant difference between the two milled denture base materials.

However, Perea-Lowery et al. [23] performed an in vitro study and found that even though the milled denture base materials showed satisfactory behavior in general, they did not have better mechanical properties than conventional denture base materials. The authors advocated use of conventional material as they showed better results.

Surface Properties

Al-Dwairi et al. [24] conducted an in vitro study comparing surface properties of milled and heat polymerized denture base materials. The authors concluded that milled denture base material had significantly higher surface hardness and hydrophobicity in comparison to the compression molded denture base material, while the compression molded denture base material showed the highest surface roughness. The authors also reported that milled denture base material brands might have variable surface hardness, surface roughness and wettability.

Alammari [25] performed an in vitro study comparing the effect of mechanical and chemical polishing methods on the surface roughness and wettability of milled, compression molded and self-cured denture base material. The author reported that mechanical polishing, which was found to be the most effective polishing method, produced lower surface roughness for milled and compression molded denture base material with a smoother surface compared to chemical polishing. Also, significant decrease of the contact angle was observed in milled denture base material with both polishing techniques compared to the other two groups, which led the author to recommend it for complete denture fabrication, especially for elderly patients with reduced salivary functions. The findings of wettability in this study contradict Al-Dwairi et al. [24] findings and highlights the influence of polishing protocol.

Al-Fouzan et al. [26] in an in vitro study investigated the association between surface roughness and adhesion of Candida albicans to milled and compression molded denture base material (n = 10). The authors found that milled denture base material exhibited less roughness and adhesion of Candida albicans compared to compression molded denture base material.

Shinawi [27] evaluated the effect of cleaning for a simulated period of three years on milled denture base material surface properties. The author found that milled denture base material exhibited a homogenous surface initially with low surface roughness that significantly increased following cleaning for a simulated three-year period. However, the author concluded that these findings were within the clinically acceptable limits and that milled denture base material exhibit favorable resistance to cleaning abrasive forces.

Totua et al. [28] described a method for incorporating TiO2 nanoparticles in printable denture base material to utilize its antibacterial and antifungal properties. The authors recommended 0.4% by weight TiO2 nanoparticles to be incorporated to achieve a printable material with the desired biological properties.

Steinmassl et al. [29] conducted a study investigating the effect of the milling process on surface characteristics of five milled denture base material and compression molded denture base material which served as the control group. The authors found that most milled denture base materials have smoother and more hydrophilic surfaces than compression molded denture base material.

Milled dentures immersed in red wine had a significantly higher color difference than injection molded dentures.

Arslan et al. [30] in an in vitro study compared the surface roughness, flexural strength and hydrophobicity of milled denture base materials with heatpolymerized polymethylmethacrylate (PMMA) after thermal cycling. They reported the milled denture base materials had significantly higher flexural strength among all groups. Surface roughness was comparable among all groups while hydrophobicity was higher for the milled denture base materials.

Color Stability

Al-Qarni et al. [31] in an in vitro study compared the staining effect on the color stability of milled CECDs with compression molded and injection molded CDs. All specimens were immersed in coffee and red wine and the control group was immersed in distilled water. Color change was measured after seven days with a spectrophotometer. All denture teeth immersed in the staining solutions showed an increase in color difference values. Monolithic milled dentures had greater resistance to stain accumulation at the tooth-denture base interface than those made with conventional processing methods. Milled dentures immersed in red wine had a significantly higher color difference than injection molded dentures. Monolithic milled dentures had similar color change to conventionally processed acrylic resins.

Denture Teeth

Cha et al. [32] in an in vitro study compared the wear of 3D printed and four types of conventionally prefabricated denture teeth against metal and zirconia antagonists. Though they found variable combinations of wear loss among groups with the antagonists used in the study, the overall conclusion was that 3D printed and the prefabricated resin denture teeth had adequate and comparable wear resistance.

Choi et al. [33] conducted an in vitro study that tested fracture toughness and bonding failure of three types of denture bases with four different types of commercially available denture teeth. Heat polymerized denture base material had the highest fracture toughness and bond strength that significantly decreased with aging. Teeth bonded to milled and 3D printed denture bases had significantly lower bond strength that did not change significantly with aging. Milled denture bases bonded with modified PMMA with nano filler denture teeth had the highest and statistically significant fracture toughness compared to all other teeth bonded to milled denture bases. In the 3D printed groups, the main mode of failure was adhesive at the tooth-bonding material interface. The main mode of failure in the milled teeth groups was cohesive within the bonding material itself. The heat cured groups showed distinct adhesive failure at the tooth-denture base interface.

Chung et al. [34] conducted an in vitro study that tested the chipping and indirect tensile fracture resistance of five different (3D printed and prefabricated) denture teeth. 3D printed resin teeth revealed comparable fracture resistance to some of the conventionally prefabricated denture teeth used in the study.

Goodacre et al. [35] conducted an in vitro study that studied the accuracy and reproducibility of prosthesis in regard to tooth movement during fabrication. They found that the milled monolithic technique was the most accurate and reproducible. The least accurate was the injection molding technique and the least reproducible was with the fluid resin technique. More tooth movement was noted for the posterior teeth than anterior teeth across all fabrication techniques, but it was not statistically different. They concluded that varying amounts of tooth movement can be expected and were dependent on the processing technique. However, they reported that the clinical significance of these differences is unknown.

Yamamoto et al. [36,37] conducted in vitro studies that compared the accuracy of bonded artificial teeth with different offset positions of milled denture bases and shapes of the milled denture teeth. They reported that a milled complete denture needs optimal offsets for accurate teeth position and the optimal offset values differ with the basal shape of different denture teeth. They suggested that dimples in the basal areas of teeth with smooth margins were the most accurate design.

Clinical Outcomes and Patient Satisfaction

Saponaro et al. [38] conducted a cross sectional study involving 48 patients. The variables examined were total number of appointments needed to insert the definitive maxillary and mandibular milled CDs, the total number of post insertion adjustment visits and any reported complications associated with the level of the clinician’s experience. The results were that the number of appointments at both predoctoral and graduate level was more than the claimed two appointments. No significant differences were found between operator’s experience and the number of appointments required to insert the dentures as well as the complications encountered. The most frequently faced complications reported in this study were the lack of denture retention on the day of denture insertion followed by increased occlusal vertical dimension.

Students preferred the milled CECD because it is easier to perform and there is no laboratory work needed.

In a retrospective survey study by Saponaro et al., [39] questions relevant to their milled CECD experience were distributed to 50 patients with only 19 responding. The factors assessed were patient satisfaction and final outcome. Seventy-eight percent of participating patients reported satisfaction with aesthetics and felt that the new milled dentures were better from their previous conventional CDs though the result was not statistically significant.

Srinivasan et al. [40] conducted a clinical study that evaluated the clinical chairside time for the predoctoral dental students to fabricate milled dentures and the heat polymerized PMMA resin dentures as well as the general cost for fabricating each of the denture protocols. The chairside time spent by the student for each patient visit during the entire course of denture construction was timed with a stopwatch and recorded by one investigator. The costs were divided into clinical fees, clinical materials and laboratory costs. Conventional complete denture protocols required significantly longer clinical time than the CECD protocols. In regard to clinical costs, CECDs were more, but overall costs of conventional dentures were significantly higher.

Kattadiyil et al. [41] conducted a clinical study and evaluated the clinical treatment outcomes (denture base contour, teeth arrangement, fit, retention, extension, stability, aesthetics, lip support and prognosis, occlusal vertical dimension), patient satisfaction and dental student preferences. Each dental student fabricated one set of compression molded denture base and one set of milled dentures. A significantly higher average of overall patient satisfaction and higher preference regarding treatment outcome was seen for the milled CECDs. Students preferred the milled CECD because it is easier to perform and there is no laboratory work needed. No significant difference was found in either patient preference in regard to aesthetics or the student preference in denture technique fabrication without faculty supervision.

In a two-year retrospective crosssectional study on 314 patients who were treated in a university setting, Clark et al. [42] reported that CECD dentures required fewer appointments to complete treatment and fewer postoperative appointments than conventionally fabricated dentures with statistical significance. However, about 50% of CECD needed four visits to finish. Less than 10% were finished in only two appointments and less than 20% in three appointments. Forty percent of conventional dentures were completed in five appointments and about 25% in six appointments.

Schlenz et al. [43] conducted a retrospective pilot study that evaluated the clinical performance of CECD in regard to survival and maintenance using the two-visit concept. Their result was all CECD survived more than the two-year observational period with no replacement recorded. For 3 out of 10 CECDs made during the initial period, the denture border had to be corrected and one CECD required relining due to inadequate retention. During the functional period, seven CECDs had to be relined and two CECDs fractured.

In a prospective study, Cristache et al. [44] assessed patient-centered 18-month outcomes of 3D printed dentures using an improved PMMA with nano TiO2. They reported significant reductions in the Oral Health Impact Profile for edentulous patients’ scores for the overall treatment groups registered at one-week, 12-month and 18-month follow-ups. No significant differences were noticed over time (18 months compared to one week). Also, the lowest mean value for the 3D printed dentures was registered at 18 months for aesthetics compared to the satisfactory initial aesthetic assessments. A microcomputed tomography examination showed satisfactory uniformity of the nano TiO2 in the PMMA matrix and no defects such as cracks or pores were seen after 18 months of use.

Schwindling and Stober [45] performed a pilot clinical trial on five patients to compare the clinical feasibility of milled and milled wax base with injection molding methods for CECDs. Complications during fabrication and quality of the milled and injected CD were reported. There was no difference in the fit between groups. The retention of the maxillary prostheses was slightly better for the milled CECDs. Aesthetic scores were comparable for both groups. Occlusion was assessed as slightly better for the milled CECDs, although no pronounced differences were reported. They concluded clinical feasibility for both types of fabrication techniques.

Participants treated with CECDs took longer to achieve satisfactory levels of comfort, but they took longer to return for unscheduled visits.

Bidra et al. [46] performed a prospective pilot study to evaluate clinical and patient-related outcomes for monolithic milled CECDs to evaluate any differences in outcomes at baseline and at a 12-month follow up and to identify any adverse clinical and patient-related outcomes related to CECDs. It was found that significant time was spent on the communication process with the laboratory. For patient-related outcomes, significant improvements in participant ratings from baseline to one year were observed regarding absence of denture sore spots and treatment time. Seventynine percent were satisfied with their milled CECDs overall. Fifty percent did not rate their CECDs as good or excellent for retention, stability and adaptation of the bases. For clinical outcomes, the excellent and good ratings for overall assessment by two evaluators was an average of 60%. No adverse clinical or patient-centered outcomes related to the milled CECDs were found, and all dentures were intact and in good condition at the one-year follow-up.

Adjustments Needed

Drago et al. [47] conducted a clinical study on 106 participants and evaluated the number of unscheduled post insertion visits after the first scheduled appointment and one to two weeks post insertion of the injection molded and milled CDs. The patients were followed up for a year. Of the participants, 22% from both groups returned for at least one unscheduled post insertion adjustment for up to a year. There was no significant difference in the number of unscheduled adjustments, but there was a significant association with the number of unscheduled appointments in patients with single dentures. Participants treated with CECDs took longer to achieve satisfactory levels of comfort, but they took longer to return for unscheduled visits.

Trial Dentures

Wimmer et al. [48] conducted a descriptive study evaluating the effect of tooth positioning in milled trial denture bases. The authors concluded that manually positioning the denture teeth into the sockets of the milled trial denture base can result in positional errors.

Stawarczyk et al. [49] in an in vitro descriptive study compared the accuracy of injection molded denture bases and milled trial denture. The authors reported that both groups showed deviations, but the milled trial dentures showed less deviation.

Applications

As a result of its improved properties, reduced production time and digital archiving, CECDs were used by clinicians to overcome some of the limitations of conventional CD treatment that could be advantageous in certain clinical situations. [50–53]

Systematic Reviews

Bidra et al. [54] presented one of the earlier systematic reviews on CECDs in 2012 that focused on the history, current status at the time and the future potential of CECDs. They reported the need for prospective clinical trials to validate the rapidly advancing CECD technology.

Wang et al. [55] performed a systematic review on the accuracy of CECDs. Selected studies were reporting on adaptation and occlusal discrepancies of milled and 3D printed CECDs. The authors concluded that CECDs have a clinically acceptable range for occlusal trueness (< 1 mm) and denture base adaptation (< 0.3 mm) that is comparable or superior to conventional denture base materials with the greatest misfit in the posterior palatal seal area. However, despite their encouraging findings, the authors cautioned that all articles reviewed had a medium (six articles) to high (eight articles) risk of bias based on their evaluation.

In a systematic review on the clinical outcomes and applications of CECDs, Kattadiyil and AlHelal [56] reported that the main advantages of utilizing digital technology in complete denture fabrication are digital achievability, improved retention, reduced number of appointments and clinical time. The findings of this review were mainly on milled CECDs.

In another systematic review on the clinical complications and the quality assessment factors with CECDs, Kattadiyil et al. [57] found that most clinical complications that were identified through the literature were patient dissatisfaction (26%), inadequate retention (21%) and aesthetic concerns (15%). Complications associated with aesthetics, occlusal vertical dimension and centric relation leading to patient overall dissatisfaction could have resulted due to the lack of a trial placement option for CECDs. Furthermore, inconveniency in reading a digital preview was recognized as a unique complication associated with CECDs. The findings of this review were mainly on milled CECDs.

There has been no report to date that has attempted to review available data to provide a consensus statement on the current status of CECDs.

Anadioti et al. [58] conducted a narrative review investigating the novel biomaterials, fabrication techniques and workflows, clinical performance and patient satisfaction of 3D printed CECDs. Due to the limited data available on 3D printed CECDs and 3D printing technology, the authors recommended that its utilization should be limited to custom tray, record bases, trial, interim and immediate dentures fabrication but not for definitive complete dentures. The authors also listed the main disadvantages of 3D printed CECDs: elimination of trial placement appointment without reliable virtual aesthetic assessment, lack of retention with printed polymers necessitating reline for clinical acceptability, inability to achieve balanced occlusion and lack of long-term color stability affecting denture aesthetics. Main advantages reported were positive patient-related outcomes and improved cost-effectiveness.

Discussion

The literature search revealed that the subtractive (milling) and additive (3D printing) CECDs have been extensively researched since the inception of the concept. There have been numerous systematic reviews that have reported the trends associated with various aspects of milled and printed CECDs related to material properties, accuracy, clinical outcomes and clinical complication. However, there has been no report to date that has attempted to review available data to provide a consensus statement on the current status of CECDs.

Even though the in vitro testing reveals significantly superior adaptation with milled denture bases compared to conventional and printed denture bases, this was not easily discerned in clinical evaluation. The resiliency of the soft tissue could have been a factor for this disparity between clinical and laboratory assessments. The clinical impact of initial superior retention of milled denture bases compared to conventional and printed denture bases has not been assessed over a longer time period. This is probably due to the technical constraints with an ideal study model. Milled denture bases revealed higher material hardness, strength, less roughness, abrasion resistance, increased wettability after polishing and reduced microbial adherence compared to printed and heat processed denture bases. The available data also seem to suggest that milled denture bases are more resistant to staining especially at the toothdenture base interface. Perea-Lowery et al. [23] reported milled denture bases did not have better mechanical properties regarding flexural strength, elastic modulus and nanohardness and surface microhardness than conventional denture base materials. This study contradicts results from other studies. [22,24,27,28] There is higher initial bond strength to denture teeth for conventional heat cured denture bases compared to milled and printed denture bases. [33] However, clinical impact of this reduced bond strength among CECDs has not been reported. [57]

It is expected that due to less wastage and lower costs associated with the 3D printing process, this mode of fabrication will become popular as the technology advances. [58] However, sufficient data is lacking currently to arrive at clinically meaningful conclusions. It is interesting to note that the initial introduction of CECDs as a two-appointment process has now been “modified” to a threeappointment protocol among most manufacturers with the “trial placement option.” The introduction of the trial placement option could have been due to increased demand for evaluating and confirming aesthetics, occlusion, phonetics and vertical dimension prior to definitive CECD fabrication. In the early protocols for capturing centric relation (CR), the manufacturers used a Gothic arch tracing to capture centric relation. Most manufacturers now provide an option of using occlusion rims to record CR at the appropriate vertical dimension. The authors expect the technology to evolve to meet the needs and demands of the increasing completely edentulous population that includes but are not limited to quality, function and reduced number of patient visits and fabrication time. A 2016 survey [59] of all postdoctoral prosthodontics program directors and prosthodontics/ restorative chairs overseeing predoctoral prosthodontic education in the U.S. related to the degree of implementation of CECDs by the postdoctoral prosthodontic students. The respondents stated that while interested in implementing the digital fabrication of CECDs, only 10% or less of CDs were actually being fabricated using digital technology. It is expected that there will be a higher shift toward CECD fabrication as the growing use of digital technology continues to increase. The authors also anticipate an increased interest in reducing laboratory procedures among the predoctoral student population due to constraints imposed by the COVID-19 pandemic. The CECD manufacturing technology can be an effective solution to such constraints and might result in increased usage. As usage increases, the environmental impact from CECD manufacturing technology (milling and printing) is a concern and needs to be studied.

Conclusion

■ Milled CECDs provide superior adaptation compared to conventional CDs and 3D printed CECDs. However, the improved adaptation of milled denture bases did not result in superior patient satisfaction or clinical experience when compared to conventional or printed dentures.

■ Maxillary CECD denture bases provide increased retention compared to conventional denture bases.

■ Milled denture base materials are reported to have improved physical and surface properties when compared to conventional and 3D printed denture bases. However, there are varying material properties among different CECD manufacturers.

■ Milled denture bases have superior color stability and biocompatibility compared to conventional denture bases.

■ CECD fabrication protocol can reduce chair time and overall costs when compared to conventional CDs.

■ A trial placement appointment before definitive CECD fabrication might assist in a more predictable and favorable outcome.

■ Patient satisfaction is not significantly better for CECDs when compared to conventionally fabricated CDs.

■ Clinical studies on long-term benefits with CECDs compared to conventional CDs are lacking.

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