Adaptation, Trueness and Precision of Inlay and Onlay Restorations Fabricated By Additive and Subtractive Manufacturing: A Systematic Review

Indirect partial-coverage restorations such as inlays and onlays remain an important treatment modality when extensive tooth structure loss cannot be predictably managed with direct restorations [1]. Compared with full crowns, these restorations are more conservative, maintain cuspal integrity and distribute functional stresses in a favorable way, thereby preserving long-term tooth vitality and strength [2]. Their clinical longevity is strongly influenced by the quality of marginal and internal adaptation. Inadequate fit has been associated with increased microleakage, recurrent caries, compromised bond durability and periodontal inflammation [3,4]. The concept of a clinically acceptable marginal gap has long been discussed, with McLean and von Fraunhofer’s in vivo estimate of approximately 120 µm often cited as a threshold [5]. However, this value represents a historically cited benchmark rather than a universally applicable clinical cutoff, as acceptable adaptation may vary according to restoration type, material, measurement technique and clinical conditions. Fabrication methods have evolved considerably over the past three decades. Conventional lost-wax techniques while reliable, are highly operator dependent and subject to inaccuracies from impression, casting and laboratory steps [6]. The introduction of Computer-Aided Design and Computer-Aided Manufacturing (CAD/CAM) facilitated a transition to subtractive milling of ceramics and hybrid materials. This method improves standardization, reduces laboratory steps and enhances reproducibility. Yet, it is limited by material waste, bur diameter restrictions and potential loss of detail in internal contours [7,8].

Additive Manufacturing (AM), often referred to as Three-Dimensional (3D) printing, has been introduced more recently into restorative dentistry. Unlike milling, AM fabricates restorations layer-by-layer, allowing reproduction of complex geometries, minimal material waste and the possibility of chairside or near-chairside workflows [9,10]. Technologies such as Stereolithography (SLA), Digital Light Processing (DLP) and Liquid Crystal Display (LCD) printing have been widely explored for dental applications [11-13]. Beyond provisional restorations, there is growing interest in permanent resin-based composites and hybrid ceramics produced by AM, with early studies demonstrating promising mechanical and esthetic properties [14-16]. Accuracy outcomes are usually reported in three main categories. Trueness refers to the closeness of the fabricated object to its digital design [17]. Precision describes the reproducibility of repeated fabrications [17]. Adaptation outcomes are assessed by measuring marginal and internal gaps [18]. In this review, “fit” refers broadly to restoration-tooth adaptation assessed through gap measurements, whereas “accuracy” is used as an overarching concept encompassing trueness and precision. Accordingly, adaptation, trueness and precision are considered related but distinct outcomes. Several investigations on crowns and veneers have shown that milled restorations often display higher trueness, whereas printed restorations achieve superior precision and, in some cases, smaller internal gaps [19-20]. Systematic reviews have reported broadly comparable adaptation between additive and subtractive methods, though variations depend on material type, printer technology and evaluation methodology [21-22].

Inlays and onlays, however, present specific challenges not fully addressed in previous reviews. Their internal design includes defined line angles and partial cuspal coverage, which demand accurate reproduction to achieve proper seating and load distribution [23]. Furthermore, inlay and onlay restorations are frequently indicated in posterior teeth under high occlusal loads, where marginal integrity is critical for clinical success [24]. Despite the increasing availability of printable restorative resins and hybrid materials, evidence focused exclusively on inlays and onlays remains limited. A systematic assessment of this subset is therefore necessary to clarify whether additive manufacturing can match or surpass subtractive and conventional techniques in terms of adaptation, trueness and precision. This systematic review aims to synthesize available in vitro evidence on the marginal and internal fit, trueness and precision of inlay and onlay restorations fabricated by additive manufacturing compared with subtractive milling and conventional methods.

Protocol and Registration

This systematic review was carried out according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) recommendations [25] and was prospectively registered in the Open Science Framework database on Sep 26, 2025 (osf.io/wpqme/overview).

Research Question

The review addressed the following focused question: Among inlay and onlay restorations, how do additive (3D-printed) techniques compare with subtractive (milled) and conventional fabrication methods regarding marginal and internal fit, trueness and precision? Subtractive milling was considered the principal comparator, while conventional fabrication methods were considered an additional comparator when reported by eligible studies.

Eligibility Criteria

Studies were considered eligible if they met the following inclusion criteria:

• Population: Natural teeth or standardized dies prepared for inlay and onlay restorations
• Intervention: Additive Manufacturing (3D printing)
• Comparator: Subtractive manufacturing (milling) and/or conventional fabrication
• Outcomes: Marginal Gap (MG), Internal Gap (IG), Absolute Marginal Discrepancy (AMD), Overall Discrepancy (OD), Trueness (RMS), Precision (TOR).
• Study Design: In vitro experimental studies
• Language: English
• Time Frame: Up to October 2025

Exclusion Criteria Included:

• Studies on crowns, veneers, or without separate inlay or onlay data
• Clinical case reports, reviews, or expert opinions
• Studies lacking evaluation of adaptation or trueness

Information Sources

A comprehensive electronic search was conducted in PubMed/MEDLINE, Embase, Web of Science and Scopus from inception to October 30, 2025. The strategy combined MeSH terms and free-text keywords related to inlays, onlays, occlusal veneers/table-tops, additive manufacturing (3D printing), subtractive manufacturing (milling/CAD-CAM) and adaptation outcomes (marginal/internal gap, absolute marginal discrepancy, trueness, precision, accuracy). Search strings were adapted for each database (Table 1). In addition, the reference lists of all included studies and relevant reviews were screened manually to identify further eligible publications. Broader partial-coverage terms, including occlusal veneers, table-tops and occlusal overlays, were incorporated into the search to maximize sensitivity; however, studies were eligible for inclusion only when inlay or onlay data were reported separately. Database searches were restricted to English-language records in accordance with the predefined eligibility criteria. All four electronic database searches were last conducted on October 30, 2025.

Table 1: Detailed Search Strategies Applied Across Electronic Databases

Study Selection

Two reviewers independently screened titles and abstracts. Full texts of potentially relevant articles were retrieved and assessed against the eligibility criteria. Disagreements were resolved by discussion until consensus was achieved. A third reviewer was available for unresolved disagreements.

Data Extraction

Data were independently extracted by two reviewers using a standardized data-extraction form. Disagreements were resolved by consensus. From each included study, the following data were extracted: Main author and year, restoration type, sample size, fabrication method, fabrication materials, measurement method (replica, micro-CT, CBCT, digital superimposition), evaluation sites (marginal gap, internal gap, absolute marginal discrepancy, trueness/precision) and main results.

Risk of Bias Assessment

The methodological quality of the included in vitro studies was assessed using the Quality Assessment Tool for In Vitro Studies (QUIN) [26], which was specifically developed and validated for methodological assessment of in vitro dental studies. This tool comprises twelve domains: clarity of aims, sample size calculation, sampling technique, use of comparison groups, methodological details and standardization, operator details, randomization, outcome measurement, assessor details, blinding, statistical analysis and presentation of results. Each domain is scored 0 (not reported), 1 (inadequately reported), or 2 (adequately reported). The overall score for each study is calculated as a percentage of the maximum possible score and then graded as low risk of bias (>70%), medium risk (50-70%), or high risk (<50%). Two reviewers performed the assessment independently; disagreements were resolved by discussion until consensus was reached.

Data Synthesis

A qualitative synthesis was undertaken because substantial heterogeneity in restoration type, restorative materials, fabrication technologies, measurement methods, evaluation sites and outcome definitions precluded meaningful quantitative pooling.

Study Selection

The electronic search yielded 866 records. After the removal of 114 duplicates, 752 titles and abstracts were screened. Of these, 742 records were excluded for not meeting the eligibility criteria. The full text of 10 articles was then assessed and three studies were excluded (two focused on restorations other than inlays or onlays and one due to ineligible study design, Supplementary file 1). Finally, seven in vitro studies involving 402 specimens fulfilled the eligibility criteria and were included in the qualitative synthesis (Figure 1).

Figure 1: PRISMA2020 Flow Diagram of the Study Selection Process

Study Characteristics

The seven included studies were published between 2019 and 2025 and all were in vitro experimental investigations [27-33]. Sample sizes ranged from 6 to 180 specimens, with restorations fabricated as either inlays or onlays. All studies compared additive manufacturing (3D printing) with either subtractive CAD/CAM milling or conventional indirect fabrication techniques. A variety of restorative materials were tested, including resin-based composites, hybrid ceramics and zirconia. Additive approaches relied mainly on Digital Light Processing (DLP), Stereolithography (SLA), or LCD-based printing, whereas subtractive methods employed CAD/CAM milling of resin nanoceramics, hybrid ceramics, or zirconia. Conventional groups, where included, involved indirect composite resins. Evaluation methods varied across studies: micro-CT was applied in three studies [27,29,31] replica with microscopic analysis in three [27-29], digital superimposition in three [28,30,32] and CBCT in one [33]. Reported outcomes focused primarily on Marginal Gap (MG) and Internal Gap (IG). Additional outcomes included Absolute Marginal Discrepancy (AMD) and Overall Discrepancy (OD) [33], as well as digital accuracy metrics such as Trueness (RMS) and Precision (TOR) [28,32]. A detailed overview of study design, materials, measurement approaches and results is presented in Table 2.

Table 2: Overview of the Included Studies

AM, Additive Manufacturing; SM, Subtractive Manufacturing; Conv./CT, Conventional Technique; MG, Marginal Gap; IG, Internal Gap; AMD, Absolute Marginal Discrepancy; OD, Overall Discrepancy; RMS, Root Mean Square deviation; TOR, Total Overlap Ratio; 3D, Three-Dimensional Printing group; MD, Mesiodistal section; BL, Buccolingual section; µm, micrometer; MPa, megapascal; TC, thermocycling; ns, not significant; TS, Tescera resin; LU, Lava Ultimate; ZR, Zolid Fx zirconia; PVC, VarseoSmile Crownplus resin; PVT, VarseoSmile TriniQ resin; MVE, VITA Enamic; CGP, Gradia Plus; Micro-CT, Micro-Computed Tomography; CBCT, Cone Beam Computed Tomography

Risk of Bias within Studies

Methodological quality was assessed using the QUIN tool (Table 3). Two studies were judged low risk of bias [32,33], while the remaining five were medium risk [27-31]. Most reports stated objectives clearly, described procedures in adequate detail, applied appropriate statistics and presented complete results. The main shortcomings were the absence of a priori sample-size calculations in all but two studies [32,33], which both reported formal power analyses. Additionally, assessor repeatability was explicitly documented only in Özden and Uygun [32]. Randomization procedures were generally under-reported and none of the studies described blinding during specimen preparation, measurement, or analysis (Supplementary file 2).

Table 3: Risk of Bias Assessment of the Included Studies using the QUIN Tool

D1, clarity of aims/objectives; D2, sample size calculation/justification; D3, sampling technique; D4, comparison/control group; D5, methodological details and standardization; D6, operator details/calibration; D7, randomization; D8, outcome measurement method (validity/reliability); D9, outcome assessor details/calibration; D10, blinding; D11, statistical analysis; D12, presentation of results

Results of Individual Studies and main Findings

Marginal Gap: All seven studies assessed marginal adaptation. Reported mean MG values for 3D-printed restorations ranged from 54 µm [28] to 290 µm [33], while milled restorations ranged from 61 µm [31] to 509 µm [27].

Conventional techniques produced MG values between 50 µm [33] and 97 µm [31]. In several studies, AM demonstrated smaller or comparable marginal gaps relative to SM, although results varied according to the material, site and measurement method. While many measurements were within the historically cited 120-µm benchmark, important exceedances were observed, particularly in Daghrery et al. [33] and in the replica-based measurements of Pasha et al. [29].

Internal Gap

Five studies reported internal adaptation. For AM, IG values ranged from 35 µm [29] to 280 µm [33]. For SM, the range was 62 µm [29] to 509 µm [27]. Conventional techniques showed IG values of 140 µm [33] to 169 µm [28]. Across studies, AM often demonstrated smaller internal gaps than SM in specific comparisons; however, considerable variability was evident among materials and measurement approaches and some AM measurements substantially exceeded the historically cited 120-µm benchmark.

Absolute Marginal Discrepancy and Overall Discrepancy

Only Daghrery et al. [33] investigated AMD and OD using CBCT. AM restorations showed AMD values between 290-420 µm, whereas SM (VITA Enamic) reached 480 µm and conventional Gradia Plus achieved the smallest value (250 µm). Similarly, OD ranged from 260 µm (AM TriniQ) to 340 µm (SM Enamic), with the conventional group again performing best. Because AMD and OD were evaluated in only one included study, these findings should be considered study-specific.

Trueness and Precision

Two studies assessed trueness using RMS-based digital analysis [28,32]. Reported RMS trueness values for AM ranged from 41 µm [28] to 114 µm [32], while the comparative groups in Lim et al. [28] ranged from 33 to 49 µm. Precision was reported using different metrics and was therefore interpreted separately. Lim et al. [28] assessed precision using RMS deviation and reported lower values for AM (20±2 µm) than the compared groups (25-65 µm), indicating greater repeatability in that study. Özden and Uygun [32] assessed precision among AM materials using Total Overlap Ratio (TOR), which ranged from approximately 80% to 95%. Cantó-Navés et al. [30] additionally reported reproducibility based on the variability of gap measurements. Because RMS, TOR and variability-based reproducibility represent different measures, they were not directly compared across studies (Table 4).

Table 4: Summary of Reported Outcomes for Inlay and Onlay Restorations Fabricated by Additive Manufacturing (AM), Subtractive Manufacturing (SM) and Conventional Techniques

AM, Additive Manufacturing (3D printing); SM, Subtractive Manufacturing (CAD/CAM milling); MG, Marginal Gap; IG, Internal Gap; AMD, Absolute Marginal Discrepancy; OD, Overall Discrepancy; RMS, Root Mean Square deviation (trueness measure); TOR, Total Overlap Ratio (precision measure); µm, micrometer

The synthesis of the included studies suggests that AM can achieve adaptation comparable to SM for inlay and onlay restorations under selected laboratory conditions. Although many reported marginal and internal gap measurements were within the historically cited 120-µm benchmark proposed by McLean and von Fraunhofer [5], substantial exceedances were also observed and this benchmark should not be regarded as a universal indicator of clinical acceptability. Considering the heterogeneity in study designs, materials and measurement approaches, the available evidence indicates a trend toward comparable (and in some individual comparisons smaller) gap measurements with AM rather than definitive superiority over SM. Trueness favored milling in some comparisons, particularly when zirconia was used. Evidence from restoration types outside the eligibility criteria is considered here only as contextual evidence and was not included in the present systematic synthesis. These findings are consistent with a growing body of literature beyond the present review. In a recent comparison of zirconia occlusal veneers, printed restorations exhibited significantly smaller internal discrepancies and absolute marginal discrepancies than CAD/CAM-milled specimens, with all values within clinically acceptable limits [34]. A recent synthesis of laminate veneers and intracoronal restorations likewise reported superior or comparable trueness and fit for AM relative to SM, noting that outcomes are influenced by material type and preparation design [35]. At the same time, broader, cross-indication meta-analyses caution that trueness may favor milling, whereas precision tends to be higher with AM and intaglio fit is often comparable between the two methods-patterns that mirror our findings for inlays/onlays [36,37].

Nevertheless, the relative performance of AM and SM should also be interpreted in the context of material composition, printer technology, milling system, resin formulation, printing orientation, post-curing conditions and restoration-design parameters, each of which may independently influence dimensional accuracy and adaptation. Recent systematic reviews and meta-analyses on tooth-supported crowns and fixed prostheses report a consistent pattern: milling tends to achieve lower trueness error, whereas 3D printing often shows higher precision/repeatability, with intaglio fit generally comparable between methods [36,37]. This pattern is echoed in individual comparative studies of monolithic zirconia crowns, where milled restorations frequently exhibit superior trueness while printed groups maintain high precision within clinically acceptable bands [38,39]. These external findings align with our synthesis, in which several included studies indicated better reproducibility for AM, whereas zirconia milling produced the lowest trueness deviations in specific comparisons [28]. Overall, the available evidence suggests that both AM and SM can achieve acceptable adaptation under selected laboratory conditions, although performance varies according to the material, workflow, measurement method and evaluated site.

Some comparisons favored AM for marginal or internal adaptation, whereas milling showed advantages in trueness in specific material systems. Evidence regarding precision and reproducibility remains limited and metric-dependent. Accordingly, the current findings support AM as a promising alternative to milling for partial-coverage indirect restorations but do not establish clinical equivalence or superiority. Importantly, laboratory gap values below a specific benchmark should not be interpreted as proof of clinical success, which is also influenced by cement thickness, bonding, material properties, biological conditions, occlusal loading and aging.

Limitations

This review was conducted in accordance with PRISMA guidelines, with comprehensive database searching, predefined eligibility criteria and standardized data extraction procedures (Supplementary file 3). However, the available body of evidence is limited to laboratory experiments, most with relatively small sample sizes and short-term testing conditions. Only two studies [32,33] reported a priori sample-size calculation and only one [33] addressed assessor calibration. Blinding was absent across all studies and randomization procedures were either insufficiently described or not applied. In addition, clinical variables such as saliva, masticatory forces, or long-term aging protocols were either simulated partially or not considered, which restricts the direct translation of these results to clinical practice. Although a comprehensive and systematic search was undertaken across four major databases and supplemented by manual screening, some limitations must be acknowledged. Only English-language publications were considered, which may have excluded relevant data in other languages. The review was also restricted to in vitro experimental studies, which, while necessary for the focused research question, prevents the inclusion of clinical outcomes that would strengthen the evidence base. Furthermore, heterogeneity in methodologies and outcome measures precluded meta-analysis and results were synthesized qualitatively. The limited reporting of randomization and absence of blinding further reduce confidence in the comparative findings and warrant cautious interpretation. Given the small number of included studies, formal assessment of publication bias was not considered meaningful; therefore, publication bias and selective reporting cannot be excluded. In addition, similarities in materials, manufacturers, or laboratory workflows across some studies may limit the independence and technological diversity of the available evidence.

Implications for Practice and Future Perspectives

Within the limits of the current evidence, AM appears to be a promising alternative to SM for inlay and onlay fabrication, offering comparable adaptation and enhanced precision. Nevertheless, milling still provides excellent accuracy with certain materials, such as zirconia and hybrid ceramics. For clinical practice, these findings suggest potential clinical applicability of 3D printing for inlay and onlay fabrication; however, routine clinical equivalence or superiority cannot be established from the current in vitro evidence.

For policy and research, the results highlight the importance of establishing standardized protocols for measuring adaptation and accuracy of indirect restorations. Standardized definitions and measurement protocols should also be adopted for MG, IG, trueness and precision to improve comparability among future studies. Future investigations should prioritize adequately powered designs, operator blinding and calibration procedures to reduce bias. Clinical studies are particularly needed to evaluate performance under functional loading and aging conditions. Comparative cost-effectiveness and chairside applicability of AM versus SM workflows should also be addressed, as these factors will ultimately determine the integration of 3D printing into routine restorative practice.

Within the limitations of this systematic review of in vitro studies, additive manufacturing appeared to have marginal and internal adaptation broadly comparable to subtractive milling under selected laboratory conditions, although results varied according to material, workflow and measurement method. Trueness favored certain milled materials in selected comparisons, whereas the limited evidence on precision and reproducibility favored AM. The available evidence does not establish clinical equivalence or superiority and confidence in these findings is limited by methodological heterogeneity and risk of bias. Future studies should adopt standardized outcome definitions and measurement protocols and include adequately powered clinical investigations assessing long-term performance.

Data Availability

All data analyzed during this study are included in this article and its supplementary files.

Author Contributions

Methodology development: ASK, RSA; Literature search: ASK, RSA; Data extraction: ASK, RSA; Quality assessment and data analysis: ASK, RSA, AD, AS, AAlt, AAlk, RA, NA, SA, MR, AAbd; Writing-original draft: ASK, RSA, AD, AS, AAlt, AAlk, RA, NA, SA, MR, AAbd; Writing - review and editing: ASK, RSA, AD, AS, AAlt, AAlk, RA, NA, SA, MR, AAbd; Project administration: ASK, RSA. All authors have read and approved the final manuscript.

Conflicts of Interest

The authors declare no competing interests.

Ethical Statement

Ethical approval was not required because this systematic review involved only previously published data and no human participants, identifiable patient information, or medical records, in accordance with applicable institutional requirements.

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Supplementary information

Supplementary file 1: Excluded studies and their reasons for exclusion

Supplementary file 2: QUIN assessment for included studies

Supplementary file 3: PRISMA 2020 checklist

Abbreviations

Supplementary File 1: List of Excluded Studies and their Reasons for Exclusion

94. Supplementary File 2: Detailed QUIN Assessment for Included Studies Ahlholm et al. [27]

134. Final QUIN %: (14 ÷ 22) × 100 = 63.6; Quality grade: Medium risk of bias

135. Lim et al. [28]

175. Final QUIN %: (13 ÷ 24) × 100 = 54%; Quality grade: Medium risk of bias

176. Pasha et al. [29]

216. Final QUIN %: (13 ÷ 24) × 100 = 54%; Quality grade: Medium risk of bias

217. Cantó-Navés et al. [30]

257. Final QUIN %: (13 ÷ 24) × 100 = 54%; Quality grade: Medium risk of bias

258. Peskersoy and Oguzhan [31]

298. Final QUIN %: (15 ÷ 24) × 100 = 62.5%; Quality grade: Medium risk of bias

299. Özden and Uygun [32]

340. Final QUIN %: 75%; Quality grade: Low risk of bias

341. Daghrery et al. [33]

381. Final QUIN %: (19 / 24) × 100 = 79.2%; Quality grade: Low risk of bias

382. Supplementary File 3: PRISMA 2020 Checklist

492. From: Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ 2021;372:n71. doi: 10.1136/bmj.n71. This work is licensed under CC BY 4.0. To view a copy of this license, visit https://creativecommons.org/licenses/by/4.0/