Marginal Gap Measurement of Pre-Cemented Metal–Ceramic Crowns: A Systematic Review

Abstract

Background/Objectives: Metal–ceramic crowns may be constructed using different techniques and coping materials. A systematic review analysing the coping material, method of construction, and instruments used for measuring the metal–ceramic crown marginal gap has not been completed. The aim of this systematic review was to appraise the literature relating to the instruments used for the in vitro marginal gap measurement of single pre-cemented metal–ceramic crowns and assess whether the crown coping material and method of coping construction influence the marginal gap. Methods: A systematic search was performed in November 2024 across the EBSCO Host, Scopus, PubMed, and Web of Science databases, adhering to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and specific eligibility criteria. The Joanna Briggs Critical Appraisal Checklist was used to assess article quality. Results: Fourteen studies evaluated marginal gaps in 402 crowns using the following techniques: direct view microscopy (eight studies), replica techniques (three studies), scanning electron microscopy (two studies), and profilometry (one study). The mean marginal gap for all the metal–ceramic crowns across all the studies was 65.97 ± 32.58 µm. The pre-cementation mean marginal gaps showed no significant difference between Computer-Aided Design–Computer-Aided Manufacturing (CAD-CAM) milled copings (87.95 ± 26.35 µm) and conventionally cast copings (90.45 ± 24.37 µm) (t = −0.197, p = 0.847). The mean marginal gaps varied significantly (F = 11.34, p < 0.001) by coping material: cobalt–chromium (Co-Cr) led to 84.28 µm, nickel–chromium (Ni-Cr) led to 70.98 µm, titanium led to 50.18 µm, and noble metal alloys led to 27.90 µm. Six studies addressed confounding factors and followed a standardised approach for measuring marginal gaps. Conclusions: Direct view microscopy was the most commonly used instrument for measuring the marginal gaps of single pre-cemented metal–ceramic crowns, yielding the smallest reported mean marginal gap of 75.00 ± 26.87 µm. Metal–ceramic crowns constructed with noble metal alloys exhibited the lowest mean marginal gaps. Metal–ceramic crowns constructed using conventional casting techniques presented similar marginal gaps to CAD-CAM crowns.

1. Introduction

Metal–ceramic crowns have been a mainstay complete coverage indirect restoration offering patients collective benefits of strength, aesthetics, and longevity. Despite an increased use of ceramic crowns, the clinical survival rate as analysed by a meta-analysis from 2016 suggests that metal–ceramic crowns perform superiorly to ceramic crowns [1]. Recent clinical research suggests that the gap is narrowing, with metal–ceramic crowns and zirconia crowns demonstrating more similar survival and complication rates [2].

The marginal gap is described as the three-dimensional space located between the axial wall of the prepared tooth and the internal surface of the restoration at the margin [2]. Among these, the vertical marginal gap is the most frequently reported parameter and represents the distance between the crown margin and the tooth preparation in the vertical axis. The vertical marginal gap plays a critical role in determining the crown’s fit and adaptation. The absolute marginal gap combines both horizontal and vertical measurements and reflects the overall marginal gap. A well-fitting crown should have a minimal and consistent marginal gap, allowing no space for bacterial or debris accumulation.

Marginal fit is related to the marginal gap and refers to the accuracy of the adaptation of the crown to the tooth at the margin. Aesthetics, fracture resistance, and marginal fit are the three most important for the success of crowns, with the last considered the most important [3,4,5]. An accurate marginal fit decreases the potential for microleakage-induced hypersensitivity and plaque accumulation [3,5,6]. An inadequate marginal fit may result in many different clinical problems including periodontal disease, caries, pulpitis, and cement dissolution resulting from exposure to the oral environment [3,5,6]. Marginal fit is influenced by the preparation design, location of the finish line, cement space, restorative material, method of fabrication, impression technique, and impression material [7,8,9,10,11,12].

Quantifying the marginal gap of pre-cemented metal–ceramic crowns is crucial for evaluating overall quality and fit, as these parameters are directly linked to clinical outcomes and long-term durability [13,14]. Consistent, precise, and reproducible measurement of the marginal gap allows clinicians and technicians to assess the quality of the manufacturing process and implement any needed refinements.

The referencing of an acceptable marginal gap has varied with time and with different types of crowns. Historically, a maximum clinical marginal gap of 120 µm [15] has been deemed acceptable, although it was noted that marginal gaps larger than 160 μm could also be clinically successful. The pioneering study by Mclean and Von Fraunhofer used in vitro measurements of in vivo seated restorations that studied cement film thickness at multiple points of interest using cross-sectioned polyether rubber films in a variety of different crowns [15]. The marginal gaps were historically reported as 100 μm or less in Dicor crowns [16,17]. This finding was later reinforced by reference to a gap of less than 100 um in ceramic crowns being clinically acceptable [18]. Marginal gaps ranging from 60 to 120 μm have been reported as clinically acceptable, while for CAD/CAM all-ceramic crowns, an acceptable marginal gap was reported to be less than 90 μm [19]. In a study of ceramic and porcelain-fused-to metal crowns, it was suggested the practical range for clinical acceptability of fit was approximately 50 to 75 μm [16]. For metal–ceramic crowns, the reported acceptable marginal gaps range from 50 to 200 µm and vary with different materials, construction methods, and measurement instruments [17,18,19]. No known study has measured the marginal gap of crowns in vitro and conducted long-term patient follow-up to assess longevity. Consequently, there is a lack of agreement on an ideal marginal gap due to a lack of supportive research [20].

Individual marginal gap measurements are commonly taken at random sites and extrapolated to reflect the total crown marginal gap [2]. To reduce measurement error, making at least 50 measurements per crown has been recommended, which was based on arithmetic mean calculations in a limited sample size [21]. This minimum number is influenced by the variability of the gap and the desired level of precision, akin to statistical power analysis for determining sample size in hypothesis testing [21]. However, there is currently no consensus on the ideal number of measurements required to produce a clinically meaningful evaluation of the crown marginal gap [22].

Instruments for measuring marginal gaps fall into two broad categories: two-dimensional (2D) or three-dimensional (3D), and destructive (DE) or non-destructive (ND). The commonly used instruments include direct view microscopy (2D, ND), scanning electron microscopy (3D, ND), and impression replicas (2D, ND). The precision of these techniques varies, with comparative studies producing inconsistent and limited results [23,24]. Some findings suggest that scanning electron microscopy (SEM) offers greater accuracy than light microscopy, while others report no significant difference, although SEM may yield more lifelike observations [23,24].

Study Rationale

Although there have been many reviews conducted on crown marginal gap assessment and influential factors [7,16,17,20,25,26,27,28], a thorough comparative analysis of the measurement instruments remained lacking across the range of single crown options available in dentistry. The current systematic review builds on the authors’ previously published series of reviews examining marginal gap measurement and assessment in all-ceramic single crowns [29], all-ceramic endodontic crowns [30], and all-metal crowns [31].

Therefore, the aim of this systematic review was to appraise the literature relating to the instruments used for the in vitro marginal gap measurement of single pre-cemented metal–ceramic crowns and assess whether the crown coping material and method of coping construction influence the marginal gap. Furthermore, this study aimed to explore the influence of variables such as the crown coping material and method of coping construction on the marginal gap. The hypothesis was that the choice of instruments would not significantly affect the reported overall marginal gap.

2. Materials and Methods

The systematic review was performed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The study protocol was registered with Open Science Framework (https://osf.io/68hkw (accessed on 2 July 2025)).

Inclusion Criteria

• In vitro studies evaluating marginal gaps of complete-coverage, single, pre-cemented metal–ceramic crowns.
• A clear description of the method used for measuring marginal gaps.
• Crowns constructed from base metals, gold alloys, noble alloys, stainless steel, or novel porcelain-fused-to-metal materials, using digital or conventional fabrication methods.
• Original research articles with cross-sectional or longitudinal experimental designs.

Exclusion Criteria

• In vivo studies, ceramic crowns, or virtual designs without physical crown components.
• Studies focusing exclusively on porcelain-bonded-to-zirconia, full-contour zirconia, or other non-metal–ceramic crowns.
• Studies measuring only post-cementation gaps, using tactile/visual assessments, or assessing only internal fit without marginal gap measurement.
• Research involving partial crowns (e.g., inlays, onlays), bridges, one-piece endodontic crowns, or ceramic layering/glazing without pre-cementation marginal gap measurement.

2.1. Databases

The following databases were used to search data: EBSCOHost Dentistry & Oral Sciences Source (DOSS), PubMed.gov, Scopus^®, and Web of Science™ (WoS). Web of Science™ captures the following databases including translations in English as appropriate: Current Contents Connect^®, Derwent Innovations Index™, KCI-Korean Journal Database™, MEDLINE^®, Russian Science Citation Index™, SciELO Citation Index™, and Web of Science Core Collection™. The searches were performed in November 2024. There was no publication date restriction. To show research only on material sciences within dentistry and healthcare, specific field restrictions were applied to the Scopus and Web of Knowledge searches.

2.2. Search Strategy

The search process was conducted independently by two reviewers (JD and THF), and data were collected independently. The Cohen kappa (K) coefficient was used to measure the inter-rater reliability. Keyword-based logic grids, Boolean logic, and individual searches of published bibliographies were performed. The database-specific search strategies are documented in

Table 1. Manuscript search strategy and keywords applied across databases.

Databases Interrogated	Search Terms
PubMed	Pubmed: ((((Fit OR Gap * OR Space OR Distance OR Length * OR Accurac * OR Precision)) AND ((“internal margin *” OR “internal discrepanc *” OR “margin * adaptation*” OR “cervical margin *” OR preparation OR “margin * integrity” OR “margin * opening *” OR “edge gap *” OR “margin * gap *”))) AND ((“Un-cemented” OR “Fixed dental”))) AND ((PBM OR “Porcelain fused to metal” OR “Porcelain bonded to zirconia” OR “PBZ” OR “Metal ceramic” or “ceramic metal”))
Scopus	(TITLE-ABS-KEY ((Fit OR Gap * OR Space OR Distance OR Length * OR Accurac * OR Precision)) AND TITLE-ABS-KEY ((“internal margin *” OR “internal discrepanc *” OR “margin * adaptation *” OR “cervical margin *” OR preparation OR “margin * integrity” OR “margin * opening *” OR “edge gap *” OR “margin * gap *”)) AND TITLE-ABS-KEY ((“Un-cemented” OR “Fixed dental”)) AND TITLE-ABS-KEY ((PBM OR “Porcelain fused to metal” OR “Porcelain bonded to zirconia” OR “PBZ” OR “Metal ceramic” OR “ceramic metal”)))
Web of Science (all databases)	(Fit OR Gap * OR Space OR Distance OR Length * OR Accurac * OR Precision) (Topic) AND (“internal margin *” OR “internal discrepanc *” OR “margin * adaptation *” OR “cervical margin *” OR preparation OR “margin * integrity” OR “margin * opening *” OR “edge gap *” OR “margin * gap *”) (Topic) AND (“Un-cemented” OR “Fixed dental”) (Topic) AND (PBM OR “Porcelain fused to metal” OR “Porcelain bonded to zirconia” OR “PBZ” OR “Metal ceramic” or “ceramic metal”) (Topic)
EBSCOHost	(TI(Fit OR Gap * OR Space OR Distance OR Length * OR Accurac * OR Precision) OR AB(Fit OR Gap * OR Space OR Distance OR Length * OR Accurac * OR Precision)) AND (TI(“internal margin *” OR “internal discrepanc *” OR “margin * adaptation *” OR “cervical margin *” OR preparation OR “margin * integrity” OR “margin * opening *” OR “edge gap *” OR “margin * gap *”) OR AB(“internal margin *” OR “internal discrepanc *” OR “margin * adaptation *” OR “cervical margin *” OR preparation OR “margin * integrity” OR “margin * opening *” OR “edge gap *” OR “margin * gap *”)) AND (TI(“Un-cemented” OR “Fixed dental”) OR AB(“Un-cemented” OR “Fixed dental”) OR DE “Dental Crowns”) AND (TI(PBM OR “Porcelain fused to metal” OR “Porcelain bonded to zirconia” OR “PBZ” OR “Metal ceramic”) OR AB(PBM OR “Porcelain fused to metal” OR “Porcelain bonded to zirconia” OR “PBZ” OR “Metal ceramic”))S

* Truncation and Wildcard prompt.

.

Eligible manuscripts were assessed using a professional systematic review management platform (Covidence.org; Veritas Health Innovations Ltd., Melbourne, Australia) that automatically removed duplicate records and supported consensus building among reviewers. The platform required the resolution of any conflicts before eligible manuscripts moved forward in the screening process, therefore facilitating complete agreement on the final articles. The Joanna Briggs Institute Critical Appraisal Checklist for Analytical Cross-Sectional Studies was used by both investigators to evaluate the quality of the included articles, although there were no studies excluded based on this assessment alone. All articles were accessed through the university library.

2.3. Data Recorded

The following information was recorded: the number of crowns fabricated, core material(s), method of crown construction, cement space, crown preparation material, tooth form measured, measurement instrument, number of marginal gap measurements per crown, mean range of marginal gaps (in µm) in any direction, and mean marginal gap (in µm).

2.4. Statistical Analysis

Normality was assessed using the Shapiro–Wilk test, and independent t-tests were conducted to compare marginal gaps between CAD-CAM milling and conventional milling methods, yielding a Shapiro–Wilk p-value of 0.108. A Welch 1-way ANOVA was performed to test for significance in differences across the marginal gap variations across different metal coping materials used in metal–ceramic crowns.

3. Results

Fourteen articles were included in the systematic review (Figure 1). Of the initial 97 articles included for screening, thirteen conflicts were resolved during a second round of screening as permitted by the platform, resulting in an agreement rating of ĸ = 0.87. Six articles were excluded following full paper reading [32,33,34,35,36,37]. Table 2 summarises the included studies and mean marginal gaps reported for pre-cemented metal–ceramic crowns. The mean marginal gap for all metal–ceramic crowns was 65.97 ± 32.58 µm. Across all studies, a total of 402 crowns were analysed, with some studies assessing multiple materials. Seven studies included a cobalt–chromium (Co-Cr) alloy (N = 175 crowns) as the crown coping material [38,39,40,41,42,43,44], while three investigated nickel–chromium (Ni-Cr) alloys (N = 40 crowns) [38,45,46], and another three studied titanium (N = 42 crowns), either in pure form or as an alloy [46,47,48]. Five studies utilised noble metal alloys (N = 145 crowns) for the coping material [46,47,49,50,51].

Table 2. Summary of studies and mean marginal gaps reported on pre-cemented metal–ceramic crowns.

Author, Year	Number of Metal Ceramic Crowns Fabricated	Core Material(s)	Method of Crown Construction	Cement Space Thickness (in µm)	Underlying Crown Preparation Material	Tooth Form Measured	Instrument Used for Measurement	Number of Marginal Gap Measurements per Crown	Mean Range of Reported Marginal Gap Measurements (in µm) in Any Direction		Mean Marginal Gap of Metal–Ceramic Crowns (in µm, Rounded to Nearest Whole Number)
Giti, 2023 [38]	20	Ni-Cr alloy (N = 10) Co-Cr alloy (N = 10)	CAD-CAM Investment casting	40	Brass die	Premolar	Direct view microscopy	16	Cast metal ceramic crowns	113.8 to 114.1	94
									CAD-CAM based metal ceramic crowns	74.0 to 75.2
Heboyan, 2022 [39]	90	Co-Cr alloy	CAD-CAM Investment casting	50	Plastic teeth	Unspecified	Direct view microscopy	4	Cast Co-Cr crowns	53.7 to 55.5	56
									CAD-CAM based Co-Cr crowns	55.8 to 57.7
Kunz, 2022 [40]	20	Co-Cr alloy	CAD-CAM Investment casting	Unspecified	Acrylic die	Mandibular molar	Replica technique	4	Cast Co-Cr crown	100.2 to 104.5	108
									CAD-CAM based Co-Cr crown	98.0 to 127.7
Tekin, 2020 [45]	10	Ni-Cr alloy	Investment casting	20	Plastic teeth	Mandibular molar	Replica technique	4	Cast Ni-Cr crowns	64.6 to 86	75
Ortega, 2017 [41]	10	Co-Cr alloy	Investment casting	50	Stainless steel die	Premolar	Scanning electron microscopy	60	Cast Co-Cr crowns	66.3 to 135.1	101
Saraswathi, 2016 [42]	10	Co-Cr alloy	CAD-CAM	Unspecified	Zirconia die	Maxillary molar	Direct view microscopy	60	Cast Co-Cr crowns	91.2 to 137.7	114
Gómez-Cogolludo, 2013 [46]	70	Noble metal alloy (N = 20) Ni-Cr-Ti alloy (N = 20) Ni-Cr alloy (N = 20) Ti alloy (N = 10)	Investment casting	Unspecified	Stainless steel die	Premolar	Direct view microscopy	120	Pd-Au alloy crowns	17.6 to 36.0	45
									Ni-Cr-Ti alloy crowns	42.5 to 48.0
									Ni-Cr alloy crowns	60.4 to 72.9
									Ti alloy crowns	39.0 to 71.2
Kim, 2013 [43]	20	Co-Cr alloy	CAD-CAM Investment casting	30	Titanium die	Maxillary molar	Replica technique and 3D superimposition	16	Cast Co-Cr crowns	81.2 to 100.6	108
									CAD-CAM based Co-Cr crowns	100.5 to 114.7
Limkang-walmongkol, 2007 [49]	32	Noble metal alloy	Investment casting	Unspecified	Extracted teeth	Premolar	Profilometry	6	Porcelain margins of metal ceramic crowns	27.9	35
									Metal margin of metal ceramic crowns	42.4
Tao, 2006 [44]	30	Au-Ti alloy (N = 15) Co-Cr alloy (N = 15)	Investment casting	Unspecified	Plastic teeth	Maxillary incisor	Direct view microscopy	4	Au-Ti alloy crowns	20 to 45	32
									Co-Cr alloy crowns	19 to 45
Nakamura, 1998 [50]	36	Noble metal alloy	Investment casting	100	Metal die	Unspecified	Direct view microscopy	12	Crowns with normal margins	19 to 39	29
									Crowns with 1.5 mm overextended margins	38 to 73
									Crowns with 3 mm overextended margins	51 to 175
Leong, 1994 [47]	18	Noble metal alloy (N = 6) Cast titanium (N = 6) Pure titanium (N = 6)	Conventional milling Investment casting	Unspecified	Plastic teeth	Premolar	Direct view microscopy	4	Pd-Cu-Ga alloy crowns	10 to 36	46
									Cast titanium alloy crowns	50 to 71
									Crowns with milled pure titanium	28 to 87
Chan, 1989 [48]	6	Noble metal alloy	Investment casting	Unspecified	Extracted teeth	Unspecified	Scanning electron microscopy	Circumferential measurement	Metal ceramic crowns	30 to 80	50
Anusavice, 1987 [51]	30	Noble metal alloy	Investment casting	5	Metal die	Premolar	Direct view microscopy	12	Crowns with coping thickness of 0.1 mm	20 to 53	31
									Crowns with coping thickness of 0.2 mm	16 to 34

Table 2. Summary of studies and mean marginal gaps reported on pre-cemented metal–ceramic crowns.

Author, Year	Number of Metal Ceramic Crowns Fabricated	Core Material(s)	Method of Crown Construction	Cement Space Thickness (in µm)	Underlying Crown Preparation Material	Tooth Form Measured	Instrument Used for Measurement	Number of Marginal Gap Measurements per Crown	Mean Range of Reported Marginal Gap Measurements (in µm) in Any Direction		Mean Marginal Gap of Metal–Ceramic Crowns (in µm, Rounded to Nearest Whole Number)
Giti, 2023 [38]	20	Ni-Cr alloy (N = 10) Co-Cr alloy (N = 10)	CAD-CAM Investment casting	40	Brass die	Premolar	Direct view microscopy	16	Cast metal ceramic crowns	113.8 to 114.1	94
									CAD-CAM based metal ceramic crowns	74.0 to 75.2
Heboyan, 2022 [39]	90	Co-Cr alloy	CAD-CAM Investment casting	50	Plastic teeth	Unspecified	Direct view microscopy	4	Cast Co-Cr crowns	53.7 to 55.5	56
									CAD-CAM based Co-Cr crowns	55.8 to 57.7
Kunz, 2022 [40]	20	Co-Cr alloy	CAD-CAM Investment casting	Unspecified	Acrylic die	Mandibular molar	Replica technique	4	Cast Co-Cr crown	100.2 to 104.5	108
									CAD-CAM based Co-Cr crown	98.0 to 127.7
Tekin, 2020 [45]	10	Ni-Cr alloy	Investment casting	20	Plastic teeth	Mandibular molar	Replica technique	4	Cast Ni-Cr crowns	64.6 to 86	75
Ortega, 2017 [41]	10	Co-Cr alloy	Investment casting	50	Stainless steel die	Premolar	Scanning electron microscopy	60	Cast Co-Cr crowns	66.3 to 135.1	101
Saraswathi, 2016 [42]	10	Co-Cr alloy	CAD-CAM	Unspecified	Zirconia die	Maxillary molar	Direct view microscopy	60	Cast Co-Cr crowns	91.2 to 137.7	114
Gómez-Cogolludo, 2013 [46]	70	Noble metal alloy (N = 20) Ni-Cr-Ti alloy (N = 20) Ni-Cr alloy (N = 20) Ti alloy (N = 10)	Investment casting	Unspecified	Stainless steel die	Premolar	Direct view microscopy	120	Pd-Au alloy crowns	17.6 to 36.0	45
									Ni-Cr-Ti alloy crowns	42.5 to 48.0
									Ni-Cr alloy crowns	60.4 to 72.9
									Ti alloy crowns	39.0 to 71.2
Kim, 2013 [43]	20	Co-Cr alloy	CAD-CAM Investment casting	30	Titanium die	Maxillary molar	Replica technique and 3D superimposition	16	Cast Co-Cr crowns	81.2 to 100.6	108
									CAD-CAM based Co-Cr crowns	100.5 to 114.7
Limkang-walmongkol, 2007 [49]	32	Noble metal alloy	Investment casting	Unspecified	Extracted teeth	Premolar	Profilometry	6	Porcelain margins of metal ceramic crowns	27.9	35
									Metal margin of metal ceramic crowns	42.4
Tao, 2006 [44]	30	Au-Ti alloy (N = 15) Co-Cr alloy (N = 15)	Investment casting	Unspecified	Plastic teeth	Maxillary incisor	Direct view microscopy	4	Au-Ti alloy crowns	20 to 45	32
									Co-Cr alloy crowns	19 to 45
Nakamura, 1998 [50]	36	Noble metal alloy	Investment casting	100	Metal die	Unspecified	Direct view microscopy	12	Crowns with normal margins	19 to 39	29
									Crowns with 1.5 mm overextended margins	38 to 73
									Crowns with 3 mm overextended margins	51 to 175
Leong, 1994 [47]	18	Noble metal alloy (N = 6) Cast titanium (N = 6) Pure titanium (N = 6)	Conventional milling Investment casting	Unspecified	Plastic teeth	Premolar	Direct view microscopy	4	Pd-Cu-Ga alloy crowns	10 to 36	46
									Cast titanium alloy crowns	50 to 71
									Crowns with milled pure titanium	28 to 87
Chan, 1989 [48]	6	Noble metal alloy	Investment casting	Unspecified	Extracted teeth	Unspecified	Scanning electron microscopy	Circumferential measurement	Metal ceramic crowns	30 to 80	50
Anusavice, 1987 [51]	30	Noble metal alloy	Investment casting	5	Metal die	Premolar	Direct view microscopy	12	Crowns with coping thickness of 0.1 mm	20 to 53	31
									Crowns with coping thickness of 0.2 mm	16 to 34

Critical appraisal revealed that 6 of the 14 studies did not adequately address confounding factors that could influence marginal gap assessments [40,42,47,49,50,51], and a further 2 studies [46,48] did not follow a standardised approach for measuring marginal gaps (Table 3). The pre-defined cement spaces also varied across studies (42.14 ± 30.26 µm).

Table 3. Critical appraisal outcomes.

Author, Year	D1	D2	D3	D4	D5	D6	D7	D8
Giti, 2023 [38]	Y	Y	Y	Y	Y	Y	Y	Y
Heboyan, 2022 [39]	Y	Y	Y	Y	Y	Y	Y	Y
Kunz, 2022 [40]	Y	Y	Y	Y	Y	U	Y	Y
Tekin, 2020 [45]	Y	Y	Y	Y	Y	Y	Y	Y
Ortega, 2017 [41]	Y	Y	Y	Y	Y	Y	Y	Y
Saraswathi, 2016 [42]	Y	Y	Y	Y	Y	N	Y	Y
Gómez-Cogolludo, 2013 [46]	Y	Y	Y	U	N	N	N	N
Kim, 2013 [43]	Y	Y	Y	Y	Y	Y	Y	Y
Limkangwalmongkol, 2007 [49]	Y	Y	Y	Y	Y	N	Y	Y
Tao, 2006 [44]	Y	Y	Y	Y	Y	Y	Y	Y
Nakamura, 1998 [50]	Y	Y	Y	Y	Y	U	Y	Y
Leong, 1994 [47]	Y	Y	Y	Y	Y	U	Y	Y
Chan, 1989 [48]	Y	Y	U	N	N	U	U	N
Anusavice, 1987 [51]	Y	Y	Y	Y	U	U	Y	Y

Y, Yes; N, No; U, Unclear. D1: Were the criteria for inclusion in the sample clearly defined? D2: Were the study subjects and the setting described in detail? D3: Was the exposure measured in a valid and reliable way? D4: Were objective, standard criteria used for the measurement of the condition? D5: Were confounding factors identified? D6: Were strategies to deal with confounding factors stated? D7: Were the outcomes measured in a valid and reliable way? D8: Was appropriate statistical analysis used?

Table 3. Critical appraisal outcomes.

Author, Year	D1	D2	D3	D4	D5	D6	D7	D8
Giti, 2023 [38]	Y	Y	Y	Y	Y	Y	Y	Y
Heboyan, 2022 [39]	Y	Y	Y	Y	Y	Y	Y	Y
Kunz, 2022 [40]	Y	Y	Y	Y	Y	U	Y	Y
Tekin, 2020 [45]	Y	Y	Y	Y	Y	Y	Y	Y
Ortega, 2017 [41]	Y	Y	Y	Y	Y	Y	Y	Y
Saraswathi, 2016 [42]	Y	Y	Y	Y	Y	N	Y	Y
Gómez-Cogolludo, 2013 [46]	Y	Y	Y	U	N	N	N	N
Kim, 2013 [43]	Y	Y	Y	Y	Y	Y	Y	Y
Limkangwalmongkol, 2007 [49]	Y	Y	Y	Y	Y	N	Y	Y
Tao, 2006 [44]	Y	Y	Y	Y	Y	Y	Y	Y
Nakamura, 1998 [50]	Y	Y	Y	Y	Y	U	Y	Y
Leong, 1994 [47]	Y	Y	Y	Y	Y	U	Y	Y
Chan, 1989 [48]	Y	Y	U	N	N	U	U	N
Anusavice, 1987 [51]	Y	Y	Y	Y	U	U	Y	Y

Y, Yes; N, No; U, Unclear. D1: Were the criteria for inclusion in the sample clearly defined? D2: Were the study subjects and the setting described in detail? D3: Was the exposure measured in a valid and reliable way? D4: Were objective, standard criteria used for the measurement of the condition? D5: Were confounding factors identified? D6: Were strategies to deal with confounding factors stated? D7: Were the outcomes measured in a valid and reliable way? D8: Was appropriate statistical analysis used?

The measurement techniques used in the included studies varied: eight used direct view microscopy, three employed the impression replica method, two used SEM, and one relied on profilometry. The mean marginal gaps recorded for these methods were 75.00 ± 26.87 µm, 91.30 ± 23.05 µm, 75.50 ± 36.10 µm, and 35.00 µm, respectively. On average, 29 measurement points were taken per crown across all studies. The most frequently used type of abutment tooth used for crown fabrication was plastic teeth. Four studies directly compared CAD-CAM-milled metal copings (87.95 ± 26.35 µm) to conventionally cast alloys (90.45 ± 24.37 µm) following porcelain veneer application, finding no statistically significant difference (t = −0.197, p = 0.847). The predominant fabrication method for metal copings was investment casting using the lost wax technique, with only four studies [38,39,40,42] using CAD-CAM technology. The marginal gaps varied significantly (F = 11.34, p < 0.001; 95% confidence interval [CI] = 48.81, 71.40 µm; Welch robust test, p < 0.001) by coping material: cobalt–chromium (Co-Cr) led to 84.28 ± 34.27 µm (CI = 29.74, 138.81 µm), nickel–chromium (Ni-Cr) led to 70.98 ± 11.28 µm (CI = 53.02, 88.93 µm), titanium led to 50.18 ± 14.50 µm (CI = 27.10, 73.24 µm), and noble metal alloys led to 27.90 ± 11.16 µm (CI = 10.13, 45.67 µm). The distribution of the measurement instruments and mean marginal gap across manuscripts, categorised by decade of publication, is presented in Figure 2.

4. Discussion

4.1. Key Findings

The present systematic review identified only 14 studies that examined the marginal gaps of completed metal–ceramic single crowns, despite their widespread clinical application. This was further limited to just six studies that addressed confounding factors and followed a standardised approach for measuring marginal gaps. A lack of in vivo studies limits direct clinical applications and directs clinicians and researchers to the results of in vitro studies; however, a lack of robust in vitro studies restricts the ability to develop acceptable guidelines.

Direct view microscopy was the most frequently employed instrument, followed by impression replica and SEM. The benefits of direct view microscopy are convenience, ease of use, practicality, and cost-effectiveness, thus explaining its widespread use. The limited use of SEM may be attributed to potential cost and equipment availability constraints. Profilometry was the least utilised instrument and, although convenient and non-destructive, raises concerns with measurement accuracy [49].

4.2. Measurement Technique and Marginal Gap

The smallest mean marginal gaps were observed with direct view microscopy and SEM, excluding one profilometry-based study that failed to account for confounding variables. The lack of comprehensive evaluations comparing the precision of various measurement instruments makes meaningful cross-study comparisons challenging. Consequently, clinicians and researchers often rely on comparing the findings of individual studies on specific materials to the commonly cited 120 μm standard [15], clinical benchmarks indicating that gaps under 80 μm are difficult to detect [16], newer guideline proposals, or even personal preferences [29]. In general, smaller marginal gaps are preferred, although the reported values vary widely [52]. Adding further complexity, some evidence suggests that the significance of the marginal gap size may be lessened by the adhesive luting of materials to the tooth structure [28]. To date, there is no universally accepted ideal marginal gap for metal–ceramic crowns. Despite the marginal gaps in this study falling within the established clinical threshold of 120 μm, variations in marginal gap may not necessarily translate into meaningful clinical differences.

Each measurement instrument has limitations that could have impacted the results [17,29]. The direct view instrument is limited to in vitro use and relies on magnification, which may result in errors when selecting measurement points or differentiating between materials. The impression replica instrument can suffer from difficulties in identifying crown margins, elastomeric film tearing, and sectioning errors, which can lead to overestimated measurements. The cross-sectioning instrument allows for direct measurements but does not permit long-term analysis or provide a complete view of crown fit. Although profilometry is a non-destructive technique, it requires exact repositioning to prevent inconsistencies.

Common confounding factors identified included an imbalanced sample distribution across groups and variations in the measurement instruments used. Moreover, discrepancies in sample preparation, inconsistent cement spaces, and different alloy compositions presented biases in the present study. A lack of consistent procedural protocols across the included studies introduced variability that impeded the direct comparison of outcomes and may have compromised the reliability and accuracy of the results. This theme appears to be consistent across most in vitro assessments of crown marginal gaps [30,31].

4.3. Measurement Locations and Sample Sizes

Across the included studies, the mean number of crowns examined per study was 37, with a mean of 29 measurements taken per crown. The majority of studies failed to achieve the previously recommended threshold of 50 measurements [21]. In the absence of explicit rationale within the studies, it appears that decisions were predominantly influenced by convenience rather than scientific justification, indicating a lack of a standardised protocol. Future research should prioritise defining scientifically grounded standards for sample sizes and measurements per sample, rather than relying solely on precedent.

4.4. Crown Coping Material and Marginal Gap

Investment casting using the lost wax technique was the primary method for crown fabrication; however, it produced similar mean marginal gaps compared with using CAD-CAM technology. The most studied metal–ceramic crown coping materials were Co-Cr and noble metal alloys, with the latter showing a significantly smaller mean marginal gap compared with all the other materials. The conventional lost wax casting technique for fabricating metal–ceramic crown copings is inherently labour-intensive and is susceptible to complications such as distorted wax patterns and imperfections during metal casting [10,11]. In contrast, CAD-CAM milling technologies, whether additive or subtractive in nature, offer enhanced precision and reproducibility by mitigating issues of porosity and casting defects associated with traditional methods [11,12]. The included studies exhibited a moderate risk of bias, primarily due to inconsistencies in cement space considerations during crown production. This finding aligns with comparable research examining the mean marginal gap in ceramic crowns [9].

4.5. Core Materials and Marginal Gap

Alloys such as Pd-Cu-Ga (10–36 µm), Pd-Au (17.6–36 µm), and Au-Ti (20–45 µm) achieved the smallest gaps, likely due to their superior castability, ductility, and thermal expansion coefficient, which provided stability during firing. However, the control of laboratory variables was not assessed in the current review. Co-Cr alloys (19–45 µm) and porcelain margins (27.9 µm) also performed well. It was seen that increased coping thickness (0.2 mm: 16–34 µm vs. 0.1 mm: 20–53 µm) improved fit, likely by enhancing rigidity. Pure titanium-based crowns showed greater variability (28–87 µm), reflecting differences between milled and cast forms. Ni-Cr (60.4–72.9 µm) and Co-Cr crowns were experimented with the most and, regardless of whether produced by casting or CAD-CAM, had broader ranges (53–137 µm) of marginal gap values. It would be interesting to assess the marginal gap of metal–ceramic crowns when newer nanoceramic layers are applied over the metal copings [53].

4.6. Limitations

This study concentrated on in vitro measurements of marginal gaps in pre-cemented metal–ceramic crowns. The statistical analysis was limited by experimental variability, the crown fabrication techniques, and inconsistencies in the instruments used, as evidenced by high mean value variations and wide standard deviations. The in vitro conditions offered a regulated setting for assessing marginal gaps; however, this setting inherently limited the evaluation of clinically relevant parameters, including tooth preparation design and the use of impression materials routinely used in clinical practice. Additionally, the identification of the finish line location was unnecessary, as distinctions between supragingival or subgingival margins are not applicable in the in vitro context. To enhance measurement reliability across future studies, future research should seek to standardise marginal gap measurement protocols, ensuring consistency irrespective of the measurement instruments employed, with the eventual goal of transitioning to in vivo analysis.

5. Conclusions

The following conclusions are drawn from this systematic review:

1.

Direct view microscopy was the most commonly utilised instrument for measuring the marginal gaps of single pre-cemented metal–ceramic crowns, yielding the smallest reported mean marginal gap of 75.00 ± 26.87 µm.

2.

Metal–ceramic crowns constructed using conventional casting techniques presented similar marginal gaps to CAD-CAM crown copings.

3.

Metal–ceramic crowns constructed with noble metal alloys reported the lowest mean marginal gaps at 27.90 ± 11.16 µm.