Dr Samruddhi Shinde, Dr Padmini Chandrasekhar, Dr Shishir Singh, Dr Rajesh Podar, Dr Roshan Shetty, Dr Avinash Salgar
Post-graduate Student,
Reader,
Dean, Professor and Head of Department,
Professor.
Department of Conservative Dentistry and Endodontics,
Terna Dental College
ABSTRACT :
Tooth-restoration margins are the most essential and critical link between the tooth structure and the restoration. Sufficient understanding regarding the margin selection and preparation is of paramount importance to ensure the restoration longevity and success. Although there are various advances in regards to the materials in restorative dentistry, selecting the margin type remains a crucial decision. This paper aims to assertively compile the appropriate margin design, configuration, and location concerning various restorative materials. Factors determining the periodontal considerations about the margins and marginal discrepancies are also addressed.
Keywords : Bevelled margins; Flares; Chamfer; Shoulder; Butt joint; Finish lines
Citations : Shinde S, Chandrasekhar P, Singh S, Podar R, Shetty R,Salgar A. Margins in restorative dentistry - a review. J Prosthodont Dent Mater 2020;1(1&2): 19-31.
INTRODUCTION
The ultimate goal of restorative dentistry is to recover biomechanics of the original tooth, prevent further decay, and restore optimal function. The selection of an appropriate restorative material with its proper clinical technique is essential for the success of the restorative treatment. An adequate seating and specific adaption of the restorative material to the preparation margins contribute to its success. This margin between the restoration and the tooth structure, which is one of the weakest links in the tooth restoration complex, should be prepared with particular attention to ensuring it transitions with the tooth.
Any gap or interference in this portion of the tooth can lead to harmful effects like difficulty in recording impression, stress build-up at the margins, increased susceptibility to fracture of the restorative material, reduced strength and esthetics, and periodontal deterioration.1-3 The marginal design effect is an underrated factor in the clinical success of the restoration. A greater understanding of the margin design and placement is essential to achieve maximum results in restorative practice.
Ideal requisites for a successful tooth restoration interface:
Preparation margins resting on enamel should fulfill all the requirements advocated by Noy4 to possess enough strength to withstand massive occlusal forces without disrupting tooth structure and the restoration. It must be smooth enough to allow proper seating of the final restoration and must be located in an area that is self-cleansing or can facilitate easy plaque removal. The margins' location should be aesthetically acceptable but must not adversely affect the periodontium's health, and hence, care should be taken to maintain the biologic width. The peripheral extension of a tooth preparation must be easy to prepare and duplicate, conservative, and provide sufficient strength to the restoring material. The restoration margin should fit as closely as possible to the finish lines of the preparation to minimize exposed cement width to prevent microleakage and secondary caries.
Features of peripheral marginal anatomy for various direct and indirect restorations:
Bevels are the extensions placed on the occlusal and the gingival enamel and dentinal walls which can function to involve the surface defects, aid in the removal of unsupported enamel rods (gingival bevel in amalgam), mask the restoration margins (for anterior composites), provide enough metal bulk (for cast metal restorations) and reduce cement dissolution.4-6 Beveling minimizes seating discrepancy by lowering the marginal metal angle.7 In intracoronal cast restorations, flares are placed on the facial and the lingual walls, which perform a function similar to that of the bevels.
Skirts are placed as thin extensions on the facial or lingual proximal margins to involve defects, impart resistance and retention, and increase retention. They are indicated when contacts and contours are to be changed in the contemplated restoration. In tilted teeth, they allow for the bulk, resistance, and retention of the occlusal cast material required in building the occlusal table.
Facial or lingual collar preparation also increases the retention and resistance forms on a weakened tooth and prevents it from cuspal fracture. A collar can accommodate both alloy and porcelain bulk and facilitate marginal seating of the castings circumferential tie. Bevels, flares, skirts, and collars form a part of the circumferential tie.
Various finish lines are used for the extra - coronal cast restorations. Based on the configuration, they could be Feather edge (knife-edge), which provides the least strength, is difficult to locate on the cast and fabricate. However, it offers an excellent marginal seal and is the most conservative type. It is indicated in full metal crowns and mandibular molars if they are lingually tilted or have convex axial surfaces.
Chamfer type is distinct and assures bulk with little tooth involvement. It is the universally used design for cast metal restorations and lingual margins of PFM crowns (Class I, II, and III cast material). It is prepared with a round end tapered diamond bur. This preparation provides a diameter of 0.5 to 1 mm.
Heavy chamfer exhibits a 90° cavosurface angle with a rounded internal angle. It is indicated for all-ceramic crowns and is prepared with a round end tapered diamond. A bevel is added for use with metal restorations.
The shoulder provides resistance to masticatory forces, produces space for healthy contours, good esthetics, and less distortion. However, it is difficult to prepare; the least conservative lacks the sliding joint fit and has inferior marginal integrity. It is best for all-ceramic crowns and facial margins of PFM crowns. It is prepared by flat end tapered diamond bur. This preparation has a diameter of 1.5 to 2 mm. (Fig.1k,2k)
Shoulder with bevel has a bevel created on the marginal end of the unprepared tooth structure. It is indicated for a proximal box of inlays, onlays, occlusal shoulders of onlays and mandibular three fourth crowns, facial aspect of PFM crowns, and acrylic facing full cast veneers. The radial shoulder is a shoulder with a rounded gingivoaxial line angle 90º cavosurface angle and is indicated for porcelain restorations. It provides lesser stress and good support for porcelain.
The sloped shoulder possesses a 120° sloped shoulder margin. It is indicated on the facial margin of the metal-ceramic crown. This configuration removes unsupported enamel but provides enough bulk for thinning of metal framework for esthetic purpose.
Preparation features specific to various restorations material-wise:
A. Margin configuration for direct restorations:
Amalgam restorations:
The location of the margin for amalgam is determined by its property of high compressive strength and weak tensile strength. Occlusal margins should be located on smooth surfaces, inclined cuspal planes, marginal ridges, and crossing ridges. Enamel cavosurface margins of 90°- 110° (butt joint) prevent enamel fracture and help marginal amalgam tolerate masticatory forces. (Fig.1a,1b,1c,2a,2b,2c) Beveled occlusal margins are not recommended as masticatory stresses acting on them will lead to elastic deformation of the tooth, resulting in tensile stress build-up at the amalgam at the bevel leading to fracture of the restoration. For the same reason, the removal of amalgam flash is also mandatory.
For class II restorations, the finished gingival margins should be located in the gingival sulcus' occlusal portion. (Fig.1c, 2c) If it is gingival to the CEJ, the gingival cavosurface angle of 90° to the external root surface is needed. Proximal clearance of 0.5 mm should be given; since it compensates for setting the amalgam. 20° declination gingivally is given; if the gingival seat is in enamel. The bevel should not be given if the gingival margin is apical to the CEJ and primary teeth. A recent instrument named "PACE" (Perfected for Amalgam Cavity Evaluation) guides the attainment of a 110° cavosurface angle, allowing the clinician to achieve a minimum of 70° amalgam margin angle.
Glass ionomer restorations:
Glass ionomers show good adhesive property with the tooth structure due to ionic bonding. Advances like resin-modified GIC has superior properties like marginal adaptation and esthetics for restoring non-carious cervical lesions.10 Therefore, bevelling of the margins is usually not recommended.
Composite restorations:
Bevelling the enamel margin in composites can be advantageous as it will help remove aprismatic enamel, increase the area for acid etching and therefore improve bonding, and reduce microleakage.
In class I, the facial and lingual margins should be extended as needed to remove all carious tooth structure and should form at least a 90˚ cavosurface margin or more. (Fig.1d,2d)
In class II, facial and lingual margins are placed. All carious structure is removed. No unsupported enamel rods exist; there is accessibility for the placement and adaptation of the restoration on cavity margins, and plaque removal from the interdental area is facilitated. Thus, clearance in the proximal box depends on the individual clinical situation. The gingival floor should be prepared flat and create a 90° cavosurface margin.5 (Fig.1e,2e)
In slot preparations, the occlusal, facial, and gingival cavosurface margins of 90˚ or greater should be placed. For a vital asymptomatic extensively carious tooth, bevel placement is not required on occlusal cavosurface margin unless there are unsupported enamel rods.
In class III and class IV, starburst bevel camouflages the restoration margin; thus, providing superior esthetics and removal of unsupported and frail enamel structure (Fig1f,2f). Preservation of enamel margin on the gingival seat is of utmost importance as it is critical for bonding.
Extensive class III and class IV preparations might need additional bevel on the enamel walls for adequate retention. If the preparation extends gingivally onto the root surface, a 90˚ cavosurface margin should be provided in class III and class IV preparations. Flame shaped or round diamond instrument can be used to create a bevel of 45˚ angle to the external surface with a width ranging from 0.5–2 mm.
For class V restorations, there is no need for enamel bevel for prevention of microleakage.14 In maxillary incisors, bevel on lingual aspect may be contraindicated if it would cause the restoration marginal interface to be placed at the incisal contact area.
B. Margin configuration for indirect restorations:
Metal inlay:
According to Marzouk, the margins of inlays should fulfill all requirements for an extension for prevention.4 Occlusal and gingival marginal bevels and placement of the secondary flare on the distolingual and distofacial walls result in a 30 to 40˚ marginal metal on the inlay. This seals and protects the margins and creates a healthy enamel margin of 140–150˚. The desirable metal angle at the inlays' margins is always 40˚ except at the gingival margins, which in 30˚facilitating burnishable metal and a lap sliding fit. The gingival bevel should blend with the secondary flare (Fig 1h,2h). Bevelling is performed by No. 8862 bur. Secondary flare is prepared by No.8862 bur or No.169L bur. It is indicated in the indirect wax pattern, broad contact areas, wide extension of caries buccolingually, and to overcome undercuts in the cervical aspect of the facial and lingual proximal walls. It is omitted in the mesiofacial wall of maxillary premolars and molars as it impacts the resistance form and smile esthetics.
Metal Onlay:
On the practical side, facial and lingual margins should be located gingivally and away from the contact area to include the facial and the lingual cuspal elements and the grooves. They should be parallel to the contour of the cusp tips and crests of the adjacent ridges. Whereas, on the non-functional side, the facial and the lingual margins should be just gingival to the tip and crest of the ridge of the involved cusps and away from the occlusal contact. Proximal margins will be located similar to that of an inlay margin. Placing skirts provides a conservative and atraumatic way to increase resistance and retention form. A slender, flame-shaped, fine-grit diamond instrument is used for giving skirts. For the same intention, collars are placed on the weakened tooth for a MOD onlay with No. 271 carbide bur at high speed. A functional cusp bevel provides sufficient metal in zones of heavy occlusal contact. It is prepared with a round-end tapered diamond at 45˚ with the axial wall and depth of 1.5 mm.
Posterior maxillary three-quarter crown:
Occlusal finish bevel of 0.5 mm is given along the occlusofacial line angle, extending from the central groove on the mesial to the central groove on the distal. This provides space for metal on the lingual-facing incline of the lingual cusp to match the space on the buccal-facing incline created by the occlusal reduction. No. 171L bur is used to smooth the bevel. Chamfer finish line provides periodontal preservation, marginal integrity.
Three-quarter crown preparation on a mandibular molar or premolar:
Occlusal finish line must be given on the facial surface, gingival to occlusal contacts. The occlusal shoulder on the buccal aspect of the buccal cusp(s) knots the grooves and strengthens the nearby Bucco-occlusal margin.
Seven-eighth crown:
The vertical distobuccal margin is placed slightly mesial to the middle of the buccal surface. Margin finishing is easy, and hygiene maintenance for patients is also facilitated.
Porcelain fused to metal:
There should be a continuity of the shoulder or the beveled shoulder margin and the chamfer margin in the interproximal region.8 This is often referred to as a winged preparation.
Ceramic inlay:
Porcelain fracture has been reported as a primary reason for restoration failure. Butt joints are preferred. Bevels are not placed in ceramic inlays because ceramics tend to fracture at the margins due to low tensile strength. Placement of margins in enamel and rubber dam isolation during the luting procedure provides a good survival rate.
Ceramic Onlay:
Beveled margins are contraindicated because the bulk is needed to prevent fracture. Cavosurface margins should be 90(Fig 1i,2i). A well-defined heavy chamfer is the recommended margin. Margins in enamel must be smooth and distinct for a ceramic restoration to fit accurately.
All-Ceramic crown
A uniform margin thickness circumferentially with a width of 1-1.5 mm is necessitated for better aesthetics. A butt joint will enable proper stress distribution and thus minimize ceramic fracture due to tensile stress. The beveled margin is not recommended as it does not support the porcelain. The margin must be smooth for appropriate fabrication.
Zirconium based crowns:
A 0.8-1mm wide, 360 deep chamfer with no sharp internal line angles is recommended as it allows room for the zirconium coping and overlying veneering ceramic.
Finishing the margins can be done by finishing grit diamonds followed by a 30-fluted carbide bur and polishing pastes. Shoulder and chamfer preparations provide adequate accuracy because they offer good detectability of the finish line for scanning devices in the case of CAD/CAM-fabricated zirconia copings.
Laminate veneers:
Veneer interproximal margins should extend into the facial and gingival embrasures without engaging an undercut and should be located just facial to the proximal contacts.
Window preparation is recommended for most direct composite veneers, indirectly fabricated veneers where the canine's outline form is intact, and the patient is canine guided. The incisal edge of the tooth is preserved. This reduces the accelerated wear of the opposing tooth.
Feather preparation is indicated in patients with a normal overbite and avoids direct contact of ceramic veneers with their antagonistic tooth structure. The incisal edge of the tooth is prepared Bucco-palatally, but the incisal length is not reduced. This can cause weak veneer, ceramic chipping, difficulty with the veneers' seating, marginal discoloration, and poor marginal adaptation.
For incisal overlap preparation, the butt joint (Fig 1l,2l) is used in worn or defective areas on the incisal edge's lingual aspect. It provides an increased adaptation of veneer to the lingual margin providing a 'lap sliding' fit.
Palatal chamfer preparation can be used when the incisal edges are thin buccolingually or when an increase in crown length is needed. This design increases the surface area for bonding, provides rounded angles and adequate ceramic thickness.
The gingival extent of the veneer's margin depends on the size of caries, discoloration, and the amount of tooth structure visible while smiling. If the tooth's cervical portion is discolored, carious, or defective, a subgingival margin needs to be placed. Margin should be placed at the crest of tissue in a high smile line, fluorosis stains for esthetic reasons.
Vertical preparation:
A preparation without a finish line is a less-invasive substitute to a horizontal margin, allowing enamel preservation in the cervical area. It is indicated when periodontally compromised teeth are used as abutments for fixed prostheses. This is referred to as the biologically oriented preparation technique (BOPT). A Batt-Bur, which is a round-ended tapered diamond bur with a non-cutting end, is used.
Fig 1. a-l
a) Class I amalgam
b) Class I amalgam palatal extension
c) Class II amalgam
d) Composite
e) Class II composite
f) Starburst bevel
g) Class II composite inlay
h) MOD metal inlay
i) Ceramic onlay
j) Margins for PFM crown
k) All shoulder margins
l) Veneer preparation
Fig 2. a-l
a) Class I amalgam
b) Class I amalgam palatal extension
c) Class II amalgam
d) Composite
e) Class II composite
f) Starburst bevel
g) Class II composite inlay
h) MOD metal inlay
i) Ceramic onlay
j) Margins for PFM crown
k) All shoulder margins
l) Veneer preparation
Effect of tooth preparation margins concerning pulpal health:
Immediate changes in dental pulp vary with different types of preparations depending upon remaining dentin thickness. However, crown preparation of up to 2 mm depth is considered within a safe limit, and it does not lead to reversible or irreversible pulpitis. The crown preparation procedure leads to dentinal tubules' exposure, which causes minute fluid shifts across dentin due to tactile, thermal, osmotic, or evaporative stimuli. This permits mechanoreceptors' activation and initiates mild to moderate inflammatory response in the pulp and odontoblast injury.
Pulpal injuries lead to the release of several inflammatory mediators that may have direct or indirect effects via modulation of trigeminal sensory nerve fibers on pulpal vasculature. Vasodilatation and increased blood flow are the two significant actions seen in the initial phase of pulpal inflammation37 Dentin hypersensitivity following tooth preparation is a common issue in dentistry. The application of dentin bonding agents can control root sensitivity. An important point to be understood here is that remaining dentin thickness, which is inversely proportional to the pulpal response tooth preparation near the pulp, should be avoided.
Effect of peripheral marginal anatomy on periodontal health:
The margin location can influence periodontal health, esthetics, finishing, and retention of the restoration and stump shade of the underlying tooth. Association between the margin placement, supracrestal fiber attachment, and location of the sulcus base is majorly vital for gingival health. Margins placed within biologic width breach the biologic principle leading to alveolar bone resorption, periodontal breakdown, aggregation of subgingival microflora, and the chronic inflammatory process followed by iatrogenic periodontal disease and restoration failure.
Supragingival finish line configuration proposed by Orban facilitates predictable impressions, ease in excess cement removal, convenience to prepare and finish by the operator, optimum periodontal health, oral hygiene maintenance and proper fitting and should be welcomed favorably in non-esthetic areas. So, margins must be placed 1-2 mm supragingival if possible. However, innovations in translucent restorative materials, adhesive dentistry, and resin cement has allowed supragingival margin placement in esthetic areas.
Equigingival finish line as suggested by Marcum accredits for marginal finish, esthetics, and accessibility. The success of this configuration is due to the presence of keratinized epithelium. They are placed at the marginal gingiva crest; they are more plaque retentive and impact the periodontium.
Subgingival margins are indicated for esthetic concerns, subgingival caries, cervical erosion, and when crown-lengthening is not shown. They are placed below the marginal gingiva and are at most significant biologic risk since they may violate the gingival apparatus. Mechanical, chemical, rotary gingival curettage, and surgical methods are employed to expose the subgingival finish line. However, "intracrevicular finish lines" are limited within the gingival sulcus and favor periodontal health and esthetics.
Provisional restoration can be placed, and tissue reaction was observed several weeks before the final restoration placement. Flemmig et al. advocated using 0.12% chlorhexidine gluconate for two weeks before the tooth preparation procedure. This aids in decreasing gingival inflammation around the teeth and will ultimately provide a healthy working environment. Healthy tissue after this indicates no violation of the biologic width and that the periodontium will respond well to the final restoration margins.
Importance of marginal fit:
Well adapted margins to finish lines of preparation provide longevity to the restoration serving marginal integrity. Configuration of finish line decides the shape and bulk of marginal metal, affects the marginal adaptation and degree of seating. The junction between a cemented restoration and the tooth is a potential site for recurrent caries. Metal casting fits within 10 μm, whereas the porcelain margin fits within 50 μm. Clinically acceptable marginal gap must be less than 120 μm. Improperly adapted margins act as an area of plaque accumulation, increase cement dissolution, and are correlated with the severity of inflammation. It may make the tooth surface susceptible to caries.
Hence, marginal adaptation is essential as it may result in biological or mechanical failure of the restoration. Finish line configuration remarkably affects the marginal seal and occlusal seating of the crowns. Shoulder margins provide a good seat but a wider marginal seal. Chamfer, long chamfer, and feather edge margin permit superior sealing of the margins regardless of poor seating. However, the finish line does affect the fit of the cemented crowns. The beveled margin does not show significant superiority in a marginal fit after cementation.
MARGINAL DISCREPANCIES:
Errors like over-extended and under-extended margins, rough, incomplete, non-uniform, J shaped margins, pseudoshoulders, margins with sharp internal line angles should be avoided for optimum results. Rough, irregular, or "stepped" margins restoration subsequently increases the overall margin dimension and decreases the restoration's adaptation accuracy. Smooth margins allow the subsequent steps like tissue displacement, impression making, laboratory communication, die formation, waxing, and finishing much more straightforward and ultimately result in longer-lasting restorations. Smooth, accurately placed preparation margins are of great importance in the repairs that are fabricated using computer-aided design and computer-aided manufacturing (CAD/CAM) process.
CONCLUSION:
The success of a dental restoration largely depends on the accurate margin placement, adaptation, and integrity. The restorative margin placed plays an essential role in the long-term prognosis and outcome of the restoration. Thus, proper knowledge of the gingival and periodontal response to the restoration from a biomechanical perspective is essential. Though more research is needed, this paper has highlighted the different tooth margins that work for the broad array of restorative materials in use. Research is progressing at a fast pace. We need to update the emerging restorative material's knowledge and their respective margins and adapt as per the latest groundwork.
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