The Metrics of Anterior Diastema Closure

INTRODUCTION
Successful closure of anterior diastemas with (fixed) prosthetic restorations requires careful pretreatment planning. Approaching case design from a physiologic philosophy enables the restorative team to use powerful tools to creatively and predictably deal with the aesthetic challenge at hand. Choice of material, preparation design, and restorative protocol are important parameters to consider prior to treatment. Based on natural anatomy, a biomimetic analysis gives the restorative team an astute understanding of aesthetic parameters to mimic. Further, a physiologic approach to restorative treatment of diastemas permits the classification of diastema closures. Diastema classification will determine the degree of difficulty the restorative team may encounter in treatment.

Figure 1. Rule of 3.	Figure 2. Varying facial planes of developmental lobes.
Figure 3. Diastema metrics.	Figure 4. Incisor contact anatomy.

ANATOMIC FOUNDATION FOR DIASTEMA CLOSURE
The foundation of a biomimetic philosophy is the anatomic form of the intact tooth in an ideal tooth-to-tooth relationship. The restorative decisions to be made should be based on a body of knowledge whereby the ceramic material will best mimic an intact physiologic dental relationship. The resultant finished restorative composition should relate well with the existing natural dentition. Treatment planning should be created by the dentist-ceramist restorative team.
Seemingly, diastema closures are simple procedures and require little planning. However, an astute understanding of anatomic shape is necessary to handle what may be potentially a difficult situation.

FOUNDATION OF ANATOMIC SHAPE: THE RULE OF THREE
Anatomically, anterior teeth are single cusped teeth. The cusp of a tooth is governed by the Rule of 3, which states that each cusp is composed of 3 developmental lobes. Termed the mesial, distal, and central developmental lobes, each lobe possesses character unto itself that defines itself and its control over its anatomic position.
An analysis of the central incisors shows 3 characteristically different developmental lobes (Figure 1), each varying in width, and its degree of buccal slope (Figure 2). If the width of each developmental lobe in a central incisor is measured, certain relative constants become evident. The mesial developmental lobe will occupy approximately one third of the central incisor’s width. The central developmental lobe and the distal developmental lobe occupy the remaining two thirds of the central incisor width, with the distal developmental lobe occupying a greater proportion of the two thirds width than the central developmental lobe. The resultant character imparted to the central incisor reveals a dominant distal developmental lobe, a mesial developmental lobe that competes for dominance, and a central developmental lobe that is inferior in width to its adjacent “brothers” (Figure 1).

BIOMIMETICS
To create a natural appearing diastema closure, the width relationships between each developmental lobe of an incisor must be properly analyzed and proportioned to the new width of the restored tooth relationship. The biomimetic rule to follow is to measure the space to be closed. Divide the space in half. Add the quotient to the existing width of each abutted tooth. This is the new width to proportion the developmental lobes. Divide the proposed new incisor width into thirds. The mesial developmental lobe will occupy one third. The central developmental lobe and distal developmental lobe will occupy the remaining two thirds with the distal developmental lobe occupying more than one third (Figure 3). Formula: Diastema Space mm/2 + Tooth Width = New Tooth Width.

USEFUL GUIDELINES FOR AESTHETIC TREATMENT PLANNING
Since width measurements are altered in diastema closure, it is logical to assume alterations in width will alter aesthetic length to width perception. Length should be considered in diastema closure to keep aesthetic compositions in proportion.¹ Length and width measurements abound in the literature.^2,3 Unfortunately, there are no ideal dimensions relative to length and width that compose natural oral aesthetics.⁴ The Golden Rule of Proportion has been suggested to relate aesthetic proportions, but has proven to not exist in natural dental forms.⁵ Measurement studies of natural teeth reveal “approximation rules” that apply relative variability to maxillary central and lateral incisor length (L) to width (W) proportions. Restored tooth length can be calculated with the following equation: L = W/tooth proportion percent, where the tooth proportion ratio ranges from 72% to 81%.⁶
The approximation rule proportions length to width measurements such that if a central incisor is 8.5 mm in width, then a proportionate length is approximately 10.5 mm. Also, the length of the clinical crown of a lateral incisor approximates the width of the central incisor. With a large variability in patient tooth length and width populations, using the approximation rule to adjust length to width proportions of central and lateral incisors can effectively create a diastema closure that relates well to the existing dentition, allowing perception based natural oral aesthetics.

CONTACT ANATOMY CONSIDERATION
Since tooth-to-tooth contact must be established in diastema closure, it is important to consider the physiologic contact position of intact anterior teeth.

Figure 5a. Class I diastema pre-op condition. Facial view.	Figure 5b. Class I diastema pre-op condition. Close-up view.

Figure 5c. Class I diastema pre-op condition. Oblique view.

Figure 6a. Class I diastema closure. Laminate preparation.	Figure 6b. Class I diastema closure. Laminate preparation. Note interproximal preparation extends to lingual line angles.

The contact of anterior teeth is located in the incisal and middle thirds of an ideal tooth-to-tooth relationship. Viewed from the sagittal, the contact is oval in shape from incisal to cervical and is located in the lingual half of the incisor (Figure 4). This knowledge can possibly pose a dilemma, if during pretreatment planning the dentist-ceramist team is considering laminate restorations for diastema closure. The laminate preparation design must extend lingually far enough to allow proper physiologic positioning of the contact.
The biomimetic restoration must have a symbiotic relationship with the supporting periodontium. There is a relationship that exists between contact position and the interdental crestal bone that permits regeneration and growth of proper height and contour of the papilla. Described by Tarnow, et al,⁷ the biomimetic rule states that the most apical location of the contact should be no closer than 3 mm from the interdental crestal bone, and no greater than 5.5 mm.⁷ This distance can be measured by sounding to bone, or by x-ray detection during the try-in appointment. A contact closure measuring less than 3 mm to the interdental crestal bone will violate the biologic width, thus creating a periodontal inflammatory response. Contact closure measuring greater than 5.5 mm will result in a black triangle. Proper contact location that is within the physiologic range of 3 mm to 5.5 mm will result in a properly developed papilla. Tarnow, et al⁷ report black triangles that exist during try-in and/or insertion, when the contact is between 2 restorations is in its proper physiologic position, the papilla will regenerate incisally to its proper position during healing.

Figure 7a. Class I diastema closure postoperative periodontal healing phase.	Figure 7b. Postoperative condition diastema metrics. Width of central approximates length of lateral (Empress crowns and veneers [Ivoclar Vivadent]).
Figure 7c. Class I diastema closure, post-op. Normalized papilla.	Figure 7d. Class I diastema closure, post-op. Normalized papilla, oblique view.
Figure 8a. Class II diastema pre-op condition. Frontal view.	Figure 8b. Class II diastema pre-op. Close-up view.
Figure 9a. Class II diastema closure post-op, demonstrating diastema metrics (Empress crowns and veneers).	Figure 9b. Class II diastema post-op; gingival porcelain closure.

Diastema closures can be placed into 3 classifications according to the biomimetic principles discussed. Class I diastema closures are closures that are effectively treated by creating physiologic contact between the clinical crowns of 2 abutted teeth whose length to width dimensions are within natural proportions and the interdental crestal bone can properly support physiologic periodontal form (Figures 5a to 7d). Class II diastema closures are closures between the clinical crowns of 2 abutted teeth whose length to width dimensions are within natural proportions and whose interdental crestal bone will not support proper periodontal form when the contact between abutted clinical crowns is created in a physiologic position (Figures 8a to 9a). Class III diastema closures are closures between the clinical crowns of 2 abutted teeth whose length to width dimensions are not within natural proportions and whose interdental crestal bone will not support proper periodontal form when the contact between abutted clinical crowns is created in a physiologic position. Class III diastema closures cannot be effectively treated without orthodontic movement to close the interdental space.
Class II diastema closures can be effectively treated by acknowledging that positioning an anatomic contact between 2 abutted teeth will result in a black triangle beneath the contact. Class II diastema closures can effectively be treated by incorporating the creation of prosthetic gingiva to mimic proper physiologic gingiva (Figure 9b).

References

1. Murthy BV, Ramani N. Evaluation of natural smile: Golden proportion, RED or Golden percentage. J Conserv Dent. 2008;11:16-21.
2. Sterrett JD, Oliver T, Robinson F, et al. Width/length ratios of normal clinical crowns of the maxillary anterior dentition in man. J Clin Periodontol. 1999;26:153-157.
3. Ash MM. Wheeler’s Dental Anatomy, Physiology, and Occlusion. 7th ed. Philadelphia, PA: WB Saunders; 1993.
4. Magne P, Gallucci GO, Belser UC. Anatomic crown width/length ratios of unworn and worn maxillary teeth in white subjects. J Prosthet Dent. 2003;89:453-461.
5. Gillen RJ, Schwartz RS, Hilton TJ, et al. An analysis of selected normative tooth proportions. Int J Prosthodont. 1994;7:410-417.
6. . Chu SJ. Range and mean distribution frequency of individual tooth width of the maxillary anterior dentition. Pract Proced Aesthet Dent. 2007;19:209-215.
7. . Tarnow DP, Magner AW, Fletcher P. The effect of the distance from contact point to the crest of bone on the presence or absence of the interproximal dental papilla. J Periodontol. 1992;63:995-996.

    

Dr. Schwartz serves as director of the Integra Institute of Center For Advanced Dental Learning, as well as assistant clinical professor at the Lousiana State University School of Dentistry Department of Prosthodontics. He is a ceramist, an innovator, author, researcher, and instructor in the field of oral aesthetics and dental ceramics. Dr. Schwartz is the creator/designer of the bleached dentition in ceramic. He is also a creator of the layering technique for milled/ pressed laminates, the gold standard for porcelain veneers. Dr. Schwartz is the inventor of the vertical shoulder preparation for porcelain laminate veneers, a technique that is accepted around the world as the definitive aesthetic prep technique for porcelain laminate veneers. He has featured articles in Practical Periodontics & Aesthetic Dentistry, Quintessence of Dental Technology, Dentistry Today, Dental Products Report, and Inside Dentistry. He can be reached at johnschwartz@drjohnschwartz.com.

Disclosure: Dr. Schwartz reports no disclosures.