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Marginal adaptation is critical in the long-term success of dental restorations and fixed dental prostheses. It has been well-documented that the compromised marginal integrity of dental restorations can increase the risk of developing complications, including periodontal disease, dental caries, and the loosening of restorations, which can eventually lead to the premature failure of dental restorations.1-4The factors that have an influence on the marginal adaptation of dental restorations have been extensively studied. These may vary depending on the type of dental restoration. In general, factors such as the marginal configuration and quality, the impression material and technique, the fabrication method and material, the cement space, and the type of cement and cementation technique are known to have an influence on the marginal adaptation of many types of indirectly fabricated dental restorations.5-7
Assessing and Adjusting Interproximal Contacts
The location, size, and tightness of a restoration's interproximal contacts should resemble those of the natural teeth.8 Although the proximal contacts between adjacent natural teeth normally appear to be closed, it has been reported that small interproximal spaces ranging from 3 to 21 μm exist between approximately 80% to 90% of natural unrestored teeth.9-11 These spaces permit micromovements of the teeth during function and parafunction, which can subsequently result in changes to the interproximal spaces.12 Other factors that are known to influence interproximal contact tightness include tooth location, tooth type, time of day, and posture.13,14 Accordingly, interproximal spaces are considered to be dynamic. In order to reproduce this dynamic interproximal relationship between adjacent teeth that are undergoing restoration, dentists should adjust the proximal surfaces of indirect restorations until passive contact or microscopic clearance is achieved.15
When evaluating the marginal adaptation of indirect dental restorations prior to cementation, it is recommended to assess and adjust the interproximal contacts first because tight interproximal contacts may prevent adequate seating of the restoration and result in excessive marginal discrepancy. If a restoration is not completely seated, it is premature to evaluate marginal adaptation or occlusion.8 The presence of the periodontal ligament permits minor movement of the teeth not only in a horizontal direction but also in an axial direction under load.16 Therefore, it is often possible to seat dental restorations in situations involving excessive interproximal tightness by applying vertical pressure. In such instances, the restoration may seem to be seated, but its ideal positioning at the microscopic level cannot be guaranteed, and the adjacent teeth may become dislodged due to excessive lateral pressure applied to their proximal surfaces. Although the recommendation that adequate adjustment of interproximal contacts should be accomplished prior to evaluating a restoration's margins is widely accepted, once seemingly adequate seating of a restoration has been accomplished, dentists often proceed to the next step without achieving completely passive proximal contacts. This may result in suboptimal treatment outcomes. Interproximal relief refers to the adjustment of overly heavy interproximal contacts such that they appear to be visibly closed but are actually only passively touching or microscopically open within the physiologic range to relieve pressure on the proximal surface of an adjacent dental restoration.15,17 When placing a crown, it is critical to achieve adequate interproximal relief to ensure proper marginal adaptation prior to any occlusal adjustment. The use of a classification system to differentiate the degree of interproximal contact between indirect restorations and their adjacent teeth is useful in this process.
Interproximal Contact Classification
The classification system for the interproximal contact between crowns and their adjacent teeth that is presented here was developed by the authors based on the number and strength of the interproximal contacts and their effect on marginal adaptation. The descriptions of and recommendations for each class are made assuming that all parts of the crown, including the margin and intaglio and occlusal surfaces, are fabricated with high accuracy. The details of the clinical presentation may vary if fabrication errors are also a factor. The clinical recommendations for each of the four classes (ie, Classes 0, I, II, and III) are intended to help clinicians select effective strategies for interproximal contact adjustments (Table 1). A class for open interproximal contacts is not included in this classification system.
Class 0
In Class 0, which is considered ideal, adequate interproximal relief is achieved for both the mesial and distal aspects of a crown (Figure 1). The interproximal contacts are visually closed with passive contact or microscopic clearance. No lateral pressure is being applied to the crown or the adjacent teeth at rest. Diagnostically, a shim stock sheet should be able to pass through both the mesial and distal interproximal spaces with minimal friction, and a thin diamond strip should be able to pass through each with equal friction. The crown should be able to be completely seated with gentle finger pressure. Once clinically acceptable marginal seating is achieved and the adjacent teeth are not displaced in any direction, the crown is ready for occlusal evaluation.
Class I
In Class I, adequate interproximal relief is achieved on only one side, and the interproximal contact on the other side is slightly heavy (Figure 2). When passing a shim stock sheet or thin diamond strip through both the mesial and distal interproximal spaces, significantly higher friction is generated on the side with the heavy interproximal contact. In Class I, a crown may still be able to be completely seated under moderate finger pressure; however, lateral pressure is being created on the side with the heavy interproximal contact, which may cause vertical or horizontal displacement of the abutment tooth or the adjacent tooth. Therefore, although the seating and marginal adaptation may seem appropriate, interproximal contact adjustment is required prior to proceeding to occlusal evaluation.
Class II
In Class II, the interproximal contacts on both sides are somewhat heavy (Figure 3). The excessive tightness of the interproximal contacts does not permit a shim stock sheet or a thin diamond strip to be passed through the mesial or distal interproximal spaces. In Class II, a crown may visually appear to completely seat when higher finger pressure is exerted; however, at the microscopic level, the forced seating results in inaccurate positioning of the crown. Furthermore, vertical or horizontal displacement of the adjacent teeth may occur to compensate for the excessive and uneven lateral pressure from one or both sides. This leads to compression of the crown margin on the side with the heavier interproximal contact, resulting in incomplete seating of the crown margin on the other side (Figure 4). Unless interproximal relief is achieved to permit complete seating, it is premature to evaluate either marginal adaptation or occlusion. Extraoral adjustment of the crown may be necessary.
Class III
In Class III, the interproximal contacts on both sides are so heavy that they do not permit complete seating of a crown, even with heavy finger pressure. A significant gap between the crown margin and the finish line is visible without magnification (Figure 5). Major extraoral adjustment of the proximal surfaces of the crown is necessary to achieve complete seating.
Strategies for Evaluation and Adjustment
Ideal interproximal contacts between dental restorations and adjacent teeth (ie, Class 0) should be neither visibly open nor overly tight. Open interproximal spaces can become food traps, potentially resulting in gingival irritation and increased plaque retention.18 Although interproximal food impaction can be a combined result of other factors, such as inadequate food escape grooves, uneven marginal ridges, and prominent opposing cusps, it is widely accepted that visibly open interproximal contacts are not ideal because they increase the risk of interproximal food impaction.19 At the same time, overly tight interproximal contacts should also be avoided. However, the creation of closed interproximal contacts has often been overemphasized in dental education, which may have led to the misconception among some clinicians that a crown should be seated with friction from slightly tight interproximal contacts to avoid open interproximal spaces. Although it is often possible to insert a crown with Class II or Class III interproximal contacts, the forced insertion of a crown in such situations may result in incomplete seating and chronic lateral stress to the adjacent teeth. The potential clinical complications of delivering a crown with heavy interproximal contacts include patient discomfort, tooth migration, and increased risk of periodontal disease and dental caries due to incomplete seating of a crown.1,17,20
Traditionally, it has been widely accepted and taught that ideal interproximal contacts should allow unwaxed dental floss to snap through them relatively easily. If floss does not pass through, the contact is considered to be too tight. If floss passes through too easily, the contact is considered to be open.8 Although this is not a very scientific way of evaluating interproximal contacts, this method has been very popular among dentists.8,20 Unfortunately, the use of dental floss is not reliable because it primarily relies on the unstandardized, subjective feelings of individual clinicians. It has been reported that the accuracy of interproximal contact evaluation using dental floss is lower when compared with other methods.20 The use of a shim stock sheet or articulating film has been demonstrated to result in higher accuracy.20 Although proven to be superior, the use of a shim stock sheet to evaluate interproximal contacts is not as popular as dental floss because it is less convenient.
The use of interproximal diamond strips has been introduced as an alternative method of both evaluating and adjusting Class I interproximal contacts.15,17This method is efficient because it obviates the need to repeatedly switch between the use of shim stock sheets and rotary instruments during the evaluation and adjustment of interproximal surfaces. The procedural steps involved in the use of interproximal diamond strips include the following:
1. Place the restoration on the prepared tooth and evaluate the marginal adaptation and proximal contacts.
2. Insert a thin interproximal diamond strip into the interproximal space with the heavier contact with the abrasive side of the strip facing the restoration.
3. While holding the restoration in place with fingers, move the diamond strip buccolingually 5 to 6 times without applying lateral pressure to the restoration. The number of required adjustments may vary depending on the tightness of the interproximal contact and the material used to fabricate the restoration.
4. Reevaluate the marginal adaptation and proximal contacts, and repeat steps 2 and 3 if necessary.
When adjusting interproximal contacts, one of the biggest fears of clinicians is that they will inadvertently create open interproximal contacts as a result of over-reduction. However, with a diamond strip, the aggressiveness of the interproximal reduction is mainly determined by the lateral pressure applied to the restoration, which is proportional to the thickness of the diamond strip and the tightness of the interproximal contacts. When a clinician uses a thin diamond strip (eg, 50 μm) to adjust an overly tight interproximal contact by passing it through the interproximal space without applying lateral pressure, the cutting efficiency of the strip decreases as the interproximal contact approaches adequate interproximal relief status. Considering that the periodontal ligament allows for the lateral displacement of natural teeth to a certain degree and that the cement space inside a crown also allows for lateral displacement of a crown at a microscopic level, passing a thin diamond strip through an interproximal space is not likely to generate sufficient lateral pressure on the interface between the diamond strip and crown to result in over-reduction unless the strip is used aggressively with intentional lateral force toward the crown. Therefore, when used properly, a thin diamond strip is not only efficient but also provides a safe way of adjusting interproximal contacts because it minimizes the risk of over-reduction.
It is the authors' personal experience that the use of digital technology has contributed to a reduction in the chair time needed for the adjustment of interproximal contacts. In the authors' practices, most conventionally fabricated crowns have resulted in either Class II or Class III relationships at delivery. However, the authors have noted a significant increase in the proportion of Class I or Class II relationships at delivery when crowns are digitally fabricated. Although this anecdotal experience cannot be generalized, it highlights that laboratory fabrication methods can influence the tightness of the interproximal contacts of indirect restorations. Heavy interproximal contacts can be easily adjusted chairside, but the presence of open interproximal contacts largely requires the restoration to be returned to the dental laboratory for adjustment. For this reason, some dental technicians may have unconsciously developed a preference for slightly heavy interproximal contacts at the time of initial try-in. Although this can be a valid strategy to minimize the need for laboratory adjustments, it also means that clinicians should not assume that adequate interproximal contacts have been achieved simply because a crown can be seated. In any case, clinicians should take full responsibility for ensuring adequate interproximal relief prior to the final cementation of indirect dental restorations.
Conclusion
Adequate interproximal contacts are critical in achieving complete seating of indirect dental restorations. The classification system of interproximal contacts suggested in this article can help dentists and dental technicians prioritize the most appropriate strategies for evaluating and adjusting interproximal contacts between dental restorations and adjacent natural teeth to achieve adequate interproximal relief.
Queries regarding this course may be submitted to authorqueries@conexiant.com
About the Authors
Daniel S. Kim, DDS
Fellow
Academy of General Dentistry
Private Practice
Vancouver, Washington
Volunteer Faculty
Department of Oral Rehabilitation and Biosciences
Oregon Health & Science University
School of Dentistry
Portland, Oregon
Hongseok An, DDS, MSD
Fellow
American College of Prosthodontists
Associate Professor
Department of Oral Rehabilitation and Biosciences
Oregon Health & Science University
School of Dentistry
Portland, Oregon
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