Crown Lengthening Surgery: A Restorative Driven Periodontal Procedure

Improper management of the periodontal tissues during restorative procedures is a common, but often overlooked, cause of failure. When a restoration is placed, the preservation of an intact, healthy periodontium is necessary to maintain the tooth or teeth being restored. Predictable long-term restorative success requires a combination of restorative principles with the correct management of the periodontal tissues.

The replacement of form, function, and esthetics is the primary goal of restorative dentistry. Equally important is doing no harm when restorations are placed. Improper management of the periodontal tissues during restorative procedures is a common, but often overlooked, cause of failure. When a restoration is placed, the preservation of an intact, healthy periodontium is necessary to maintain the tooth or teeth being restored. The restorative dentist must attempt to eliminate all factors that could lead to the accumulation of bacterial plaque and its subsequent effects on the gingival tissues, root surfaces, and underlying alveolar bone.

Predictable long-term restorative success requires a combination of restorative principles with the correct management of the periodontal tissues. One factor that is of particular importance is the potentially damaging result to the periodontium when margins are placed below the gingival margin. To avoid these potential problems to the supporting structures of the teeth, clinical crown lengthening can provide adequate clinical tooth structure to enable the placement of margins either coronal to or at the gingival margin.

Anatomy of the Gingival Complex

Clinical doctrines and common myths regarding the gingival response to restorative materials and the limitations of intracrevicular tooth preparation make it difficult to place restorative margins subgingivally. Because tissue components cannot be visualized, the guidelines of sulcus depth are often misunderstood and clinically mismanaged. Therefore, before discussing crown lengthening, it is important to review the anatomy of the supra-alveolar tissues of the healthy periodontium and the relevance of these dimensions to the position of restoration margins.

The supra-alveolar tissues include the gingival sulcus and the gingival attachment. The gingival attachment joins the gingiva to the tooth and is made up of the connective tissue attachment and the epithelial attachment. The gingival attachment is made up of connective tissue fibers that are embedded in cementum. The epithelial attachment is an adhesion of epithelial cells through hemidesmosomes.

The concept of a minimum dimension of tissue from the alveolar crest to the bottom of the gingival sulcus is based on a study by Garguilo and colleagues.¹ They examined 30 cadavers with clinically healthy periodontia and reported on the average histologic dimensions of the connective tissue attachment, the epithelial attachment, and the gingival sulcus. They found that there appears to be a proportional relationship between the crest of the alveolar bone, the connective tissue attachment, and the epithelial attachment. The investigators found that the average histologic dimension of the connective tissue fibers was 1.07 mm, the average histologic dimension of the epithelial attachment was 0.97 mm, and the average histologic dimension of the sulcus was 0.69 mm. The combined dimension of the junctional epithelium and connective tissue attachments is referred to as the "biologic width"² Subsequent authors^3,4 have also shown that a definite dimensional relationship exists among the alveolar crest, the supra-alveolar connective tissue attachment, the junctional epithelium, and the base of the gingival sulcus.

Understanding and clinically managing the concept of biological width and the level of the osseous crest is key to maintaining periodontal health in the presence of dental restorations. The location of a restorative margin relative to the crest of the alveolar bone is more critical for preserving gingival health than its distance below the free gingival margin.⁵ The restorative dentist must be able to determine the height of the osseous crest and width of the gingival attachment before placing intracrevicular margins. This is done to prevent impingement of the soft tissue attachment, otherwise referred to as violation of the biologic width.

Violations of Biologic Width

Several studies have shown that the position of the crown margin in relation to the gingiva can significantly affect the gingival index, as well as the pocket depth and the position of the epithelial attachment.^6-10 It was shown that crown margins positioned subgingivally were associated with the most gingival inflammation, whereas supragingivally located crown margins were associated with the least gingival inflammation.^6,9-10 Valderhaug^7,8 showed that loss of periodontal attachment was significantly higher around teeth with subgingivally located crown margins than around similar teeth with crown margins located supragingivally. Furthermore, subgingivally prepared teeth exhibited deeper pockets than teeth prepared with the margin at the height of the gingiva or supragingivally. The same study also showed that after five years, 30 percent of the subgingivally located crown margins were associated with gingival recession.

Figure 2a Chronic gingival inflammation around anterior crowns. Figure 2b Elevation of a flap shows crown margins in close proximity to alveolar crest and violation of gingival attachment or "biologic width". Figure 2c New restorations after surgery to re-establish room for gingival attachment.

There are also reports in the periodontal literature that claim that in the presence of inflammation, restorations that impinge upon the gingival attachment or biologic width will trap bacterial plaque, induce inflammation, and increase the severity of periodontal breakdown^3-5,11-14 (Figure 2). Maynard and Wilson3 reported that violation of the biologic width will lead to a progressive inflammation with downgrowth of the epithelial attachment and loss of connective tissue attachment. Allen¹³ reports that wherever the biologic width is violated, there is a reaction by the periodontium. Alveolar bone will resorb inconsistently in an attempt to provide space for a new connective tissue attachment, which will result in an increase in probing depth. Re-establishment of the periodontal attachment at a more apical position and a deepened sulcus, combined with a deep subgingival restorative margin, frequently lead to chronic inflammation and localized periodontal breakdown. This is especially applicable in periodontally susceptible patients.

It is possible that the gingival inflammation associated with restorations that impinge on the gingival attachment is not from a physical insult, but from a bacterial insult. Crown margins are inherently imperfect and will eventually collect bacterial plaque. The average marginal fit of gold restorations is 57 m and ceramic restorations is 48 m.¹⁵ Christensen16 has stated that all cast restorations have cement lines and that most studies show these lines to be at least 20 to 40 m thick. Since the size of a typical microorganism is only about 1 m thick, it can be assumed that most crown margins will eventually harbor bacterial plaque. Several studies have shown that subgingival crown margins interfere with gingival health.^6-10

Waerhaug¹⁷ claimed that the inflammatory lesion radiates 1 to 2 mm from the plaque front. Therefore, it is probable that a 1 to 2 mm zone of inflammation is going to be contiguous with a subgingival restorative margin. Waerhaug¹⁸ also stated that the loss of connective tissue attachment rarely occurred when the plaque front was less than 1.2 mm from the apical border of the junctional epithelium and that there was no loss of bone when the plaque front was greater than 2.7 mm coronal to the bone. It has been claimed that the rationale for obtaining 3 mm of distance from the expected restorative margin to the alveolar bone should not be to allow for the gingival attachment and sulcus, but instead to position the restorative margin, with its anticipated plaque deposit, beyond the 2.7 mm danger zone from the bone.¹⁹
Regardless of the etiology of the inflammation and bone loss, to ensure periodontal health in the presence of subgingival restorations, there must be a minimum amount of sound tooth structure coronal to the alveolar crest. Most authors^3-5,12 recommend that when supragingival margins are not feasible, restorative margins should be placed at least 3 mm from the alveolar crest. This dimension allows a distance of 1 mm for each part of the gingival attachment (connective tissue attachment and epithelial attachment), for a total of 2 mm. The additional 1 mm is for a healthy gingival sulcus. Other authors have recommended 4 mm or even 5 mm for the working dimension of biologic width.20,21 However, these authors take into consideration an additional 1 to 2 mm of tooth structure for a restorative margin.

It is also important to remember that the above guidelines are averages. Each clinical situation varies and should be examined prior to margin placement.

Another factor to take into consideration is the relationship of the position of the biologic width relative to the cementoenamel junction. In the normal crestal to soft tissue relationship, the junction of the connective tissue fibers and the epithelial attachment is located at the cementoenamel junction. In what is termed a "high crest," the alveolar crest is at the cementoenamel junction, resulting in a minimal connective tissue attachment and what is often called "delayed passive eruption." A "low crest" is when the osseous crest is more than 2 to 3 mm apical to the cementoenamel junction. This most often results in a long junctional epithelial attachment, with the junction of the epithelial attachment and connective tissue fibers on cementum.

Ochsenbein and Ross²² further defined tissue types as either flat or scalloped. A scalloped architecture will have a low crest and is usually found with a thin periodontium, causing a tendency for soft tissue recession. A flat architecture is usually found in patients with thicker tissue. Only by sounding to bone under local anesthesia can these relationships be determined.

Figure 3a Radiographs of maillary left showing decay under existing restorations Nos. 12 and 13 and supra-eruption No. 14. Figure 3b Buccal view of maxillary left showing subgingival margins and soft tissue cratering. Figure 3c Palatal view of maxillary left showing short clinical crowns and extent of subgingival decay removal. Figure 3d Buccal view of maxillary left pre-osseous resection. Figure 3e Buccal view of maxillary left post-osseous resection. Figure 3f Palatal view of maxillary left pre-osseous resection. Figure 3g Palatal view of maxillary left pre-osseous resection. Figure 3h Buccal view of healing post-crown-lengthening surgery showing amount of clinical crown length gained on buccal. Figure 3i Palatal view of healing post-crown-lengthening surgercy showing amount of clinical crown length gained on palatal. Figure 3j Buccal view of final restoration (final restoration by Dr. Michelle IKoma). Figure 3k Palatal view of final restoration.

Measurements should be made from the free gingival margin to the osseous crest with a periodontal probe. It is also important to remember that the straight and interproximal surfaces have different requirements. In health, the facial aspect has approximately a 3 mm depth from the gingival margin to the osseous crest. The interproximal surfaces have a gingival margin to osseous crest depth ranging from 3 to 4.5 mm. The interproximal depth varies depending on the amount of the scallop of the gingival tissue relative to the scallop of the interproximal alveolar bone. The gingival scallop is always equal to or greater than the underlying osseous scallop, which is greatest in the anterior teeth and flattens out posteriorly. Furthermore, it is important to remember that the osseous crest parallels the cementoenamel junction circumferentially and that the biological width follows the shape of the osseous crest.

Surgical Crown Lengthening

Crown lengthening is a surgical procedure performed on a healthy periodontium that requires exposure of adequate tooth structure for restorative purposes (Figure 3). Several techniques are available, depending upon the proposed location of the restorative margin, the location of the alveolar crest and gingival margin, the width of the keratinized attached tissue, and the amount of exposed tooth structure available.

Indications

Indications for surgical crown lengthening are periodontal, restorative, and esthetic. Periodontal considerations include cases of "delayed passive eruption" and where intracrevicular placement of the restorative margin encroaches on the gingival attachment and may lead to inflammatory periodontal disease. Restorative considerations include lack of retention due to short clinical crowns; treatment of overerupted teeth to correct the occlusal plane; presence of subgingival caries; and presence of a subgingival crown or tooth fracture, root perforation, or subgingival root resorption (Figure 4). Esthetic considerations include changing a "gummy smile," and marked discrepancies in the height of the gingiva around teeth in the esthetic zone.

Esthetic needs may also demand orthodontic eruption prior to surgical crown lengthening to maintain existing gingival contours. This is most common in the anterior maxilla with a high smile line (Figure 5).
The traditional forced-eruption technique causes the gingival tissues to erupt with the tooth (Figure 6). A minor periodontal surgical procedure is then necessary to return the gingival margin to its proper location. There are several advantages that forced eruption prior to surgical crown lengthening provide as opposed to surgery alone that may be significant in varied clinical situations. In the maxillary anterior zone, forced eruption places the gingiva and underlying bone around the erupted tooth at a more coronal position. When surgical crown lengthening is completed, the surgeon is able to place the gingival tissues and osseous crest at a level that will be more conducive to a cosmetic result.

Another significant advantage to orthodontically erupting a tooth prior to surgical crown lengthening is that postsurgically, the crown-to-root ratio remains virtually the same or is improved compared to that obtained with surgery alone. In addition, supporting bone from the adjacent teeth does not have to be sacrificed to obtain sufficient clinical crown length for the tooth requiring treatment.

Figure 4a Buccal view of mandibular left shwoing short clinical crowns. Figure 4b Lingual view of mandibular left showing short clinical crowns. Figure 4c Buccal view of mandibular left showing gain in clinical crown length after osseous resective surgery. Figure 4d Surgical crown lengthening to gain clinical crown length allows the restorative dentist to gain retention without placing restorative margins subgingival. Note the tissue health obtained when restorative margins are placed at thte ginfival margin (final restoration by Dr. Blake Mueller). Figure 5a Patient presents with short clinical anterior teeth, resulting in a "gummy smile." Figure 5b Close-up view of patient's short clinical anterior teeth (not flat architecture of periodontium). Figure 5c Surgical crown lengthening was completed to lengthen anterior clinical crown length. Figure 5d Postsurgical results showing significantly improved anterior cosmetics. Figure 6a Chronic gingival inflammation around crown restoration No. 8. Figure 6bREmoval of the restoration No. 8 shows restorative margin placed deep into gingival sulcus. Figure 6c Orthodontic extrusion was completed to coronally position osseous and gigival tissues prior to crown lengthening surgery. Figure 6d Final restoration No. 8 just slightly below gingival margin shows improved gingival health (final restoration by Dr. Gary Solnit).

The disadvantage of forced eruption is that the diameter of the root decreases as the preparation moves apically. The final restoration will therefore exhibit a greater degree of taper from the gingival margin to the incisal edge. This will require greater attention to the gingival areas to avoid overcontoured margins. In addition, tooth preparation of the smaller root segment will require modification if one is to achieve a healthy blending of restorative materials, gingival health, and esthetics. Teeth with moderate to severe tapering of the root form are contraindicated for forced eruption.

Rapid orthodontic forced eruption14 (two to six weeks) can be followed by surgical crown lengthening as long as the tooth is stabilized. It is not always necessary to wait for bone maturation of the attachment that follows with the eruption.

Another indication for forced eruption prior to crown lengthening with osseous resection is teeth adjacent to implants or future implant sites. Random alveolar crest reduction of partially edentulous ridges to achieve a flat positive bone architecture adjacent to the treated tooth is contraindicated.

Contraindications

Contraindications for surgical crown lengthening include teeth that are nonrestorable, teeth or adjacent teeth that would be compromised either functionally or esthetically, and teeth whose value is not compatible with the procedures necessary to save it. The advantages of retaining a tooth in terms of its significance to the overall treatment plan must be weighed against the extent of the procedures needed to properly restore the tooth. This is especially important today with the accessibility to highly predictable dental implants. Other factors to evaluate when considering crown lengthening procedures are the crown-to-root ratio after the tooth is restored and the ability of the patient to maintain the periodontium in a state of health after the restorative procedures have been completed.

Restorative Requirements

It is extremely common to find short clinical crowns, which pose a problem to the restorative dentist during crown and bridge procedures. Schwartz and colleagues23 found that loss of retention was the second most frequently encountered complication following caries development in fixed partial dentures. When undiagnosed for some time, loss of retention can lead to serious problems. In an attempt to reduce the risk of loss of retention, the abutments should be prepared following a carefully chosen design and not prepared further subgingival. A review of the ideal tooth preparation requirements follows:²⁴

* Adequate length for proper retention and resistance. The ideal preparation must have at least 4 mm of axial wall height with a minimum of 2 to 3 mm of sound tooth structure circumferentially and a maximum convergence angle of 6 to 10 degrees.

* Sufficient axial reduction for adequate esthetic rendition. Metal-ceramic and all-ceramic crowns require 1.5 to 2 mm of tooth reduction to allow proper thickness of the ceramic veneer.

* Sufficient occlusal reduction for occlusal function and anterior guidance. Metal ceramic crowns require 2 mm of occlusal reduction, less if full cast gold restorations are utilized.
In the anterior maxilla, Kois²⁵ has advocated having at least 2 mm of solid tooth structure on the buccal and lingual surfaces of the tooth preparation. Kois recommends finishing the interproximal margin parallel to the cementoenamel junction without being concerned with the 2 mm rule of solid tooth structure, as long as the margin is on sound tooth structure. The scalloped tissue form follows the cementoenamel junction and underlying osseous crest, which is more coronally positioned in the interproximal zones. During crown preparation, it is easy to continue the interproximal crown margin at the same level as the buccal or lingual margin and violate the biologic width interproximally. This is most common when preparing anterior teeth, as crown margins are placed subgingivally for cosmetic purposes, and the underlying osseous scallop changes so dramatically from the straight to the interproximal surfaces.

Kois recommends avoiding the use of flat-end diamond burs when extending beyond the line angles. The tendency is to follow the flat shoulder margin from the straight buccal surface past the line angle into the interproximal. Adjustment in the depth of the interproximal margin using a round-end tapered diamond bur is recommended. This will prevent violation of the biologic width in the interproximal area, which is the most susceptible area.

Treatment Plan and Sequence

Prior to crown lengthening procedures, a combined periodontal and restorative treatment plan is essential. The sequence of therapy is very important to achieve desired clinical results. Diagnostic procedures include periodontal probing depths and radiographs to determine root form, root proximity, and bone levels. An esthetic examination includes evaluation of the smile line (position of the upper lip relative to incisal edge position and gingival facial levels, i.e., the amount of gingival exposure during speech as well as smiling).¹³ The tissue type and the amount of keratinized tissue along with the patient's esthetic concerns, desires, and expectations are extremely important. The prognosis of the tooth or teeth to be treated along with the adjacent teeth is required. Compromising the adjacent dentition to save a tooth with a poor prognosis is a contraindication to treatment. Alternative options such as implants or fixed partial dentures should be considered. Mounted casts with a diagnostic wax-up of the future restorative plan are always indicated. Once the ideal future crown contours have been established, the restorative and periodontal team can work in reverse to achieve the desired clinical results.

Initial therapy includes scaling and root planing and providing the patient with oral hygiene instructions. A provisional restoration should be placed prior to the surgical procedure. When inadequate restorations exists, they should be removed and proper provisional restorations fabricated. This will help reduce the inflammatory component of the dentogingival complex and permit re-evaluation of tissue response before deciding what type of surgical correction is necessary. If endodontic treatment is indicated, root canal therapy should be completed prior to the surgery. This will prevent later surprises, especially if extensive decay exists under old restorations. It is always better and easier to make a judgment regarding prognosis when the restoration and decay have been removed. Removal of the provisional restorations at the time of surgery facilitates access to the interproximal areas and allows the periodontist to make the important decision about how much ostectomy (removal of supporting bone) is enough. It is important to remember that this is not a one-tooth procedure. Creating a level osseous contour that allows the soft tissue to follow is an important concept for long-term periodontal stability and maintenance.

If the margin is close to the osseous crest (as is often the case with fractured teeth or extensive decay) resulting in excessive ostectomy that will compromise the adjacent teeth, orthodontic forced eruption of single rooted teeth can be considered.27 Decay extending close to furcations on molars can be evaluated for possible root amputation procedures. The restorative requirements postsurgery in the remaining furcations is the same as described. i.e., 3 mm margin to osseous crest. If this cannot be achieved, the nonrestorable tooth should be extracted.

Combined implant and conventional restorative treatment in the anterior zone should always be carefully analyzed. If crown lengthening is required on the teeth adjacent to implant sites, this procedure should precede the implant placement. Once healing has occurred with a stable desired gingival architecture, the implants can be placed with the hex platform at the desired vertical height relative to the teeth. It is very easy to misjudge this if the amount of osseous reduction has not already been determined and completed. Once the implant is placed, it is impossible to correct. If bone level changes are not anticipated, then implant placement can precede soft tissue alterations. This can usually be performed simultaneously with the second stage implant procedure.

Healing

In non-esthetic (posterior) areas, the patient should be re-evaluated six weeks postsurgery prior to continuing with final restorative procedures. Margins should be kept at the gingival margin. In the anterior esthetic zone, a longer healing period is recommended. Wise recommends waiting 21 weeks for soft tissue gingival margin stability.²⁸ Kois has suggested waiting longer.²⁵

During final tooth preparation, margins should be placed supragingival or, if cosmetic concerns direct, at the gingival margin. If cosmetics dictate subgingival margin placement, the dentist preparing the teeth should not only be aware of the cementoenamel junction and soft tissue form, but should also again sound to bone. Studies in the periodontal literature indicate that the postsurgical dimension of biologic width will approximate the amount present prior to surgery.^29,30

It is as important postsurgically as it is prior to surgery to keep in mind that the location of a restorative margin relative to the crest of the alveolar bone is more critical for preserving gingival health that its distance below the free gingival margin. (Kois recommends keeping the margin 3 mm from the osseous crest.⁵) Prior to final margin placement, the restorative dentist should identify the level of the soft tissue in relation to the osseous crest before the retraction cord is placed. If this relationship is not properly identified, as the biologic width redevelops, the preparation margin can easily end up being too far subgingival. As described earlier, this relationship can set the stage for progressive periodontal breakdown.

Use of a rotating instrument beneath the gingival margin traumatizes the gingival, sulcular epithelium, and possibly even the gingival attachment. The trauma caused by a rotating instrument may be reversible.³¹ However, in some cases, permanent loss of periodontal attachment may result.¹⁰

Tissue retraction is a traumatic procedure requiring gentle manipulation of the soft tissues. Loe10 showed that with normal pressure, part of the retraction cord could impinge into the biological width. Excessive tearing of the dentogingival complex and inappropriate use of chemicals (buffered 14 percent aluminum chloride, Hemodent solution) used to control gingival crevicular fluid and bleeding will induce recession, exposing crown margins. It is safe to leave the impregnated cord up to 15 minutes. Depending on the thickness of the gingival tissue and the sulcus depth encountered, Chiche32 recommends a single string or selective double-string technique. It is important to distinguish between thick and thin gingival tissue because the tissue behaves differently when it is surgically manipulated. It is easier to maintain papilla height postsurgically with thick tissue. Thin tissue tends to shrink more. In respect to crown margin placement, it is easy to hide a margin subgingivally in a thick periodontium as compared to thin tissue. Depending on the thickness of the tissue, a certain amount of rebound of the soft tissue is expected. It can take one to three years for the final mature tissue architecture to reform. It is not practical to keep patients in provisional restorations for so long. The important aspect of this concept is to place a final restoration that does not impinge the gingival embrasure and allows space for the interproximal gingival tissue to rebound. The final restoration should be fabricated to allow for final maturation of the gingiva and rebound of the interproximal gingival tissue. As long as the apical end of the final crown restoration is 5 mm or less from the interproximal bone, Tarnow33 found that a papilla was always maintained or reformed postsurgically.

Conclusion

In the past, esthetics usually demanded subgingival margin placement. Current porcelain materials should allow more restorative margins to be finished at the level of the soft tissue. If the ideal relationship between bone and margin is respected, a healthy periodontium will follow.

Authors

David F. Levine, DDS, is an assistant clinical professor at the University of Southern California School of Dentistry. He also maintains a private practice in Toluca Lake, Calif.
Mark Handelsman, DDS, is an assistant clinical professor at the USC School of Dentistry. He maintains a private practice in Beverly Hills.
Nicolas A. Ravon, DDS, is a clinical instructor for the ISP Program at USC School of Dentistry. He maintains a private practice in Garden Grove, Calif.

References

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To request a printed copy of this article, please contact: David F. Levine, DDS, 3808 W. Riverside Drive, #3051, Burbank, CA 91505-4325.