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Crown Lengthening

Have you experienced the frustration of wanting to save a tooth but having no clinical crown with which to work? A good example of this situation was seen recently. In the lower arch a three unit bridge replacing a second bicuspid had experienced recurrent caries at the gingival margin of the first bicuspid. Caries had destroyed the tooth structure at the apical base of the crown. An explorer would move from the mesial to the distal beneath the crown. To the practitioner it was obvious that, following endodontics, there would be inadequate tooth structure to prepare a finishing margin and still obtain a workable model for the bridge construction. And yet, the remaining root in bone was adequate to support a fixed bridge. All to frequently, attempts to save these teeth are rewarded with deep subgingival margins which cannot be impressioned well due to the gingival hemorrhage from periodontal inflammation developed during the time the tooth structure was carious. Retraction of the gingiva around these teeth is extremely difficult. Since there is little root length as a guide, the laboratory has difficulty creating adequate crown contours. Such a situation may be dealt with best through the use of crown lengthening procedures. This newsletter provides information about the indications, rationale and advantages of crown lengthening procedures.

Besides carious destruction, when you see cuspal fractures extending apical to the gingival margin, crown lengthening procedures may be indicated. Crown lengthening may be of benefit in the treatment of endodontic perforations near the alveolar crest. Crown lengthening can also increase a restoration’s retention on teeth exhibiting short crown lengths due to gingival hyperplasia, less than full eruption, or severe wear secondary to bruxism. Supraerupted teeth can often be reduced to conform to the occlusal plane to allow better prosthetic treatment. These teeth often will need lengthening of the tooth following the shortening process to improve interproximal embrasure spaces as well as regain retentive form for the crown.

What is the rationale and how does one determine the need for crown lengthening procedures? This question can be answered by considering two factors, the biology of the gingival tissues coronal to the alveolar crest and the objectives in margin placement.

Have you ever placed subgingival margins and been surprised to see the tissues develop additional depth at the base of the crown as well as show constant edema? These reactions could be related to the adaptation of the gingival tissues to the root surface. Beginning at the alveolar crest, the normal gingival unit of the periodontium will have connective tissue inserted into the root of the tooth an average of one millimeter above the bone crest. This is termed the cemental-fibrous interface and is present even in the diseased state. The distance will be recreated at the expense of alveolar bone if violated during tooth preparation. The recreation of this zone would reduce the adaptation of the gingival tissues to the tooth at a more coronal level and encourage pocket formation. In the normal state, the epithelial attachment begins at the coronal aspect of the cemental-fibrous interface and extends for an additional millimeter coronally. This is the area where immature epithelial cells are attached to the tooth through chemical bonding. Although it can be much longer, the epithelial attachment averages one millimeter in length. If violated, this zone produces a reaction similar to the cemental-fibrous interface. Loss of the integrity of the epithelial attachment by crown margin impingement virtually ensures pocket formation as plaque accumulation at the crown margin is impossible for the patient to remove. Coronal to the epithelial attachment is the sulcus. Sulcular depths are from one to three millimeters. For cleansibility and the avoidance of irreversible trauma to the epithelial attachment during retraction it has been previously recommended that preparations not be extended into the sulcus more than on half of its original depth in the healthy state. Using these distances the requirement for tooth structure above the alveolar crest for subgingival margin placement would be 1.0 mm for the cemetal-fibrous interface, 1.0 mm for the epithelial attachment, and 1.0 mm for sulcus penetration of the subgingival margin (one-half penetration of a two millimeter sulcus) plus 1.0 mm for the tooth structure to prepare the finishing margin. The total of four millimeters represents a good guideline for tooth length above the alveolar crest. In anterior teeth, many practitioners would prefer to have a three millimeter sulcus when doing subgingival margin preparation to allow for adequate subgingival extension as well as retraction.

An additional value of crown lengthening in the anterior teeth is the ability to control to some degree the thickness of the labial gingival tissues. If these tissues are thin, recession often results after tooth preparation, impressions and temporization. This tendency can be managed if adequate tissue thickness is encouraged during initial incision for the surgical procedure. In many cases prerestorative gingival bulk can be established through gingival grafting.

Without periodontal pocketing present or at least three millimeters of soft tissue coronal to the bone, tooth length can only be altered at the expense of the alveolar bone and requires osseous surgery. Gingival resection would only represent violation of the biologic width and uncleansibility of the prosthesis. Electrosurgical techniques have often been recommended for crown lengthening procedures. When the thickness of gingival tissues above bone exceeds three millimeters this may be a viable alternative in the posterior regions of the mouth. As reported repeatedly in the literature, extreme caution is necessary to avoid contact with bone or tooth root when using a rectified current as extensive bone necrosis and pulpal death have been reported. Sounding of gingival depth under anesthesia would be helpful to determine the need for osseous surgery.

Should the need for crown length occur on the facial or lingual surfaces, the need for bone removal can be confined to that single surface by accentuating the normal soft tissue contour. Without periodontal pockets interproximal inadequate tooth length always requires osseous reduction. If the reduction will be extensive, the adjoining teeth could experience support loss too severe to justify this approach. Fortunately, there is another approach for this situation. The tooth which requires additional crown length could be forcefully erupted orthodontically. This type of movement is rapidly achieved. Provided the soft tissues can be kept healthy surrounding the tooth root, the alveolar bone will move coronally as the tooth does. This initially produces a vertical bone defect on both the mesial and distal of the supra-erupted tooth. The osseous structure will slant from a coronal location on the erupted tooth to its original location on the adjoining teeth. Subsequent osseous periodontal surgery can be employed to establish the tooth length and normal bone contours at the same time. The support level on the adjoining teeth is preserved.

There are several areas where crown length can be achieved through surgical intervention to make restorative dentistry easier. A common area which comes to mind is the distal surfaces of maxillary and mandibular second molars. These teeth often have less than two to three millimeters of tooth structure between the marginal ridge and the gingival crest making retention form in the preparation very difficult to achieve. A second group off teeth which are candidates for crown lengthening includes those teeth where the subgingival margin of an existing restoration makes retraction for impressions difficult. Often the base of the restoration can be felt three millimeters below the crest of the gingiva. These areas could also be improved through partial reduction of the subgingival extension of the existing restoration by crown lengthening or gingival tissue reduction.

With any surgical procedure there are contraindications. Crown lengthening is no exception. Medical or psychological factors contraindicating routine periodontal surgery would also apply in crown lengthening. In addition, crown lengthening would be contraindicated in teeth already weakened by extensive periodontal involvement. Projected exposure of furcations would cause one to look for other alternatives. Unfortunately, extensive caries in the furcation region of molars cannot be effectively treated through crown lengthening. As in pocket reduction surgery, one must weigh the value of the individual tooth if its retention means extensive reduction of adjoining tooth support. While each case is an individual study, the wider the interproximal space, the more osseous reduction which can be tolerated.

Our office hopes the crown lengthening process can make your life a little easier and the patient’s prosthesis the healthiest possible. We would look forward to working with you to achieve your desired prosthetic result and appreciate the opportunity of sharing this information with you.
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