Crown lengthening is any procedure by which the amount of tooth exposed supragingivally is increased. This may be achieved orthodontically or surgically. This paper will discuss some of these indications as well as introducing some of the methods by which crown lengthening may be achieved.
To understand why crown lengthening may be desirable, a review of periodontal anatomy is in order. Unfortunately, space does not permit this here, so the reader is directed to standard veterinary and human dental texts for information on the subject. Review anatomy and the relationships between the various dental and paradental tissues.
One concept that bears special mention is Biological Width. Each of the zones within the cervical area of the tooth has a purpose and the body strives to maintain them. Biological width describes this relationship. In periodontally healthy humans, the gingival sulcus is approximately 1 millimeter deep, the junctional epithelial collar is about 1 millimeter wide, and so is the zone of connective tissue attachment. Therefore, the distance from the free gingival margin to the alveolar crest is approximately 3 millimeters. If the free gingiva is amputated, the alveolar crest, connective tissue attachment, and junctional epithelium will all migrate 1 millimeter down the root to re-create a gingival sulcus and re-establish the relationship prescribed by biological width. In periodontally healthy dogs, the zones between the free gingival margin and alveolar crest may be up to 3 millimeters each, depending on the size of the dog and which tooth is being considered.
Biological width is a natural defense of the periodontium, which will strive to maintain a certain distance between any restoration and the alveolar crestal bone. In the dog, there should be at least 1 millimeter of connective tissue attachment to root cementum coronal to the alveolar crest. Coronal to this should be 1 to 2 millimeters of junctional epithelium. Coronal to this should be a gingival sulcus of 1 to 2 millimeters depth. Finally the gingival cavomargin should be 1 millimeter coronal to the free gingival margin. Therefore, the gingival cavomargin should be 4 to 6 millimeters coronal to the alveolar crest. If it is closer than this, inflammation will ensue to cause resorption of bone and apical migration of the gingival attachments until the biological width is re-established. Though it is ideal to have restorations totally supragingival, it is possible to place them subgingival if no alternative exists.
INDICATIONS FOR CROWN LENGTHENING
If a tooth remains only partially erupted, there will be more of the crown subgingivally than normal. As neither the gingiva nor periodontal ligament attach to the subgingival enamel this creates a deep periodontal defect. As this defect becomes infected with plaque bacteria, a pericoronitis ensues. One option is to extract the under-erupted tooth. However, for strategically significant teeth, a crown lengthening procedure might resolve the periodontal concerns while preserving the tooth.
Dental Fractures That Extend Subgingivally
Dental fractures are a common finding in both dogs and cats. In many cases, the damage is restricted to the clinical crown supragingivally. In these cases, restoration of the defect has little impact on the periodontal status of the tooth. However, many dental fractures involve damage that extends subgingivally and may go beyond the cementoenamel junction.
For all practical purposes, gingiva and periodontal ligament only attach to cementum, they do not attach to dentin or restorative materials. Any fracture that extends onto the root will involve loss of the cemental covering and exposure of the underlying dentin. If the defect is not restored, the periodontal ligament and/or gingival tissues in contact with the exposed dentin may just lie passively against the dentin or may attack it with odontoclasts in an attempt to reabsorb the tooth. If the tooth is restored, the gingiva and ligament will still not attach, so a periodontal defect persists and there may still be external resorption at the tooth-restoration interface if the restoration has not been finished properly.
To improve the periodontal prognosis for a tooth with a crown-root fracture, a crown lengthening procedure may be employed to place the defect and its restoration completely supragingival.
To Increase Retention Of A Prosthetic Crown
There is a variety of situations in which it is desirable to place a cast metal crown on a damaged tooth. In order for this prosthesis to stay in place, there must be adequate surface area for retention. For a dog canine tooth, there should be a minimum of 4 millimeters of natural crown height covered by the prosthetic crown. Sometimes the fracture of the tooth is such that there is less than this amount of clinical crown remaining. In those cases, a crown lengthening procedure can be used to increase the amount of tooth available for retention of the prosthesis.
There are three types of crown lengthening procedures. Depending on the tooth involved, there is a variety of ways of achieving the desired results.
Type-2: Apically repositioned flaps.
Type-3: Forced eruption with some type of orthodontic device (not discussed here).
Type 1: Crown Lengthening
For teeth with abundant gingiva, such as the maxillary canine teeth in a large dog, 2 to 3 millimeters of clinical crown can be gained simply by performing a circumferential gingivoplasty. A periodontal probe is used to measure the depth of the gingival sulcus and the outer gingival epithelium is pricked with a needle to create a line of bleeding points, 1 to 2 millimeters coronal to the sulcus floor. A beveled incision is then made with a No. 11 scalpel blade, connecting the dots around the tooth and the severed gingiva is removed. By keeping the incision coronal to the floor of the gingival sulcus, there is no encroachment on the biological width and so the periodontal status of the tooth is not compromised.
Type 1 crown lengthening need not involve the entire circumference of the tooth. If the defect to be exposed is isolated to one face of the tooth, then only the gingiva covering that area need be removed. Be certain to blend the incision with the surrounding gingiva to avoid sharp steps and angles in the free gingival margin.
Type 2: Crown Lengthening
The specific procedure depends on the tooth involved. Single rooted teeth with no furcations are the easiest to manage periodontally but multi-rooted teeth can also be treated in this manner. Various flap designs are possible, but all must:
Respect biological width.
Maintain an adequate crown/root length ratio (root at least 1.5 times longer than crown).
Allow placement of the flaps with no tension at the suture lines.
Allow for maintenance of periodontal health.
Maxillary Canine Tooth
Full thickness, mucoperiosteal envelop flaps are elevated buccally and palatally. The incision for the flaps starts on the buccal face of the lateral incisor and severs the gingival attachment to that tooth. It then runs distally through the diastema between the incisor and canine tooth, around the gingival sulcus on the buccal side of the canine, through the diastema between the canine and first premolar, around the gingival sulcus on the buccal side of the first premolar, and so on. The incision is extended as far distally as necessary to allow adequate reflection of the buccal flap. Palatal flap is elevated after severing gingival attachment of the involved teeth in the palatal side.
Alveolar bone is removed from the root surface using bone chisels or dental burs in a high-speed dental hand piece. Great care must be taken to preserve the exposed cementum so that the gingival flap can reattach to the tooth surface. The bone is contoured so that the new alveolar crest is at least 1 millimeter apical to the defect. The new alveolar crest should be shaped to a smooth and sharp edge that meets the tooth at a very acute angle, much like the original alveolar crest in a periodontally healthy situation. A dental curette works well for this.
Once sufficient bone has been removed to expose a band of cementum (for gingival reattachment) all the way around the tooth, the bone palatal, mesial and distal to the tooth is contoured so there are gradual slopes and transitions to the surrounding bone. Then debris is removed from the surgical field in preparation for repositioning of the flaps.
The buccal flap is laid down on the tooth with the free gingival margin placed about 4 millimeters coronal to the new alveolar crest and the palatal flap is laid down flat on the hard palate. It may be necessary to trim some redundant tissue from the flap margins to allow the flaps to lie flat without buckling. The flaps are sutured to each other interdentally with a fine (5-0) absorbable material such as Monocryl™.
The patient is fed softened food for two weeks and denied access to hard toys. Systemic antibiotics and antiseptic oral rinses are indicated at the discretion of the surgeon.
Mandibular Canine Tooth
Full thickness mucoperiosteal buccal and lingual flaps are developed as for the maxillary canine tooth. The gingiva between the mandibular canine tooth and the first mandibular premolar tooth is a thin band along the dorsal ridge of the interdental bone. Try to keep the incision within this narrow band of gingiva as gingiva holds a suture better than oral mucosa.
Bone reduction is done in a manner similar to that described above. One difference is the proximity of the mandibular lateral incisor to the mesial face of the mandibular canine tooth. In order to achieve proper osseous contouring for the canine, it is sometimes necessary to remove considerable alveolar bone from the lateral incisor tooth as well. Though the purpose of advanced periodontal procedures is to preserve teeth, it may be deemed appropriate to sacrifice the lateral incisor in favor of improving the prognosis for the more significant canine tooth. This will be a decision based on the periodontal status of the individual, the amount of bone support remaining for the incisor, and the perceived importance of that tooth for the patient (show dog versus family pet).
Once the alveolar crest has been shaped and smoothed and surrounding bone reduced to blend gradually with the new alveolar crest, the area is debrided as above. The buccal and lingual flaps are laid down on the tooth and sutured interdentally. Again, it may be necessary to trim the flap margins to remove redundant tissue.
The buccal flap has a tendency to ride up on the canine tooth and find its way back to its original height. This can be dealt with by placing a couple of tuck sutures. Using the same, fine absorbable material, place the needle through the gingiva near the mucogingival junction, run it subperiosteally to the ventral border of the mandible, and then out through the oral mucosa at the bottom of the labial vestibule. Then tie the knot, pulling it tight enough to draw the gingival ventrally until the free gingival margin is about 4 millimeters coronal to the new alveolar crest. After-care is as for the maxillary canine tooth.
Maxillary Fourth Premolar Tooth
These teeth, and all multi-rooted teeth, pose an extra challenge. In dogs, the furcation is close to the cemento-enamel junction. Therefore, a defect does not have to extend far subgingivally before there is serious furcation involvement. The teeth, the patient in whose mouth they are found, and the owner who presented the patient must all be carefully evaluated to decide if the tooth is treatable or should be extracted. You must evaluate the extent of the damage, the animal’s periodontal status, the owner’s commitment to oral hygiene, and your own technical skills.
Typically, the defect to be managed is a buccal slab fracture. Rarely is there any involvement of the palatal side of the tooth. Therefore, it is usually only necessary to elevate a buccal mucoperiosteal flap. Given the close approximation of the posterior teeth, an envelop flap may not be sufficient; one or more vertical releasing incisions may be needed. After elevating the flap, sufficient alveolar bone is removed to expose the entire defect as well as at least 1 millimeter of intact cementum. If the defect involves only one root, it may be possible to do this reduction without removing bone from the furcation.
Once the new alveolar crest has been created and the surgical field debrided, the flap can be sutured in place. In many cases, it is not possible to leave the entire defect subgingivally while still covering the furcation with the flap. In these cases, the defect should be restored prior to placement of the flap. This will result in the apical margin of the restoration being placed subgingivally, which is not desirable, but it is better than leaving the furcation exposed.
The flap is repositioned apically and sutured to the gingiva mesial and distal to the flap. It is important to that the free gingival margin be about 2 millimeters coronal to the furcation so that the gingiva can cover and protect this periodontally significant anatomy.
Since the flap can only be sutured mesially and distally and may not lie flat against the tooth along the entire buccal wall, some tissue adhesive may be used. After the flap is sutured, it is pushed against the alveolar bone and tooth to force any blood and fluid from surgical field. Then a drop of cyanoacrylate is placed on the free gingival margin of the flap and allowed to flow over onto the enamel of the crown, tacking the flap to the tooth. Care should be taken that the glue does not flow under the flap where it would act as a foreign body and delay the reattachment of the flap to the alveolar bone and exposed cementum.