Summer 2000 Presentation Summary:

Although single lower in cisor extraction is not a commonly used orthodontic treatment approach, there are certain indications for this treatment option. Dr. Moore has had extensive experience with single lower incisor extraction treatment. In his presentation he provided the sub tleties of case selection and diagnosis, tips on case finishing, and potential compromises with this approach.

When a patient presents with a chief complaint of lower anterior crowding, Class I occlusion and well-balanced facial profile, the treatment options to consider are (1) extraction of premolars, (2) nonextraction with orthodontic expansion, or (3) extraction of a single lower incisor. If a nonextraction approach is not feasible due to excessive expansion and instability, extraction is necessitated. The decision to choose the single lower incisor extraction treatment option is based on facial balance, where extraction of premolars would have a negative effect, and when the amount of crowding in the mandibular arch is greater than that in the maxillary arch.

The following are advantages of a single lower incisor extraction over premolar extraction:

• Minimize the effect on profile
• Maintain posterior occlusion
• Minimize tooth movement
• Shorten treatment time
• Improve stability of lower incisor alignment

Which Incisor Should Be Extracted?

As a general rule, you want to remove the central incisor with the poorest prognosis, closest to the area of crowding. Extraction of a single lower incisor will negatively effect the maxillary: mandibular anterior tooth size ratio. Central incisors are typically .5mm narrower than lateral incisors and, therefore, he preferred choice for extraction. However, if any incisor is periodontally or restoratively compromised extraction of this tooth would be indicated.

Diagnostic Criteria and Indications for Single Lower Incisor Extraction

1. Class I posterior occlusion, well-balanced facial profile, mandibular anterior crowding greater than maxillary anterior crowding

2. Mandibular incisor crowding greater than 4.5 mm

• If there is less than 4.5mm of crowding, it can be resolved with stripping, reproximation. Mild expansion and extraction are not indicated
• A mandibular central incisor is approximately 5mm in width, making extraction appropriate.

3. Favorably shaped mandibular incisors

• Square-shaped mandibular incisors are preferred as they will better fill gingival embrasures following extraction. Triangular, or bell shaped lower incisors leave more dark triangular spaces following extraction and orthodontic alignment.

4. Favorably shaped maxillary incisors

• Triangular or bell shaped maxillary incisors are preferred as they are more favorable for interproximal stripping, which is almost always indicated to compensate for the tooth size discrepancy created with a single lower incisor extraction.
• Dr. Moore noted that the preferred tooth shape for the mandibular arch and maxillary arch are opposite, and often do not occur in the same patient, which can lead to compromises in case selection.

5 .Bolton discrepancy (mandibular 6 anterior > maxillary 6 anterior) greater than 2 mm

• In the Bolton tooth size analysis, the mandibular anterior width is 77% of the maxillary anterior width. Mandibular anterior excess is much more common than maxillary anterior excess and, in fact, is thought to occur 90% of the time. This situation favors single lower incisor extraction when the Bolton discrepancy is greater than 2mm.

Gingival Embrasures

In a recent study at the University of Washington, it was found that open gingival embras ures occur 38% of the time following orthodontic treatment. With single lower incisor extraction orthodontic treatment, open gingival embrasures are an even greater concern. In some patients these triangular spaces show with smiling, a tendency that increases with age.

Open gingival embrasures are related to gingival volume and interproximal spaces. Gingival volume is constant and can decrease with disease; it does not increase. Interproximal space, however, is influenced by the location of the contact point, crown shape, bone support, and root angulation. These factors can be manipulated favorably in orthodontic finishing to influence gingival embrasures. Shaping the contact point to move the contact point gingivally will decrease the gingival embrasure, a s will decreasing root divergence. Dr. Moore recommends careful evaluation of lower anterior root angulation.

Diagnostic Set-Up

Dr. Moore advises the diagnostician to section the upper and lower anterior teeth from the model and reset them in wax, removin g the proposed extraction tooth. Using this diagnostic set-up as a treatment goal will resolve any questions about inherent tooth size problems or excessive gingival embrasures. Some possible techniques to resolve such problems:

1. Labiolingual inclination of the maxillary and mandibular incisors

To reduce the tooth size discrepancy, the maxillary anterior teeth can be positioned upright to reduce the circumference and the mandibular anterior teeth flared to increase the circumference.

2. Reproximation/ Stripping

If overjet is excessive with the above setup, you can revise the set up to determine the exact amount of upper interproximal reduction required to normalize the overjet. Set up the teeth with ideal overjet/overbite, spacing the lower incisors as necessary. The mm of spacing produced equals the amount of upper stripping needed. Divide the stripping goal he by the 10 available tooth surfaces (mesial of maxillary right canine to mesial of maxillary left canine). This gives you the mm amount to remove from each surface. Stripping of the distal of canines is not recommended as it places canines in an increased Class III relationship without improving the anterior tooth size discrepancy.

Dr. Moore uses a series of single and double sided disks along with s trips for interproximal reduction. Isolation of the teeth is done with cotton rolls, and Stimudents are used to depress the gingiva during the procedure.

3. Retorations

Restoration of the lower anterior can be done to increase mesiodistal width, buccal li ngual thickness, and change tooth shape (reduce gingival embrasure space) for a more ideal occlusal and esthetic result. Maxillary restorations may also be indicated in certain cases.

Often there is a compromised functional relationship with a single lower incisor extraction treatment approach. Either the maxillary canines are more Class III than ideal and have excessive overjet, or the anterior overjet/overbite relationship is excessive. However, with the proper case selection and the finishing technique s Dr. Moore described, one can successfully treat lower incisor crowding with an acceptable esthetic and functional result, gaining space with the removal of one lower incisor.

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