Taking the guesswork out of mandibular symphyseal distraction osteogenesis

ORIGINAL ARTICLE

Taking the guesswork out of mandibular symphyseal distraction osteogenesis Stanley Braun, DDS, MME,a William P. Hnat, PhD,b Timothy W. Hnat,c and Harry L. Legan, DDSd St. Louis, Mo, and Louisville, Ky Mandibular symphyseal distraction osteogenesis has recently been introduced as a means of resolving arch length deficiencies in the anterior segment and as a method of reducing large vestibular spaces related to a narrow mandible. Accurately relating the required distraction for a given anterior tooth mass and desired future anteroposterior location of the central incisors has not been possible until recently. The relationship between these 3 controlling factors has been mathematically described by the hyperbolic cosine function and a computer program designed for easy use by the clinician. Two clinical cases illustrate the application of the program. A Web site where the program can be downloaded at no cost is mentioned. (Am J Orthod Dentofacial Orthop 2001;119:121-6)

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andibular symphyseal distraction osteogenesis followed by movement of the incisors into the distracted area has recently been introduced as a means of resolving arch length deficiencies in the anterior teeth, as well as reducing excessive bilateral vestibular space associated with an unusually narrow mandible.1-6 Questions arise about the amount of distraction that should be obtained to reconcile a specific anterior arch length deficiency and, most important, about the effect on the anteroposterior position of the incisors, with related implications regarding facial esthetics and dental stability. Additionally, from a clinical viewpoint, where and how should the distraction be measured? Without an accurate predetermination of targeted outcomes, the result becomes somewhat of a guessing game. Fortunately, the arc of the 6 mandibular anterior teeth has been previously described by the hyperbolic cosine function with a correlation coefficient (r) of 0.951.7 This mathematical description permits a precise relationship to be expressed between the crossarch width (a function of distraction), the tooth mass of the 6 anterior teeth, and their arc depth (anteroposterior position of either the left or the right central incisor) (Fig 1). This has been accomplished in an iterative computer program that can be used in treatment planning to obtain predictable outcomes of mandibular

aClinical Professor of Orthodontics, Vanderbilt University Medical Center, and St Louis University. bAssociate Professor of Mechanical Engineering, University of Louisville. cProgramming Consultant. dProfessor and Chairman, Orthodontics, Vanderbilt University Medical Center. Reprint requests to: Stanley Braun, 7940 Dean Rd, Indianapolis, IN 46240. Submitted, January 2000; revised and accepted, May 2000. Copyright © 2001 by the American Association of Orthodontists. 0889-5406/2001/$35.00 + 0 8/1/110986 doi:10.1067/mod.2001.110986

symphyseal distraction. Two clinical examples are presented to illustrate the use of the computer program. The computer screen, ready for data, is seen in Fig 2. PATIENT 1

A pretreatment mandibular cast is seen in Fig 3. The pretreatment dimensions are shown, as well as the desired posttreatment anteroposterior position of the left central incisor. The sum of the mesiodistal widths of the 6 anterior teeth, the cross-arch canine width measured at the normal distal contacts, the existing anterior segment depth, and the desired change in the anterior segment depth (a positive [+] measurement if the incisor is to be advanced anteroposteriorly, and a negative [–] measurement if the incisor is to be retracted anteroposteriorly), carefully measured from the models, are all input to the nearest 0.1 mm. When the “calculate” button is activated, the required change in cross-arch canine width, measured at the normal distal contacts, appears (+8.2 mm) (Fig 4). The distraction mechanism can now be appropriately activated until the cross-arch dimension of 36.8 mm is obtained (28.6 + 8.2 = 36.8). This dimension will accommodate the anterior tooth mass and the predicted anteroposterior position of the left central incisor. PATIENT 2

This patient’s pretreatment mandibular cast is seen in Fig 5. The right second premolar is unerupted and trapped by the first premolar along with the skewed anterior segment toward the right. A significant anterior segment asymmetry exists. The treatment objective is to obtain sufficient arch length with symphyseal distraction osteogenesis to correct the asymmetry and then to recover the second premolar. The second premolar 121

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Fig 1. Variables defining arc of 6 mandibular anterior teeth.

Fig 2. Computer screen ready for data input.

requires 8.4 mm of space, as determined from a radiograph. The pretreatment measurement of the cross-arch width at the normal distal contacts of the canines (23.8 mm) is made symmetrically, eliminating most or all of the asymmetry. This is necessary to maintain the accu-

racy of the hyperbolic cosine function on which the computer program is based. The pretreatment anterior segment depth of 11.8 mm will be maintained. The anterior segment depth is defined by the future anteroposterior position of the left central incisor. It will be

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Fig 3. Patient 1 pretreatment mandibular cast and its dimensions to be entered into computer program.

Fig 4. Patient 1 dimensions entered into computer program.

maintained to preserve facial esthetics and to maintain assumed stability. The sum of the mesiodistal dimensions of the 6 anterior teeth, measured from the models, is 40.3 mm. When these dimensions are entered

into the computer program (Fig 6), a cross-arch width increase of 4.8 mm is determined to be necessary. This yields a final distracted measurement of 28.6 mm (23.8 + 4.8 = 28.6) at the distal contacts of the canines. The

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Fig 5. Patient 2 mandibular cast and its dimensions to be entered into the computer program.

future position of the right canine is determined in order to make this measurement. Fig 7 shows the patient’s mandibular cast after distraction, with mechanics underway to obtain the 8.4 mm required for bringing the unerupted second premolar into the arch. Note that the anterior segment depth is currently 11.5 mm; it will become 11.8 mm when the full space needed for the unerupted second premolar is obtained. CONCLUSION AND DISCUSSION

Two examples have been presented for which a computer program was used to quantitate the mandibular symphyseal distraction necessary to accommodate a given tooth mass (sum of mesiodistal widths of the 6 anterior teeth) and a desired future anteroposterior position of either the right or the left central incisor. If, in a given

patient, distraction is undertaken to reduce large vestibular spaces, the effect on the anteroposterior location of the central incisors can be accurately forecast for a given anterior segment tooth mass. The ability to accurately describe the effects of symphyseal distraction osteogenesis before treatment has not been possible until now. The computer program is based on the hyperbolic cosine function that describes the arc of the 6 mandibular anterior teeth with a correlation coefficient (r) of 0.951. 7 This program may be downloaded at no cost from the following Web site: http://athena.louisville.edu/~wphnat01/index.htm. We wish to thank Dr Enrique Hernandez for his invaluable assistance in the preparation of the models and illustrations.

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Fig 6. Patient 2 computer screen illustrating the cross-arch required distraction measured at normal distal contacts of the canines (4.8 mm).

Fig 7. Patient 2 mandibular cast following symphyseal distraction osteogenesis. Additional orthodontic activity is underway to create space for unerupted right second premolar.

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REFERENCES 1. Guerrero CA. Mandibular expansion. J Venez Orthod 1990;48:1-2. 2. Guerrero CA, Contasti G. Transverse (horizontal) mandibular deficiency. In: Bell WH, editor. Modern practice in orthodontic and reconstructive surgery. Vol 3. Philadelphia: WB Saunders; 1992. p. 2383-402. 3. Guerrero CA, Bell WH, Flores A, Modugno VL, Contasti G, Rodriguez AM. Distraction osteogenica mandibular intraoral. Odontol Dia 1995;11:116-32. 4. Weil TS, Van Sickels JE, Payne CJ. Distraction osteogenesis for

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correction of transverse mandibular deficiency: a preliminary report. J Oral Maxillofac Surg 1997;55:953-60. 5. Guerrero CA, Bell WH, Contasti G, Rodriguez AM. Mandibular widening by intraoral distraction osteogenesis. Br J Oral Maxillofac Surg 1997;35:383-92. 6. Samchukov ML, Cope JB, Harper RP, Ross JD. Biomechanical considerations of mandibular lengthening and widening by gradual distraction using a computer model. J Oral Maxillofac Surg 1998;56:51-9. 7. Braun S, Hnat WP. Dynamic relationships of the mandibular anterior segment. Am J Orthod Dentofacial Orthop 1997;111:518-24

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