Long-term outcome study of bilateral mandibular distraction: a comparison of Treacher Collins and Nager syndromes to other types of micrognathia.

Abstract

A long-term follow-up study of patients who underwent bilateral mandibular distraction is presented, and the results of patients with Treacher Collins syndrome and Nager syndrome are compared with results for other forms of congenital micrognathia. It was hypothesized that the factors responsible for the predetermined, syndrome-specific shape of the mandible in patients with Treacher Collins and Nager syndromes would alter the long-term results of linear (uniplanar) distraction of the mandible. Thus, over time, the mandibles would remodel to preoperative form while maintaining the increase in volume. To investigate this hypothesis, all patients treated with bilateral mandibular distraction who had at least 1.5 years of follow-up, including satisfactory cephalometric examinations, were retrospectively reviewed. Two groups were identified. Group 1 (n = 6) were Treacher Collins and Nager syndrome patients (ages, 2 to 13 years; mean, 5.2 years) and group 2 (n = 6) included other forms of bilateral, congenital micrognathia (ages, 1.5 to 19 years; mean, 8.4 years). Serial cephalometric measurements were recorded before distraction, after distraction, and at least 18 months after distraction. Mandibular mean linear distraction distance (as recorded on the device) averaged 24.5 mm in group 1 and 26.2 mm in group 2. In group 1, the antegonial angle (angle from the mandibular plane to the top of the antegonial notch) decreased after distraction by 3.8 degrees, and the antegonial notch height was reduced by 1.6 mm. The posttreatment morphologic change was modified significantly over time, with a 3.7-degree increase of the antegonial angle and a 1.2-mm deepening of the antegonial notch. In group 2, the immediate reduction in height of the antegonial notching was subtler; however, long-term recurrence of the antegonial notching was also observed. At the end of distraction, the mean group 1 gonial angle became 8 degrees more obtuse. In contrast, patients in group 2 developed a more acute angle (mean, 8 degrees). The mandibles of the Treacher Collins syndrome patients (group 1) maintained their more obtuse postdistraction gonial angle during the period of follow-up, whereas over time this change was reversed in group 2 patients. In conclusion, experience with bilateral mandibular distraction has demonstrated that long-term determination of mandibular form is more complex than either the amount of distraction or the direction of the distraction vector. The underlying genotype and the musculoskeletal milieu must be taken into account when planning distraction, as these factors tend to remodel the mandible into its preoperative shape over time, despite the fact that the increased mandibular volume and projection are maintained.