Treacher Collins syndrome and hemifacial microsomia are phenotypically related conditions involving the mandible. Treacher Collins syndrome exhibits a clockwise rotation, steep mandibular plane, and retrognathia.1 Hemifacial microsomia demonstrates a short ramus, deviation, and retrognathia.2
The severity of hemifacial microsomia is graded emphasizing the ramus-condyle unit.3 The Treacher Collins syndrome mandible has not been classified, but phenotypic stratification exists. Three-dimensional analysis has been performed to characterize the mandibular phenotype in these conditions.4,5
The purpose here is to compare mandibular volumes in Treacher Collins syndrome and hemifacial microsomia. We hypothesize that there is a greater proximal mandibular hemifacial microsomia deficiency compared with a global Treacher Collins syndrome deficit. We expect a volumetric similarity between Treacher Collins syndrome and hemifacial microsomia type III mandibles.
Mixed dentition children with Treacher Collins syndrome, hemifacial microsomia, and controls were analyzed using three-dimensional computed tomography. The mandible was segmented digitally at the central incisor–midsymphyseal and antegonial notch–ramal intersections (Materialise) (Fig. 1).5 Volumetric calculations were obtained in cubic millimeters, and statistics involved analysis of variance and Tukey's honestly significant difference test.
Computed tomographic scans were obtained for 10 Treacher Collins syndrome patients (20 sides) (four girls and six boys), 24 hemifacial microsomia patients (25 sides) (15 boys and nine girls), and 14 controls (28 sides) (seven boys and seven girls). Mean ages were 66.5, 70, and 68.8 months, respectively (Tables 1 through 3).
Treacher Collins syndrome mandibles were smaller than hemifacial microsomia hemimandibles. Treacher Collins syndrome proximal segments were significantly smaller than hemifacial microsomia segments. There was no difference in Treacher Collins syndrome and hemifacial microsomia dentate segments. Subdividing hemifacial microsomia, Treacher Collins syndrome hemimandibles and proximal segments were smaller than those of type I/IIa hemifacial microsomia, but not versus type IIb/III hemifacial microsomia. The Treacher Collins syndrome dentate segment was significantly less than that of hemifacial microsomia I/IIa but similar to that of hemifacial microsomia IIb/III.
A hypoplastic, malformed mandible characterizes both Treacher Collins syndrome and hemifacial microsomia1–5 but with noteworthy phenotypic differences. Treacher Collins syndrome is typically bilaterally symmetric with a clockwise rotation, steep mandibular and occlusal plane, and symmetric chin point deficiency.1,5 Hemifacial microsomia is usually asymmetric, with a more “affected side” and characteristic chin-point deviation, occlusal cant, and sagittal deficiency.2 Both Treacher Collins syndrome and hemifacial microsomia exhibit diminished mandibular structure compared with controls4,5; however, the global morphology differs and these conditions have never been directly compared.
We objectively compared Treacher Collins syndrome and hemifacial microsomia mandibular segmental volumes. The Treacher Collins syndrome mandible was smaller than hemifacial microsomia as a group, trending toward significance. This may be secondary to the greater number of type I/IIa versus IIb/III hemifacial microsomia hemimandibles (14 and 11, respectively). Interestingly, though hemifacial microsomia is characterized by ramus-condyle unit deficiency, the Treacher Collins syndrome proximal segment was statistically decreased versus hemifacial microsomia. Also, the dentate segments were not statistically different between Treacher Collins syndrome and hemifacial microsomia, despite the expectation that the Treacher Collins syndrome body would be smaller than hemifacial microsomia, because of perceived global Treacher Collins syndrome microretrognathia.
Comparing individual hemifacial microsomia subtypes, stratified by Pruzansky score, to Treacher Collins syndrome gleaned additional information. Not surprisingly, the Treacher Collins syndrome hemimandibles and proximal segments were smaller compared with those of type I/IIa hemifacial microsomia. Interestingly, the dentate segments were statistically equivalent but trended toward significant diminution in Treacher Collins syndrome. There were no statistical differences comparing the Treacher Collins syndrome hemimandible and subsegments to those of type IIb/III hemifacial microsomia. Numerically, the Treacher Collins syndrome hemimandible, proximal segment, and dentate component fell between the type IIb and III hemifacial microsomia mandibular components.
The Treacher Collins syndrome mandible is volumetrically diminished compared with hemifacial microsomia as a whole, with the greatest affect being proximal, contrary to our hypothesis. The dentate segment is equivalent between the two groups. With increasing hemifacial microsomia Pruzansky score, the volumetric difference is obviated compared with Treacher Collins syndrome. The Treacher Collins syndrome mandible exhibits a volume in between a type IIb and a type III hemifacial microsomia mandible.
Robert Travieso, B.A.
Jordan Terner, B.A.
Plastic Surgery, Yale University School of Medicine
Christopher Chang, M.D.
Edward Teng, M.D.
Plastic Surgery, Yale University, New Haven, Conn.
Alexander Gougoutas, M.D.
Scott P. Bartlett, M.D.
The Children's Hospital of Philadelphia, Philadelphia, Pa.
Derek M. Steinbacher, M.D., D.M.D.
Plastic and Craniomaxillofacial Surgery, Yale University, New Haven, Conn.
The authors have no financial interest to declare in relation to the content of this article.
1. Steinbacher DM, Bartlett SP. Relation of the mandibular body and ramus in Treacher Collins syndrome. J Craniofac Surg. 2011;22:302–305.
2. Kaban LB, Mulliken JB, Murray JE. Three-dimensional approach to analysis and treatment of hemifacial microsomia. Cleft Palate J. 1981;18:90–99.
3. Pruzansky S. Not all dwarfed mandibles are alike. Birth Defects Orig Artic Ser. 1969;1:120–129.
4. Steinbacher DM, Gougoutas A, Bartlett SP. An analysis of mandibular volume in hemifacial microsomia. Plast Reconstr Surg. 2011;127:2407–2412.
5. Terner JS, Travieso R, Chang C, Bartlett SP, Steinbacher DM. An analysis of mandibular volume in Treacher Collins syndrome. Plast Reconstr Surg. 2011;129:751e–753e.
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