A Morphological Classification Scheme for the Mandibular Hypoplasia in Treacher Collins Syndrome

Background: Mandibular hypoplasia is a hallmark of Treacher Collins syndrome (TCS), and its severity accounts for significant functional morbidity. The purpose of this study is to develop a mandibular classification scheme. Methods: A classification scheme was designed based on three-dimensional computed tomography (3D-CT) scans to assess 3 characteristic features: degree of condylar hypoplasia, mandibular plane angle (condylion-gonion-menton), and degree of retrognathia (sella-nasion-B point angle). Each category was graded from I to IV and a composite mandible classification was determined by the median value among the 3 component grades. Results: Twenty patients with TCS, aged 1 month to 20 years, with at least one 3D-CT prior to mandibular surgery were studied. Overall, 33 3D-CTs were evaluated and ordered from least to most severe phenotype with 10 (30%) Grade 1 (least severe), 14 (42%) Grade 2, 7 (21%) Grade 3, and 2 (7%) Grade 4 (most severe). Seven patients had at least 2 longitudinal scans encompassing an average 5.7 (range 5-11) years of growth. Despite increasing age, mandibular classification (both components and composite) remained stable in those patients over time (P = 0.2182). Conclusion: The authors present a classification scheme for the TCS mandible based on degree of condylar hypoplasia, mandibular plane angle (Co-Go-Me angle), and retrognathia (SNB angle). While there is a natural progression of the mandibular morphology with age, patients followed longitudinally demonstrate consistency in their classification. Further work is needed to determine the classification scheme's validity, generalizability, and overall utility.