Date of Award

January 2016

Document Type


Degree Name

Medical Doctor (MD)



First Advisor

Derek M. Steinbacher


Malar hypoplasia is the hallmark of Treacher Collins syndrome (TCS), but an underemphasized feature of hemifacial microsomia (HFM). The purpose of this study is to morphologically classify and volumetrically characterize the zygomata in each dysostosis, commenting on associated masseter deficiency, mandibular underdevelopment, and left-right symmetry. We hypothesize that both the TCS and HFM zygomata will be smaller than controls and ipsilateral to hypoplastic masseters and mandibles, respectively.

In both investigations, demographic and computed tomography (CT) data were recorded at the outset. The CT scans were converted into three-dimensional (3-D) facial renderings, and the zygomatic morphology was grossly evaluated. For Treacher Collins facial bones, a classification was reported based on malar structure and presence/absence of normal zygomaticomaxillary complex articulations. For hemifacial microsomia patients, the Pruzansky grade of each mandible was determined. Then the zygoma and masseter muscles were digitally isolated using 3-D planning software (Materialise, Leuven, Belgium). Volumes and sidedness ratios were calculated and compared using two-sided t-tests, ANOVA and Tukey’s HSD.

In the TCS investigation, 58 sides were identified (24 TCS: 34 controls), mean age of 60.0 months and normally distributed. The phenotypic dysmorphology was graded as mild, moderate or severe (I, II and III, respectively). TCS malar and masseteric volumes were significantly smaller than controls (p < 0.0001 in both cases). The TCS zygomatic side-side symmetry ratio was 0.66 ± 0.28, compared to 0.97 ± 0.02 for controls (p= 0.002). The TCS masseteric side-side ratio was 0.74 ± 0.20, compared to 0.92 ± 0.09 for controls (p= 0.001).

In the HFM investigation, 72 total sides were evaluated (44 hemifacial: 28 control). For each group, mean age was 67.5 months and the malar volumetric data were normally distributed. Gender stratification showed 12M:10F and 9M:5F for HFM and controls, respectively. The affected zygomata were significantly smaller than the unaffected sides: 2790.05 versus 3396.64 mm3 (p= 0.0427). The type III subgroup showed the greatest diminution compared to both type I/II and controls: 1876.03 versus 3285.45 and 3255.31 mm3 (p= 0.0093 and 0.0047, respectively). There was no statistical distinction between the unaffected zygoma and controls: p= 0.6563. The sidedness ratio was less than 1 comparing affected:unaffected, except in a small subgroup.

In summary, whether known as a bilateral syndrome (TCS) or an asymmetrical disorder (HFM), left and right sides are not equally affected. The decrease in TCS zygomatic volume occurs in concert with masseteric hypoplasia, while HFM malar underdevelopment is most severe with type III mandibular deformity. Using a standardized hypoplasia classification permits pattern-recognition for uneven growth. However, significant volumetric differences portend differential reconstructive augmentation.


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