A Range of Zygomatic Hypoplasia Exists in Treacher Collins Syndrome

INTRODUCTION:  Bilateral zygomatic hypoplasia characterizes Treacher Collins syndrome (TCS) and the at-risk malar subcomponents have been previously tabulated.¹  We sought to morphologically classify the pathology, volumetrically assess the TCS malar body versus norms, and analyze for symmetry.  

METHODS:  Demographic and computed tomography data were recorded.  The zygomata were digitally segmented using three-dimensional planning software (Materialise Surgicase CMF).  Each zygoma was classified into one of three groups, based on the amount and location of agenesis.  Volumes (mm³) were calculated and sidedness ratios compared using two-sided t-tests.

RESULTS:  56 sides were identified (24 TCS: 32 controls), both groups a mean age of 60.5 months and normally distributed.  The dysmorphology ranged from small but structurally continuous cheekbones, to nearly-complete aplasia (Figure 1).  Based on our classification, the degree of zygomatic deformity in our cohort was 8% type I (mild), 29% type II (moderate) and 63% type III (severe).  The mean TCS zygoma was significantly smaller than controls:  735 ± 777mm³ versus 3057 ± 1115mm³ (p<0.0001).  The sidedness ratio was 0.61 ± 0.31 for syndromic patients (Figure 2), compared to 0.97 ± 0.02 for controls (p<0.002).     

CONCLUSION:  As an aid for anatomical assessment and surgical planning, this is the first study to classify the spectrum of Treacher Collins malar deformity, quantitatively evaluate volume, and compute bilateral asymmetry.  We found that an increasing grade of deficiency has an inverse correlation with volume.  Structure is absent or significantly hypoplastic in a progressively sequential fashion, beginning with the 1) zygomatic arch, followed by the 2) zygomaticosphenoid process, and lastly, in the most severe circumstances, the 3) zygomaticofrontal and zygomaticomaxillary processes.  The malar bones, especially type III, are volumetrically deficient compared to controls.  The type III subtype was most frequent in our series, in line with the near-total zygomatic agenesis thought to represent the syndrome.  Though bilaterally affected, the amount of TCS zygomatic hypoplasia from side-to-side is surprisingly asymmetric.

REFERENCES:

1.  Marsh JL, Celin SE, Vannier MW, Gado M. The skeletal anatomy of mandibulofacial dysostosis (Treacher Collins syndrome). Plast Reconstr Surg. 1986;78:460-470.

Figure 1. TCS patient with type I (right) and type III (left) zygomata.

Figure 2. Zygomatic asymmetry and variability.

Presented at the American Cleft Palate-Craniofacial Association meeting on April 20, 2012.  Not previously published.