Craniofacial morphology in twins discordant for cleft lip and/or palate.

The craniofacial structure in 44 pairs of like-sexed twins discordant for cleft of the lip and/or palate was investigated using lateral cephalometric radiographs. While only a slight rotation of the mandible was evident in the twins having cleft lip, more extensive rotation of the mandible was found in the twins discordant for unilateral cleft lip and palate. Larger, though generally non-significant, changes were recorded for the twins affected with bilateral cleft lip and palate. In the twins discordant for cleft palate only, statistically significant differences were found for maxillary length, mandibular corpus length, cranial base flexure (Ar-S-Na), horizontal and vertical positioning of the posterior maxilla, mandibular plane angle, and gonial angle. Although the effect of sex on the within twin differences was not statistically significant, the effects of age, zygosity, and cleft type were. It is suggested that there is an increasing degree of dysmorphism, with increasing severity of clefting from cleft lip only to bilateral cleft lip palate.