Review: the Michigan cleft twin study

The combined effects of clefting of the lip and/or palate and of their surgical repair have been examined for a cross-sectional sample of 20 MZ and 25 DZ like-sexed twins discordant for clefting, in the age range of 4 to 17 years. The findings of four studies published since 1975 on that sample are explored for the primary, contiguous and pleiotropic effects of clefting. The primary effects on facial structures of repaired cleft lip appear to be minimal. However, for twins with repaired cleft of the palate (with or without cleft of the lip), the maxilla was both deficient antero-posteriorly and was positioned more posteriorly than in their non-cleft co-twins. The maxillary first molars were usually slightly less erupted in the cleft twins than in the non-cleft twins. The cleft palate only twins had larger cranial base angles than their non-cleft co-twins. A contiguous effect of the posteriorly positioned maxilla was a mandibular rotation downward and backward in the cleft palate and bilateral cleft lip and palate groups but not in the unilateral cleft lip and palate group. The slightly larger cranial base angles found for cleft palate only, although contiguous in location, are more likely pleiotropic effect. The pleiotropic effects include a deficit in height and weight for cleft subjects which appears only after puberty. Although the teeth of the cleft twins tended to be smaller than those of the non-cleft twins, the amount was not clinically significant.