Etiological aspects on craniofacial morphology in twins with cleft lip and palate.

The craniofacial morphology of 39 Finnish twin pairs concordant or discordant for cleft of the lip (CL), unilateral cleft lip and palate (UCLP), or cleft palate only (CP) was studied by means of lateral cephalometric radiographs. The intrapair differences between various cleft and twin subgroups were compared by means of univariate analysis; multivariable analysis of cleft discordant pairs and the variance components were estimated with the TWINAN program. Larger values for intrapair differences were found, especially in linear measurements in older twin pairs. Heritability estimates for the cranial base measurements and, to a lesser extent, for the mandibular measurements were high. Comparisons on the basis of cleft type revealed more differences between twin pairs when clefting was more severe. Concordant and discordant CP twin pairs showed similarity in their intrapair craniofacial morphology, indicating that surgery seemed to have no major effect on their craniofacial morphology. A similar comparison in discordant UCLP showed that, in addition to clefting itself, external factors, i.e., the repaired cleft's direct effects on craniofacial morphology, have a strong influence on craniofacial morphology, especially in the maxillas of the UCLP twins. The craniofacial morphology in these Finnish twins supports the hypothesis of different genetic behavior for cleft lip and cleft lip and palate (CL(P)) and CP in Finland.