A Study of Nasopharyngeal Airway Size and its Relationship to Maxillary Morphology and Position in Monozygotic Twins

Abstract

This investigation was undertaken to study possible relationships between the size of the nasopharyngeal airway space and the morphology of the growing face. With reduction of the nasopharyngeal airway space, mouth breathing often becomes necessary. If mouth breathing becomes the dominant form of respiration it is believed to cause morphologic alterations in the cranioskeleton, as in the so-called "adenoid facies" individual. This environmental alteration in facial growth pattern, in contrast to the facies derived from genetic predisposition, can be tested only by using a sampling of monozygotic (identical) twins. It is assumed that since both twins have identical growth determinants any variances in their facial morphology would be due to environmental factors. Thirty-five pairs of monozygotic twins were divided on the basis of having a significantly differing intrapair airway size or a similar intrapair airway size. Thus, the independent variable in this investigation was the airway size difference between identical twins in a pair. Seven parameters were chosen to identify any maxillary morphologic changes among twin pairs. From lateral cephalometric radiographs and study casts of the twin pairs, data were accumulated for statistical analysis. It was hypothesized that significant differences between the variances in the maxillary traits of the two groups would be related to the size differences of the airways used in dividing these groups. Lower facial heights and intermolar distances were significantly different between the two groups of twins. The findings of this investigation support the opinion that nasal obstruction is related to changes in the morphology of the maxilla. The relationship is such that with decreased nasopharyngeal airway size the lower facial height tends to increase and the maxillary intermolar widths decrease. No significant changes appear to occur concomitantly in the maxillary antero-posterior relationship to the cranial base, cant of the maxilla, maxillary incisor flaring, upper facial height, and posterior vertical height of the nasomaxillary complex.