An investigation of craniofacial asymmetry using the serial twin-study method

Abstract In certain cases, asymmetry of the craniofacial complex can be a problem of concern to the orthodontist, in both diagnostic and treatment procedures. Few studies have been made of this problem, the main reason being the lack of a suitable tool for the accurate measurement of right-left differences in three planes of space. By the application of solid geometry design for correction of size distortion, the two-film roentgenographic cephalometric technique introduced by Broadbent 6 has been used to record measurements of right and left structures within the craniofacial complex. By suitable mathematical treatment, the two-film procedure yields three-dimensional distances, thereby expressing both form and position of right and left structures. On the basis of recent findings regarding the relative stability of certain cranial and extracranial points, three registration points were selected, and distances from them to appropriate right and left structures were calculated. Values for both right and left sides were secured (1) relative to the cranial base, (2) for those within the maxilla, relative to anterior nasal spine , and (3) for those within the mandible, relative to the mandibular point, menton . Indexes were constructed which could be comparable both in time and within the sample on an individual-to-individual basis. Cross-sectional indexes represented observations at two particular moments in time, 1957 and 1960. Serial indexes included the interval between these two times, reflecting incremental changes in terms of asymmetry. By retention of the appropriate arithmetical sign, the preservation of direction of change , if any, was recorded. The sample employed in this study consisted of six same-sex sets of triplets of North American Caucasian descent, aged 9 to 15 years. By within-set pairings, six identical twin sets and twelve fraternal twin sets were secured. Through the use of serial records, taken in 1957 and again in 1960, and employing the twin-study method, many questions could be investigated with a small sample that could not be answered satisfactorily with a large population sample: 1. 1. Are craniofacial asymmetries predominantly controlled by heredity? Or is environment more influential in the production of rightleft differences? 2. 2. Do all regions of the craniofacial complex exhibit the same degree of asymmetry? Or, for example, is asymmetry of the maxillary region greater than asymmetry of the mandible? 3. 3. Is degree of asymmetry constantly proportional throughout the development of the individual? Or does the difference between right and left sides undergo changes with age? 4. 4. What are the ranges or limits of asymmetry in the craniofacial complex? To answer these questions, the data were then subjected to statistical treatment which permitted interpretation of possible differences (1) between identical and fraternal twin members, (2) between age groupings, (3) between times of observation, and (4) between regions within the craniofacial complex. The main statistical procedure used was multifactorial analysis of variance. Once differences were recognized, additional treatment was performed to determine means, ranges, and standard deviations. Means were then tested against each other to identify those which were truly different and those whose difference was small enough to have occurred by chance in the sampling process. The findings of this study were as follows: 1. 1. Regardless of method of analysis, I have failed to show significant differences between identical- and fraternal-twin groups. Thus, according to this study, heredity is not the controlling agent in the production of craniofacial asymmetry (excluding hereditary syndromes). 2. 2. Cross-sectional evaluation shows that there are decided differences in amounts of asymmetry of the various craniofacial regions and landmarks, but serial evaluation shows no differences. Therefore, although there may be initial differences between right and left sides, these differences are not intensified with dimensional changes occurring with increase in age. 3. 3. Although age differences of mean asymmetry exist on a cross-sectional basis, there is no effect of age on the individual's mean asymmetry (when measured serially). 4. 4. Ranges or limits of asymmetry have been established for the various craniofacial landmarks included in this study. An approach which utilizes conventional oriented lateral and posteroanterior roentgenograms for the measurement of craniofacial asymmetry has been offered. This procedure has been applied to answer basic questions concerning the nature of asymmetry. Three-dimensional assessment of gross asymmetries, such as cleft lip and cleft palate, condylar and facial dysplasia, and craniofacial hereditary syndromes, can now be made. Moreover, pretreatment and posttreatment evaluation can be made of those patients with gross asymmetries who would seek orthodontic, surgical, or prosthetic treatment.