Need for orthognathic surgery in cleft patients from Northern Finland.

OBJECTIVE: Northern Finland has a unique distribution of clefts compared to the rest of Europe and Finland. This may reflect the need for orthognathic surgery among Northern Finland's patient pool. The aim of this study was to compare previously operated patients aged 18 years or older with cleft lip, cleft lip and alveolus, cleft lip and palate, cleft palate and to evaluate the need for orthognathic surgery in order to achieve a stable and functional occlusion.  Materials and methods: The study group consisted of all 18-years-old cleft patients treated in the Oulu Cleft Center. The total amount of patients was 110. The patients were compared retrospectively using patients' hospital records. The majority of patients did not have any cleft-associated syndrome. The need for maxillary or bimaxillary orthognathic or corrective-jaw surgery was evaluated by the Oulu Cleft Team. A descriptive and statistical analysis was used to determine the need for orthognathic surgery according to sex and cleft type. RESULTS: There were nineteen patients of the total of 110 patients who met the criteria requiring corrective-jaw surgery (17,3%). In total 12 males (25,0%) and 7 females (11,3%) were evaluated for the need of orthognathic surgery. Sixteen of the 19 patients had palatal involvement of the cleft. CONCLUSIONS: The need for orthognathic surgery was greater in the cleft lip palate and cleft palate patient groups compared to cleft lip alveolus or cleft lip groups. This study also found that males from Northern Finland need surgery more often than females.

The association between dental and facial symmetry in adolescents.

INTRODUCTION: Facial aesthetics have become one of the most important objectives of orthodontic treatment. The correction of dental arches should be performed in accordance with the face. This study explored the association between occlusal and facial asymmetries in adolescents, particularly emphasizing a Class II subdivision. METHODS: Eighty-one adolescents (43 males, 38 females) with a median age of 15.9 (interquartile range, 15.17-16.33) years were enrolled. Of these patients, 30 had a Class II subdivision (right side, n = 12; left side, n = 18). Three-dimensional facial scans were analyzed using surface- and landmark-based methods. Chin asymmetry was determined using the chin volume asymmetry score. Three-dimensional intraoral scans were analyzed to assess occlusal asymmetry. RESULTS: The surface matching scores were 59.0% ± 11.3% for the whole face and 39.0% ± 19.2% for the chin. Chin volume was larger on the right side than on the left side in most patients (n = 51, 63%), and it was associated with a dental midline shift to the corresponding subdivision side. A correlation between dental and facial asymmetries was noted. In addition, the dental midline shifted to the left in patients with a Class II subdivision, regardless of the side, and to the right in those with a symmetrical Class II subdivision. However, several patients did not possess asymmetrical occlusal traits sufficient for statistical analysis. CONCLUSIONS: Dental asymmetry was weak but significantly correlated with facial asymmetry.

A longitudinal study of facial asymmetry in a normal birth cohort up to 6 years of age and the predisposing factors.

OBJECTIVES: This prospective, population-based cohort study aimed to investigate the development of facial asymmetry up to 6 years of age using a three-dimensional (3D) soft tissue imaging method in a normal population. In addition, the study sought to identify potential predisposing factors to facial asymmetry. METHODS: A total of 102 newborns were enrolled in the study at birth. 3D stereophotogrammetric images of the head and face were analysed at the ages of 12 months (T1), 3 years (T2), and 6 years (T3). The surface-based analysis involved the calculation of the average distance (mm) and the symmetry percentage (%) between the original and mirrored surfaces. For landmark-based analysis, the distance of facial landmarks to the facial midline was examined. RESULTS: The final analysis included 70 (68.6%) subjects. Surface-based analysis showed a significant improvement of facial symmetry from T1 to T3 in all facial areas. Landmark-based analysis showed that upper facial landmarks were located, on average, slightly on the left and lower facial landmarks slightly on the right in relation to the facial midline (P < 0.001). LIMITATIONS: The size of the study population was limited. Facial posture may affect the reliability of the results, especially in younger children. CONCLUSION: Facial asymmetry is detectable in early childhood and tends to reduce with age in young children. The lower face deviates slightly to the right, and the upper face to the left in relation to the facial midline. Possible predisposing factors for facial asymmetry at the age of 6 years include deformational plagiocephaly, sleeping position, and previous facial asymmetry.

Occlusal characteristics and oral health-related quality of life in adults operated due to sagittal synostosis in childhood: a case-control study with 26 years of follow-up.

PURPOSE: The aim of this case-control study was to investigate occlusal characteristics, received orthodontic treatment, oral health-related quality of life (OHRQoL), and satisfaction with dental esthetics in adults operated due to sagittal synostosis. METHODS: The study group consisted of 40 adults (25 males, 15 females, mean age 27.4 years, range 18-41) who were operated due to isolated sagittal synostosis in childhood. The control group comprised 40 age and gender-matched adults. Occlusal characteristics were evaluated clinically during study visits. Information on the previous orthodontic treatment was collected from dental records. OHRQoL was measured using the 14-item Oral Health Impact Profile (OHIP-14), and satisfaction with dental esthetics was evaluated using a visual analogue scale. RESULTS: No statistically significant differences were found between the patient group and the controls in malocclusion traits (overjet, overbite, molar relationships, crossbite, scissor bite), previous orthodontic treatment, pre-treatment malocclusion diagnoses, OHIP variables, or satisfaction with dental esthetics. However, there was a tendency toward increased overjet and overbite in scaphocephalic patients. CONCLUSION: It seems that adults with scaphocephaly operated in childhood do not differ from the average population in terms of occlusion, received orthodontic treatment, or oral health-related well-being.

Prevalence of sagittal molar and canine relationships, asymmetries and midline shift in relation to temporomandibular disorders (TMD) in a Finnish adult population.

OBJECTIVE: To examine the prevalence of sagittal relationships, asymmetries and midline shift, and their associations with temporomandibular disorders (TMD) in the Northern Finland Birth Cohort 1966 (NFBC1966). MATERIALS AND METHODS: 1845 subjects participated in a clinical examination at the age of 46. Occlusal measurements were performed using 3D models. Symptoms of TMD were screened using validated questions, and signs and diagnoses of TMD were assessed using a modified protocol of the Diagnostic Criteria for TMD (DC/TMD) and questionnaires. Associations between variables of occlusion and TMD were evaluated with χ2-test and Fisher's exact test and using logistic regression analyses, adjusted for self-reported general health, mental health, bruxism, and rheumatoid arthritis. RESULTS: The most common sagittal relationships were Class I and normal canine relationship. Half-cusp Class II and post-normal canine relationship were more frequent in females, and Class III and pre-normal canine relationship in males. Deviations from normal cuspid or molar relationships showed a weak but statistically significant association with TMD, especially in females. Half-cusp Class II and Class II relationships were more frequent in relation to joint-related TMD signs and diagnoses while missing canines were associated with pain-related TMD diagnoses. CONCLUSIONS: The present study findings gave some indications that canine relationships are associated with pain-related TMD, whereas Angle II sagittal occlusal relations may associate with joint-related TMD. Occlusal characteristics should therefore be taken into account as one possible associating factor in subjects with TMD.

Facial asymmetry and chewing sides in twins.

OBJECTIVE: To resolve how the preferred chewing side (PCS) affects facial asymmetry in twins, whether there are differences between monozygotic (MZ) and dizygotic (DZ) twins, and whether the twins with PCS have more asymmetric faces compared to symmetrically chewing twins. MATERIAL AND METHODS: The study included 106 Lithuanian twin pairs of the same sex, 59 MZ and 47 DZ pairs. The data were analysed from facial 3D images and manually added landmarks. 3D images were analysed by Rapidform2006 software and statistical analyses were done by using the R software environment version 4.1.0. RESULTS: The contralateral effect of PCS and larger chin side was dominant among right and non-right side chewing twins. Being female increased the whole face symmetry. CONCLUSION: The volume of the chin becomes larger on the side opposite to the twins' habitual chewing side. As the results are quite similar in both twin types, functional factors are more prominent than heredity.

Associations of sagittal malocclusions with dental arch characteristics and crowding in Northern Finland Birth Cohort 1966.

OBJECTIVE: To analyse the associations between the widths and forms of dental arches, malocclusions and crowding in middle-aged adult population. MATERIALS AND METHODS: The study material is part of the Northern Finland Birth Cohort 1966 (NFBC1966); the subjects were examined at the age of 46 years. A total of 781 adults with full dentition and no orthodontic treatment history were included. Overjet, overbite, cross-bite and scissor bite were examined during clinical examination. The widths and forms of dental arches, molar relationships and crowding were measured using three-dimensional (3D) dental models. RESULTS: Dental arches were significantly larger in males than in females. Class II (CII) occlusion and increased overjet (≥6 mm) were associated with narrow maxilla (P < .001, P < .05 respectively). Subjects with lateral cross-bite had significantly narrower maxilla and wider mandible (P < .05) than subjects with normal lateral occlusion. In subjects with CII occlusion, square or tapered maxilla and tapered or ovoid mandible were significantly more frequent compared to subjects with Class I (CI) occlusion (P < .05). CII occlusion was significantly associated with crowding in the maxilla (P < .01). CONCLUSION: Narrow and tapered-shaped maxilla was related to CII occlusion, which was associated with dental crowding in the maxilla. Malocclusions were more likely to be found together with other malocclusions than alone.

Craniofacial characteristics and cosmetic satisfaction of patients with sagittal and metopic synostosis: a case-control study using 3D photogrammetric imaging.

PURPOSE: The aim of this study was to investigate the craniofacial and aesthetic characteristics of adult metopic and sagittal craniosynostosis patients operated on in early childhood compared to controls. The goal was to find objective measurements that would correlate with the patient's subjective self-evaluation of their own cosmetic appearance. METHODS: The study population consisted of 49 patients from whom 41 had premature fusion of the sagittal and in 8 of metopic suture. There were 65 age and gender matching controls from The Finish National Register. The 3D photogrammetric models were created from all patients and controls. The images were analysed using Rapidform 2006. Facial landmarks were set by the standard Farkas points. Facial symmetry parameters were calculated by using the landmarks and the mirror shell of the face. Aesthetic evaluation was done from standard photographs using panels. Subjective satisfaction with one's own appearance was evaluated using questionnaires. RESULTS: Patients had the greatest asymmetry in the forehead area when compared to controls (symmetry percentage 59% versus 66%, p = 0.013). In the control group, the gap between the eyes was smaller than in the case group, resulting in an absolute 2 mm difference (p = 0.003). The area of the chin and the landmarks were more located on the left side in the patient group, resulting in up to a 1.1 mm difference between the groups (p = 0.003). Only a weak association was found between craniofacial symmetry and appearance evaluations. CONCLUSION: Patients operated on because of sagittal and metopic synostoses were found to have facial asymmetry at long follow-up. However, the differences were < 3 mm and not clinically important. The long-term aesthetical outcome of the surgery performed because of sagittal or metopic craniosynostosis based on the 3D image evaluation was good.

Effect of cervical headgear on dental arch area, shape and interarch dimensions : A randomized study.

PURPOSE: The goal was to study the effects of early cervical headgear treatment on maxillary and mandibular dental arch area, shape and interarch dimensions. METHODS: The total study group comprised 67 children aged 7.6 years (standard deviation 0.3) with Angle class II malocclusion collected between 1992 and 1996. The children were randomly divided into two groups of equal size. In the first group, cervical headgear treatment was started immediately and undertaken for 2 years. The remaining patients served as untreated controls. Dental casts were taken and scanned at the beginning of treatment (T0) and at the 2­year (T1) and 4­year follow-up (T2). Three-dimensional landmarks describing the positions of maxillary and mandibular incisors, canines, first and second premolars and first molars were used to calculate and visualize the maxillary and mandibular dental arch area and shape using the polynomial equation y = Ax6 + Bx2. RESULTS: Significant changes in the shape and area of both maxillary and mandibular dental arches were induced with cervical headgear. The headgear increased dental arch area, sagittal dimensions at the mid-sagittal line and transversal dimensions at all of the measured levels in both dental arches compared to the control group. CONCLUSIONS: Cervical headgear is an effective treatment device to gain space in both dental arches. Furthermore, when used as an early phase treatment, relapse is relatively small compared to the gained space.

Reproducibility of Natural Head Position assessed with stereophotogrammetry.

OBJECTIVE: The aim of this study was to assess the stability of Natural Head Position (NHP) over time using the 3dMDface System. SETTING AND SAMPLE POPULATION: This was an experimental study. Three-dimensional facial images of 40 students were captured on two different occasions, with an interval of at least two weeks. MATERIALS AND METHODS: The images were taken using a stereophotogrammetric device (3dMD). The mirror positioned NHP was obtained in a standing position and then replicated in a sitting position for capturing. The self-balanced NHP was taken in a sitting position. Rapidform 3D software was used for position angle calculations. The angle changes between the positions were calculated for rotations around the x-, y- and z-axes. RESULTS: The differences between NHP in the self-balanced and mirror positions recorded on the first and second occasions were 2.43 and 1.75 degrees, respectively, around the x-axis. The average changes in NHP around the x-axis between the self-balanced and mirror balanced positions exceeded 3 degrees at the two-week interval. The differences were smaller for the rotations around the y- and z-axes. Some subjects consistently tended to hold their heads in a more extended position when self-balanced, while others did this when mirror balanced. There was no difference in the reproducibility of NHP between men and women. CONCLUSION: The reproducibility of NHP for consecutive stereophotogrammetric captures is generally acceptable. The reproducibility of NHP using the mirror position was slightly better compared with NHP in the sitting self-balanced position.

A 3D Follow-Up Study of Cranial Asymmetry from Early Infancy to Toddler Age after Preterm versus Term Birth.

Preterm infants are at higher risk for both symmetrical and asymmetrical head molding. This study involved 3D stereophotogrammetry to assess the cranial growth, molding, and incidence of deformational plagiocephaly (DP) in preterm children compared to term born children. Thirty-four preterm infants and 34 term born controls were enrolled in this study from Oulu University Hospital, Finland. Three-dimensional head images were obtained at the age of 2-4 months (T1), 5-7 months (T2), 11-13 months (T3), and 2.5-3 years (T4) from the term equivalent age (TEA). There was no statistically significant difference in oblique cranial length ratio (OCLR), cephalic index (CI), or weighted asymmetry score (wAS) between the two groups. Occipital flattening, defined by flatness score (FS) was statistically significantly greater in the preterm group than in the term group at T1-T4 (p < 0.05). In both groups, OCLR improved gradually over time. There were no instances, in either group, of severe DP and no moderate DP after T2. Results indicate that DP affects preterm and full-term children almost equally during the first three years of life, and cranial asymmetry resolves at a similar rate in both preterm and term groups after three months of corrected age. Preterm infants present with more occipital flattening than full-term children.

Relations of laterality and chewing sidedness in twins.

OBJECTIVES: To study, whether there are associations between chewing side preference and other lateralities, whether there is a genetic origin for preferred chewing side (PCS), relations to sex, birth order and orthodontic treatment. MATERIALS AND METHODS: The study included 113 twin pairs, 57 pairs monozygotic, 47 pairs same sex dizygotic and 9 pairs of opposite sex. The lateralities of preferred chewing side, footedness and eyedness were assessed by functional tests and handedness was checked by asking. RESULTS: Right-handed, -footed and -eyed as separated groups, and true-right sided (combined), were evenly distributed by preferred chewing side. By contrast, left-handed, -footed, -eyed and non-right sided used more left or both sides when chewing. Birth order affected preferred chewing side among monozygotic: the first-born twin was more likely to have the preferred chewing side on the non-right, whereas second-born twins used right side. Gender, zygosity and orthodontic treatment were not statistically significant factors. LIMITATIONS: Examination of chewing sidedness could have been done in several different ways to provide a definite result. Sample size of opposite sex twins was very small. CONCLUSIONS: Preferred chewing side generally seemed to follow the side where other lateralities occur; however, monozygotic twins seemed to be more receptive to it. True-right sided were more evenly distributed than non-right sided by the preferred chewing side.

Craniofacial Asymmetry from One to Three Years of Age: A Prospective Cohort Study with 3D Imaging.

Deformational plagiocephaly (DP) is considered a risk factor for facial asymmetry. This cohort-based, prospective, follow-up study used three-dimensional (3D) stereophotogrammetry to assess the development of facial asymmetry in a normal birth cohort and to investigate the impact of DP on facial asymmetry for the age range of one to three years. The study sample consisted of 75 children: 35 girls (47%) and 40 (53%) boys recruited from Oulu University Hospital. A total of 23 (31%) subjects had a history of DP in infancy. 3D facial images were obtained at the mean (SD) age of 1.01 (0.04) year old at T1 and 3.02 (0.14) years old at T2. To determine facial asymmetry, both landmark-based and surface-based facial symmetry methods were used. As measured with the surface-based methods, upper facial symmetry improved from T1 to T2 (p < 0.05). As measured with the landmark-based methods, facial symmetry improved on the upper and lower jaw from T1 to T2 (p < 0.05). The asymmetric effect of DP on the upper parts of the face tends to correct spontaneously during growth. Results indicate that previous DP does not seem to transfer to facial or occlusal asymmetry at the age of three years old.

Accuracy of measurements used to quantify cranial asymmetry in deformational plagiocephaly.

OBJECTIVE: Various measurements are used to quantify cranial asymmetry in deformational plagiocephaly (DP), but studies validating cut-off values and comparing the accuracy of such measurements are lacking. In this study, we compared the accuracy of four different measurements in classifying children with and without DP diagnosed by visual assessment, and sought to determine their optimal cut-off values. STUDY DESIGN: Two experts rated 407 3D craniofacial images of children aged between 3 and 36 months old using the Argenta classification. We then measured the following asymmetry-related variables from the images: Oblique Cranial Length Ratio (OCLR), Diagonal Difference (DD), Posterior Cranial Asymmetry Index (PCAI), and weighted Asymmetry Score (wAS). We created receiver operating characteristic curves to evaluate the accuracy of these variables. RESULTS: All variables performed well, but OCLR consistently provided the best discrimination in terms of area under the curve values. Subject's age had no clear effect on the cut-off values for OCLR, PCAI, and wAS; however, the cut-off for DD increased monotonically with age. When subjects with discrepant expert ratings were excluded, the optimal cut-off values for DP (Argenta class ≥ 1) across all age-groups were 104.0% for OCLR (83% sensitivity, 97% specificity), 10.5% for PCAI (90% sensitivity, 90% specificity), and 24.5 for wAS (88% sensitivity, 90% specificity). CONCLUSION: We recommend using OCLR as the primary measurement, although PCAI and wAS may also be useful in monitoring cranial asymmetry. The threshold of relative asymmetry required for a deformation to appear clinically significant is not affected by the child's age, and DD has no additional utility in monitoring DP compared to using only OCLR.

Increases in Cranial Volume with Posterior Cranial Vault Distraction in 31 Consecutive Cases.

BACKGROUND: Posterior cranial vault distraction (PCVD) is a technique widely used in surgical treatment of craniosynostosis when cranial expansion is required. It has proven to be safe and to allow a significant increase of intracranial volume. OBJECTIVE: To evaluate increases in intracranial volume as a result of PCVD performed in Oulu Craniofacial center using 2 different methods based on 3-dimensional (3-D) photogrammetric imaging or plain skull radiographs. METHODS: All children less than 16 yr of age who were treated by PCVD (n = 31) from 2009 to 2015 at the Oulu Craniofacial Center were included. All patients were followed at outpatient clinics with plain radiographs performed for follow-up. In 5 patients, additional 3-D photogrammetric imaging was done pre- and postoperatively. RESULTS: The mean intracranial volume increase was 25.0%, ranging from 16.9% to 39.4%. In 5 patients, the increase in volume was calculated from the photogrammetric 3-D images comprising a mean of 17.4%. Volume calculations from cephalograms in the same patients gave a mean of 20.8%. Whether the distraction was a primary operation or patient had undergone previous cranioplasty did not influence the achieved volumetric results. There were no statistically significant differences in the distraction results between different diagnostic groups. CONCLUSION: PCVD is an effective surgical method to increase intracranial volume in a variety of clinical entities. Volumetric results of this procedure could be easily evaluated using 3-D photogrammetric imaging or plane radiographs that expose the patients to only low ionizing radiation doses.

The course of positional cranial deformation from 3 to 12 months of age and associated risk factors: a follow-up with 3D imaging.

Deformational plagiocephaly is reported in up to 46.6 % of healthy infants, with the highest point prevalence at around 3 months of age. Few prospective studies on the natural course of skull deformation have been conducted, and we know of no studies using 3D imaging starting from the highest point prevalence period. In this prospective, population-based cohort study, we describe the course of cranial asymmetry and shape in an unselected population using 3D stereophotogrammetry and investigate factors associated with late cranial deformation and failure to recover from previous deformation. We evaluated 99 infants at 3, 6, and 12 months of age. We acquired 3D craniofacial images and performed structured clinical examinations and parental interviews at each visit. Eight outcome variables, representing different aspects of cranial shape, were calculated from a total of 288 3D images. Scores of asymmetry-related variables improved throughout the observation period. However, the rate of correction for cranial asymmetry decreased as the infants grew older, also in relation to the rate of head growth, and a significant amount of asymmetry was still present at 12 months. Positional preference at 3 months predicted an unfavorable course of cranial asymmetry after 3 months, increasing the risk for DP persisting. What is known: • The prevalence of deformational plagiocephaly spontaneously decreases after the first months of life. • Limited neck range of motion and infant positional preference increase the risk of deformational plagiocephaly during the first months of life. What is new: • Positional preference at 3 months predicts an unfavorable spontaneous course of deformation also from three to 12 months of age, presenting a potential target for screening and treatment. • The spontaneous rate of correction for cranial asymmetry decreases after 6 months of age, also in relation to the rate of head growth.

Analyzing infant head flatness and asymmetry using kernel density estimation of directional surface data from a craniofacial 3D model.

Infant skull deformation is analyzed using the distribution of head normal vector directions computed from a 3D image. Severity of flatness and asymmetry are quantified by functionals of the kernel estimate of the normal vector direction density. Using image data from 99 infants and clinical deformation ratings made by experts, our approach is compared with some recently suggested methods. The results show that the proposed method performs competitively. Copyright © 2016 John Wiley & Sons, Ltd.

Preventing deformational plagiocephaly through parent guidance: a randomized, controlled trial.

Deformational plagiocephaly (DP) occurs frequently in otherwise healthy infants. Many infants with DP undergo physiotherapy or helmet therapy, and ample treatment-related research is available. However, the possibility of preventing DP has been left with little attention. We sought to evaluate the effectiveness of intervention in the newborn's environment, positioning, and handling on the prevalence of DP at 3 months and to investigate the causal relationship between DP and cervical imbalance. We carried out a randomized controlled trial, with healthy newborns randomized into two groups at birth. All families received standard positioning instructions to prevent SIDS. Additionally, the intervention group received detailed instructions regarding the infant's environment, positioning, and handling, with the goal of creating a nonrestrictive environment that promotes spontaneous physical movement and symmetrical motor development. Two- and three-dimensional photogrammetry served to assess cranial shape and goniometry to measure cervical motion. At 3 months, the prevalence of DP was lower in the intervention group in both 2D (11 vs 31 %) and 3D analyses (15 vs 33 %), and the asymmetry was milder in the intervention group. Infants with DP at follow-up had also developed more torticollis. CONCLUSION: An early educational intervention reduces the prevalence and severity of DP at 3 months. WHAT IS KNOWN: •Deformational plagiocephaly, often with associated torticollis, is common in healthy infants. •Parental education is frequently recommended for preventing deformational plagiocephaly, although information regarding the effectiveness of preventive strategies is scarce. WHAT IS NEW: •Early parent guidance effectively reduces the prevalence and severity of DP and improves the cervical range of motion at three months. •Educating both parents and professionals about proper infant positioning on a national scale could help minimize public healthcare costs.

Comparing Facial 3D Analysis With DNA Testing to Determine Zygosities of Twins.

The aim of this study was to compare facial 3D analysis to DNA testing in twin zygosity determinations. Facial 3D images of 106 pairs of young adult Lithuanian twins were taken with a stereophotogrammetric device (3dMD, Atlanta, Georgia) and zygosity was determined according to similarity of facial form. Statistical pattern recognition methodology was used for classification. The results showed that in 75% to 90% of the cases, zygosity determinations were similar to DNA-based results. There were 81 different classification scenarios, including 3 groups, 3 features, 3 different scaling methods, and 3 threshold levels. It appeared that coincidence with 0.5 mm tolerance is the most suitable feature for classification. Also, leaving out scaling improves results in most cases. Scaling was expected to equalize the magnitude of differences and therefore lead to better recognition performance. Still, better classification features and a more effective scaling method or classification in different facial areas could further improve the results. In most of the cases, male pair zygosity recognition was at a higher level compared with females. Erroneously classified twin pairs appear to be obvious outliers in the sample. In particular, faces of young dizygotic (DZ) twins may be so similar that it is very hard to define a feature that would help classify the pair as DZ. Correspondingly, monozygotic (MZ) twins may have faces with quite different shapes. Such anomalous twin pairs are interesting exceptions, but they form a considerable portion in both zygosity groups.