Mean variation
To our knowledge, to date, only Hermann et al. [
14] have addressed variation due to 3D facial expression, in a pilot study in infants, although stereophotogrammetry has often been applied in previous studies with non-compliant infants and less compliant children. In Hermann et al. [
14], alignment of 3D facial images of 10 infants (mean age 3.9 months, seven with UCLP and three with bilateral cleft lip, alveolus and palate) was performed on the forehead via an iterative closest point algorithm, and the results were compared with data derived from 10 adults and a static mannequin head. In that study, deformation generally increased moving downward from the forehead to the chin, and mean variation was 0 mm at the forehead, 1.1 mm at the nose, 1.7 mm at the cleft region, 3.1 mm for the lower lip and 3.5 mm for the chin. In the current study, mean variation was 0.88 mm at the nasolabial region in UCLP subjects at the age of 3 months, and mean variation was 0.66 for the full face. The value of 0.88 mm is half of the value of 1.7 mm that Hermann et al. [
14] reported at the cleft region. This difference may be due to differences in consistency of the image capture procedure, i.e. the experience of the photographer. Experience of the physician in case of direct anthropometry has been demonstrated to be a factor of influence as well, as it is likely to be with any other technique [
17]. Other potentially contributory factors are that the two studies used different methods for the alignment of 3D facial images, and there were differences in the size and homogeneity of the samples.
In conjunction with the selected methods of alignment, it can be expected that the error in a particular facial area is related to its distance to the area of reference, i.e. mapping on the forehead will lead to a larger error in the area of the chin. On the other hand, it is reasonable to assume that aligning on the full facial surface will average variation around various facial regions, i.e. regression to the mean. In the adult control sample in Hermann et al. [
14], there was a similar increase in deformation moving downward from the forehead to the chin; however, the values were a factor of 10–20 times less (0.09 mm at the forehead to 0.25 mm at the chin) than those derived from the infant sample. These values are suggested to be clinically acceptable and indicate that adults are sufficiently able to present a neutral facial expression. In a static mannequin, however, head deformation did not increase moving downward from the forehead to the chin (0.032 mm at the forehead to 0.027 mm at the chin) [
14] indicating that alignment on the forehead is a perfectly reliable method in a static mannequin. Notably, it also indicates that even in compliant adults, minor deformation in the surface of the forehead exists.
Variation of the face at rest using 3D stereophotogrammetry in adults has been described by Maal et al. [
12] and Lubbers et al. [
13]. Maal et al. [
12] used surface-based registration of 25 3D images of the same adult individual taken with and without wax bite in maximal occlusion at T
0 and repeated the process after 6 weeks. They reported a mean variation of 0.25 mm and no large differences between different time points when a wax bite was used. There were small variations in anatomical regions of the face, the smallest variation being in the nose and forehead and the largest variation in the mouth and eyes. In the current study, the mean variations of the face were 0.40 mm in the control sample and 0.44 mm in the UCLP sample, which are larger values than that reported by Maal et al. [
12]. This is to be expected, due to the non-compliant nature of infants with regard to maintaining a neutral facial expression during 3D image acquisition. However, the differences at 3, 12 and 18 months of age were small in the current study, and not clinically relevant. Lubbers et al. [
13] examined calliper distances of labelled landmarks in two adult subjects and reported a mean global error of 0.41 mm, of which technical error accounted for 0.09 mm and involuntary facial movements accounted for 0.32 mm. This is comparable to our findings in controls and UCLP subjects at the ages of 3, 12 and 18 months.
To estimate the clinical significance of error due to changing facial expressions, it is relevant to compare this error with the system error and with normative data of craniofacial growth. The accuracy of the 3dMD system compared to direct facial anthropometry for linear measurements reportedly ranges from 0.21 to 0.94 mm [
9,
10,
18,
19], and for landmark identification it ranges from 0.49 to 0.83 mm [
11,
20,
21]. The mean
r value representing the correlation between linear measurements derived from the 3dMD and direct anthropometry was reportedly 0.88 in one study, indicating a very high correlation [
22]. The error due to involuntary facial expressions in the current study ranged from 0.38 to 0.50 mm, and actually includes the system error itself. Therefore, the influence of involuntary facial expressions seems to be clinically insignificant. However, when summing up all potential factors like system error, landmark identification error and error due to involuntary facial expression a clinically significant error is possible.
Furthermore, studies that produced normative data of craniofacial growth from 0 until 2 years of age in North American Caucasians measured with direct anthropometry have shown that linear cranial growth was 11 to 20 mm in width and 21 to 27 mm in height; linear facial growth was 5 to 9 mm in width and 10 to 17 mm in height; linear growth of the nose was − 0.2 to 1.7 mm in width, 7.2 to 8 mm in height and 0.6 to 1.8 mm in length; and linear growth of the lips and mouth was 4.4 to 5.2 mm in width and 0.6 to 4.1 mm in height [
23]. Compared to these increments for linear measurements due to craniofacial growth, the error due to involuntary facial expressions in the current study (0.38 to 0.50 mm) seems to be clinically insignificant except for measurements of the width and length of the nose and height of the lips and mouth. Future studies on longitudinal 3D facial growth in young children should take the mean variation in facial expression into consideration in relation to the mean variation due to facial growth.
Assessment of variation in pairs of selected 3D images at ages 3, 12 and 18 months
The mean, p50, p90 and p95 of the variation in the face and nasolabial area of controls and UCLP subjects did not differ significantly in the current study. Age was not related to variation in facial expression in children with or without UCLP at the ages of 3, 12 and 18 months. The explanation for this is that a carefully instructed photographer can capture multiple 3D facial images of an infant aged between 3 and 18 months with the same level of neutral facial expression. Results reported by Hermann et al. [
14] indicate that the ability of a photographer to recognise a neutral facial expression is one of many factors related to variations in 3D images. In the current study, 43% of all available 3D facial images of controls and UCLP subjects were included (Table
1). There was a tendency towards a higher percentage of included 3D facial images in subjects at 3 months of age (58%) and a tendency towards a lower percentage of included 3D facial images in subjects at 12 months of age (32%) and 18 months of age (38%). An explanation for the higher percentage of included 3D facial images of subjects aged 3 months might be that they are less aware of the process of image capturing, and therefor unconsciously more cooperative or easier to district than subjects aged 12 and 18 months. We recommend a thorough review by the photographer after 5–8 3D images of the same child have been captured, in order to maximise the number of high-quality 3D images with a neutral facial expression. Furthermore, the quality of the process of selecting eligible 3D images has a strong influence on outcome. To achieve a good selection of eligible 3D images of the same child, two observers were calibrated for the independent selection of eligible 3D images. The inter-observer agreement of 0.90 indicates that selection of high-quality 3D images can be reliably performed. This is a crucial factor for both reliability of results within a single study and for comparison of results derived from different studies.
Assessment of variation in pairs of selected 3D images: the whole face vs. the nasolabial area
Statistically significant differences were found in the mean, p50, p90 and p95 variation of the face compared to the nasolabial area for all ages in controls and at age 3 months in UCLP subjects. As expected, variation was larger in the nasolabial area than in the full face due to movement of the lips and mandible. In the total subject pool (including controls and UCLP subjects), the p95 of the mean variation of the face ranged from 1.01 to 1.38 mm, whereas the p95 of the mean variation of the nasolabial area in UCLP at age 3 months was up to 3.00 mm with a sd of 2.47 mm, indicating that in this specific region, reliability was questionable. Conversely, mean variation in the nasolabial area was 0.88 mm at most, which is well below 1 mm and therefore clinically acceptable. In general, the upper quartile was below a mean variation of 1 mm. Exceptions to this were the nasolabial region in UCLP subjects at age 3 and 18 months and the nasolabial region of controls at age 12 months. Also, outliers in the higher mean variation were present indicating an unsteady character of the nasolabial region due to variation in involuntary facial expression.