IDEAS AND INNOVATIONS

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1 IDEAS AND INNOVATIONS Determination of Facial Symmetry in Unilateral Lip and Palate Patients From Three-Dimensional Data: Technical Report and Assessment of Measurement Errors Emeka Nkenke, M.D., D.M.D., Ph.D., Bernhard Lehner, M.D., Manuel Kramer, Gerd Haeusler, Dr.Ing., Ph.D., Stefanie Benz, Maria Schuster, M.D., Friedrich W. Neukam, M.D., D.M.D., Ph.D., Eleftherios G. Vairaktaris, M.D., D.M.D., Ph.D., Jochen Wurm, M.D. Objective: To assess measurement errors of a novel technique for the threedimensional determination of the degree of facial symmetry in patients suffering from unilateral cleft lip and palate malformations. Design: Technical report, reliability study. Setting: Lip and Palate Center of the University of Erlangen-Nuremberg, Erlangen, Germany. Patients: The three-dimensional facial surface data of five 10-year-old unilateral cleft lip and palate patients were subjected to the analysis. Distances, angles, surface areas, and volumes were assessed twice. Main Outcome Measures: Calculations were made for method error, intraclass correlation coefficient, and repeatability of the measurements of distances, angles, surface areas, and volumes. Results: The method errors were less than 1 mm for distances and less than 1.5 for angles. The intraclass correlation coefficients showed values greater than.90 for all parameters. The repeatability values were comparable for cleft and noncleft sides. Conclusion: The small method errors, high intraclass correlation coefficients, and comparable repeatability values for cleft and noncleft sides reveal that the new technique is appropriate for clinical use. KEY WORDS: measurement error, optical 3D imaging, plane of symmetry, 3D cephalometry, unilateral cleft lip and palate It is generally accepted that differences between the dimensions of the left and right halves of the human face are common even in aesthetically pleasing faces. Differences of up to Dr. Nkenke is with the Department of Oral and Maxillofacial Surgery, University of Erlangen-Nuremberg, Erlangen, Germany. Dr. Lehner is with the Department of Oral and Maxillofacial Surgery, Kantonsspital Lucerne, Switzerland. Mr. Kramer is with the Department of Oral and Maxillofacial Surgery, University of Erlangen-Nuremberg, Erlangen, Germany. Dr. Haeusler is Chair for Optics, University of Erlangen-Nuremberg, Erlangen, Germany. Ms. Benz is with the Department of Oral and Maxillofacial Surgery, University of Erlangen-Nuremberg, Erlangen, Germany. Dr. Schuster is with the Department of Phoniatrics and Pedaudiology, University of Erlangen-Nuremberg, Erlangen, Germany. Dr. Neukam is with the Department of Oral and Maxillofacial Surgery, University of Erlangen-Nuremberg, Erlangen, Germany. Dr. Vairaktaris is with the Department of Oral and Maxillofacial Surgery, University of Athens, Athens, Greece. Dr. Wurm is with the Department of Otorhinolaryngology, University of Erlangen-Nuremberg, Erlangen, Germany. The Deutsche Forschungsgemeinschaft supported the study (Special Research Sector 603, Model-Based Analysis and Visualization of Complex Scenes and Sensor Data Subproject C4). Submitted September 2004; Accepted July mm between the two facial halves are considered to be within a normal range (Nkenke et al., 2003a). Studies on facial laterality report a larger right side hemiface in a normal population (Burke, 1971, 1979; Shah and Joshi, 1978; Farkas and Cheung, 1981; Koff et al., 1981, 1985; Peck et al., 1990). Congenital anomalies such as cleft lip and palate malformations are causal factors that predispose to the development of facial asymmetry with a larger noncleft side (Bishara et al., 1994). It has been stated that a greater degree of asymmetry hinders unobtrusive participation in the social life of cleft lip and palate patients. This seems to be especially true for the nasolabial appearance (Tan and Pigott, 1993). Therefore, one of the major goals of cleft lip and palate surgery is the establishment of facial symmetry. Address correspondence to: Dr. Emeka Nkenke, Department of Oral and Maxillofacial Surgery, University of Erlangen-Nuremberg, Glueckstr. 11, Erlangen, Germany. emeka.nkenke@mkg.imed.uni-erlangen.de. 129

2 130 Palate Craniofacial Journal, March 2006, Vol. 43 No. 2 Each cleft lip repair technique has its own advantages and disadvantages. The surgical options available have advanced the care of affected children to a point where new techniques and developments are likely to bring about only small changes (Atack et al., 1997). Consequently, more sophisticated methodology is required to detect improvements (Roberts et al., 1991). It is difficult to assess facial appearance in a valid and reliable way. In particular, the quantification of asymmetry is often of a subjective nature (Bearn et al., 2002a, 2002b). Because it is a three-dimensional (3D) phenomenon with transverse, vertical, and sagittal components, a comprehensive assessment of asymmetry requires a method to investigate all three components simultaneously (Ras et al., 1994a, 1994b; Ras et al., 1995). In recent years, several studies have been carried out that adopted 3D imaging techniques for the analysis of the facial surface of cleft lip and palate patients. Unfortunately, the determination of the plane of symmetry often was confined to a limited number of landmarks, not taking advantage of all points on the facial surface (Ferrario et al., 1994; Nkenke et al., 2003a). Moreover, few studies have quantified surface areas and facial volumes in unilateral cleft lip and palate (UCLP) patients as a part of the symmetry analysis (Russell et al., 2000; Ferrario et al., 2003; Nkenke et al., 2003b). Therefore, it was the aim of the present study to introduce a more comprehensive technique of 3D analysis of facial symmetry of patients with UCLP malformations and to assess the measurement errors of the different parameters that were determined. MATERIAL AND METHODS An optical 3D sensor (CAM 3D, 3D-shape GmbH, Erlangen, Germany; was used for acquisition of the facial surfaces. The sensor is based on a modification of the phase-measuring triangulation method. A sequence of phase-shifted fringe patterns of structured light is projected on the region of interest. Two CCD cameras record the data from different directions. Subsequently, the images of the phase-shifted patterns are evaluated by means of a fourshift algorithm to receive the 3D shape of the object s surface. The 3D sensor takes advantage of an astigmatic optical device for the projection of precise sinusoidal intensity coded fringe patterns instead of a commonly used Ronchi-grating fringe projection (Häusler and Gruber, 1992). The optical 3D sensor was calibrated for a measurement volume of 300 mm 3, adapted to the average dimensions of a human head. The required measurement time for data acquisition was 640 milliseconds. During data assessment, the position of the patients was adjusted reproducibly using a cephalometric head holder to prevent movement artifacts and soft tissue distortion caused by altered inclination of the head. The optical data were assessed with the patient s lips at rest. The acquired 3D images could be viewed immediately on the computer screen from any angle desired. FIGURE 1 Method of assessment of the plane of symmetry that bisects the distance (horizontal line) between P i (original point) and P i (mirror point after rigid transformation). The Frankfort horizontal plane and the plane of symmetry were used as reference for anthropometric analysis of the facial surface data. The Frankfort horizontal plane was defined by a line connecting tragion and orbitale landmarks of both hemifaces. Determination of the plane of symmetry of the facial surface began with generation of mirror images of the optical 3D surface data (Fig. 1). Thereby, point P i of the original facial surface received a corresponding point P* i in the mirror data set. Coarse and fine registrations were performed to superimpose original and mirror images. With the registration algorithms, a rigid transformation was calculated that minimized the distance between the two data sets. During this procedure, for each point of the original data set, the closest point in the superimposed mirror data set was determined. If the distance between these two points was larger than 2 mm, both were excluded from calculation of the rigid transformation. Subsequently, the calculated rigid transformation was applied to all points P* i of the mirror data set, leading to a modified mirror data set consisting of points P i. Each original data point P i now corresponded with a point P i in the modified mirror data set subjected to the rigid transformation. By bisecting the distances between all corresponding pairs P i and P i, the plane of symmetry was assessed. Further detail on the procedure has been provided previously (Benz et al., 2002).

3 Nkenke et al., MEASUREMENT ERRORS OF 3-D FACIAL ANALYSIS 131 FIGURE 3 Nostril landmarks and angles (for the definition of the landmarks, see Table 2; angle of the long axis of the noncleft side nostril with the plane of symmetry, angle of the long axis of the cleft side nostril with the plane of symmetry, green noncleft side, red cleft side). FIGURE 2 Facial landmarks (for the definition of the landmarks see Table 2; green noncleft side, red cleft side). Anthropometric facial landmarks that were determined are shown in Figures 2 and 3, and Table 1. La med is the medial intersection of the long axis of the nostril with the nostril rim. La lat is the lateral intersection of the long axis of the nostril with the nostril rim. The short axis of the nostril was defined as a line perpendicular to the long axis of the nostril showing the largest distance between two opposing points on the nostril rim. Sa sup is the superior intersection of the short axis of the nostril and the nostril rim. Sa inf is the inferior intersection of the short axis of the nostril and the nostril rim. The definitions of the axes of the nostril and the adjacent landmarks were chosen according to Yamada et al. (2002). The distances of the different landmarks from the plane of symmetry were calculated when projected on the plane of symmetry in a coronal plane that was perpendicular to the plane of symmetry. Differences in vertical height of corresponding landmarks of the cleft and noncleft sides were determined by projecting them on an axial plane through the columella base (Col) parallel to the Frankfort horizontal plane. Moreover, the distance between corresponding landmarks of the two hemifaces projected on the plane of symmetry was determined in an anterior-posterior direction parallel to the Frankfort horizontal plane. Additionally, the angle between the long axis of the nostril and the plane of symmetry was calculated (Fig. 3). Nostril areas and vermillion areas were assessed separately for cleft and noncleft sides (Figs. 4 and 5). The visible, virtual volumes of the midface, the upper lip, and the nose were determined without considering underlying voids and bony structures of the oronasal cavity (Fig. 6). As a border for the volume determination of each half of the nose of the two hemifaces, a line along nasion, endocanthion, nasal sulcus, and columella base was used. For the upper lip volume, the line proceeded along the labial fissure, cheilion, alar base, and columella base. For each midfacial half, the difference in volume between cleft and noncleft sides was determined with a border along nasion, exocanthion, cheilion, and stomium. By TABLE 1 Definition of Landmarks Extracted From the Optical Three-Dimensional Images FH N En Prn Col Ch Lb Lt Sto Abbreviation G base G lat G sup La lat La med Sa inf Sa sup Nostril length Nostril width Nostril angle Nostril area Landmark Frankfort horizontal plane, plane through tragion and orbitale landmarks Soft tissue nasion Endocanthion Pronasale Columella base, point with maximum vertical curvature bisecting the columella caudally Cheilion, point located at each labial commissure Bottom of the cupid s bow Top of the cupid s bow Stomium, point bisecting the distance between the cheilion points of both hemifaces Most inferior point of the alar groove Most lateral point of the alar groove Most superior point of the alar groove Lateral point of the long axis of the nostril Medial point of the long axis of the nostril Inferior point of the short axis of the nostril Superior point of the short axis of the nostril Distance between La med and La lat (long axis of the nostril) Distance between Sa inf and Sa sup (short axis of the nostril) Angle of the long axis of the nostril formed with the plane of symmetry Area of the nostril in a plane through the most anterior points of the nostril

4 132 Palate Craniofacial Journal, March 2006, Vol. 43 No. 2 FIGURE 4 Nostril areas (green noncleft side, red cleft side). FIGURE 5 Vermillion area. projecting all single data points of the border lines of the volumes onto the plane of symmetry in frontal planes perpendicular to the Frankfort horizontal plane and the plane of symmetry, the posterior boundary areas of the volumes of interest were generated. This facial analysis technique was applied to the facial surface data of five 10-year-old patients suffering from complete UCLP malformations. The data assessment was repeated after a time interval of 6 months by the same examiner. Statistics Method errors of the different parameters were determined using the Dahlberg formula S e d 2 2n where S e is the standard deviation of the differences of each of the replicates from its mean, n is the number of patients, and d is the difference between the first and the second recordings (Dahlberg, 1940). coefficients of the repeated measurements were calculated according to Bland and Altman (1996b). The higher the correlation coefficient between repeated measurements of a parameter, the better this parameter discriminates between individuals. The repeatability of the determination of the different parameters and the agreement were assessed according to Bland and Altman (1996a). Of the differences between the first and second measurements, 95% are expected to be less than the repeatability. All calculations were done using the StatsDirect Version (http// Method errors calculated that the Dahlberg formula showed values less than 1 mm for linear measurements and values less than 1.5 for angular measurements. The intraclass correlation coefficients were greater than.90 for all parameters. The repeatability was comparable for cleft and noncleft sides for all parameters. DISCUSSION Satisfactory functional results can be achieved after primary repair of the malformations in the majority of the UCLP patients. From this point of view, there is only a limited need for corrections and repeat operations. However, morphologic results tend to be less satisfactory (Breitsprecher et al., 1999). Despite the many advances in surgery, the cleft lip nose continues to be a stigma of cleft surgery. Many patients require secondary repair because of nasolabial asymmetries (Kane et al., 2000). It is well known that the method used to reproduce facial characteristics (two-dimensional [2D] or 3D) and the method RESULTS Determination of the plane of symmetry of the facial surface of the five patients yielded exactly the same coordinates of the plane for first and second measurements. The results of the determination of distances, angles, areas, and volumes are given in Tables 2 through 7. FIGURE 6 Facial volumes (yellow plane of symmetry, blue surface bordering the midfacial volume, red surface bordering the nasal volume, green surface bordering the lip volume).

5 Nkenke et al., MEASUREMENT ERRORS OF 3-D FACIAL ANALYSIS 133 TABLE 2 Facial Landmarks (Distance of Facial Landmarks From the Plane of Symmetry Determined in an Axial Plane Perpendicular to the Plane of Symmetry and Parallel to the Frankfort Horizontal Plane; Negative Values Reveal a Deviation to the Side, Positive Values Reveal a Deviation to the Side; for Definition of the Landmarks, See Table 1) Endocanthion * Endocanthion Pronasale Columella Base Cheilion Cheilion Method error coefficient agreement Repeatability * noncleft side; cleft cleft side; Pat. patient; Meas. measurement; Method error method error calculated by the Dahlberg formula applied to quantify asymmetry (global or localized analysis) may play a major role in the predominant site of asymmetry (Ferrario et al., 1994). Most studies involving the quantitative assessment of facial asymmetry in living persons have been performed on 2D reproductions of hard tissue (radiographs) or soft tissue (photographs) morphology. Both methods project a complex 3D structure onto a 2D plane, thus causing one of the facial dimensions (usually facial depth) to be lost. It is obvious that a correct and comprehensive evaluation of the surface of the face should involve all three spatial planes simultaneously (Ras et al., 1994b; O Grady and Antonyshyn, 1999; Ferrario et al., 2001). To evaluate the efficacy of the many different treatment methods with a standardized and quantitative method and to decide whether a surgical technique is adequate to improve the appearance of patients with cleft lip and palate deformities, it is desirable to objectively assess different facial features threedimensionally (Coghlan et al., 1987; Laitung et al., 1993; Russell et al., 2000). Therefore, it was the aim of the present study to introduce a new comprehensive method for the 3D analysis of facial symmetry of patients with UCLP malformations and to assess the measurement errors that occur during the determination of different parameters such as distances, areas, angles, and volumes. Although some studies have been carried out with different 3D imaging techniques, most of them have been confined to the determination of anthropometric landmarks. When single landmark measurements are used for symmetry analysis, endless comparisons between the two facial halves can be made. The position of a localized asymmetry can be easily appreci- TABLE 3 Nasal Landmarks (Distance of Nasal Landmarks From the Plane of Symmetry Determined in an Axial Plane Perpendicular to the Plane of Symmetry and Parallel to the Frankfort Horizontal Phase; Negative Values Reveal a Deviation to the Side, Positive Values Reveal a Deviation to the Side; for the Definition of the Landmarks, See Table 1) G lat * G lat G sup G sup G base G base Method error coefficient agreement Repeatability * noncleft side; cleft cleft side; Pat. patient; Meas. measurement; Method error method error calculated by the Dahlberg formula

6 134 Palate Craniofacial Journal, March 2006, Vol. 43 No. 2 TABLE 4 Nostril Landmarks (Distance of Nasal Landmarks From the Plane of Symmetry Determined in an Axial Plane Perpendicular to the Plane of Symmetry and Parallel to the Frankfort Horizontal Plane; Negative Values Reveal a Deviation to the Side, Positive Values Reveal a Deviation to the Side; for the Definition of the Landmarks, See Table 1) La lat * La lat La med La med Method error coefficient agreement Repeatability * noncleft side; cleft cleft side; Pat. patient; Meas. measurement; Method error method error calculated by the Dahlberg formula ated, but a global evaluation of the face is lost (Ferrario et al., 2001). Recently, a new technique of symmetry analysis has been introduced and has been validated, experimentally as well as clinically (Benz et al., 2002; Nkenke et al., 2003a). Instead of using single landmarks for the determination of the plane of symmetry, the technique takes advantage of all data points of a 3D facial surface image. This plane of symmetry is not dependent on manually defined landmarks and, therefore, is highly reproducible (Benz et al., 2002). In the present study, the determination of the plane of symmetry in five patients at two different points of time yielded exactly the same coordinates for each pair of corresponding planes of symmetry. The new method for the assessment of symmetry is available now in daily routine. In addition to the symmetry analysis, a global evaluation of the face should include the assessment of differences in facial surface areas and visible soft-tissue volumes between cleft and noncleft sides. These data allow an improved determination of the influence of surgery on facial growth (Russell et al., 2000; Ferrario et al., 2003). To date, there are no studies taking advantage of all the different aforementioned parameters during the evaluation of discrepancies between the two hemifaces of UCLP patients. In the present study, such a comprehensive analysis has been performed for the first time. The determination of well-defined facial surfaces areas for symmetry analysis has been described previously by the use of photographs or video imaging (Hurwitz et al., 1999; Russell et al., 2001). With both techniques, 3D structures are projected on 2D planes. Therefore, it seems that the areas assessed in the present study are more precise, because the curvature of TABLE 5 Nostril Dimensions, Nostril Area, and Nostril Angle Formed With the Plane of Symmetry (for the Definitions of the Parameters, See Table 1) Nostril Length * Nostril Length Nostril Width Nostril Width Nostril Angle (degrees) Nostril Angle (degrees) Nostril Area (mm 22 ) Nostril Area (mm 2 ) Method error coefficient agreement Repeatability * noncleft side; cleft cleft side; Pat. patient; Meas. measurement; Method error method error calculated by the Dahlberg formula

7 Nkenke et al., MEASUREMENT ERRORS OF 3-D FACIAL ANALYSIS 135 TABLE 6 Labial Landmarks (Lb and Lt lat, Respectively Distance of the Labial Landmarks Lb and Lt From the Plane of Symmetry Determines in an Axial Plane Perpendicular to the Plane of Symmetry and Parallel to the Frankfort Horizontal Plane; Negative Values Reveal a Deviation to The Side, Positive Values Reveal a Deviation to the Side; Lt vert Shortest Distance of the Labial Landmark Lt vert From an Axial Plane Through Col Parallel to the Frankfort Horizontal Plane; Lt ap Shortest Distance of the Labial Landmark Lt From a Coronar Plane Through Col Perpendicular to the Plane of Symmetry; Negative Values Reveal That the Landmark Lies Behind the Coronal Plane, Positive Values Reveal That the Landmark Lies in Front of the Coronal Plane; for the Definitions of the Landmarks, See Table 1) Lb Lt lat * Lt lat Lt vert Lt vert Lt op Lt ap Method error coefficient agreement Repeatability * noncleft side; cleft cleft side; Pat. patient; Meas. measurement; Method error method error calculated by the Dahlberg formula the planes are preserved by the 3D imaging technique, whereas it is lost when photographs or video imaging are used. Because of their 2D origin, photographs, as well as video images, are susceptible to parallax errors when the images are not assessed from the correct direction. It is not possible to correct later for these parallax errors. When 3D surface data are used, as in the present study, parallax errors do not play any role during image acquisition and the real information is not obscured. Unfortunately, previous studies of Hurwitz et al. (1999) and Russell et al. (2001) did not provide a complete metrical analysis of the assessed surfaces; they did not give absolute data in mm 2 for the areas, but counted only the number of pixels or gave only the ratio of the cleft side area to the noncleft side area to characterize the dimension of the measured areas. Therefore, a relevant comparison of these area data to the data of the present study, where quantification in mm 2 has been completed, is not possible. In the current literature, techniques for the determination of facial volumes have been proposed (Ferrario et al., 2003). However, they give only rough approximations of the volume, because they do not take advantage of the complete facial surface bordering the volume of interest (Ferrario et al., 1998). The volume determination is based on the generation of geometrical structures (tetrahedra) using only single landmarks from the facial surface as their vertices (Ferrario et al., 1998). The lateral surfaces of the tetrahedra are defined by these ver- TABLE 7 Area of the Vermillion and Virtual Facial Volumes Area vermillion * (mm 2 ) Area vermillion (mm 2 ) Volume midf. Volume midf. Volume nose Volume nose Volume lip Volume lip Method error coefficient agreement Repeatability * noncleft side; cleft cleft side; Pat. patient; Meas. measurement; Method error method error calculated by the Dahlberg formula

8 136 Palate Craniofacial Journal, March 2006, Vol. 43 No. 2 tices. They do not follow the convex and concave facial surface. Therefore, a large amount of individual information on the facial surface of the single patient is lost in the course of this kind of volume determination. In the present study, all data points of the surfaces surrounding the volumes of interest were used for the volume determination. The technique of volume determination had been validated (Nkenke et al., 2003b). Therefore, the new technique seems to provide better basic conditions to assess minimal differences between cleft and noncleft sides than the methods proposed previously. Previous studies revealed that for the calculation of measurement errors, a sample of five patients is sufficient to gain relevant results (Nkenke et al., 2003b). In the present study, the calculation of measurement errors has shown that they were small for all the different parameters. The method errors calculated by the Dahlberg formula were less than 1 mm for distances and less than 1.5 for angles. Although relative errors calculated from these data can vary distinctively, depending on the landmark that has been chosen, it has been stated that absolute errors in this range do not have any clinical importance independent of the relative deviation (Houston, 1983; Sandler, 1988). Therefore, the method introduced in the present study seems to be appropriate for the assessment of distances and angles in clinical use. coefficients of greater than.90 indicate a good relationship between the two sets of measurements, with little random error (Murphy and Willmot, 2005). All repeatability data fell between a narrow 95% limit of agreement, which means that the method of assessing distances, angles, areas, and volumes can detect useful clinical differences (Bland and Altman, 1996b). 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