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Published online before print October 24, 2002, 10.1148/radiol.2253010942
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(Radiology 2002;225:852-858.)
© RSNA, 2002

Pediatric Imaging

Facial Canal Anatomy in Patients with Microtia: Evaluation of the Temporal Bones with Thin-Section CT1

Hideki Takegoshi, MD, Kimitaka Kaga, MD, Shigeru Kikuchi, MD and Ken Ito, MD

1 From the Department of Otorhinolaryngology, Faculty of Medicine, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8655, Japan. Received May 21, 2001; revision requested July 9; final revision received March 28, 2002; accepted May 13. Address correspondence to H.T. (e-mail: takegosi@saitama-med.ac.jp).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
PURPOSE: To ascertain the location of the facial nerve in patients with microtia by using thin-section computed tomography (CT).

MATERIALS AND METHODS: Thin-section CT was performed in 66 ears of patients with microtia (unilateral, n = 12; bilateral, n = 34) and mandibulofacial dysostosis (MFD, n = 20). Findings were compared with those in 22 ears with normal auricles (control group) by using the Dunnett two-sided t test.

RESULTS: The facial nerve at the mastoid portion in patients with MFD was 2 mm more lateral and 3 mm more anterior than that in control subjects (P < .01). The same portion in patients with microtia was 3 mm more anterior than that in the control subjects (P < .01). The distance between the facial nerve and the most lateral point of the temporal bone in patients with MFD was 10 mm shorter and that in patients with bilateral microtia was 3 mm shorter than that in the control subjects (P < .01).

CONCLUSION: The facial nerve in patients with microtia was not more lateral from the Bill bar (vertical crest) than that in control subjects. The facial nerve in patients with MFD was different from that in patients with microtia who had no other head anomalies.

© RSNA, 2002

Index terms: Bones, dysostoses, 126.1664, 243.1664 • Ear, abnormalities, 2123.1493, 2123.1664 • Ear, CT, 2123.12118 • Nerves, facial


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
The most frequently encountered developmental abnormality of the external ear is microtia. The incidence of microtia has been estimated to be approximately one in 10,000 births (1). Bilateral microtia is observed in 10% of affected patients (2). Microtia can be repaired by the age of 10 years by a plastic surgeon, and hearing can be improved by means of external canal and ossicular reconstruction between the ages of 5 and 12 years by an otologic surgeon, because most patients with microtia have aural atresia and middle-ear anomaly. Thin-section computed tomography (CT) is the preferred technique for preoperative evaluation of aural atresia. There are several reports on the usefulness of thin-section CT for preoperative planning in patients with congenital aural atresia (3–6).

Facial nerve injury is the most substantial complication after surgery for congenital aural atresia. Thus, it is essential to determine preoperatively the course of the facial nerve in these patients. The facial nerve has been reported to be more laterally and anteriorly displaced in these patients (4,5,7–10). However, there are few studies on the course of the facial nerve in patients with congenital aural atresia. The objective of this study was to ascertain the location and course of the facial canal with thin-section CT in patients with microtia, including those with unilateral and bilateral microtia and mandibulofacial dysostosis (MFD), and to compare the results with those in subjects with normal ears.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Patients
Between 1996 and 2000, 40 patients with microtia who were younger than 15 years of age underwent thin-section CT at the University of Tokyo. We retrospectively evaluated all patients. The patients were classified into three groups: those with unilateral microtia and those with bilateral microtia, both without any associated maxillofacial anomalies, and those with MFD. There were 12 patients (11 boys and one girl with a mean age of 111.4 and 85.0 months, respectively; seven right ears and five left ears in patients with a mean age of 118.0 and 96.8 months, respectively) with unilateral microtia, 18 patients (14 boys and four girls with a mean age of 90.2 and 114.8 months, respectively) with bilateral microtia, and 10 patients (four boys and six girls with a mean age of 57.5 and 95.2 months, respectively) with MFD.

Two authors (H.T., K.K.) graded the severity of microtia according to Marx’s classification system (11). Grade I microtia corresponds to a normal-shaped but smaller pinna, grade II to a residual vertical ridge of tissue, and grade III to complete absence of the pinna or the presence of only rudimentary soft tissue.

For comparison, 22 ears of subjects with normal auricles were evaluated as the control group. These subjects were consecutive patients younger than 15 years of age who underwent thin-section CT for other unilateral diseases of the ear between 1996 and 2000 at the University of Tokyo. There were 14 boys and eight girls (mean age, 89.6 and 129.0 months, respectively). We evaluated the contralateral ear rather than the diseased ear (12 ears with cholesteatoma, five with chronic otitis media, three with conductive hearing loss, one with sensorineural hearing loss with no associated aplasia of the middle and inner ear, and one with head trauma without temporal bone fracture). No temporal bone abnormality was detected with thin-section CT. In addition, the contralateral ear was also evaluated in 11 patients (10 boys and one girl with a mean age of 113.4 and 85.0 months, respectively) with unilateral microtia. One patient with unilateral microtia whose contralateral ear was not evaluated with thin-section CT was excluded.

Informed consent was obtained from all patients and control subjects, and the protocol was approved by the institutional review board.

CT Imaging
We obtained transverse CT images of the temporal bone with a HiSpeed Advantage unit (GE Medical Systems, Milwaukee, Wis). The images were obtained parallel to the orbital roof at 120 kVp and 160 mA with a 1.0-mm section thickness and a thin-section bone algorithm.

Image Evaluation and Analyses
The images were viewed at a window width of 4,000 HU and a window level of 400 HU. The equipment used for the facial canal measurements consisted of a personal computer (Power Macintosh-G3; Apple Computer, Calif) and a high-resolution color monitor (Multiscan 17c1; Sony, Tokyo, Japan). The software used for the study was Photoshop 5.0 (Adobe Systems, Calif).

The Bill bar (vertical crest) was defined as the basic point, and the CT image on which the Bill bar was found was defined as the basic scan. To provide an easily reproducible internal landmark, the Bill bar was selected as the reference point on which all other measurements are based. A line connecting the basic point and the midpoint of the porus acusticus was defined as the basic line. The central long axis of the internal auditory canal was selected as the basic line from which all other angles were calculated. The following were measured for analyzing the labyrinthine, tympanic, and mastoid segments (Fig 1): the line between the basic point and the geniculate fossa (GF) midpoint (BG-D), the angle between the basic line and BG-D (B-A), the angle between the labyrinthine segment and the proximal tympanic segment of the facial canal (BGF-A), the line between the GF midpoint and the most posterior point of the facial canal (GF-D), the angle between the basic line and GF-D (F-A), and the line between the most posterior point of the facial canal and the most lateral point of the temporal bone (FL-D), which makes an angle of 135° with GF-D (Fig 2). FL-D reflects the development of the temporal bone from tympanomastoid suture to petrosquamosal suture. One author (H.T.) measured these points on each of the images obtained up to 8 mm below the basic scan.



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  		Figure 1. Abbreviations used throughout the article.

 


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  		Figure 2a. (a) Transverse thin-section CT image obtained at the level of the basic scan. basic point = the Bill bar, basic line = line between the basic point and midpoint of the porus acusticus, G = GF midpoint, BG-D = line between the basic point and GF midpoint, B-A = angle between the basic line and BG-D, BGF-A = angle between the labyrinthine segment and the proximal tympanic segment of the facial canal. (b) Transverse thin-section CT image obtained at the level of the tympanic segment of the facial nerve. basic line = line between the basic point and midpoint of the porus acusticus, G = GF midpoint, F = most posterior point of the facial canal, GF-D = line between G and F, F-A = angle between the basic line and GF-D, L = most lateral point of the temporal bone on the line that makes a 135° angle with GF-D, FL-D = line between F and L.

 


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  		Figure 2b. (a) Transverse thin-section CT image obtained at the level of the basic scan. basic point = the Bill bar, basic line = line between the basic point and midpoint of the porus acusticus, G = GF midpoint, BG-D = line between the basic point and GF midpoint, B-A = angle between the basic line and BG-D, BGF-A = angle between the labyrinthine segment and the proximal tympanic segment of the facial canal. (b) Transverse thin-section CT image obtained at the level of the tympanic segment of the facial nerve. basic line = line between the basic point and midpoint of the porus acusticus, G = GF midpoint, F = most posterior point of the facial canal, GF-D = line between G and F, F-A = angle between the basic line and GF-D, L = most lateral point of the temporal bone on the line that makes a 135° angle with GF-D, FL-D = line between F and L.

 
For GF-D, F-A, and FL-D, each measured point was averaged from three fields. Three fields included the level from the GF to the oval window (GO level), the level from the oval window to the round window (OR level), and the level under the round window (UR level) (Fig 3). Each measurement was performed twice by the same person, and the mean values were used. By using an orthogonal coordinate system, O-xy, with the basic point as the origin and the basic line as the x axis, the x and y values of each ear were calculated. The system can be expressed by using these formulas: x(GF) = BG-D x cos(180 - B-A), y(GF) = BG-D x sin(180 - B-A), x(each level) = x(GF) + [GF-D x cos(F-A)], and y(each level) = y(GF) - [GF-D x sin(F-A)].



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  		Figure 3. Diagram from the right lateral view indicates the level from the GF to the oval window (GO level), the level from the oval window to the round window (OR level), and the level under the round window (UR level). Figure shows the horizontal semicircular canal (1), malleus (2), incus (3), stapes (4), round window (5), and the labyrinthine (a), tympanic (b), and mastoid (c) portions of the facial nerve.

 
Statistical Methods
These measurements in the control group were compared with those in the microtia groups by using the Dunnett two-sided t test. In addition, the age distribution and the severity of microtia in each group were compared with those of the other groups by using the Tukey multiple comparisons test. Comparison between age and measurements in the control group was performed by using regression analysis. Statistical significance was assumed when P was less than .01. All data were analyzed by using statistical software (SPSS, version 9.0; SPSS, Chicago, Ill).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Age Distribution and the Severity of Microtia
Patient age ranged from 51 to 154 months (mean age ± SD, 109.2 months ± 26.1) in the unilateral microtia group, from 32 to 149 months (mean age, 96.6 months ± 36.2) in the bilateral microtia group, and from 1 to 163 months (mean age, 80.1 months ± 48.0) in the MFD group. The age of the subjects in the control group ranged from 38 to 158 months (mean age, 104.0 months ± 39.1). The age of the patients in the unilateral microtia group in which the contralateral ear was also evaluated ranged from 51 to 154 months (mean age, 110.8 months ± 26.7). There was no significant difference among the groups with regard to age distribution (P > .05).

The severity of microtia in the unilateral group was classified as grade I for three ears, grade II for five ears, and grade III for four ears. We graded 34 ears in the bilateral group, except for two ears that were not evaluated with thin-section CT: Three ears were classified as grade I, 17 ears as grade II, and 14 ears as grade III. In the MFD group, six ears were grade I, seven ears were grade II, and seven ears were grade III. There was no significant difference among the groups with regard to the severity of microtia (P > .05).

Measurements
Table 1 shows the mean values and SDs of BG-D, B-A, and BGF-A for each group. With regard to BG-D, there were no significant differences between the control group and the other groups. However, B-A and BGF-A in the MFD group were more obtuse than those in the control group (P < .01).


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  		TABLE 1. BG-D, B-A, and BGF-A Data in the Five Groups

 
Table 2 shows the mean values and SDs of F-A for each group. At the OR level, the F-A in the MFD group was more acute than that in the control group (P < .01). At the UR level, F-A in the bilateral microtia and MFD groups was more acute than that in the control group (P < .01).


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  		TABLE 2. F-A Data in the Five Groups

 
Table 3 shows the mean values and SDs of GF-D for each group. At OR and UR levels, GF-D in all the microtia groups was significantly shorter than that in the control group (P < .01).


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  		TABLE 3. GF-D Data in the Five Groups

 
Table 4 shows the mean values and SDs of FL-D for each group. On all images, FL-D in the MFD group was shorter than that in the control group (P < .01). At the UR level, FL-D in the bilateral microtia group was shorter than that in the control group (P < .05).


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  		TABLE 4. FL-D Data in the Five Groups

 
Tables 5 and 6 show the mean x and y values for each group. With regard to the x values, the MFD group showed significant differences from the control group at the UR level (P < .01). With regard to the y values, all the microtia groups showed significant differences from the control group at OR and UR levels (P < .01). We reconstructed the facial nerve course from the mean x and y values in each group (Fig 4).


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  		TABLE 5. x Values in the Five Groups

 

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  		TABLE 6. y Values in the Five Groups

 


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  		Figure 4. Graph based on data from transverse reconstructed images shows the facial nerve course from the mean x and y values in each group. Uni-MN = unilateral microtia, bilateral examination; Uni-M = unilateral microtia, unilateral examination; Bi-M = bilateral microtia and examination.

 
Age and Measurements in the Control Group
With respect to the control group, there was no significant difference between age and measurements except for FL-D (Table 7).


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  		TABLE 7. Quantitative Relationship between Age and Measurements in the Control Group (n = 22)

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
There are several reports on the role of CT in congenital aural atresia (3–6) and MFD (12–16). The facial nerve has been reported to show abnormal lateral and anterior displacement in these patients (4,5,7–9,15,16). However, few investigators report quantitative objective evidence indicating that in patients with congenital aural atresia, the facial nerve runs more laterally and anteriorly than that in control subjects (5,9). To our knowledge, there are no reports of comparison of the facial nerve course in patients with microtia, including those with unilateral and bilateral microtia and MFD. The objective of this study was to determine the location and course of the facial canal by means of thin-section CT in patients with microtia and compare them with those in control subjects.

In past reports of normal facial nerve anatomy, the angle between the internal auditory canal and the labyrinthine segment of the facial canal is 125°–132°, the angle between the labyrinthine segment and the proximal tympanic segment of the facial canal is 58°–75°, and the labyrinthine and tympanic segments of the facial canal are 3–5 and 10–12 mm in length, respectively (17,18,22,23). In the present study, the mean values of B-A, BGF-A, BG-D, and GF-D of the OR level of the control group agree approximately with those.

With regard to the distance between the basic point and the GF midpoint, there were no significant differences between the control group and the other groups. The mean B-A was 19.7° more obtuse in the MFD group than in the control group. This indicates that the GF midpoint is more posterior and more lateral in patients with MFD than in other groups. With regard to the x and y values, however, there were no significant differences in the GF midpoint between the MFD and control groups. BGF-A in patients with MFD was a mean of 86.8°, which is more obtuse than that in the control group.

From the data of F-A, GF-D, and FL-D, it is considered that the tympanic and mastoid portions of the facial nerve run more laterally and anteriorly in patients with MFD than do those in control subjects. From the x and y values, the mastoid portion of the facial nerve was a mean of 2 mm more lateral and 3 mm more anterior in patients with MFD than that in the control group. From the y values, it was determined that the mastoid portion of the facial nerve in patients with microtia was a mean of 3 mm more anterior than that in the control group. In this study, the x values in the microtia groups showed no significant differences from those in the control group. These results indicate that the facial nerve course in patients with microtia is not more lateral than that in control subjects when the Bill bar is considered the origin. However, FL-D in patients with MFD was a mean of 10 mm shorter and that in patients with bilateral microtia was a mean of 3 mm shorter than that in the control subjects. It is considered that FL-D reflects the development of the mastoid part of the temporal bone.

These results show that the mastoid bone in patients with MFD and bilateral microtia was more poorly developed than that in control subjects. The results also indicate that the facial nerve is found to be unexpectedly more laterally placed when the mastoid part of the temporal bone is drilled for repair of aural atresia in patients with microtia. Jahrsdoerfer (9) reported that the facial nerve at the level of the round window in patients with aural atresia may be as much as 4 mm more lateral compared with its position at the round window in subjects with normal ears.

In the present study, the unilateral microtia group showed no significant differences from the control group with regard to either x values or FL-D. However, there were four ears in the unilateral microtia group that showed an FL-D at least 2 mm shorter than that in the control group.

MFD is an autosomal dominant genetic disorder. The gene has been mapped to chromosome 5 (5q32-33.1) (19). The fully expressed phenotype is characterized by typical dysmorphic features involving the face, eyes, and mandible, structures derived from the first and second branchial arches. In the present study, all the patients with MFD had bilateral microtia. There was a significant difference between the facial nerve course in the MFD group and that in the bilateral microtia group. This implies that facial nerve development in patients with MFD is clearly different from that in patients with microtia, for both those with and those without any other maxillofacial anomalies.

The labyrinthine and tympanic portions of the facial canal in children are about the same size as that in adults in terms of both length and width (20), and these portions develop in the bone within or near the otic capsule, which reaches its adult size by the 21st week of intrauterine life (21). Weiglein (20) has reported that the most marked changes with development are the changes in the length of the mastoid portion of the canal. In the present study, there was some difference between age and FL-D in the control group. However, the age distribution was not significantly different among the groups.

In conclusion, the results of the present study show that the mastoid portion of the facial nerve was a mean of 2 mm more lateral in patients with MFD and 3 mm more anterior in patients with microtia than in control subjects on the basis of the Bill bar. Although the relationship to deeper structures of the otic capsule is fairly constant, there is less distance to reach the facial canal during a lateral surgical approach compared with that in subjects with normal ears. These data are expected to be useful for safe reconstruction surgery in patients with microtia and atresia.


   FOOTNOTES
 
Abbreviations: GF = geniculate fossa, MFD = mandibulofacial dysostosis

Author contributions: Guarantor of integrity of entire study, K.K.; study concepts, H.T.; study design, H.T., S.K.; literature research, H.T.; clinical studies, K.K., K.I.; data acquisition, H.T., K.I.; data analysis/interpretation, H.T.; statistical analysis, H.T.; manuscript preparation, H.T.; manuscript definition of intellectual content, K.K., H.T.; manuscript editing, K.K., H.T.; manuscript revision/review, K.K., S.K.; manuscript final version approval, K.K., H.T.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 

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