DOI: 10.1148/radiol.2361040223
(Radiology 2005;236:559-564.)
© RSNA, 2005
Temporomandibular Joint Disk Position Assessed at Coronal MR Imaging in Asymptomatic Volunteers1
Marc Schmitter, DDS,
Bodo Kress, MD,
Christina Ludwig,
Andreas Koob, DDS,
Olaf Gabbert, DDS and
Peter Rammelsberg, DDS, PhD
1 From the Departments of Prosthodontics (M.S., C.L., A.K., O.G., P.R.) and Neuroradiology (B.K.), University of Heidelberg, Im Neuenheimer Feld 400, 69120 Heidelberg, Germany. Received January 5, 2004; revision requested April 13; final revision received August 6; accepted September 29.
Address correspondence to M.S. (e-mail: Marc_Schmitter{at}med.uni-heidelberg.de).
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ABSTRACT
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PURPOSE: To assess the normal position of the temporomandibular joint (TMJ) disk relative to the condyle by using coronal magnetic resonance (MR) imaging in asymptomatic volunteers.
MATERIALS AND METHODS: This study was approved by the review committee for human research, and all subjects signed an informed consent form. Thirty symptom-free volunteers without histories of TMJ disorders underwent standardized clinical examinations. Afterward, bilateral sagittal oblique and coronal oblique MR images were acquired with the patient's mouth opened and closed. The coronal oblique opened- and closed-mouth images were analyzed by using computer software. The medial and lateral edges of both the TMJ disk and the condyle were marked for these imaging examinations by using the section through the posterior 3 mm of the disk. To eliminate the effect of different magnifications and/or distortions, the distance between these points was measured automatically and divided by the largest mediolateral dimensions of the condyle. To assess the reliability of the measurements, four observers evaluated the position of the disk in the coronal plane. To assess the changes in position of the posterior 3 mm of the disk in the coronal plane in the closed- and opened-mouth positions, the Wilcoxon signed rank test for matched pairs was used. Interobserver measurement reliability was evaluated by using interclass correlation coefficients (ICCs).
RESULTS: Analysis of the coronal closed-mouth disk position revealed a medial position of the TMJ disk relative to the condyle in 11 (21%) of 52 analyzed joints. In the opened-mouth position, the medial location of the disk was more frequent: 29 (85%) of 34 analyzed joints exhibited a medial position of the disk relative to the condyle in this plane. This increasingly medial position of the disk was statistically significant (P 
.001). Measurement reliability assessment revealed sufficient results (ICC 
0.7).
CONCLUSION: At both closed- and opened-mouth MR imaging, a medially located TMJ disk seems to be within the normal range of variation. The disk seems to shift even more medially when the mouth is opened.
© RSNA, 2005
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INTRODUCTION
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Diagnoses of temporomandibular joint (TMJ) disorders include muscle disorders on the one hand and joint abnormalities on the other (1). Various procedures for the clinical examination of affected patients have been described (2,3). Studies involving different imaging modalities (4,5) have yielded inconsistent results: In some studies, there has been inadequate agreement between the clinical examination and imaging findings, and in others, there has been good agreement (6–8).
Magnetic resonance (MR) imaging is accepted as the most advanced imaging modality for the diagnosis of TMJ abnormalities (9). It is noninvasive and has the potential to yield high-quality tomographic images in any plane with bone as well as soft-tissue spatial resolution. Additionally, the patient is not exposed to ionizing radiation or any known biologic hazards. Other advantages of MR imaging are its sensitivity, specificity, and diagnostic accuracy (9). MR imaging has therefore become the reference standard for examination of the TMJ. The standard plane for MR imaging of the TMJ has been the sagittal plane. In various studies involving symptom-free subjects, investigators have described the variation in the normal TMJ disk position in this plane at closed- and opened-mouth MR imaging (10–12). Thus, the distinction between a normal and an abnormal disk position has become possible.
Some study investigators have found that auxiliary information can be obtained by using coronal MR images (12–14) and concluded that medial and lateral TMJ disk displacements are an important aspect of internal derangement (14). However, these studies have lacked a clear-cut definition of the normal TMJ disk position in the coronal plane, which has not been established with either opened-mouth or closed-mouth MR imaging. Only subjective descriptions of the normal disk position in closed-mouth coronal planes are available, and subjective descriptions of the normal disk position in opened-mouth coronal planes are absent. Therefore, assessments of the disk position in these planes have become inconsistent and unreliable and consequently have resulted in questionable diagnoses. Thus, the aim of this pilot study was to assess the position of the TMJ disk relative to the condyle in asymptomatic volunteers by using MR imaging in the coronal plane.
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MATERIALS AND METHODS
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Volunteers
This study was approved by the review committee for human research, and all subjects signed an informed consent form. To avoid age-related bias, not only young healthy volunteers but also older symptom-free healthy volunteers were recruited for participation. The study group consisted of 15 women aged 19–74 years (mean age, 32 years) and 15 men aged 22–79 years (mean age, 38 years). There were no statistically significant age-related differences between the male and female volunteers (Mann-Whitney U test). All subjects were recruited from among the staff (including retired persons) and student body of the University of Heidelberg.
Study inclusion criteria were as follows: no history of TMJ disorders, no diseases affecting the joints, no previous trauma to the head or face, no acute dental or ear diseases, no painful muscle sites, and no conditions contraindicating MR image acquisition.
Clinical Examination
All volunteers were physically examined before MR imaging examination by an author (M.S., with 2 years of experience in specialized clinical examinations and MR imaging of the TMJ), according to documented research diagnostic criteria for TMJ disorders (1). This examination includes assessment for the presence or absence of joint sounds and pain, palpation of the intra- and extraoral masticatory muscles with use of defined pressure, and measurement of the range of mandibular motion. This procedure yields defined cutoff limits for muscle- and joint-related diagnoses. The procedures involved in this examination, which is compliant with research diagnostic criteria for temporomandibular disorders, are described in detail elsewhere (1).
MR Imaging
A 1.5-T tomograph (Symphony; Siemens, Erlangen, Germany) with TMJ surface coils (Siemens) was used to acquire bilateral sagittal oblique and coronal oblique images (hereafter referred to as sagittal and coronal images, respectively) of the subjects both with their mouths opened and with their mouths closed. MR images of 60 TMJs were obtained in the 30 subjects.
To adjust the sagittal MR images, we obtained transverse scout images of the TMJs in the closed- and opened-mouth positions. Subsequently, five two-dimensional sagittal intermediate-weighted fast low-angle shot sections (208/10.2 [repetition time msec/echo time msec], 120 x 120-mm field of view, 256 x 256 matrix, 3-mm section thickness, acquisition time of 5.3 minutes) were obtained: one at the lateral edge of the condyle, one at the medial edge of the condyle, and three between the lateral and medial edges of the condyle. All sagittal sections were oriented perpendicular to the long axis of the condyle in the transverse plane. These sagittal images were used as localizer images for coronal MR imaging, as described by Hollender et al (15). Five coronal two-dimensional intermediate-weighted fast low-angle shot sections (208/10.2, 120 x 120-mm field of view, 256 x 256 matrix, 3-mm section thickness, acquisition time of 3.5 minutes) perpendicular to the axis of the TMJ disk were obtained: one through the posterior 3 mm of the disk, one behind that point, and three anterior to that point.
In addition, every effort was made to locate the penultimate section through the posterior edge of the disk: Because the section thickness was 3 mm, the posterior 3 mm of the disk was imaged. For both sagittal and coronal opened-mouth MR imaging, a mechanical mouth opener (Burnett BiDirectional TMJ Device; Medrad, Pittsburgh, Pa) was used to stabilize the opened-mouth position and therefore reduce image blurring due to mandibular motion. All MR imaging examinations were performed by the same radiologist (B.K., with 3 years of experience performing TMJ-related MR imaging examinations) under standardized conditions.
Image Evaluation
All sagittal MR images were evaluated independently by two observers: one radiologist (B.K.) and one dentist (M.S). The observers were evaluated beforehand during a calibration session, at which interobserver reliability assessment revealed acceptable agreement (
= 0.7 for evaluation of the dichotomous variables: presence or absence of disk displacement and/or arthrosis). Differences in the interpretation of the sagittal MR images were resolved by consensus. The sagittal sections were evaluated by using the criteria described by Rammelsberg et al (16). The images of one symptom-free subject had to be excluded from further analysis because of image blurring due to movement. Thus, we evaluated only 58 of the 60 TMJs.
Afterward, the coronal MR images were evaluated by using a standardized assessment protocol: The position of the TMJ disk relative to the condyle was measured on the penultimate section (ie, section through the posterior 3 mm of the disk) by using a computer program (Computer-aided Design and Measurement, version 6; Malz++Kassner, Braunschweig, Germany).
The MR imaging data were downloaded into the program, and the widest mediolateral distance (in the penultimate section) of the condyle was marked (line a in Fig 1). Afterward, perpendicular lines through the lateral and medial edges of the condyle (line b) were constructed automatically, and lines parallel to these perpendicular lines (line c) were drawn tangentially to the medial and lateral edges of the TMJ disk. The lateral distance between lines b and c (distance e) and the medial distance between lines b and c (distance f) were measured automatically. If line b was medial to line c at the medial edge of the disk, distance f became negative. Thus, if line b was lateral to line c at the lateral edge, distance e became negative.
To eliminate the effect of different magnifications and/or distortions that probably would have led to falsified linear measurements, all disk positions were expressed in terms of their relation to the mediolateral extension of the condyle: e` = e/a and f` = f/a, where a is the distance of line a (in millimeters); e and f are the distances of lines e and f (both in millimeters), respectively; e` is the position at the lateral distance between lines b and c in relation to the extension of the condyle; and f` is the position at the medial distance between lines b and c in relation to the extension of the condyle. Thus, the observers had only to mark the medial and lateral edges of the condyle and the disk. The closed- and opened-mouth MR images were measured as described above. These detailed construction schemes are illustrated in Figure 1. These schemes make the interpretation of the resulting values clear: If e` is greater than 0, then the lateral aspect of the disk covers the lateral aspect of the condyle. If e` is less than 0, then the lateral edge of the disk does not cover the lateral aspect of the condyle. For f`, the same formula is applied: When f` is greater than 0, the disk covers the medial aspect the condyle. Thus, if e` is less than 0 and f` is greater than 0, then the disk is medial to the condyle. In contrast, if e` is greater than 0 and f` is less than 0, then the disk is lateral to the condyle.
To assess the reliability of the measurements, four observers (B.K. and C.L. with 1 year of experience, A.K. with no experience, and O.G. with year of experience), all of whom were blinded to the clinical findings and the sagittal image evaluation results, independently performed the computer program–supported measurements in a randomized sample of 20 (out of the total of 58 joints) coronally imaged joints. There were no significant differences in age or sex (P > .1, Mann-Whitney U test) between the group of subjects with the 20 joints and the group of subjects with the remaining 38 joints. For the randomized sample of 20 joints, 10 (50%) subjects were male and 10 (50%) were female. For the sample of 38 joints, 18 (47%) subjects were male and 20 (53%) were female. The 38 coronally imaged joints were analyzed by the initially described two observers (B.K., M.S). Among the MR images of the 58 joints, images were excluded from further analysis if the positions of the medial and lateral edges of the disk were not definitive. Because the worst-case situation should have been simulated, none of the readers was calibrated—that is, trained with respect to measurement of the coronal MR images.
Statistical Analyses
Mediolateral alterations of the relationship between the disk and the condyle in the posterior 3 mm of the disk, as depicted on the opened- and closed-mouth MR images (of both joints), were analyzed by using the Wilcoxon signed rank test for matched pairs. However, the assumption of independence of the left and right joints is not necessarily acceptable. Consequently, a generalized estimation equation, or, GEE, model (mixed procedure performed by using SAS, version 8.2, software [SAS Institute, Cary, NC]) also was used.
The interobserver reliability of measurements was assessed by using interclass correlation coefficients (ICCs), and dichotomous variables were assessed by using statistics. All analyses, including descriptive statistics but with the exception of the analyses performed by using the generalized estimation equation model, were performed by using a statistical software program (SPSS, version 11.5, 2002; SPSS, Chicago, Ill).
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RESULTS
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TMJ Disk Position at Sagittal Opened- and Closed-Mouth MR Imaging
We were able to evaluate the normal disk position in 52 (90%) of the 58 TMJs at sagittal MR imaging. Six (10%) of the 58 joints had anterior disk displacement, whereas one volunteer had bilateral disk displacement. Therefore, five (17%) of 29 subjects had anterior disk displacement. Arthrosis (degenerative alterations including osteophytes) was found in three (10%) of the 29 subjects, whereas no osteochondral defects (large degenerative alterations without osteophytes) were found in the symptom-free volunteers. All subjects who were found to have arthrosis also had anterior disk displacement. These findings were confirmed on the coronal MR images.
TMJ Disk Position at Coronal Closed-Mouth MR Imaging
For six joints, no measurement was possible at either opened-mouth or closed-mouth coronal MR imaging because, owing to an anterior displaced disk, there was no longer an association between the disk and the condyle. The positions of the lateral aspect of the disk in relation to the condyle (e`) ranged from –0.50 to 0.23 (mean, 0.02; 2 standard deviations [SDs], ±0.34). The positions of the medial edge of the disk in relation to the condyle (f`) ranged from –0.10 to 0.25 (mean, 0.11; 2 SDs, ±0.14). In 11 (21%) of 52 joints, the disk was located medially (e` < 0, f` > 0). Position values for the left and right joints combined are illustrated in Figures 2 and 3. An example of a coronal MR image of a TMJ in the closed-mouth position is given in Figure 4.
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Figure 2. Graph illustrates distribution of positions of medial aspect of TMJ disk relative to condyle in closed-mouth position in left and right TMJs. f` is the distance between the line perpendicular to the longest mediolateral distance of the condyle and the line parallel to that perpendicular line and tangential to the TMJ disk at the medial aspect of the TMJ. Almost all measurements are positive. This finding indicates that the medial edge of the disk is medial to the condyle.
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Figure 3. Graph illustrates distribution of positions of lateral aspect of TMJ disk relative to condyle in closed-mouth position in left and right TMJs. e` is the distance between the line perpendicular to the longest mediolateral distance of the condyle and the line parallel to that perpendicular line and tangential to the TMJ disk at the lateral aspect of the TMJ. About 21% (11 of 52) of the measurements are negative. This finding indicates that the lateral edge of the disk is medial to the condyle.
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Figure 4. Two-dimensional coronal oblique intermediate-weighted fast low-angle shot MR image (208/10.2, 120 x 120-mm field of view, 256 x 256 matrix, 3-mm section thickness, acquisition time of 3.5 minutes) of TMJ in closed-mouth position in 20-year-old female volunteer. Both the medial (left *) and the lateral (right *) edges of the TMJ disk are medial to the condyle (c).
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TMJ Disk Position at Coronal Opened-Mouth MR Imaging
Although the opened-mouth position was stabilized by using a mechanical mouth opener, detailed analysis was impeded in some cases because of slightly blurred images: at the medial aspect of the disk in 15 joints and at the lateral aspect of the disk in 16 joints. Therefore, 37 opened-mouth MR images were analyzed with respect to the medial aspect of the disk, and 36 opened-mouth MR images were analyzed with respect to the lateral aspect.
The positions of the lateral edge of the TMJ disk in relation to the condyle (e`) ranged from –0.31 to 0.34 (mean, –0.1; 2 SDs, ±0.28). The positions of the medial aspect of the disk in relation to the condyle (f`) ranged from 0.11 to 0.38 (mean, 0.25; 2 SDs, ±0.14). Position values for the left and right joints combined are illustrated in Figures 5 and 6. Because these results are presented pairwise for closed- and opened-mouth positions, only 34 pairs are available; the position values are presented graphically. The increasingly medial position of the TMJ disk between the closed- and opened-mouth positions was verified by using the Wilcoxon signed rank test for matched pairs, and P values were less than .05 for the mouth position–based differences in values at the medial and lateral aspects of the disk.
Use of the generalized estimation equation model (ie, mixed procedure performed with SAS software) yielded a P value of .013 for the opened-mouth–versus–closed-mouth difference at the medial aspect of the disk and a P value of .114 for this difference at the lateral aspect. The comparisons and resulting P values are outlined in the Table. An opened-mouth coronal MR image of a TMJ is presented in Figure 7.
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Figure 7. Two-dimensional coronal oblique intermediate-weighted fast low-angle shot MR image (208/10.2, 120 x 120-mm field of view, 256 x 256 matrix, 3-mm section thickness, acquisition time of 3.5 minutes) of TMJ in opened-mouth position in female volunteer in Figure 4. Both the medial edge (black *) and the lateral edge (white *) are medial to the condyle (c). These findings combined with those in Figure 4 emphasize the medial shift of the TMJ disk.
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Interobserver Reliability Assessment
Results of interobserver reliability assessment indicated acceptable to excellent agreement according to the benchmarks described by Fleiss (17). Interobserver agreement regarding mediolateral measurements especially was remarkable (ICC = 0.9). The ICC for measurement of the lateral edge of the disk was 0.7, which indicated acceptable agreement. Agreement regarding the position of the medial edge of the disk was excellent (ICC = 0.8).
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DISCUSSION
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Alterations in the position of the TMJ disk may be accompanied by joint sounds, joint pain, and/or restricted mandibular motion. To assess these alterations, the normal position of the disk has to be established at MR imaging examination of healthy subjects who do not have signs or symptoms of TMJ disorders. Investigators in several studies (10,16) have assessed the position of the TMJ disk in the sagittal plane and found wide variations among healthy subjects. The results of these studies have supported the interpretation of sagittal MR images and helped observers avoid giving false-positive diagnoses.
However, there is incomplete information regarding the position of the articular disk in the coronal plane, and all of these descriptions are subjective: At both MR imaging (13,14) and anatomic examination (18), the coronal position of the disk in only the closed-mouth position is described. For the opened-mouth position, even subjective information is not available. Thus, only two dimensions ofa three-dimensional anatomic structure have been objectively assessed by using metric measurements. However, coronal alterations of the position of the disk, which are not explicable when only sagittal images are assessed, may explain the clinical findings (13). In addition, coronal MR images are suitable for assessment of the bone outline (19) of the condyle and may reveal a TMJ disk with a partial anterior displacement that is not seen in the sagittal plane. Thus, important information about the status of the TMJ is lost when only sagittal images are assessed (13).
A useful description of the position of the disk has to fulfill two major criteria: First, it has to be a simple and effective measurement. Second, it should allow different observers to assess the position of the disk as reliably as possible. In the present pilot study, the observers had to determine the positions of only the medial and lateral edges of the disk and the condyle. Reliability assessment revealed extraordinary agreement among four observers, even though these raters were not calibrated with respect to the assessment of the coronal images. If a calibration session had been performed, it can be assumed that even better reliability coefficients would have been calculated. Consequently, medial and lateral edge position measurements are suitable for assessment of the disk position in the coronal plane.
Steenks et al (18) found angulated coronal MR images to be preferable for assessment of the position of the disk in the coronal plane. Hollender et al (15) described a protocol for acquiring coronal MR images of the TMJ that yielded excellent images. Consequently, angulated coronal MR images obtained by using the protocol described by Hollender et al (15) were used in the present study. However, the major problem with obtaining measurements in the opened-mouth position seems to be movement-induced artifacts, which impede the accurate determination of the medial and lateral edges of both the disk and the condyle, especially in the coronal plane.
Despite this problem, the present study results demonstrate that in the closed-mouth position, a medial position of the disk in the coronal plane seems to be physiologic: It was seen in about 21% (11 of 52) of the joints in our investigation. This was evident by the negative values of e` and the positive values of f` (see Materials and Methods). A central position relative to the condyle seems to be an exceptional finding: In most cases in our study, the disk was located medially relative to the condyle ([f` – e`] > 0). The medial position was even intensified when the mouth was open, as expressed by the e` and f` values: e` became more negative (change in mean e`, 0.12), and f` became more positive (change in mean f`, 0.14). This means that the disk shifted, on average, about one-sixth of a part of the mediolateral condyle dimension to the medial aspect of the joint. On the other hand, a medially located disk was seen more frequently in the opened-mouth position: 29 (85%) of 34 joints showed a negative e` and a positive f`. Results of the Wilcoxon signed rank test for matched pairs indicated that this increase was significant. This result was confirmed by using the generalized estimation equation model with respect to the medial aspect of the disk. Therefore, a shift in the posterior part of the disk to the medial aspect of the condyle during mouth opening seems to be physiologic.
However, the findings of the present pilot study should be verified by using a larger number of volunteers. Furthermore, a comparison between the coronal disk position in healthy volunteers and that in patients who present with joint disorders without abnormalities in the sagittal MR imaging plane would be helpful for assessing the differences between these groups.
In the present study, 17% of the symptom-free subjects had anterior disk displacement at sagittal MR imaging. This finding is not exceptional: The prevalence of disk displacement in the symptom-free individuals observed in other studies has fluctuated between 12% and 34% (20).
Within the limitations of this pilot study, it can be concluded that a medially positioned TMJ disk in both the closed-mouth and the opened-mouth positions in asymptomatic subjects seems to be physiologic. Additionally, the disk seems to shift even more medially when the mouth is opened.
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FOOTNOTES
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Abbreviations: ICC = interclass correlation coefficient • SD = standard deviation • TMJ = temporomandibular joint
Authors stated no financial relationship to disclose.
Author contributions: Guarantor of integrity of entire study, M.S.; study concepts and design, M.S.; literature research, C.L., M.S.; clinical studies, B.K., M.S.; data acquisition, C.L.; data analysis/interpretation, M.S., B.K.; statistical analysis, M.S.; manuscript preparation, O.G., M.S., A.K.; manuscript definition of intellectual content and editing, P.R.; manuscript revision/review, M.S., B.K., P.R.; manuscript final version approval, M.S., B.K., A.K., P.R., O.G.
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TOP
ABSTRACT
INTRODUCTION
= distance between b and c at lateral aspect of joint, 
= distance between b and c at medial aspect of joint, l = lateral, m = medial.
) and closed-mouth (
) positions with respect to medial edge of TMJ disk for left and right joints. When the mouth is opened, as compared with when it closed, the medial edge of the disk is located more medially relative to the condyle. f` is the distance between the line perpendicular to the longest mediolateral distance of the condyle and the line parallel to that perpendicular line and tangential to the TMJ disk at the medial aspect of the TMJ.