Sonographic Evaluation of the Temporomandibular Joint: Uses and Limitations

To the Editor: We read with great interest the article “Dynamic High-Resolution Sonography Compared to Magnetic Resonance Imaging for Diagnosis of Temporo mandibular Joint Disk Displacement” by Habashi et al,1 which was published earlier this year in the Journal of Ultrasound in Medicine. We agree with the authors on the importance of investigating sonography as an inexpensive and widely available imaging tool to supplement clinical examination findings in patients with suspected temporomandibular joint (TMJ) disorders. At our institutions, we currently have ongoing studies in the evaluation of the use of point-of-care sonography of the TMJ in pediatric patients with juvenile idiopathic arthritis. Our question and comment concerning the article published by Habashi et al1 regards the accuracy in the depiction of the anatomy of the TMJ on their sonograms and how this anatomy corresponds to TMJ anatomy as seen on magnetic resonance imaging (MRI). First of all, in Figures 1–3, the authors do not indicate in the figure legends whether these are axial or coronal scan planes.1 The first figure appears to be in the coronal plane given the appearance of the mandibular condyle; the second and third figures are more difficult to tell which scan plane is being used. We ask that the authors clarify this point. Second, and more importantly, what the authors are labeling as a normally positioned intra-articular disk in Figure 1 is superficial (closer to the transducer) to what they are labeling as the mandibular condyle.1 We propose that if this structure truly were the intra-articular disk, it would not be normally positioned but would be laterally displaced. A normally located disk should be cranially positioned in relation to the mandibular condyle and would thus be located to the left of the condyle on the sonogram (left being more cranial assuming this is a coronal scan plane, but as mentioned above, the authors do not indicate the scan plane in the legend). 1,2 However, we believe that this structure is incorrectly labeled and is not in fact the intra-articular disk. In our experience, because the TMJ is bordered inferiorly by the mandibular condyle and superiorly by portions of the temporal bone, either the mandibular fossa (in the normal closed-mouth position) or the articular eminence (in the normal open-mouth position), sonograms only depict the lateral aspect of the joint because of dense shadowing deep to these bony structures, which obscures the central and medial portions of the joint. Therefore, if any portion of the intra-articular disk is visualized, it is only the lateral portion of the disk, which is located between the mandibular condyle and the temporal bone, unless there is lateral displacement of the disk. We have provided a figure with a coronal sonogram, which has been rotated to be in a similar orientation as a coronal MR image (see our Figure 1). We believe that the hypoechoic area lateral to the condyle, which Habashi et al1 have labeled as the intraarticular disk, corresponds to the lateral extension of the joint capsule of the TMJ (see our Figure 1). In this regard, we are in agreement with other authors, who have referred to the measurement of this hypoechoic area as the capsulecondyle distance.3 Accurate depiction of the anatomic structures within the TMJ on sonography is essential when investigating the utility of sonography in the evaluation of this joint. It is particularly important given the growing literature on the use of TMJ sonography.4–7 Katzberg2 has written an excellent review of the difficulties of imaging the TMJ with sonography, providing diagrammatic, gross pathologic, and surfacerendered computed tomographic images to correlate what is actually being depicted on sonograms of the TMJ. He points out that the systemic reviews and meta-analysis articles assessing the diagnostic efficacy of sonography in detecting TMJ disk displacement have shown widely ranging sensitivities (13%–100%) and accuracies (51.5%–100%). 2,4,6