About the difficulty in interpreting ultrasonographic images of temporomandibular joint

C. Mastaglio and F. Fantini1

Rheumatology Unit of the Moriggia-Pelascini Hospital, Gravedona, and 1 Chair of Rheumatology, University of Milan, Italy

Correspondence to: C. Mastaglio, U.O.S. di Reumatologia Ospedale Moriggia-Pelascini, v. Pelascini 1, 22015 Gravedona (Como), Italy. E-mail: claudio.mastaglio{at}libero.it

SIR, We have read with interest the paper ‘A comparison of ultrasonography and magnetic resonance imaging in the evaluation of temporomandibular joint involvement in rheumatoid arthritis and psoriatic arthritis’ by Melchiorre et al. [1].

We would like to dwell upon the method of ultrasonographic (US) examination, unfortunately not described in the paper, for a few observations that, in our opinion, could help to better understand the images obtained with this technique.

The echographic study of the temporomandibular joint (TMJ) consists of different scans in coronal, axial and oblique plans [2, 3]. The exploration of the bone profile of the mandibular condyle can give very precise information about the condition of this structure, which is visualized through a window of 120° in the coronal scans and 40° in the axial scans. Further information can be obtained with the dynamic scans: they allow us to observe the condyle posterosuperior surface, to measure the anterior translation of the condyle, having as a landmark the tragus cartilage, and finally to study the articular and peri-articular soft tissues, included part of the morphology and the movement of the disc (Figs 1 and 2).



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FIG. 1. Coronal scan of a normal TMJ. Main findings: a, skin layers; b, masseter muscle; c, articular space and lateral capsule; d, acoustic shadow of the zygomatic arch; e, contour line of condyle; f, temporal squama and muscle; g, space between condylar head and zygomatic arch (disc); h, space between the contour line of the condyle and tragus cartilage; i, tragus cartilage.

 


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FIG. 2. Axial scan of a normal TMJ. Legend as in Fig. 1.

 
As for the study of the articular space in the axial scans, our experience suggests that the image of the capsulosynovial thickening should be considered as the slightly echoic structure between the convex bone profile of the condyle and the convex hyperechoic line of the capsular external surface.

In the axial scans we can consider as an effusion in the joint only the anechoic space that can be seen anteriorly or posteriorly just below the condylar convexity near to the mandibular condylar neck, particularly when it shows the unambiguous hallmarks of the effusion, mainly the distal acoustic enhancement (see Fig. 3).



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FIG. 3. Posterior effusion and synovial thickening, axial scan.

 
In the coronal scan, an effusion can be detected superficially and caudally to the condylar convexity. Close to the condylar convexity, the compression can often hide the fluid: at this level only a major effusion can be detected [4].

The measure of the capsulosynovial thickening must be obtained perpendicularly to the tangent line to the convexity, to avoid over- or underestimating the real values. It is also quite difficult to measure this thickening if a low-frequency probe (under 10 MHz) is used. The instrument we use to assess the TMJ, a Toshiba Nemio with a multifrequency linear PLM-1204AT probe (8–14 MHz), provides an axial resolution of 0.14 mm and a lateral resolution of 0.35 mm.

Studying the condylar profile by US, it is possible to assess the following features: erosions of the cortical profile on the lateral side, superior irregularities of the condylar profile, its flattening and the sharpening of the convexity at the point of passage from the lateral to the superior condylar surface in the coronal scan (see Fig. 4). The last alterations are typical of the condylar resorption, similar to the radiographic images [5].



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FIG. 4. Erosion of the condylar head, coronal scan. Legend as in Fig. 1.

 


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FIG. 5. Our interpretation of Fig. 2 of the paper by Melchiorre et al. [1], shown specularly as previous images. Legend as in Fig. 1. The distance between the two marks indicates the space between the condylar profile and the capsular wall and does not represent the width of the joint capsule. A doubtful small effusion is present distally to the condylar head (small arrow).

 
According to our observations, in the reported US image in the work of Melchiorre et al. [1], which presumably is an oblique scan showing the condylar convexity, the neck and a small part of the mandibular branch, the capsular synovial space is clearly thickened and hypoechoic and shows a convexity along the plane of scanning (probably due to a thin effusion). The maximum thickness is detectable at the neck, i.e. distally to the point taken as a reference in the paper. The zygomatic arch generates the typical acoustic shadowing, which in the picture is appreciable proximally to the condyle. The subsequent hypoechoic image, defined as pseudocystic in the paper, is produced by the muscular fibres of the temporal muscle. These fibres appear hypoechoic because of the anisotropic effect as they pass below the zygomatic arch and are oblique to the surface of the probe (see Fig. 5).

All these remarks come from our study of the TMJ in patients with juvenile chronic arthritis, which we started in 1999 at the Gaetano Pini Institute in Milan. Some preliminary data have been previously presented [6].

The authors have declared no conflicts of interest.

References

  1. Melchiorre D, Calderazzi A, Maddali Bongi S et al. A comparison of ultrasonography and magnetic resonance imaging in the evaluation of temporomandibular joint involvement in rheumatoid arthritis and psoriatic arthritis. Rheumatology 2003;42:673–6.[Abstract/Free Full Text]
  2. Mastaglio C, Gattinara M, Gerloni V et al. Ultrasonography as a diagnostic aid in temporomandibular joint (TMJ) involvement of juvenile chronic arthritis (JCA): description of echographic method. Ann Rheum Dis 2003;62(Suppl 1):143.
  3. Gateno J, Miloro M, Hendler BH, Horrow M. The use of US to determine the position of the mandibular condyle. J Oral Maxillofax Surg 1993;51:1081–6.
  4. Fantini F, Mastaglio C. Videoclip ‘TMJ sonography in juvenile chronic arthritis" 3rd EULAR Sonography Course, Milan, 20–22 April, 2001.
  5. Pedersen TJ, Jensen JJ, Melsen B, Herlin T. Resorption of the temporomandibular condylar bone according to subtypes of juvenile chronic arthritis. J Rheumatol 2001;28:2109–15.[ISI][Medline]
  6. Mastaglio C, Fantini F. Ultrasonography of joints and soft tissues. Minicourse in the 10th European Pediatric Rheumatology Congress, Annual Scientific Meeting of PRES, Stresa, Italy, 2–5 October 2003.
Accepted 8 October 2004





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