Magnetic resonance imaging of the temporomandibular joint: diagnostic difficulty caused by extensive pneumatization of the mastoid air cells

Objective. During routine MR imaging of temporomandibular joints we have ob served that an area of low signal is sometimes identified within the tempor al bone overlying the glenoid fossa on proton-density sequences. This findi ng at times simulates a mass. CT in this situation has demonstrated marked pneumatization of the mastoid air cell extending to that portion of the tem poral bone overlying the glenoid fossa. We undertook a clinical study to de termine the frequency and appearance of such a finding. Design and patients. We reviewed the images of 12 patients who underwent bo th MR and CT examinations of their temporomandibular joints. The authors, b y consensus, graded all 24 joints using a four-point scale for the presence and appearance of a low-signal area overlying the glenoid fossa on the MR images prior to seeing the corresponding CT images. We then compared our gr ading with the findings on CT to assess for the presence and extent of pneu matization of the temporal bone overlying the glenoid fossa. Results. Of the 24 joints reviewed there was identical extent of low signal on MR imaging and extent of pneumatization on CT in 22 of 24 joints. In tw o of the 24 joints assessed there was overestimation of pneumatization due to extensive sclerosis secondary to surgery. Conclusion. The findings of an area of low signal overlying the glenoid fos sa on a T2-weighted scan of a nonoperative temporomandibular joint is often due to extensive temporal bone pneumatization of the mastoid air cells. Re cognition of the nature of this finding will avoid mistaking it for a fibro us or osseous lesion. Patients with previous operative history may, however , require further investigation with CT.