MRI findings of traumatic and degenerative rotator cuff tears and introduction of the “cobra sign”

Abstract

A rotator cuff tear (RCT) is a common shoulder diagnosis and its etiology may be acute, traumatic, or chronic degenerative. Differentiation between the 2 etiologies may be important for multiple reasons, but remains difficult based on imaging. Further knowledge about radiographic and magnetic resonance findings to distinguish traumatic from degenerative RCT is needed. We analyzed magnetic resonance arthrograms (MRAs) of 96 patients with traumatic or degenerative superior RCT, which were matched according their age and the affected rotator cuff muscle into the 2 groups. Patients older than 66 years of age were excluded from the study to avoid including cases with pre-existing degeneration. In the case of traumatic RCT, the time between the trauma and MRA had to be less than 3 months. Various parameters of the supraspinatus (SSP) muscle-tendon unit were assessed (tendon thickness, presence of a remaining tendon stump at the greater tubercle, magnitude of retraction, layer appearance). The retraction of the 2 SSP layers were individually measured to determine the difference of retraction. Additionally, edema of the tendon and muscle, the tangent- and kinking-sign as well as the newly introduced Cobra-sign (bulging of the distal part of the ruptured tendon with slim configuration of the medial part of the tendon) were analyzed. Edema within the SSP muscle (sensitivity 13%, specificity 100%, P=.011) or the tendon (sensitivity 86%, specificity 36%, P=.014) are more frequent in traumatic RCT. The same association was found for the kinking-sign (sensitivity 53%, specificity 71%, P=.018) and the Cobra sign (sensitivity 47%, specificity 84%, P=.001). Even though not statistically significant, tendencies were observed toward thicker tendon stumps in traumatic RCT, and greater difference in retraction between the 2 SSP layers in the degenerative group. The cohorts had no difference in the presence of a tendon stump at the greater tuberosity. Muscle and tendon edema, as well as tendon kinking appearance and the newly introduced cobra-sign are suitable MRA parameters to distinguish between traumatic and degenerative etiology of a superior RTC.