There is a well-defined, encapsulated lesion at the anterior radial side of the proximal forearm. It is hyperechoic in comparison to the adjacent muscle along with the presence of linear internal echogenic strands. There is no calcification/ cystic changes/ vascularity. The approximate size of the lesion is about 31 x 29 x 15 mm. With compression, the lesion is deformable.
The lesion is between the superficial and deep heads of the supinator muscle causing anterior displacement and compression of the posterior interosseous nerve resulting in nerve edema and hypoechogenicity.
The asymptomatic side posterior interosseous nerve was examined for comparison which shows normal coarse, normal echopattern without any perineural mass lesion.
Extensor pollicis longus tendon is intact. There are possibly two tendon slips just distal to Lister's tubercle. The extensor muscles show moderate diffuse fatty infiltration and moderate volume loss as compared to the asymptomatic side.
The common extensor tendon shows normal echopattern without changes of tendinosis or tear.