Teres Minor Muscle - Imaging

Imaging

Atrophy of the teres minor muscle is often a consequence of a rotator cuff tear. But also uncommon isolated teres minor atrophies have been found. A so-called quadrilateral / quadrangular space syndrome causes excessive and or chronically compression of the structures which pass thru this anatomical tunnel. The axillary nerve and the PHCA posterior humeral circumflex artery pass posteriorly thru the space. The concerned patients note shoulder pain and paresthesia down the arm first and foremost in abduction, extension, external rotation and overhead activity. Selective atrophy of the musculus teres minor has been seen and pulled together directly with compression of the corresponding axillary nerve branch or PHCA. Fibrous bands, inferior paralabral cysts, lipoma or dilated veins can occupy the quadrilateral space pathologically. Similar symptoms are common with anterior shoulder dislocation, humeral neck fracture, brachial plexus injury and thoracic outlet and inlet syndrome. It is important to include those pathologies for a complete as possible differential diagnosis.
Ultrasonography is a wide spread, low cost, harmless and useful tool to detect a fatty degenerative atrophy of the teres minor and shows in affected muscles increased echogenicity and betimes a slight reduction in muscle bulk. MR imaging helps to consolidate the diagnosis of neurogenic muscle atrophy. Extracellular edema after traumatic events causing neural damage show an increased signal intensity on T2-weighted MRI sequences and normal intensity on T1-weighted sequences. PHCA compression and reduced blood flow in stressful arm positions and or maneuvers can easily be diagnosed thru a dynamic ultrasonographic Color Doppler blood flow examination. The nerve should be detected adjacent to the vessel. In an elevated arm position the axillary neurovascular bundle can be seen at the posterior axillary fold just before it perforates the deltoideus. While the posterior course is well visible in the neutral position. As all but always, the imaging assessment has to be bilateral comparative and there are asymptomatic arterial occlusions. For a detailed assessment of the artery, a MR angiography is required. The major task of an ultrasonographic examination is to rule out any space occupying mass. Additional electromyography is helpful to reveal any decelerated nerve conduction velocity, and thus denervation of the concerned muscle.
First to identify via ultrasonography is the characteristic shape of the infraspinatus muscle tendon unit. On the longitudinal view it reveals itself due to the central tendon and triangular shape. Subsequently the transducer has to be slided inferiorly, maintaining the same orientation in order to reach the teres minor muscle tendon unit. Following the unit leads to the broad insertion on the humerus. Rotating the transducer 90° allows the transversal view, on which the musculus teres minor insertion has a characteristic elongated shape.