Teres minor muscle
|Teres minor muscle|
|Teres minor muscle (shown in red), seen from behind.|
|Muscles on the dorsum of the scapula, and the Triceps brachii muscle: #3 is Latissimus dorsi muscle
#5 is Teres major muscle
#6 is Teres minor muscle
#7 is Supraspinatus muscle
#8 is Infraspinatus muscle
#13 is long head of Triceps brachii muscle
|Latin||musculus teres minor|
|Gray's||subject #123 441|
|Origin||lateral border of the scapula|
|Insertion||inferior facet of greater tubercle of the humerus|
|Artery||posterior circumflex humeral artery and the circumflex scapular artery|
|Actions||laterally rotates the arm, adducts the forearm, stabilizes humerus|
It arises from the dorsal surface of the axillary border of the scapula for the upper two-thirds of its extent, and from two aponeurotic laminæ, one of which separates it from the infraspinatus muscle, the other from the teres major muscle.
Its fibers run obliquely upward and laterally; the upper ones end in a tendon which is inserted into the lowest of the three impressions on the greater tubercle of the humerus; the lowest fibers are inserted directly into the humerus immediately below this impression.
The tendon of this muscle passes across, and is united with, the posterior part of the capsule of the shoulder-joint.
The muscle is innervated by the posterior branch of axillary nerve where it forms pseudoganglion. Pseudoganglion has no nerve cells but only nerve fibres are present. Damage to the fibers innervating the teres minor is clinically significant.
The infraspinatus and teres minor attach to head of the humerus; as part of the rotator cuff they help hold the humeral head in the glenoid cavity of the scapula. They work in tandem with the posterior deltoid to externally (laterally) rotate the humerus, as well as perform adduction, extension and transverse extension.
Sometimes a group of muscle fibres from teres minor may be fused with infraspinatus.
There are two types of rotator cuff injuries: acute tears and chronic tears. Acute tears occur as a result of a sudden movement. This might include throwing a powerful pitch, holding a fast moving rope during water sports, falling over onto an outstretched hand at speed, or making a sudden thrust with the paddle in kayaking. A chronic tear develops over a period of time. They usually occur at or near the tendon, as a result of the tendon rubbing against the underlying bone.
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 through this anatomical tunnel. The axillary nerve and the PHCA posterior humeral circumflex artery pass posteriorly through 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 by 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.
- Bahr, Ronald. Ed. Clinical Guide to Sports Injuries. Gazette bok. ISBN 0-7360-4117-6.
- Brestas P.S. et al.. Ultrasound findings of teres minor denervation in suspected quadrilateral space syndrome. J Clin Ultrasound. 2006 Sep;34(7):343-7.
- Kim HM., Sonography of the teres minor: a study of cadavers., AJR Am J Roentgenol. 2008 Mar;190(3):589-94.