Quadrilateral Space

The quadrilateral space is bounded by the teres minor muscle superiorly, teres major muscle inferiorly, the long head of the triceps muscle medially, and by the surgical neck of the humerus laterally. The axillary nerve and the posterior humeral circumfiex artery exit through this space.

This space is important because mass effect on it can lead to compression of the axillary nerve and posterior circumflex humeral artery, resulting in a combination of signs and symptoms referred to as the quadrilateral space syndrome

• Poorly localized shoulder pain exacerbated by abduction and external rotation.
• Paresthesia along the lateral shoulder and upper posterior arm (innervated by the axillary nerve)
• Discrete point tenderness in the lateral aspect of the quadrilateral space
• Weakness and atrophy of the teres minor and/or deltoid muscles (innervated by the axillary nerve).

The characteristic MRI finding is focal atrophy of or abnormal signal in the teres minor muscle and/or portions of the deltoid muscle. Large paralabral cysts can sometimes be seen in the quadrilateral space and are associated with labral tears. Fibrous bands in the region of the quadrilateral space can also be a cause, but I'm not sure if these can be resolved on MRI.

Differential considerations include:

• Parsonage-Turner syndrome: Look for involvement of more than one branch of the brachial plexus.
• Isolated teres minor tear: Very rare. Look for disruption of the tendon.

References

• Linker CS, Helms CA, Fritz RC. Quadrilateral space syndrome: findings at MR imaging. Radiology. 1993;188(3):675-6.
• Cothran RL Jr, Helms C. Quadrilateral space syndrome: incidence of imaging findings in a population referred for MRI of the shoulder. AJR Am J Roentgenol. 2005;184(3):989-92.