Tuesday, January 17, 2012

Bubbles in a Paralabral Cyst

Photo by Artolog
SLAP lesions and paralabral cysts are well-known lesions of the shoulder. Although classically associated with the throwing athlete, they can be seen with relatively sedentary patients as well. Any busy radiologist will see dozens on SLAP lesions every year, with some of these lesions harboring paralabral cysts.

In this case, a 38 year-old male with a history of a prior distal clavicular resection presented to his orthopedic surgeon with persistent shoulder pain. He was referred for an MR arthrogram, to elucidate the cause of his pain.

A coronal T2 fatsat image demonstrates a SLAP lesion (red arrow) as well as a large paralabral cyst (yellow arrows) that occupies the suprascapular and spinoglenoid notches:


An adjacent coronal image again depicts the SLAP lesion (red arrow), and also reveals an oval focus of decreased signal (green arrow) within the paralabral cyst:

 

A coronal T1 fatsat image demonstrates additional hypointense foci within the paralabral cyst (green arrows); the paralabral cyst (pink arrows) is primarily hypointense on this pulse sequence, but a portion of the cyst does fill with gadolinium (yellow arrow):

 

An axial intermediate image depicts the overall anatomy nicely, showing the posterosuperior component of the SLAP lesion (red arrow), paralabral cyst (yellow arrows) and hypointense material (green arrow) within the cyst:


What is the hypointense material within the cyst? Possible explanations include areas of calcification or air. A CT scan of the shoulder shows that the hypointense material is air within the paralabral cyst:




Tung et al. (AJR 174, 1707-1715, 2000) described the MRI features of shoulder paralabral cysts.  They observed that:

--> paralabral cysts are strongly associated with labral tears.

--> paralabral cysts can exert local mass effect, and cause a compressive neuropathy, typically affecting the suprascapular nerve.

--> prevailing theory is that these cysts form after the capsulolabral complex is torn or avulsed.

There are few reports of air in paralabral cysts in the literature. Lozano Calderon et al. recently reported a case of a paralabral cyst containing air in a middle-aged woman (Am J Orthop 38, E107-E109, 2009). Air can be more commonly seen in synovial cysts arising from spinal facet joints, but is an uncommon finding in shoulder paralabral cysts. The etiology of air formation in a paralabral cyst is unclear, although the mechanism of air formation in joints has been elucidated (Unsworth et al., Ann Rheum Dis 30, 348, 1971).

The differential diagnosis for hypointense material associated with a paralabral cyst is air, calcification, and prior surgery. Conventional radiographs and/or CT scan will reliably differentiate between these possibilities.