Magnetic resonance imaging of shoulder impingement.
MRI is an accurate technique for diagnostic evaluation of a broad spectrum of shoulder pathology. In particular, rotator cuff injuries are a frequent cause of pain and disability. The mechanism of injury to the supraspinatus tendon usually begins with microscopic tears of fibers of the triple helix collagen molecule. This leads to increased motion of the bound water within the collagen molecule that lengthens the effective T2 of the tendon, allowing short TE imaging sequences to detect signal in abnormal tendons. Clinically significant disease probably does not occur without frank collagen rupture, and the associated collection of free water within the severely diseased tendon presents as high signal on long TE images. Thus, the clinical significance of tendon disease is best evaluated on long TR, long TE image acquisitions. Any process that accelerates microscopic tear formation or delays repair can increase a patient's risk of supraspinatus tendon tears. Impingement is the most important process accelerating microscopic tear formation. MRI's multiplanar, tomographic imaging ability markedly improves the ability to sensitively and specifically detect bony impingement. Hopefully, this will allow earlier arthroscopic decompression and improved patient prognosis with impingement.