Muscle strains are probably the most common type of injury to the myotendinous unit (MTU). A muscle strain is an acute stretch-induced injury secondary to excessive indirect force generated by eccentric muscular contraction. Muscle strains may occur anywhere in the body, but the most frequent muscles involved are the quadriceps femoris, biceps femoris, semimembranosus, semitendinosis, and gastronomies-soleus complex. Muscles that cross two joints and have a high proportion of fast twitch fibers are more prone to muscles stabilizing the hip, shoulder, and elbow joints. The pain elicited from an acute muscle strain is typically experienced during an athletic activity or immediately at its termination. The pathologic changes in an acutely strained muscle include disruption of the muscle fibers near the myotendinous junction along with edema and hemorrhage. The grade of a muscle strain depends on the degree of fiber disruption and the clinical findings.
The appearance of a grade 1 muscle strain on MRI is similar to the findings of a grade 1 muscle contusion. There may be enlargement of the muscle due to interstitial edema and hemorrhage and, on a spin-echo T2-weighted or STIR sequence, there will be increased signal intensity within the muscle. Muscle strains are frequently located near the muscle’s myotendinous junction. The tendon of a multipennate muscle extends into the muscle belly; therefore, the symptoms elicited by a strain may be located anywhere within a muscle and not merely at its ends. MRI has provided excellent documentation of the extent and position of these injuries. Fleckenstein et al reported on the MRI appearance of the natural history of acute muscle strains. Acutely, the abnormal signal intensity was identified throughout the muscle, but on follow-up studies the abnormal signal intensity was most prominent in the periphery of the muscle. In one patient there was persistent abnormal signal intensity within the muscle after complete resolution of symptoms.
A grade2 muscle strain manifests clinically as muscle pain associated with a loss of strength. Pathologically there is a macroscopic partial tear of the MTU. On an MRI study, there will be a partial tear of the muscle fibers associated with edema and/or hemorrhage. With a grade 3 strain there is a complete disruption of the MTU. Plain films provide little useful information in the evaluation of most muscle strains. Only if there is a grade 3 strain that results in gross instability or malalignment (e.g., a quadriceps rupture) will plain films be helpful. CT has also been used to evaluate muscular strain injuries, but it provides less useful clinical information compared to an MRI examination.
In addition to the evaluation of acute or delayed muscle injuries, MRI is an ideal imaging modality to follow the evolution of the inflammatory and reparative processes within a muscle. With MRI it is possible to detect any sequelae from a MTU injury (e.g., muscle atrophy or fibrosis). Clinically it can be extremely difficult to determine when a muscle has completely healed, and if an athlete or worker returns to his or her athletic activity or job too soon after injury, he or she may be predisposed to repeat injury. MRI has detected acute MTU injuries that were superimposed on sub acute or chronic injuries that may be predisposed the muscle to reinjury.