Muscle Contusions and Tears
Muscle injuries may result from a direct or indirect application of force to the muscle. A direct blow to a muscle may cause a muscle contusion with disruption of muscle fibers. Acute disruption of muscle fibers and capillaries may precipitate soft tissue hemorrhage and a hematoma along with a secondary inflammatory response. With the acute pain associated with muscle injury, it may be difficult on a physical examination to determine the precise location, extent, and severity of an injury. Prior to the implementation of MRI, radiologic imaging studies were of little value in the evaluation of acute muscle injuries. On plain films there may be obscuration of the fat planes surrounding an injured muscle secondary to the perimuscular edema. With CT there may be an alteration of the size or contour of a muscle but detection of intramuscular hemorrhage, edema, or a hematoma is difficult. With the excellent soft tissue contrast resolution provided by MRI, it is now possible to obtain the following important clinical information related to a muscle injury: (1) the extent of muscle edema and/or hemorrhage; (2) is a focal hematoma is present, including its size and location; (3) the degree and extent of muscle fiber disruption; (4) if there is complete disruption of the muscle, whether there is associated muscle retraction; (5) whether there is interruption of the overlying fascia and if there is a muscle herniation; (6) the degree of muscle swelling and the detection of a possible concomitant compartment syndrome; and (7) whether single or multiple muscles are injured. Muscle contusions occur most frequently in the lower extremities, particularly involving the quadriceps mechanism.
On an MRI examination, a muscle contusion is detected by abnormal signal intensity and morphology of the muscle. On spin-echo sequences, normal muscle demonstrates intermediate signal intensity on T1-weighted sequences and intermediate to low signal intensity on T20weighted sequences. Because hemorrhage infiltrates through the muscle, and mixes with the interstitial edema, it is not possible to separate it from the edematous muscle tissue. With a grade 1 contusion (i.e., microstructural fiber failure) there may be a slight increase in the size of the muscle and the margins of the muscle may have a feathery appearance due to the extension of interstitial edema into the perimuscular tissue. Edematous changes in the adjacent subcutaneous fat are also frequently detected. With a grade 2 muscle contusion (ie, partial tear) there will be a focus of disrupted muscle fibers in addition to the altered signal intensity from the interstitial edema and hemorrhage. A grade 3 muscle contusion will appear similar to a grade 2 contusion, except there will be complete disruption of the muscle fibers. With a muscle hematoma, there will be a focal accumulation of blood within a muscle. A hematoma demonstrates intermediate or high signal intensity on a T1-weighted sequence, depending on the chemical composition of the hematoma, and high signal intensity on a T2-weighted sequence. The sequelae of a muscle contusion may include muscle atrophy, fibrosis, calcification, or ossification.