Magnetic resonance imaging (MRI) is the highest standard in diagnostic imaging of muscle injuries(1). Diagnosing muscle injuries of the hip and thigh is a relevant issue in professional sports(2). Radiologists must familiarize themselves with typical MRI findings to accurately detect and classify muscle injuries. Proper interpretation of the findings is crucial, especially among elite athletes. At 35% of all soccer players’ injuries, hip and thigh muscle injuries are the most common reason for missing a game(3). Such injuries often occur during a match. Meanwhile, around 96% of muscle injuries have an indirect mechanism of an accident, usually due to muscle tears(4). Direct trauma is about 2% of the reasons for a muscle injury. Eccentric contraction usually induces indirect trauma(5). In MRI, indirect muscle injuries typically include hematoma (blood pooling outside the blood vessel), muscle edema (swelling), and fiber disruption. In contrast to indirect injuries, muscle contusions usually occur after direct trauma(6). MRIs can almost clearly detect direct traumas that induce muscle bleeding(7). Generally, muscle contusions heal quickly, allowing athletes to return and play within a week. Knowing the typical imaging findings and distinguishing between the different kinds of muscle injuries is essential from a radiological standpoint. Radiologists with significant knowledge on the subject may better identify the widely varying prognoses among the different types and recommend the appropriate therapy options.
Ultrasound and MRI are suitable methods of evaluating muscle injuries. There is a large variety in imaging patterns of hip and thigh muscle injuries. Such variations range from very subtle muscle strain changes to complete tears with muscle retraction and large hematoma(8-9). Radiologists may use ultrasound as the first modality in assessing muscle injuries due to its wide availability. Fast, easy-to-use, and affordable ultrasound machines are usually available in the immediate area of several sports arenas(10). However, ultrasound can be very user-dependent, making it difficult to determine the muscle tear’s length. Ultrasound cannot immediately identify small hematomas, which is imperative to find within the first 24 hours after the injury. Meanwhile, MRI may excellently depict the most relevant findings in muscle injuries(11). MRI also has a high sensitivity in identifying acute and chronic soft tissue alteration(12). MRI is a more elaborate imaging technique that experts consider the reference standard for muscle injuries. Radiologists assess muscle injuries on MRI by acquiring fluid-sensitive short-tau inversion recovery (STIR) or proton density (PD) fat-saturated sequences and T1-weighted sequences. Fluid-sensitive sequences allow radiologists to visualize edema, muscle tears, hematoma, and bone bruises. STIR is less sensitive to patient movement and metal artifacts. Meanwhile, PD fat-saturated sequences provide more anatomical information and have a higher signal to noise ratio (SNR). T1-weighted sequences can assess stress fractures, including hematoma and bony avulsions. Assessing anatomical features on non-fat-suppressed images is a common practice. Dixon sequences or chemical-shift imaging can be useful for their ability to provide separate water-only and fat-only images. This technique provides more uniform fat suppression(13-14).
Complications of Muscle Injuries of the Hip and Thigh
The following conditions are possible complications of muscle injuries(15):
Muscle hernias manifest in herniation (abnormal protrusion) of the muscle tissue through a small fascial defect concerning a prior blunt or penetrating muscle trauma. Muscle hernias may increase in size during activity and can sometimes be undetectable during rest periods. Thus, dynamic or standing ultrasound is a requirement(16-19).
Acute or Chronic Exertional Compartment Syndrome
Muscle anoxia causes compartment syndrome due to a pressure increase within the compartment(20). A chronic exertional compartment syndrome is caused by an increased compartment pressure during exercise and resolves with rest. Meanwhile, an acute compartment syndrome is a surgical emergency presented in pain, disproportionate to the injury.
Myositis ossificans or heterotopic ossification is a common sequela of muscle injuries. This condition is not an inflammatory process. Initially, edema and hematoma are present. There is a high signal intensity on MRI on fluid-sensitive images and an increased enhancement after contrast administration. After six weeks, radiologists may find typical imaging features with a peripheral rim of ossification that progresses towards the center(21).
Calcific myonecrosis is a rare benign condition that mainly affects a single leg compartment’s muscles. Experts believe that the condition follows a history of trauma with a latent period of years. Patients may present with a slowly growing mass(22).
Morel-Lavallée lesions result from a closed degloving injury due to shear forces associated with severe trauma. Such conditions result in the separation of the subcutaneous tissue from the fascia(23). Assessing this condition using MRI often comes with recurrent fluid accumulation while the fluid is of variable signal intensity.