A PICTORIAL ESSAY OF BASIC SPORTS INJURIES

– Written by Guillaume Bierry, France

KNEE

Anterior Cruciate Ligament Injuries

In a normal knee, the ACL (anterior cruciate ligament) appears as a continuous, dark (hypointense) band on both T1- and (fat saturated) T2-weighted images.

A complete tear may show as a disruption or discontinuity of this ligament, with irregular or wavy fibers instead of the usual taut band (Figure 1)¹.

On fat-saturated T2-weighted or proton density fat-saturated (PD-FS) images, acute ACL tears often appear as a hyperintense (bright) area due to edema and hemorrhage within or around the ligament. This increased signal may appear throughout the ACL or be localized in the area of injury, usually at the proximal femoral insertion.

ACL tears are often associated with characteristic bone bruises on the lateral femoral condyle and posterior aspect of the lateral tibial plateau, reflecting their impaction during forced anterior tibial translation (“pivot shift injuries”). These bone bruises appear as high signal intensity areas on fluid sensitive sequence images due to edema². The lateral femoral can demonstrate the “anterior femoral notch sign”, a traumatic loss of its concave shape on sagittal images.

On sagittal views of the knee, the tibia may be positioned slightly forward (anterior translation) relative to the femur, which can indicate instability due to an ACL tear.

Partial tears are more difficult to diagnosis, in most the case, the ligament appears enlarged, irregular and edematous. The ligament may still appear partially intact but with areas of increased signal, suggesting some fiber disruption. The ACL may appear wavy or buckled instead of having the usual taut, cord-like appearance.

Medial Collateral Ligament Injuries

The MCL is located on the inner (medial) side of the knee and consists of two layers: the superficial MCL and the deep MCL³. It normally appears as a low-signal intensity (dark) structure on T1-weighted and fluid-sensitive MR images.

MCL tears can be classified into three grades (Figure 2):

• Grade I (Mild Sprain): mild injury without fiber disruption, slight increase in MR signal in fluid-sensitive sequences around the ligament due to edema, but the ligament fibers remain intact.
• Grade II (Partial Tear): partial disruption of the MCL fibers with high MR signal intensity within the ligament, indicating partial tearing and inflammation, and thickening or irregularity of the ligament
• Grade III (Complete Tear): complete disruption of the ligament fibers, often with significant edema and hematoma formation; the ligament may appear completely discontinuous with a high-signal fluid gap where the ligament fibers should be.

MCL injuries are most common proximal, near the femoral attachment; distal Tear occurs near the tibial attachment and may sometimes pull away a small bone fragment (avulsion fracture).

Bone bruising can sometimes be seen, especially on the medial femoral condyle or tibial plateau, due to valgus forces.

Lateral Collateral Ligament Injuries

The LCL runs from the lateral femoral condyle to the fibular head, stabilizing the knee against varus (inward) forces⁴. It typically appears as a low-signal intensity (dark) band on T1- weighted and fluid-sensitive sequences MR images, located on the lateral side of the knee.

As for the MCL, injuries range from mild sprain to complete tear in which a gap between the torn ends can be seen (Figure 3). Most tears are distal and occur at the fibular attachment and may sometimes involve an avulsion fracture of the fibular head (“arcuate sign”).

Injuries to the lateral collateral ligament are rare, occurring in severe damage to the knee, usually dislocation. Isolated tears of the LCL are uncommon, as it is part of the posterolateral Corner (PLC) Injury, which includes the popliteus tendon, popliteofibular ligament, and lateral capsule. Injury to the LCL can often involve other structures in the PLC, especially in severe trauma of LCL tear.

Meniscus Injuries

Menisci are wedge-shaped, fibrocartilaginous structures situated between the femoral condyles and tibial plateau. On MRI, normal menisci appear as homogeneous, low-signal (dark) structures on T1 weighted and fluid-sensitive MR images⁵.

A meniscal tear appears as a high-signal (bright) line or area that reaches the meniscal surface.

If the high signal does not extend to the meniscal surface, it is likely degenerative rather than a true tear. An abnormal meniscal morphology is usually associated, truncated and with irregular edges.

Different types of meniscal tears exist³:

• Horizontal Tear: A cleavage plane develops horizontally within the meniscus, often associated with degeneration (Figure 4).
• Longitudinal Tear: Occurs along the length of the meniscus, often in the posterior horn. A particular form of vertical tear is the so called “ramp lesion”, a meniscocapsular separation at the posterior aspect of the medial meniscus (Figure 5).
• Radial Tear: Extends perpendicularly from the inner free edge of the meniscus, disrupting its hoop stress function. Radial tears involve mostly the posterior root of the medial meniscus, in the setting of degenerative changes (Figure 6). A partial meniscal defect on coronal views often indicates a radial or complex tear “truncated triangle sign”.
• Complex Tear: A combination of multiple tear types within the meniscus, often associated with advanced degeneration.

Some tears may lead to displaced meniscal tissue within the joint space⁶. A “flap tear” may have a fragment displaced from the main body, often flipping up or folding. In large longitudinal tear, the inner part of the free edge can migrate within the intercondylar notch, forming a “bucket handle tear” (Figure 7). The fragment is often seen as double lines or as a second dark band mimicking the PCL on sagittal MR images (“double LCP sign”).

Extensor System Injuries

Patellar dislocation

MRI is a highly effective imaging modality for evaluating patellar dislocation, as it helps visualize the extent of soft tissue injuries, bone contusions, and cartilage damage that may occur during the dislocation event.

The classical imaging pattern after patellar dislocation is bone bruising typically occurring on the anterolateral aspect of the lateral femoral condyle and the medial aspect of the patella due to the impaction during dislocation (Figure 8). These bruises appear as high-signal areas on fluid-sensitive images, indicating edema within the bone marrow. In the most severe forms, osteochondral fractures can occur and free fragment expulsed within the cavity⁷.

Medial patellofemoral ligament (MPFL) which runs from the medial femoral condyle to the medial patella, is the primary soft tissue stabilizer of the patella. MPFL tears are common with patellar dislocations, often at its patellar or femoral attachment. On MRI, this may appear as discontinuity or increased signal within or around the ligament on fluid-sensitive images.

Patellar tendinopathy

Patellar tendinopathy or patellar tendinitis (also known as of “jumper’s knee”) is classically seen in athletes who engage in repetitive knee extension (jumpers, basket players, etc)⁸.

The proximal portion of the patellar tendon, where it attaches to the inferior pole of the patella, often appears thickened with Increased signal intensity on fluid-sensitive MR sequences (Figure 9).

Degenerative changes may cause irregular or disrupted tendon fibers, which are sometimes visible as disorganized areas within the tendon.

Some cases may show bone marrow edema in the patella itself, indicating chronic stress or inflammation at the tendon insertion site. Unlike some other tendonopathies, jumper’s knee typically does not show calcifications in the tendon.

Bone stress injuries

Bone stress injuries occur due to repetitive stress (especially in athletes or individuals who engage in repetitive loading) resulting in localized weakness and pain.⁹.

Bone Marrow Edema is an early sign of a bone stress injury and appears as increased signal intensity on T2-weighted or STIR (Short Tau Inversion Recovery) images and decreased signal on T1-weighted images. This edema indicates inflammation and early bone injury, even before a fracture line is visible (Figure 10).

Several MR imaging-based grading systems were proposed for bone stress injuries¹⁰. One of the most utilized systems was proposed for tibial bone stress injuries by Fredericson et al (see Illustration 1)¹¹. The Fredericson MRI grading scale categorizes injuries from Grade 1 to Grade 4, with higher grades indicating greater severity. Grades 1 through 3 are classified as stress reactions. When a visible fracture line is present, the injury is typically designated as a Grade 4 bone stress injury (BSI) and regarded as a true stress fracture.

Periosteal reaction presents edema or thickening, indicative of new bone formation in response to stress. Adjacent soft tissue edema or fluid collection can also be seen, reflecting inflammation in nearby structures.

ANKLE

Lateral collateral ligament injuries

A sprain to the lateral collateral ligament is one of the most common injuries sustained by athletes and can involve the anterior talofibular ligament (ATFL), calcaneofibular ligament (CFL), and, less frequently, the posterior talofibular ligament (PTFL)¹².

Normal ligaments are low-signal-intensity structure on T1 and T2-weighted images.

MRI imaging can help differentiate between grades of sprain (Figure 11):

• Grade I (Mild Sprain): Shows ligament thickening and increased signal intensity within the ligament, indicating edema or mild strain.
• Grade II (Partial Tear): Partial discontinuity or irregularity of the ligament fibers, with surrounding edema.
• Grade III (Complete Tear): Complete discontinuity of the ligament with extensive edema and hemorrhage around the ligament insertion points.

In addition, in a lateral ankle sprain, the peroneal tendons—the peroneus brevis and peroneus longus—are often affected due to the force of inversion and twisting.

Although the primary structures injured are the lateral ligaments, the peroneal tendons can sustain secondary injuries, which may complicate recovery and increase the risk of chronic instability. Injury ranges from peroneal tendons displacement from the fibular groove due to a torn or damaged retinaculum, to partial or complete split.

Ankle Impingement

Ankle impingement refers to soft tissue or bone structures becoming compressed in the ankle joint, leading to pain, limited range of motion, and sometimes swelling. The most common form is the anterolateral type where cicatricial scar tissue or synovitis, from chronic injury to the anterior talofibular ligament (ATFL), is entrapped into the latter gutter. Depending of its fibrous content, cicatricial tissue presents as areas of mid to high signal on fluid sensitive sequences.

Osteochondral Lesion of The Talar Dome

Forced motion of the talus during a sprain can result in an osteochondral lesion of the talar dome (OCLs)¹³.

MRI findings for osteochondral lesions (OCLs) of the talar dome typically show injury to the cartilage and underlying subchondral bone of the talus, typically on the superomedial or superolateral aspect of the talus (Figure 12).

Cartilage may present various degree of severity can be seen fissuring (the articular cartilage may appear disrupted, with fissures or fragmentation), defects (partial or complete thickness defects), displaced fragment (in severe cases, the cartilage fragment may detach and be visible as a loose body within the joint). Mild to extensive sub chondral edema is usually present.

In chronic lesions, small cyst-like lesions or subchondral cysts may form representing areas of bone resorption.

Fluid accumulation in the ankle joint is commonly associated with acute osteochondral lesions, particularly if there is associated inflammation or injury to adjacent structures.

Syndesmosis Injuries

Injury to the syndesmotic ligament complex (including the anterior and posterior tibiofibular ligaments and the interosseous membrane) often occurs due to external rotation or dorsiflexion of the ankle and is commonly associated with high ankle sprains¹⁴.

Syndesmosis injuries can vary from mild sprains to complete ligament tears, and MRI helps to grade the severity and guide treatment (Figure 13):

• Mild sprain with ligament thickening and edema but intact ligaments.
• Partial tearing, usually involving one or more ligaments with increased signal and some irregularity but without diastasis (widening) between tibia and fibula.
• Complete tearing of multiple syndesmotic ligaments, with or without diastasis. If diastasis is present, it often indicates a need for surgical stabilization.

In the case of interosseous membrane involvement, edema or tearing may extend up into the leg, particularly in higher-grade injuries, where the entire membrane between the tibia and fibula may be affected.

Achille Tendon Disorders

The Achilles tendon is particularly prone to injury in athletes.

Achilles tendinopathy typically occurs at the mid-portion of the tendon (2–6 cm proximal to the calcaneal insertion) or at the insertional area with tendon thickening, often with increased signal intensity within the tendon on fluid sensitive sequence.

Achilles enthesopathy demonstrates tendon thickening and irregularity at the calcaneal insertion site, with increased signal at the enthesis on fluid sensitive sequence, with or without associated retrocalcaneal bursitis.

Partial tear of the tendon demonstrates partial thickness disruption of tendon fibers with focal areas of high signal intensity within a thickened tendon (Figure 14)¹⁵.

In complete tear, there is discontinuity of the tendon fibers with retraction of tendon ends, the gap between tendon stumps often filled with fluid.

The Achilles tendon is not covered by a synovial sheath but by a thin fibrous structure called the paratenon. Paratenonitis, or inflammation of this membrane, appears as an area of high MR signal around the tendon, which remains otherwise normal (Figure 15).

Guillaume Bierry MD, PhD, MBA

MSK imaging

Strasbourg University Hospital

Strasbourg, France

Contact:

guillaume.bierry@chru-strasbourg.fr

References

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