Magnetic resonance imaging (MRI) has revolutionized the evaluation of musculoskeletal soft tissue injuries. Nowhere is this more evident than in the evaluation of internal derangements of the knee. MRI is an accurate and cost-effective means of evaluating a wide spectrum of knee injuries, ranging from cruciate-collateral ligament injuries to cartilage deficiencies.
For interpreting radiologists and clinicians, evaluation of an injured knee using MRI requires knowledge of the proper imaging techniques, normal and aberrant anatomy, and the clinical significance of detected abnormalities. 
A coronal, proton-density (PD) MRI scan of the medial collateral ligament (MCL) is seen below.
See Football Injuries: Slideshow, a Critical Images slideshow, to help diagnose and treat injuries from a football game that can result in minor to severe complications.
MCL tears rarely are isolated. More commonly, they are associated with other soft tissue injuries of the knee, such as anterior cruciate ligament (ACL) tears and medial meniscal tears (O'Donoghue's unhappy triad). Of complete MCL tears, 73% are associated with additional significant knee injuries, usually an ACL tear. Other associations include meniscocapsular separations and bone bruises.
Similar to MCL tears, isolated injuries of the lateral collateral ligament (LCL) are uncommon, typically occurring in association with ACL or posterior cruciate ligament (PCL) tears. Injuries of the lateral compartment are complex, usually with injuries to multiple components; they are often more disabling than injuries of the medial structures because of the greater forces to which lateral structures are subjected during normal gait. 
The grading system for MCL and LCL tears, which is the same as that used for other ligaments evaluated by MRI, is as follows:
Grade 1 - Microscopic tears
Grade 2 - Partial tears
Grade 3 - Complete tears
Examples of these injuries are seen in the images below.
MRI is the preferred modality for examining MCL and LCL injuries. Detection of associated internal derangements of the knee makes MRI superior to ultrasonographic imaging; with isolated injuries, however, the accuracy of ultrasonography is comparable to that of MRI. [3, 4, 5, 6, 7, 8] The usual limitations of MRI pertain to MRI evaluation of the MCL and LCL. The usefulness of MRI is limited in patients with claustrophobia, who are obese, or who have a pacemaker. Its usefulness is also limited by the presence of artifacts created by nearby orthopedic hardware. The use of open MRI units, as well as dedicated extremity units, has decreased the number of patients for whom MRI cannot be used because of claustrophobia or obesity.
For excellent patient education resources, visit eMedicineHealth's First Aid and Injuries Center. Also, see eMedicineHealth's patient education articles Knee Injury and Magnetic Resonance Imaging (MRI).
Calcification, particularly in the proximal portion of the MCL (visualized in the radiograph below), may be seen in persons with chronic MCL tears; it is termed Pellegrini-Stieda disease.
Avulsions of the fibular head (seen below) or of the lateral tibial metaphysis may be seen with injuries of the LCL/biceps femoris tendon or lateral capsule, respectively.
Capsular avulsion of the lateral tibial metaphysis (presented in the image below) is termed a Segond fracture and is highly associated with ACL tears.
Findings similar to those observed on plain radiographs may be seen on computed tomography (CT) scans. In addition, soft tissue injuries of the MCL and LCL may be detected, although not with the accuracy or contrast resolution seen with MRI.
Magnetic Resonance Imaging
Routine MRI sequences for the evaluation of the knee vary among institutions and scanners. The knee should be imaged in all 3 planes—sagittal, coronal, and axial. At a minimum, scans should include sequences to define anatomy, edema, and cartilage. [9, 10, 11, 12, 13, 14, 15, 16]
Sequences for anatomic definition include spin-echo (SE) and fast spin-echo (FSE), proton-density (PD) sequences. Fluid-sensitive sequences, such as SE/FSE PD fat-suppressed or short tau inversion recovery (STIR), detect edema. Cartilage may be characterized by fat-suppressed FSE PD sequences; fat-suppressed, gradient-echo (GRE) sequences; or spoiled gradient, fat-suppressed sequences.
Coronal images with anatomy-defining and fluid-sensitive sequences optimally demonstrate the medial and lateral supporting structures. Additional useful information may be gleaned from sagittal and axial images of these structures.
The anterior vertical component and the posterior oblique component of the MCL are depicted consistently on coronal T1-weighted or SE/FSE T2-weighted sequences. As seen on the image below, the MCL is seen as a thin, taut, well-defined, low T1/T2-signal structure extending from the medial femoral epicondyle to the medial tibial metaphysis.
Surrounded by high T1-weighted signal – fibrofatty tissue throughout its full extent, the MCL is parallel to and closely applied to the medial femoral epicondyle and medial tibial metaphysis. The anterior vertical or superficial component is best visualized at the level of the intercondylar notch in the vicinity of the distal insertion of the ACL.
The MRI appearance of acute MCL tears depends on the degree of tearing. With grade I, or microscopic, ligamentous tears (seen below), an intact ligament of normal thickness surrounded to a variable degree by intermediate T1-weighted and high T2-weighted signals (indicative of surrounding edema) is seen. The ligament remains closely applied to the underlying cortical bone.
With grade II tears (seen below), thickening and/or partial disruption of the fibers of the MCL is demonstrated, along with an increase in the amount of surrounding intermediate T1-weighted and high T2-weighted signals, indicative of increased edema and concomitant hemorrhage.
With grade III tears (seen below), complete disruption of the ligament with corresponding surrounding hemorrhage and edema is seen.
Distinguishing between MRI grade II and grade III tears is difficult. Clinical evaluation of the presence (grade II) or absence (grade III) of an end point to valgus laxity is helpful. As discussed earlier, the presence of a concomitant ACL tear is suggestive of a complete disruption of the MCL.
A chronic MCL tear is seen as an ill-defined, thickened ligament with low T1-weighted and T2-weighted signals. Occasionally, the MCL ossifies, and normal bone marrow signal may be seen within its proximal portion (see the first image below). With healing of subacute tears, a thickened, low T1/T2-signal ligament is demonstrated; the ligament reaches approximately 50% of its original strength at 12 months (see the second image below).
To include the LCL, the lateral supporting structures are depicted consistently on posterior coronal and far lateral sagittal T1-weighted or SE/FSE T2-weighted sequences. The arcuate popliteofibular and fabellofibular ligaments are visualized inconsistently.
Because of its posterior course, the entire LCL rarely is visualized on a single coronal image. Specialized coronal oblique (parallel to typical course of normal LCL) and sagittal, 1-mm, 3-dimensional, volume-rendered sequences depict the LCL and posterior lateral corner (PLC) structures particularly well. The LCL is seen as a thin, taut, well-defined, low – T1/T2-signal structure extending laterally and posteriorly from the lateral femoral epicondyle to the fibular head (see the images below).
Unlike MCL tears, the appearance of an LCL tear on MRI depends less on the degree of tearing. The LCL is extracapsular; therefore, it excludes accumulated extravasated joint fluid and, as a result, does not demonstrate the high surrounding T2-weighted signal seen with MCL tears.
In contrast to MCL tears, an acute LCL tear is seen as a serpiginous or lax ligament with discontinuous fibers (or avulsed fibular head), often without significant thickening of the ligament. As previously discussed, LCL tears rarely are isolated, and an LCL tear becomes more likely as associated PLC and cruciate ligament injuries increase in severity. Characteristics of acute LCL tears are demonstrated in the images below.
A chronic LCL tear (seen below) appears as a thickened, low – T1/T2-weighted signal ligament.
Degree of confidence
The degree of confidence is high with MRI of tears of the collateral ligaments and rises with the increasing grade of the tear. A prospective study of normal knees and knees with surgically verified grade III LCL injuries demonstrated a sensitivity, specificity, and accuracy of 94.4%, 100%, and 95%, respectively, for MRI.
The sensitivity, specificity, and accuracy of MRI for MCL injuries are less well established because of the nonsurgical nature of the injury, but they may be assumed to be similar to those of the LCL.
Loose, high – T1-weighted areolar tissue interposed between the 2 layers of the MCL is a normal finding that may mimic disease.
The normal MCL appears as 2 parallel, hyperechoic bands with loose, hypoechoic areolar tissue imposed between them. In the average individual, its thickness varies from approximately 2-4 mm along its length. [17, 3]
An MCL tear appears as a thickened ligament with decreased echogenicity. A complete disruption appears as a discontinuity in the ligament.
The normal LCL appears as a single hyperechoic band just deep to the biceps femoris tendon. Similarly to an MCL tear, an LCL tear appears as a discontinuity in the ligament or as thickening and loss of echogenicity.
Ultrasonography is approximately 94% sensitive for MCL tears.