Residency · Residency · Diagnostic Radiology

Internal Derangement of the Knee: MRI Interpretation

Overview

Indications for Knee MRI

Knee MRI is indicated for suspected meniscal tear (mechanical symptoms such as locking and catching), suspected ligament injury (instability, pivot shift, acute hemarthrosis), unexplained knee pain not responding to conservative management, and pre-operative planning for arthroscopy.

Standard MRI Protocol

The standard knee MRI protocol includes sagittal proton density (PD) or PD fat-saturated, coronal PD or PD fat-saturated, axial PD fat-saturated or T2 fat-saturated, and sagittal T2 fat-saturated or STIR sequences. Field strength is 1.5T or 3T, with 3T providing superior signal-to-noise ratio.

Meniscal Tears

Normal Meniscal Anatomy

The medial meniscus is C-shaped, larger, and less mobile, and it is attached to the deep MCL. The lateral meniscus is more circular, smaller, and more mobile, with the popliteal hiatus located posterolaterally. Both menisci are triangular (bow-tie shaped) on cross-section and demonstrate low signal on all sequences. Normal meniscus shows homogeneous low signal intensity.

Grading of Meniscal Signal

Grade I signal is globular and intrameniscal, does not reach the articular surface, and represents mucoid degeneration. Grade II signal is linear and intrameniscal, does not reach the articular surface, and represents degeneration. Grade III signal extends to at least one articular surface and defines a tear.

Tear Morphology

A horizontal tear runs parallel to the tibial plateau, is often degenerative, and may lead to a parameniscal cyst. A vertical longitudinal tear is perpendicular to the tibial plateau and parallel to the long axis of the meniscus. A bucket-handle tear is a displaced vertical longitudinal tear in which the fragment flips into the intercondylar notch. The key signs include the double PCL sign (the displaced fragment parallels the PCL on sagittal images), the absent bow-tie sign (fewer than two body segments visible on sagittal images), and visualization of the fragment in the intercondylar notch on coronal images. A radial tear is perpendicular to the free edge and long axis, disrupting the circumferential hoop stress fibers. It produces the ghost meniscus or cleft sign on sagittal images and a truncated meniscus on coronal images. A complex tear combines multiple tear patterns and is often degenerative. A root tear occurs at the meniscal root attachment and is functionally equivalent to total meniscectomy because it disrupts the hoop stress mechanism.

Root Tears

Medial meniscal posterior root tears are common in middle-aged women and are associated with rapid cartilage loss. Meniscal extrusion exceeding 3 mm on coronal images correlates with root tear. The ghost sign on sagittal images through the root attachment and a radial cleft at the root on axial or coronal images are diagnostic findings.

Pitfalls

The meniscofemoral ligaments (Humphry anteriorly and Wrisberg posteriorly) can mimic a tear of the posterior horn of the lateral meniscus on sagittal images. The transverse intermeniscal ligament can mimic an anterior horn tear. A meniscal flounce is a wavy contour of the inner free edge that represents a normal variant, not a tear. Magic angle artifact causes increased signal at 55 degrees to B0 and affects the posterior horn of the lateral meniscus on sagittal images.

Cruciate Ligaments

Anterior Cruciate Ligament (ACL)

The ACL originates from the posterolateral femoral condyle (medial wall of the lateral femoral condyle) and inserts at the anterior intercondylar tibial eminence. It is best seen on sagittal oblique images paralleling the intercondylar roof (Blumensaat line). The normal ACL appears as a taut, low-signal band paralleling the intercondylar roof line.

ACL Tear Signs

Primary signs include discontinuity, abnormal signal (edema or hemorrhage), non-visualization, and abnormal angulation. Secondary signs reflect the bone bruise pattern from the pivot-shift mechanism: bone bruises at the posterolateral tibial plateau and mid-lateral femoral condyle ("kissing contusions"), anterior tibial translation exceeding 7 mm, the deep lateral femoral notch sign (sulcus exceeding 1.5 mm depth), the Segond fracture (lateral capsular avulsion off the proximal tibia, which is pathognomonic for ACL tear), and a buckled PCL. Partial versus complete tears can be challenging to distinguish, as partial tears show some intact fibers.

Posterior Cruciate Ligament (PCL)

The PCL originates from the anterolateral medial femoral condyle and inserts at the posterior tibial sulcus. It is thicker and stronger than the ACL and normally appears as a smooth arc of low signal on sagittal images. PCL tears manifest as increased signal, thickening, and discontinuity. The typical injury mechanism is the dashboard injury (posterior tibial translation), and the bone bruise pattern involves the anterior tibial plateau.

Collateral Ligaments

Medial Collateral Ligament (MCL)

The MCL has superficial and deep components. Injuries are graded as Grade I (sprain) with periligamentous edema and intact fibers, Grade II (partial tear) with partial disruption, thickening, and high signal, and Grade III (complete tear) with full discontinuity, waviness, and retraction. MCL tears are often associated with ACL tear and medial meniscal tear (O'Donoghue's unhappy triad, though the lateral meniscus is more commonly injured than originally described).

Lateral Collateral Ligament (LCL) / Fibular Collateral Ligament

The LCL is a cordlike structure extending from the lateral femoral epicondyle to the fibular head and is part of the posterolateral corner (PLC) complex. The PLC includes the LCL, popliteus tendon, and popliteofibular ligament. The arcuate sign, an avulsion fracture of the fibular styloid, suggests PLC injury.

Bone Bruise Patterns and Mechanism

Pivot-Shift (ACL Tear)

The pivot-shift mechanism produces bone bruises at the posterolateral tibial plateau and mid-lateral femoral condyle and is the most common bone bruise pattern associated with ACL tear.

Hyperextension

Hyperextension produces bone bruises at the anterior tibial plateau and anterior femoral condyle in a "kissing" pattern and is associated with ACL and/or PCL tears.

Dashboard (PCL Tear)

The dashboard mechanism produces bone bruises at the anterior proximal tibia and is associated with PCL tear with or without posterior dislocation.

Clip Injury (Valgus)

The clip or valgus mechanism produces bone bruises at the lateral femoral condyle with MCL injury and is associated with ACL and medial meniscal tears.

Bone Bruise Pattern Summary

<image>A sagittal PD fat-saturated MRI of the knee showing a bucket-handle meniscal tear. The displaced meniscal fragment is seen in the intercondylar notch anterior to the PCL (double PCL sign, arrows). An inset coronal image shows the fragment flipped into the notch. A second inset demonstrates the absent bow-tie sign with only one body segment visible on the sagittal section. The normal contralateral meniscus is shown for comparison.</image>

<image>A sagittal MRI of the knee demonstrating an acute ACL tear. The ACL fibers are discontinuous and replaced by amorphous high signal intensity. The PCL shows a buckled appearance (secondary sign). Bone marrow edema (bruising) is visible in the posterolateral tibial plateau and mid-lateral femoral condyle, consistent with the classic pivot-shift injury pattern. Arrows label the torn ACL, bone bruises, and buckled PCL.</image>

<image>An illustration showing the major bone bruise patterns associated with different knee injury mechanisms. Four knee diagrams showing: (1) Pivot-shift: bruises at posterolateral tibial plateau and lateral femoral condyle with ACL tear. (2) Hyperextension: bruises at anterior tibia and anterior femur with cruciate injuries. (3) Dashboard: bruise at anterior proximal tibia with PCL tear. (4) Clip/valgus: bruise at lateral femoral condyle with MCL tear. Each mechanism is labeled with the associated ligament and meniscal injuries.</image>

Clinical Pearls

A meniscal tear requires signal that unequivocally extends to at least one articular surface on at least two consecutive images, which prevents overcalling Grade II signal as a tear. The bucket-handle tear produces the "double PCL sign" and "absent bow-tie sign," which are the most reliable signs for a displaced meniscal fragment. The meniscofemoral ligament (Wrisberg or Humphry) is the most common mimic of a posterior horn lateral meniscus tear, and it should be identified as a separate structure near the PCL. A Segond fracture (lateral capsular avulsion) is essentially pathognomonic for an ACL tear. Medial meniscal posterior root tears are functionally equivalent to meniscectomy and lead to accelerated cartilage loss; meniscal extrusion exceeding 3 mm on coronal images should raise this concern. When reading a knee MRI, all four corners should be evaluated: the ACL, PCL, MCL, LCL/PLC, both menisci, articular cartilage, and the extensor mechanism.

References

• Helms CA. Fundamentals of Skeletal Radiology. 5th ed. Elsevier, 2020
• Stoller DW. Magnetic Resonance Imaging in Orthopaedics and Sports Medicine. 3rd ed. Lippincott, 2007
• De Smet AA, et al. "MRI of Meniscal Tears." Radiologic Clinics of North America, 2014
• LaPrade RF, et al. "The Posterolateral Corner of the Knee." JAAOS, 2003

Read this lecture as Markdown