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ECR 2018 / C-3062
Posterior medial meniscal root lesions and its different patterns: an imaging approach
Congress: ECR 2018
Poster No.: C-3062
Type: Educational Exhibit
Keywords: Trauma, Inflammation, Athletic injuries, Perception image, Imaging sequences, Diagnostic procedure, MR, Musculoskeletal system, Musculoskeletal joint, Anatomy
Authors: B. S. Carleial1, R. D. C. LUNA2, L. P. PEREIRA2, C. L. D. M. Filho2, A. C. S. torres2; 1295, CE/BR, 2Fortaleza/BR

Findings and procedure details

The meniscal roots are ligament-like structures that anchor the meniscal body and horns to the tibial plateau and provide added mechanical stability for femorotibial gliding [1,3]

The semicircular medial meniscus has a wide posterior horn, narrows anteriorly, and has a more open C-shaped configuration than the more circular lateral meniscus.  The attachment of the
posterior horn of the medial meniscus is located at the posterior intercondylar fossa of the tibia.  It can be identified between the attachment of the posterior horn of the lateral meniscus and the PCL. [1] (Figs. 1,2)

The menisci are important structures for the knee, sharing the force load by increasing the contact surface area and providing uniform distribution of weight bearing across the articular surfaces. [11]

The meniscus increases the contact surface and resists radial extrusion from axial loading. In addition, the meniscal roots provide secondary stability to the knee; the posterior root of the medial meniscus provides support against tibial external rotation and lateral translation leading to varus deformity of the articular cartilage. [3]

The medial meniscus is less mobile than the lateral meniscus and is firmly fixed to the joint capsule. The relatively limited mobility of the medial meniscal attachment to the deep layer of the MCL and capsule render the medial meniscus susceptible to injury. [1,3]

Meniscal body and horn tears occur in numerous tear types, including radial, longitudinal, bucket-handle, and degenerative tears with each tear pattern requiring specialized treatment approaches. [10](Fig.3)

Root tears had been defined as “an avulsion of the tibial insertion of the meniscus or a radial tear close to the meniscal insertion”.  Various numbers of authors have defined meniscal root tears as tears within 9 to 10 mm of the root attachment. In addition, biomechanical studies have described that complete radial tears up to 9 mm from the root attachment significantly alter the native biomechanics of the posterior meniscal roots. [10, 11]

Laprade et al. were able to classify meniscal root tears into 5 tear types: partial stable root tears (type 1), complete radial tears  within 9 mm from the bony root attachment (type 2), bucket-handle tears with complete root detachment (type 3), complex oblique or longitudinal meniscal tears with a complete root detachment (type 4), and avulsion fractures of the meniscal root attachment (type 5) [10] (Figs. 4)

Posterior meniscus root tears typically result in loss of meniscal circumferential hoop stress. Equivalent peak tibiofemoral contact pressure has been shown between a complete posterior medial meniscal root tear and a total medial meniscectomy. [3, 11]

PMMRT is the most common lesion, with a prevalence of 10 to 21% of meniscal surgeries. More frequently these are chronic injuries associated with meniscal extrusion, reduced shock absorption, joint degeneration and ultimately osteoarthritis. [6]

In clinical practice, the incidence of these lesions is often underestimated. The clinical diagnosis is often limited by unspecific symptoms. Presenting symptoms are posterior knee pain; Joint pain (at the joint line);  Clicking;  Effusions; essential—but not always present—clinical diagnostic tool is the presence of a popping sound during light activities such as doing housework, going upstairs/downstairs, rising from a chair, and squatting. [1,5]

The gold standard for diagnosis of a meniscal root lesion is under direct visualization during arthroscopy. In most cases, however, the diagnosis of meniscal root tears should occur prior to proceeding to the operating room

MRI has profoundly changed our ability to image the meniscal root, thereby allowing us to more accurately define both normal and pathologic anatomy.

The typical parameters used in menisci MRI include a field of view of 16 cm or less, a matrix size of at least 192 × 256 (phase-encoding × frequency-encoding directions), and a section thickness of 3–4 mm [7]. The accuracy of MR imaging for tear detection is comparable at field strengths ranging from 0.1–7.0 T; however, higher field strengths often improve spatial resolution and reader confidence and reduce image acquisition time. For detection of medial meniscal root tears, coronal T2-weighted images show higher accuracy (96%, compared with 85% for PD-weighted images). More recently, three-dimensional (3D) sequences with isotropic resolution have been developed that provide thinner sections and reduce partial volume averaging.  [4]

If an PMMRT is identified, one must determine whether there is associated medial compartment degenerative cartilage wear, to identify the subset of patients who might benefit from root repair. [10]

MR imaging criteria for meniscal tears include abnormal morphologic structure or abnormal fluid-sensitive signal intensity extending to the articular or osseous surface [3,5]

The addition of the “two-slice-touch” rule of seeing the tear on two or more MR images increases the positive predictive value for a medial meniscal tear to 94% from 91% and increases the positive predictive value for a lateral meniscal tear to 96% from 83% .
Meniscal tears isolated to the posterior root of the medial meniscus can be detected with a high sensitivity of 86%–90% and a high specificity of 94%–95% [3,4]

Direct signs for radial meniscal root tear include a radial linear defect on axial MR images perpendicular to the long axis of the meniscus, a “cleft” sign on coronal MR images, and a “truncated triangle” or “ghost meniscus” on sagittal MR images. The ghost sign consists in the absence of an identifiable meniscus in the sagittal plane, or an increased signal replacing the normally dark meniscal tissue signal in more than 3 contiguous MRI cuts. [5] (Figs. 5, 6)

Secondary or indirect signs of a meniscal tear are MR imaging findings that can accompany meniscal tears. In technically limited or equivocal cases, these signs can increase the reader’s diagnostic confidence. Although these indirect signs have low sensitivity, they have high specificity and high PPVs for an underlying tear. [4] (Fig. 7,9)

Meniscal extrusion is an indirect and concerning sign that may be observed on MRI. Medial meniscal extrusion, which is defined as a partial or total displacement of the meniscus from the tibial articular cartilage, is a finding highly correlated to the presence of a root tear. If greater than 3 mm of displacement of the medial meniscal body in the midcoronal plane is considered pathologic extrusion, often associated with radial tear of the medial meniscus and, specifically, MRT. Other indirect signs that can be found are parameniscal cysts, joint effusion and subchondral marrow edema.  [5,7,10] (Fig. 7)

In addition, a higher risk of spontaneous osteonecrosis of the knee (SONK) has been described in association with PMMRT, compared with simple radial tears of the meniscus. PMMRT is associated with SPONK in 80% of patients, while medial meniscal tears in. [6] (Fig. 8)

At our institution, we describe the grade of the tear—degeneration, partial tear, or complete tear; the orientation of the tear—radial or longitudinal or complex; and the distance of the tear from the root attachment and possible extension into the posterior horn. These factors have implications in the preoperative planning for root repairs. Similar kind of description is also viewed in another institutions. [3]

Over the last few years, there has been an increasing interest in surgical repair of tears or avulsions at the meniscal root. All of the described techniques are done arthroscopically or in
an arthroscopically assisted manner to avoid posterior dissections of the knee. [8]

Repair is usually indicated for patients with acute tears, with or without associated soft tissue injury to the knee, and those with chronic or acute on chronic tears with minimal arthritis within the knee. Depending on the tear pattern, repair techniques include pull-out (transosseous), suture anchors, and side to side repair. Additional posteromedial and posterolateral portals can be established to facilitate the procedure. [3,6, 8]

There have been limited clinical studies following MRTs and repairs. Complete healing has been defined as continuity of the repaired root in all three planes (axial, coronal, and sagittal), partial healing as the loss of continuity in one plane, and a repeat tear as the loss of continuity in all three planes. In general, an intact repair is characterized by root continuity and no T2 hyperintensity extending to the articular surface a partial repeat tear is characterized by partial discontinuity and T2 hyperintensity extending to the articular surface, and a complete repeat tear is characterized by complete discontinuity and T2 signal void. [3]

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