Meniscal Posterior Root Tears

Clinical History: A 61-year-old female presents with left posteromedial and posterolateral knee pain for three weeks after hyperextending the knee while lying in bed. The pain was gradually resolving when she suffered a misstep off the sidewalk and a twisting injury two days before presentation. Axial (1A), contiguous coronal (1B), contiguous sagittal (1C), contiguous sagittal (1D), and coronal (1E) fat-suppressed proton-density-weighted images of the knee are provided. What are the findings? What is your diagnosis?

 

Findings

 

Diagnosis

Recent complete radial tear posterior horn medial meniscus 5 mm from the root insertion with 4 mm extrusion and an ill-defined partial tear posterior root lateral meniscus about 4 mm from the root insertion with an intact meniscofemoral ligament

 

Introduction

The role of the meniscus in maintaining joint and especially chondral integrity is multifactorial, including shock absorption, load distribution, lubrication, nutrition, and secondary stability. The meniscal roots are ligament-like structures that anchor the menisci to bone and aid in translating axial loads into circumferential hoop stress. Meniscal root tears are biomechanically akin to high-grade radial tears and total meniscectomy because the loss of hoop stress drastically diminishes the ability of the joint to bear axial loads and increases contact forces incurred upon the cartilage, thereby predisposing to premature osteoarthritis.

 

Anatomy

The posterior root medial meniscus attaches anteromedial to the PCL. The anterior root attachment is more variable along the anterior tibial slope intercondylar crest, discussed in further detail in the February 2023 Web Clinic.

The anterior and posterior lateral meniscal root insertions are only about 10 mm apart. The anterior root lateral meniscus inserts beneath the ACL, and some of the fibers interdigitate.¹ The posterior root attaches anterolateral to the medial meniscus posterior root and anterior to the PCL (Figure 3).

Biomechanically, the anterior lateral root is the strongest, and the anterior medial root is the weakest, however, the posterior roots are most frequently torn and thus the focus of our discussion.²

 

Pathophysiology

A root tear is classically defined as a tear within 9mm of the root insertion.³ If the root itself is avulsed, it should be described as a root avulsion. If the tear is located within 9 mm of the root insertion, it should be described as a root-equivalent tear.

Statistically, the posterior roots are more commonly torn than the anterior roots because they are less mobile and bear larger loads, especially when the knee is flexed at 90 degrees.⁴ The posterior root medial meniscus is especially prone to injury as the posteromedial corner absorbs a greater degree of compressive forces, and medial meniscal posterior horn mobility is further constrained by the meniscotibial ligament.^5,6

Posterior root medial meniscus tears often occur during daily tasks such as rising from a seated position or squatting with the knee hyperflexed. Patients often report posterior knee pain with a pop. In the United States, the population most affected includes middle-aged women with an elevated BMI. Varus malalignment is also a risk factor, and many of these patients present with some degree of underlying chondromalacia. On physical exam, patients may report joint line tenderness and pain with deep flexion.

In contrast, posterior root lateral meniscus tears often occur in younger patients with a history of trauma and a concomitant ACL tear or multi-ligament injury.

Anterior root tears are less common and may be degenerative, traumatic, or iatrogenic in etiology. Disruption of the lateral meniscus anterior and posterior roots may occur due to tibial tunnel reaming or misplacement during ACL reconstruction.^7,8 Disruption of the anterior root medial meniscus may occur during anterograde intramedullary tibial nailing.⁹

 

MRI Findings

Meniscal root avulsion and root-equivalent tears are an important MRI diagnosis because initial clinical examination can be non-specific. In addition, meniscal extrusion is an imaging-driven diagnosis.

Root tears have a varied appearance on MRI therefore, familiarity with the anatomy and interrogation in all three planes is essential. On MRI, normal meniscal roots sometimes demonstrate a striated appearance or nonspecific increased signal (Figure 4).

 

The root itself may be torn, or the tear may be located up to 9 mm from the root insertion (i.e., a root-equivalent tear). Root avulsion and root-equivalent tears may also be partial or complete, and most tears are radial or oblique. Root tears are also generally described as acute versus chronic. The most widely accepted classification system describing tear morphology is an arthroscopic classification, which radiologists should be aware of. However, it is best to describe the tear and leave the specifics of a type I-V designation to the arthroscopists.³ Additionally, a separate classification system to address posterior root lateral meniscus tears in the setting of an ACL tear with specific attention to the status of the meniscofemoral ligaments has also been proposed.¹⁰

 

Medial meniscus posterior root tears

Edema in the posterior root medial meniscus and the underlying bone generally signifies that the tear is more recent, whereas the absence of edema suggests chronicity, and an ossicle embedded in the torn meniscus clinches the diagnosis (Figure 5).

 

Secondary findings frequently associated with medial meniscus posterior root tears include meniscal extrusion due to an increase in contact pressure across the joint, an edematous swollen posterior horn, and subchondral marrow edema or a subchondral fracture along the weight-bearing surface signifying joint overload.

The most accepted definition of extrusion is protrusion of the meniscus by 3 mm or more beyond the outer margin of the tibial plateau.^11,12 The preferred method to measure medial meniscus extrusion is a mid-coronal image along the posterior margin of the MCL, and if a bone spur is present, it should be included in the measurement (Figure 6).¹³

 

If the root appears abnormal due to edema or degeneration, bone marrow edema immediately subjacent to the root insertion can suggest an occult or impending tear and should be described as such (Figure 7).^14,15 Similarly, ill-defined partial tears can progress to complete tears in a short time (Figure 8). Traumatic medial meniscus posterior root tears are less common but also occur in the setting of valgus injury (Figure 9).

 

 

 

Lateral meniscus posterior root tears

The lateral meniscus posterior root insertion and adjacent posterior horn are more challenging to assess on MRI given an oblique course, which is not always appreciated in all three imaging planes. As mentioned above, these tears are more often identified in a younger population with antecedent trauma (Figures 10 and 11).

 

 

The integrity of the meniscofemoral ligaments and any evidence of meniscal extrusion should also be reported, as the meniscofemoral ligaments can confer a degree of stability even in the setting of root avulsion or a root-equivalent tear. Interestingly, lateral meniscus root tears are more prevalent in patients with an ACL tear and lateral femoral notch impaction >2 mm.¹⁶

 

Treatment

The mainstay of treatment is preserving joint contact pressures and preventing or prolonging osteoarthritis progression. Nonoperative treatment is more often recommended in older patients with an elevated BMI and Outerbridge grade 3-4 chondromalacia or Kellgren-Lawrence grade 3-4 osteoarthritis.¹⁷ This may consist of activity modification, NSAIDs, injections, and unloader bracing, among other interventions. For patients with a well-maintained joint space and minimal to no chondromalacia in the affected compartment, meniscus root repair is recommended. Surgical intervention is recommended in a timely fashion, as it has been shown that chronicity (>12 months) leads to increased meniscal extrusion and advanced cartilage wear.¹⁸ It should be noted that arthroscopic debridement of the meniscus root tear alone does not provide symptomatic relief.¹⁹

Therefore, an arthroscopic root repair utilizing a transtibial pullout technique is the standard of care for patients in whom operative intervention is indicated (Figure 12).²⁰ Moreover, the treating surgeon should always obtain pre-operative long leg alignment radiographs and consider a concomitant off-loading osteotomy when indicated.

 

Summary

The meniscal roots anchor the menisci to bone, and root tears eliminate the joint’s ability to control hoop stress which drastically increases contact pressure, leads to joint overload, and osteoarthritis progression. A root tear is classically defined as a tear within 10 mm of the root insertion. If the root itself is avulsed, it should be described as root avulsion. If the tear is located within 9 mm of the root insertion, it should be described as a root-equivalent tear. Lateral meniscus posterior root tears are more commonly associated with acute trauma and concomitant ligament injury. Medial meniscus posterior root tears are more commonly reported while performing ordinary daily tasks, and more often associated with bone marrow edema and extrusion. Meniscal extrusion 3 mm beyond the outer tibial margin is generally agreed upon as pathologic. Posterior root lateral meniscus tears may be stable if the meniscofemoral ligament is intact. When describing meniscal root tears, it is also important to mention any chondromalacia or osteoarthritis that may affect treatment decisions.

 

 

 

 

References