Meniscal tears are among the most common injuries encountered in the knee.1 Diagnosis is typically made through the use of magnetic resonance imaging (MRI) as a noninvasive means to assess meniscal integrity and tear pathology.2 In addition to a detailed history and physical examination, meniscal tear location, size, and type factor into the decision regarding operative or nonoperative management.3–5 Therefore, proper characterization of the meniscal tear is an important early step in assessing the optimal method of treatment.
Parrot beak flap tears are complex oblique tears of the meniscus that begin as central radial tears and progressively extend toward the periphery, creating a partially detached and displaced fragment. Displaced fragments are of particular clinical significance because of the pain and instability associated with the dislodged tissue. Although the radiologic characteristics of inferiorly displaced horizontal meniscal tears have been identified in the literature,6 there is no prior description of a displaced parrot beak flap in the meniscotibial recess. In the authors' experience, these tears are commonly encountered and can be identified preoperatively on MRI. These tears must be recognized because they can cause persistent pain if treated nonoperatively. Although these tears may be encountered with osteoarthritis, they are more likely than traditional degenerative tears to respond to operative intervention because of the mechanical symptoms that they produce. In addition, these tears can frequently be mistaken for meniscal extrusion owing to their similar appearance on MRI.
The purpose of this study was to present the “meniscal comma sign”—the MRI findings of an inferiorly displaced parrot beak flap tear of the meniscus that becomes wedged in the meniscotibial recess—and to evaluate whether this finding is associated with coexisting osteoarthritis.
Materials and Methods
Institutional review board approval was obtained before data collection. Patients were retrospectively identified in out-patient imaging between January 2008 and December 2012 and were included in the study if they had available MRIs, had an identifiable meniscal comma sign (found using search terms including “meniscotibial recess” and “meniscus perched over the medial tibial margin”), and went on to have an orthopedic evaluation at the authors' institution. Magnetic resonance images were examined for the presence and grade of medial compartment osteoarthritis based on the modified Outer-bridge classification system.7
Typical Patient Presentation and Surgical Technique
The typical patient is between 40 and 60 years old and is experiencing medial-sided knee pain with or without superimposed degenerative joint disease. These patients frequently present with pain at or just below the joint line on the medial tibial plateau. Plain radiographs appear to have normal findings or may reveal mild or moderate joint space narrowing. Typical MRI findings are illustrated in Figure 1 and Figure 2. Patients present with what the authors refer to as the meniscal comma sign, which appears to be an extrusion of the medial meniscus that becomes displaced inferiorly, producing the typical downward curved appearance of the meniscus on coronal (Figure 1) and sagittal (Figure 2) MRI sections. On arthroscopic evaluation, the displaced piece of meniscus originates from a parrot beak flap tear that has become tucked underneath the meniscus and displaced medially, lodged between the tibial plateau and the medial collateral ligament. Images obtained during arthroscopy (Figure 3) illustrate the manner in which the flap, which normally remains in the boundaries of the joint, has become displaced. Using a probe, the flap tear is reduced from beneath the meniscus onto the joint surface, thus reversing the displacement (Video). A partial meniscectomy is then performed, removing the previously dislodged tissue (Figure 4).
Coronal T2 magnetic resonance image showing inferiorly displaced meniscal fragment with downward curved “comma” appearance (arrow) (A). Arthroscopic image showing the inferiorly displaced meniscal fragment within the meniscotibial recess (B).
Sagittal T2 magnetic resonance image showing the inferiorly displaced meniscal fragment (arrow).
Arthroscopic image showing the displaced meniscal fragment after reduction onto the joint surface with the probe.
Arthroscopic image after partial meniscectomy of the previously displaced tissue.
When searching their institution's knee MRI database from 2008 to 2012, the authors identified 4872 knees with a medial meniscal tear. Of these, 100 (2.1%) knees were found to have a meniscal comma sign. All MRIs with the comment “displaced fragment in meniscotibial recess” were secondarily reviewed to ensure that they met the radiographic criteria of a meniscal comma sign. Three patients were excluded because only MRI reports, not actual MRIs, were available. Of the remaining 97 patients, 9 (9.3%) had normal articular cartilage, 5 (5.1%) had grade 1 chondrosis, 15 (15.5%) had grade 2 chondrosis, 27 (27.8%) had grade 3 chondrosis, and 41 (42.3%) had grade 4 chondrosis.
The authors have identified a previously undescribed radiological finding that they call the meniscal comma sign. It represents an inferiorly displaced parrot beak flap tear of the medial meniscus that has become rolled underneath the meniscus, pushed inferiorly, and wedged between the tibial plateau and the medical collateral ligament. This tear pattern may be misinterpreted on MRI, potentially leading to a delay in treatment, because many of these patients also have cartilage loss due to superimposed osteoarthritis. In this study, 88 (90.7%) of 97 patients with a meniscal comma sign had an associated chondral defect. Furthermore, more than 70% of the patients in this study with this finding had grade 3 or 4 medial compartment chondral defects. Lewandrowski et al8 reported that 821 (76.3%) of 1076 patients with a medial meniscal tear were found to have an associated chondral defect. Therefore, patients with a meniscal comma sign may have an increased prevalence of osteoarthritis. Patients who present with a meniscal comma sign benefit from surgical treatment that removes the offending flap tenting the deep medial collateral ligament fibers. Because of the complex nature of the tear, its location in the nonvascular region of the meniscus, and the potential for the flap to revert back to its displaced configuration, the authors have favored the use of partial meniscectomy with the intent to remove the parrot beak flap and contour the remaining meniscus while preserving as much unaffected tissue as possible. Tears within 2 to 4 mm of the meniscal vascular rim are considered to be good candidates for repair.1,9 However, individuals displaying the meniscal comma sign typically exhibit complex tears of the nonvascular “white zone,” for which partial meniscectomy is preferred.
Descriptions of horizontal flap tears that have become displaced inferiorly or superiorly exist in the literature.6,10 However, the descriptions do not provide the incidence of osteoarthritis associated with these findings. Although a displaced horizontal flap tear may be missed more easily than a displaced parrot beak flap tear on arthroscopy, the latter might lead to a higher risk of chondral damage and therefore plays an important role in the decision regarding operative vs nonoperative treatment of degenerative meniscal lesions.
Patients who present with the meniscal comma sign and undergo surgery to remove the displaced parrot beak flap may experience a significant reduction in pain even if underlying osteoarthritis is present. Further research is warranted to validate these results in this unique patient population presenting with these characteristic imaging findings.
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