Hypermobile Lateral Meniscus

Clinical History: A 16 year-old male presents with persistent lateral knee pain and snapping for 10 months.  What are the findings? What is your diagnosis?

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Figure 1:

(1a) A proton density-weighted sagittal image and two (1b, 1c) fat-suppressed proton density-weighted coronal images are provided.

Findings

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Figure 2:

The proton density-weighted sagittal image reveals anterior displacement of the lateral meniscus (arrows) relative to the lateral tibial plateau. Normal attachments between the posterior horn and the popliteus tendon (arrowheads) are absent.

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Figure 3:

The meniscal body (arrow) is noted to be abnormally enlarged on the fat-suppressed proton density-weighted coronal view.

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Figure 4:

A more posterior coronal slice demonstrates that the posterior horn of the lateral meniscus is also medially displaced with a peripheral gap (arrow). The ligament of Wrisberg attachment to the root of the posterior horn is visible (arrowhead).

Diagnosis

Unstable Wrisberg type discoid lateral meniscus.

Introduction

The Wrisberg type discoid lateral meniscus is but one of several unstable lateral meniscal entities that may be encountered on MR imaging. This Web Clinic will discuss the anatomical features that stabilize the lateral meniscus, in particular the popliteomeniscal fascicles, and will review presentations of lateral meniscal instability seen with MRI and at arthroscopy.

Relevant Anatomy

The menisci are C-shaped fibrocartilage structures vital for shock absorption and stability at the knee. Both menisci have firm attachments to the tibial plateau at their anterior and posterior roots. Peripheral attachments of the lateral meniscus are less extensive and generally more lax than on the medial side¹, allowing greater mobility of the lateral meniscus, and likely contributing to the relatively increased frequency of medial meniscal tears². The coronary ligaments (meniscotibial ligaments) provide an attachment from the posterior horn of the lateral meniscus to the tibia, and are thought to assist in keeping the lateral meniscus adherent to the tibial plateau³. The meniscal roots and the coronary ligaments can be thought of as static stabilizers of the lateral meniscus.

The meniscofemoral ligaments of Humphrey and/or Wrisberg extend from the medial femoral condyle to the posterior horn of the lateral meniscus. Because of the connection between the femur and the tibia, the meniscofemoral ligaments cause posterior movement of the lateral meniscus during flexion and tibial internal rotation⁴.  The popliteomeniscal fascicles arise from capsular extensions that blend with the popliteus musculotendinous junction. They attach to the peripheral posterior horn of the lateral meniscus. Consequently, contraction of the popliteus muscle also results in posterior movement of the lateral meniscus during flexion. The movement of the lateral meniscus is felt to maintain congruency of the meniscus with the articular surfaces and to protect the meniscus from shear forces during knee flexion. A MRI study by Thompson found normal posterior excursion of the lateral meniscus during flexion of 11.2mm⁵, though other authors have reported lesser values.

The popliteomeniscal fascicles form the popliteal hiatus and create a passageway for the popliteus tendon to pass from an intra-articular to an extra-articular location. Most studies of popliteomeniscal fascicle anatomy have described at least two components, the anteroinferior and the posterosuperior fascicles⁶, ⁷ . A third fascicle, the posteroinferior popliteomeniscal fascicle, is more controversial, and less consistently present. Resnick found using MR arthrographic analysis of ten cadaveric knees that anteroinferior and posterosuperior fascicles were present in all of the specimens and that the posteroinferior fascicle was found in 4/10⁸. A gross cadaveric study of 42 knees by Feipel found the posteroinferior popliteomeniscal fascicle in 17% of specimens⁹. The posteroinferior fascicle is more medially located, and extends from a medial aponeurotic extension of the popliteus tendon to attach to the lateral meniscus near the attachment of the meniscofemoral ligaments.

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Figure 5:

3D posterior oblique views demonstrate posterolateral restraints of the lateral meniscus including the posterosuperior popliteomeniscal fascicle (PSF), anteroinferior popliteomeniscal fascicle (AIF), meniscofemoral ligament (MFL), and the coronary ligament (CL). The popliteofibular ligament (PFL) is also labeled. Illustration by Dr. Michael Stadnick.

Anatomy on MR imaging

Although 45 degree oblique coronal images have been recommended for visualization of the popliteomeniscal fascicles¹⁰, these structures are often well visualized during the review of routine sagittal acquisitions of the knee. In the following examples, non fat-suppressed proton density-weighted sagittals images depict the normal appearance of the popliteomeniscal fascicles (Figures 6-9). Additional images demonstrate the normal appearance of the meniscofemoral ligament of Wrisberg (Figure 10) and the coronary ligament (Figures 11a, 11b).

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Figure 6:

Beginning at the lateral aspect of the lateral compartment, the anteroinferior popliteomeniscal fascicle (arrow) is seen to course from the inferior aspect of the lateral meniscus posteroinferiorly to blend with fibers of the popliteofibular ligament. The popliteus tendon is indicated (arrowhead). This fascicle is typically seen on sagittal slices where the fibular head (asterisk) is also visible.

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Figure 7:

In another patient, a component of the anteroinferior popliteomeniscal fascicle (arrow) which courses directly to the popliteus tendon (arrowhead) is visible. This component is inconsistently visible on routine sagittal MR images.

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Figure 8:

More medially, the posterosuperior popliteomeniscal fascicle (arrow) extends from the superior margin of the peripheral posterior horn of the lateral meniscus to attach to the posterior joint capsule superior to the popliteus tendon (arrowhead).

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Figure 9:

The posteroinferior popliteomeniscal fascicle (arrow) is the least consistently visualized of the fascicles. When present, it is seen near the root of the posterior horn and courses from the inferior margin of the meniscus posteroinferiorly to attach to a capsular extension of the popliteus tendon.

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Figure 10:

A proton density-weighted coronal image demonstrates the posterior meniscofemoral ligament of Wrisberg (arrow) coursing obliquely downward from the medial femoral condyle to attach to the posterior horn of the lateral meniscus (arrowhead).

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Figure 11:

(11a) The coronary (meniscotibial) ligaments are found along either side of the popliteal hiatus. The coronary ligament anterior to the popliteal hiatus is easiest to identify, and best seen on posterior coronal images. This fat-suppressed proton density-weighted coronal image demonstrates the typical distal attachment of the lateral meniscal coronary ligament (arrow), with the distal insertion seen several millimeters distal to the articular surface. (11b) Because of the distal attachment of the coronary ligaments, this structure may result in visualization of a fluid filled recess (asterisk) as on this posterior coronal image.

Patterns of Lateral Meniscal Instability

A meniscal flounce is thought to be a normal meniscal variant in which the meniscus appears “wavy” on sagittal or coronal MR images (Figure 12). Orthopaedic surgeons have long realized that a meniscal flounce may be seen at surgery with the application of external force upon the knee¹¹. Indeed, the presence of a normal medial meniscal flounce at arthroscopy has been found to signify that the meniscus is likely intact¹². A meniscal flounce is a relatively rare finding on MR, and is known to be much more common medially than laterally¹³. The presence of a flounce on MR images may be variable and dependent upon positioning factors such as knee flexion and external rotation¹⁴.

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Figure 12:

A STIR sagittal image reveals a wavy appearance of the lateral meniscal body compatible with a meniscal flounce.

When a meniscal flounce is encountered on MR images, it can typically be described as a normal variant. However, flounce is associated with ligamentous injuries and meniscal tears can result in a flounce-like appearance, and thus a careful search for associated pathology should be performed. Because of the relative rarity of flounce at the lateral meniscus, identification of a lateral meniscal flounce should prompt particular scrutiny for a meniscal tear or an abnormality of the capsular attachments (Figure 13).

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Figure 13:

Same patient as Figure 12. An adjacent sagittal image reveals abrupt truncation of the anteroinferior popliteomeniscal fascicle (arrow) shortly beyond its attachment to the posterior horn of the lateral meniscus, compatible with a tear. The posterosuperior popliteomeniscal fascicle appeared small and ill-defined and the posteroinferior fascicle was not visualized (images not shown).

A discoid lateral meniscus is a not uncommon anatomical variant, found in up to 3% of the population¹⁵.  Patients with discoid lateral menisci have variable enlargement of the meniscal body, often with thickening of the meniscus as well. All discoid menisci are felt to be at greater risk for tearing due to increased shearing forces across the enlarged meniscus¹⁶ (Figure 14). The Wrisberg type discoid lateral meniscus lacks the typically stabilizing attachments of the posterior horn, having only an attachment from the meniscofemoral ligament of Wrisberg¹⁷. As in the test case of this clinic, patients with the Wrisberg type discoid lateral meniscus are more likely to demonstrate signs of meniscal instability such as snapping or locking.

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Figure 14:

A fat suppressed proton density-weighted sagittal image of a 17 year old male with lateral pain, swelling, and instability reveals a large horizontal tear (arrows) extending to the tibial surface of a discoid meniscus. The posterosuperior and the proximal aspect of the anteroinferior popliteomeniscal fascicles are visible (arrowheads), indicating that this is not a Wrisberg type discoid meniscus.

The hypermobile lateral meniscus is an entity that has received relatively little attention in the radiology or orthopaedic literature. Patients with symptomatic hypermobile lateral menisci present with knee pain and/or locking in the absence of a discrete meniscal tear or a discoid meniscus¹⁸. Hypermobile lateral menisci are thought to result from either congenital absence of posterior capsular attachments, similar to the Wrisberg type discoid meniscus, but with an absence of discoid morphology¹⁹, or from tears of posterior capsular attachment, in particular the popliteomeniscal fascicles²⁰.  The hypermobile lateral meniscus ranges in severity from asymptomatic patients (Figure 15) to those with severe pain and mechanical locking.

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Figure 15:

(15a) 39 year-old with medial pain and no mechanical symptoms. The sagittal image reveals anterior displacement of the posterior horn of the lateral meniscus (arrow) relative to the tibial plateau. No popliteomeniscal fascicles were visible in this case. (15b) A STIR axial image provides fortuitous visualization of the lateral meniscus (outlined with dots). The posterior horn is confirmed to lie significantly more anteriorly than is typical with a resultant “tight” configuration of the meniscal C-shape.

The large majority of patients with hypermobile lateral menisci in the literature received negative MR results of their lateral menisci on initial review. In rare cases patients able to reproduce locking were scanned in the locked position, allowing verification of meniscal displacement²¹.  Detection of a hypermobile lateral meniscus is thus highly dependent upon the discovery of the characteristic clinical presentation of lateral pain, locking, and episodes of giving way. LaPrade described a provocative maneuver known as the Figure-4 test which places the affected knee in flexion, varus and external rotation. This test may reproduce locking in patients with a hypermobile lateral meniscus²².

Because of variability in the ability to visualize the popliteomeniscal fascicles and also in part due to variability in scrutiny of these structures among interpreting radiologists, the hypermobile lateral meniscus is rarely diagnosed prospectively on MR examinations. But a study by Suganuma of 238 knees with a history of mechanical locking and arthroscopically proven meniscal subluxation revealed that close inspection of MR images will reveal abnormalities of the popliteomeniscal fascicles in virtually all such patients²³. Suganuma found that a high incidence of abnormal posterosuperior popliteomeniscal fascicles is present in patients with recurrent lateral meniscal subluxation, and that when compared to controls, abnormal anteroinferior fascicles were found only in patients with recurrent subluxation, suggesting that an abnormal anteroinferior popliteomeniscal fascicle is an essential lesion to allow lateral meniscal subluxation.

Our experience at Radsource also finds that many of the MR examinations of patients with a hypermobile lateral meniscus are interpreted as normal upon initial review. However, close inspection of the lateral capsular attachments in such patients will frequently reveal abnormalities not previously identified (Figures 16-18).

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Figure 16:

Fluid (arrow) extends between the posterior horn of the lateral meniscus and the popliteal capsular extension in a 25 year-old male whose surgeon suspected lateral meniscal dislocation. Ill-defined soft tissue (arrowhead) lies at the periphery of the meniscus but normal low signal intensity popliteomeniscal fascicles were not apparent. At arthroscopy, peripheral detachment of the posterior horn was confirmed, and the meniscus was freely movable into the joint with the arthroscopic probe.

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Figure 17:

Consecutive sagittal fat-suppressed proton density-weighted images in a 16 year-old with lateral pain and intermittent knee locking for 3 years. The anteroinferior popliteomeniscal fascicle (arrow) is thickened, and abnormal hyperintense signal is seen at the posterior horn attachment (arrowheads) on both images. Abnormal signal extended through the peripheral meniscocapsular junction on multiple additional views (not shown).

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Figure 18:

(18a) A view through the lateral arthroscopic portal demonstrates the normally positioned posterior horn (PH) and body (B) of the lateral meniscus. Considerable free edge fibrillation (arrowheads) is present at the meniscus, unusual in a 16 year-old, and likely reflecting repetitive stress. (18b) In a subsequent arthroscopic photo, a probe has been placed into the popliteal hiatus and the posterior horn has been pulled anteriorly. The free edge of the meniscus (arrowheads) has flipped up and the entire posterior horn lies anterior to the lateral femoral condyle (LFC), confirming a hypermobile meniscus. Arthroscopic images courtesy of Allen F. Anderson, MD, Tennessee Orthopaedic Alliance, Nashville, TN.

Treatment of the unstable lateral meniscus

An asymptomatic meniscal flounce or discoid meniscus requires no treatment. However, patients with the rare lateral meniscal flounce should be carefully evaluated as the appearance may be a sign of meniscal instability. When mechanical symptoms are present or the meniscus is torn, non-Wrisberg type discoid lateral menisci are amenable to partial meniscectomy. Identification of the Wrisberg type discoid meniscus pre-operatively is important for surgical planning as total meniscectomy has been traditionally used for treatment of this variant. However, particularly in younger patients, partial meniscectomy with concomitant repair of the capsular attachment deficiencies may be performed²⁴. In patients with a hypermobile lateral meniscus, the current approach favors meniscal preservation, with stabilization obtained by fixation of the meniscus to the posterior capsule. Although success has been found with open technique and primary repair of the popliteomeniscal fascicles, many surgeons favor arthroscopic repair (Figure 19). Perhaps because in many cases the patients are young and have no intrinsic meniscal tear or abnormality of hyaline cartilage, operative results have in general been excellent.

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Figure 19:

Same patient as in Figure 18. The posterior horn was reattached to the posterior capsule utilizing vertically stacked sutures from above and below (3 on the femoral surface are visible in this photo). Meniscal stability was restored. Arthroscopic image courtesy of Allen F. Anderson, MD, Tennessee Orthopaedic Alliance, Nashville, TN.

Conclusion

The lateral meniscus is inherently more mobile than the medial, but meniscal motion is a normal process required for proper biomechanical function of the joint. Entities exist in which excessive mobility of the lateral meniscus results in patient pain and/or mechanical symptoms. Some abnormalities such as the Wrisberg type discoid lateral meniscus are well known. The hypermobile lateral meniscus is a less well-recognized abnormality in which a careful history and physical examination are critical for accurate diagnosis. From a practical standpoint, variability in the appearance of the popliteomeniscal fascicles makes it inadvisable to suggest a hypermobile lateral meniscus in every case in which the structures are not clearly visualized. However, in the setting of high clinical suspicion, a critical review of this anatomic region often provides previously unrecognized evidence to support the diagnosis, resulting in a higher likelihood that appropriate operative therapy will be provided.

References