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?
Unstable Wrisberg type discoid lateral meniscus.
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.
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.
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).
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.
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).
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.
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.
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).
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.
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.
- G.J. Romanes, Cunnigham’s Textbook of Anatomy, Oxford University Press, London, UK, 10th edition, 1964. ↩
- Thornoton DD, Rubin DA, Magnetic resonance imaging of the knee menisci, Semin Roentgenol, Jul 2000;35(3):217-30. ↩
- G. Y. El-Khoury, H.Y. Usta, and R.A. Berger, “Meniscotibial (coronary) ligament tears,” Skeletal Radiology, vol. 11, no.3, pp.191-196,1984. ↩
- d’Entremont AG, Wilson DR: Joint mechanics measurement using magnetic resonance imaging. Top Magn Reson Imaging 2010, 21:325-334. ↩
- Thompson WO, Thaete FL, Fu FH, et al Tibial meniscal dynamics using three-dimensional reconstruction of magnetic resonance images. Am J Sports Med 19 210-216, 1991. ↩
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- Feipel V, Simonnet ML, Rooze M. The proximal attachments of the popliteus muscle: a quantitative study and clinical significance. Surg Radiol Anat 2003; 25:58–63. ↩
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- Williams AM, Myers PT, Watts MC et al. The meniscal flounce: a valuable arthroscopic sign. The Knee 2006; 13(4):337-341). ↩
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- Ryu KN, Kim IS, Kim EJ, et al. MR imaging of tears of discoid lateral menisci. AJR Am J Roentgenol. Oct 1998;171(4):963-7. ↩
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- Kim EY, Choi SH, Ahn JH, Kwon JW. Atypically thick and high location of the Wrisberg ligament in patients with a complete lateral discoid meniscus. Skeletal Radiol. Sep 2008:37(9):827-33. ↩
- Van Steyn MO, Mariscalco MW, Pedroza AD, et al. The hypermobile lateral meniscus: a retrospective review of presentation, imaging, treatment, and results. Knee Surgery, Sports Traumatology, Arthroscopy 2014 (Dec). ↩
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- LaPrade RF, Konowalchuk BK. Popliteomeniscal fascicle tears causing symptomatic lateral compartment knee pain: diagnosis by the figure-4 test and treatment by open repair. AJSM 2005;33(8):1231-36. ↩
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