Arthroscopic Visualization of Abnormal Movement of Discoid Lateral Meniscus With Snapping Phenomenon

Abstract

Discoid lateral meniscus with snapping phenomenon is a rare pathologic condition. The purpose of this article is to present an arthroscopic technique for the treatment of discoid lateral meniscus with snapping phenomenon. The patient is placed in the supine position for confirmation of snapping. As the patient's knee bends, it can be confirmed by arthroscopy that the posterior horn of the discoid lateral meniscus moves posteriorly and the central portion of the discoid lateral meniscus moves anteriorly at the same time with snapping at deep flexion angles. The anterior segment of the discoid lateral meniscus is found to be redundant and is often folded. On the contrary, as the patient's knee extends, the central portion is returned to the original position accompanied by snapping at nearly full extension. After excision of the central portion, the movement of the meniscus is evaluated again and the disappearance of the snapping phenomenon can be confirmed. Although it includes limitations, this application is easy and would certainly help surgeons to treat snapping knee with discoid lateral meniscus.

The discoid lateral meniscus (DLM) was first described in cadaveric specimens by Young¹ in 1889. DLM is an anatomic variant of the meniscus in which the meniscus is thickened and discoid shaped, and the DLM covers a greater area of the tibial plateau than the normal meniscus. It is more prevalent among Asians than white persons.² The actual incidence of discoid menisci is difficult to estimate because of the high rate of asymptomatic patients. A previous study indicated that insufficient meniscal attachment to the tibia could increase meniscal excursion during flexion and extension.³ Therefore DLM frequently will become symptomatic despite the fact that DLM is a rare pathologic condition. Popping and snapping were reported to occur in 56% of patients as subjective symptoms.⁴ Although it is important to know the real movement of the DLM in a snapping knee, little attention has been paid to evaluating the direct confirmation of the snapping phenomenon using arthroscopy. We describe in detail the movement of the DLM with the snapping phenomenon before and after partial resection using arthroscopy.

Technique

In a patient with a symptomatic and snapping DLM, magnetic resonance imaging usually shows a horizontal tear (Fig 1). Basically, in our experience, most of the snapping knees are classified as the “no shift” type according to magnetic resonance imaging findings.⁵

Fig 1.

(A) Fat-suppressed T2-weighted coronal image of the right knee in a 13-year-old boy. This magnetic resonance image shows a horizontal tear of the discoid lateral meniscus. (B) T1-weighted sagittal image of the right knee of the same boy. This image shows the “no shift” type of discoid lateral meniscus with a horizontal tear.

The patient is positioned supine with a standard leg holder allowing full range of motion. With the patient under general anesthesia, the flexion and extension angles when the knee is snapping should be confirmed before arthroscopy. In general, as we bend the patient's knee with or without application of a compressive load on the lateral side, we are able to see and feel an abnormal movement of the meniscus on the anterolateral side with snapping at deep flexion angles. In addition, as we extend the patient's knee, we confirm snapping again at 20° to 30° of knee flexion (Video 1, Table 1).

Table 1.

Indications and Surgical Steps of Arthroscopic Visualization of Abnormal Movement of Discoid Lateral Meniscus

Indications

Snapping knee with discoid lateral meniscus

Surgical steps

Preparation

Position the patient supine with a standard leg holder allowing full range of motion.

Confirm the flexion and extension angles when the knee is snapping with the standard position.

Arthroscopic visualization of abnormal movement of discoid lateral meniscus

Perform routine arthroscopy using the standard anterolateral and anteromedial portals.

Use the anterolateral portal for viewing.

Evaluate the movement of the meniscus when the knee is snapping with the standard position. (Do not use the figure-of-4 position.)

Saucerization

Change the knee position from the standard position to the figure-of-4 position.

Use the anteromedial portal for viewing and the anterolateral portal for working.

Excise the central portion using 12-cm scissors for the anterior aspect and an oval punch for the middle and posterior aspects of the discoid lateral meniscus.

Debride the piecemeal meniscus using a 3.5-mm shaver.

Arthroscopic visualization of remaining meniscus

Change the knee position from the figure-of-4 position to the standard position.

Confirm the disappearance of snapping with the standard position using the anteromedial or anterolateral portal for viewing.

Thereafter routine arthroscopy is performed by use of the standard anterolateral and anteromedial portals. In the same way, as we bend the patient's knee with or without application of a compressive load on the lateral side, we can directly see the abnormal movement of the DLM with snapping using arthroscopy through the anterolateral portal. Specifically, as we bend the patient's knee, it can be confirmed by arthroscopy that the posterior horn of the DLM moves posteriorly and the central portion of the DLM moves anteriorly at the same time with snapping at deep flexion angles. The anterior segment of the DLM is found to be redundant and is often folded (Fig 2). On the contrary, as we extend the patient's knee, the central portion is returned to the original position accompanied by snapping at 20° to 30° of knee flexion. Therefore it is predictable that snapping of the DLM will disappear after excision of the central portion.

Fig 2.

Supine positioning of patient, allowing full range of motion with a standard leg holder. Routine arthroscopy is performed by use of the standard anterolateral and anteromedial portals. We can see the discoid lateral meniscus of the right knee in a 13-year-old boy through the anterolateral portal before excision. The snapping phenomenon with discoid lateral meniscus of the right knee is observed in deep flexion before excision with the standard position (not the figure-of-4 position). This image shows that the central portion of the discoid lateral meniscus moves anteriorly with snapping at deep flexion angles (arrowheads).

The knee position is changed from the standard position to the figure-of-4 position. Anteromedial and anterolateral portals are used for viewing and working, respectively. The central portion is excised using 12-cm scissors for the anterior aspect and an oval punch (Smith & Nephew, Andover, MA) for the middle and posterior aspects of the DLM (Fig 3). Debridement of the piecemeal meniscus is performed with a 3.5-mm shaver (Smith & Nephew). After partial meniscectomy of the central portion is performed, the movement of the DLM should be confirmed again in the standard position. Contrary to before excision of the central portion, we can see the normal movement of the remaining meniscus as we bend and extend the patient's knee without snapping (Fig 4).

Fig 3.

The knee position is changed from the standard position to the figure-of-4 position. Anteromedial and anterolateral portals are used for viewing and working, respectively. The central portion is excised using 12-cm scissors for the anterior aspect (white arrows) and an oval punch for the middle and posterior aspects (black arrows) of the discoid lateral meniscus. Debridement of the piecemeal meniscus is performed with a 3.5-mm shaver.

Fig 4.

After partial meniscectomy of the central portion is performed, the movement of discoid lateral meniscus should be confirmed again in the standard position. Contrary to before excision of the central portion, we can see the normal movement of the remaining meniscus as we bend and extend the patient's knee without snapping. This image shows discoid lateral meniscus in a right knee after excision of the central portion.

The patient follows a routine postoperative rehabilitation protocol. Range-of-motion exercises and weight bearing without restriction are encouraged immediately after the operation.

Discussion

It is well known that the menisci serve as load distributors and shock absorbers and play an important role in knee joint stability. A meniscus-deficient knee carries a high risk of early cartilage degeneration and early degenerative changes. The lateral meniscus seems to contribute to load bearing more than the medial meniscus. DLM tears are known to be related to articular cartilage lesions.⁶ However, the biomechanical characteristics of DLM are still unknown, and thus orthopaedic surgeons should know whether the remaining DLM is still functional. According to previous reports, total or subtotal meniscectomy has been favorable for symptomatic DLM.⁷ Although arthroscopic saucerization and repair are currently topics of discussion,⁸ there is no consensus concerning the area of excision for symptomatic DLM so far.

We successfully performed partial meniscectomy of the central portion of symptomatic DLMs with snapping phenomenon in 5 knees in 4 male patients. Our patients had a mean age of 13 years (range, 12 to 13 years). In all cases we confirmed the abnormal movement of the DLM using arthroscopy. However, peripheral rim instability was not observed in all cases. Because we have observed the abnormal movement of the symptomatic DLM with the snapping phenomenon, it was possible to determine the area requiring excision. In 1948 Smillie³ first described the cause of snapping knee and reported that the actual sound would be produced by backward or forward movement of the meniscus at the moment when the femoral condyle rides over (1) the thick intact strip of fibrocartilage that forms the anterior boundary of the facet produced by the femoral condyle or (2) the thick anterior peripheral margin of the meniscus itself as the result of sudden alteration in the usual relation between the meniscus and the femoral and tibial condyles. According to more recent reports, the snapping knee in DLM is typically caused by peripheral rim instability, and the whole discoid meniscus is moved to the intercondylar notch and reduced to the anatomic position with a loud snap during knee flexion and nearly full extension.^9,10 However, how to achieve direct confirmation of abnormal movement in patients with snapping DLM is still unknown. Usually, the lateral meniscus is investigated using the figure-of-4 position to open the lateral joint space; however, this position provides little information concerning the abnormal movement of the DLM (Table 2). This is a key point of our technique. Our arthroscopic findings showed that an unstable central portion with a stable peripheral rim was observed in each case with a snapping knee. Therefore repair of peripheral tears was not required in our cases.

Table 2.

Key Points, Advantages, and Limitations of Procedure From Our Experience

Key points

The discoid lateral meniscus should not be investigated with the figure-of-4 position; we can never confirm the snapping phenomenon in this position.

A lateral compressive load is sometimes required.

Advantages

Easy evaluation of abnormal movement of discoid lateral meniscus

Easy confirmation of disappearance of snapping

Safe procedure for patient

Limitations

The long-term clinical results are unknown.

Repair of the peripheral rim may be required if a rupture of the rim is observed.

Although this technique has many advantages, several limitations should be noted (Table 2). First, this is a technical note, and further follow-up will be necessary to investigate the long-term clinical results of partial meniscectomy. Second, our patients were all boys, and thus tears of the peripheral rim had not yet occurred. Presumably, repetitive abnormal movement of the DLM with snapping during daily activities will lead to a tear of the peripheral rim. Therefore we would consider performing a repair of the peripheral rim if a rupture of the rim is observed. Nevertheless, the described technique can facilitate visualization of the abnormal movement of the DLM with the snapping phenomenon.

Supplementary Data

Video 1

Arthroscopic visualization of abnormal movement of discoid lateral meniscus.