Arthroscopic management of a rare free-edge medial meniscal cyst: A case report

Abstract

BACKGROUND

Meniscal cysts are a relatively uncommon form of meniscus pathology and often coexist with meniscus tears or degeneration. They are typically diagnosed as intra-meniscal cysts or para-meniscal cysts. The majority of para-meniscal cysts protrude towards the joint capsule side of the meniscus, and to date, no cases have been documented at the free edge of the meniscus. The present case of free-edge medial meniscal cysts will contribute to a deeper understanding of meniscal cysts.

CASE SUMMARY

A 30-year-old male presented with intermittent right knee pain six months ago. Diagnostic arthroscopic examination revealed a medial meniscus tear accompanied by a meniscal cyst. The patient's clinical findings, imaging results, surgical procedures, relevant images and outcomes are described. The unique aspect of this case is the occurrence of the meniscal cyst at the free-edge of the meniscus, providing valuable insights for the classification and further study of meniscal cysts.

CONCLUSION

This first case of meniscal free-edge cyst exemplifies the expansion of the spectrum of morphological types of meniscal cysts.

Key Words: Meniscus tear; Meniscal cyst; Knee arthroscopy; Magnetic resonance imaging; Case report

Core Tip: Meniscal cysts represent a relatively uncommon form of meniscal pathology and are typically classified as intra-meniscal or para-meniscal cysts. Para-meniscal cysts generally arise on the side of the meniscus adjacent to the joint capsule. However, this report presents a rare case of a medial meniscus free-edge cyst, which, to the best of current knowledge, has not been previously documented. This case contributes to a broader understanding of meniscal cysts.

INTRODUCTION

Meniscal cysts, a relatively uncommon form of meniscal pathology, exhibit incidence rates ranging from 1% to 8%, as reported in the literature[1]. These cysts frequently coexist with meniscal tears[1,2] or degenerative changes; however, their underlying pathogenesis remains incompletely understood. A unidirectional valve mechanism is believed to facilitate the accumulation of synovial fluid[3]. Medial meniscal cysts are more prevalent than lateral meniscal cysts, with a reported prevalence ratio ranging from 2:1 to 4:1[4]. The most common site for these cysts is the posterior horn of the medial meniscus[4]. Meniscal cysts are typically diagnosed using magnetic resonance imaging (MRI), with approximately 5% identified as intra-meniscal cysts and 95% as para-meniscal cysts[5]. Most para-meniscal cysts protrude toward the joint capsule side of the meniscus. However, to date, no cases of para-meniscal cysts occurring at the free edge of the meniscus have been reported. This report presents a cyst located at the free edge of the medial meniscus—a very rare presentation—which contributes to a deeper understanding of meniscal cysts.

CASE PRESENTATION

Chief complaints

A 30-year-old male with no significant medical history presented with intermittent right knee pain that began 6 months prior.

History of present illness

The patient initially experienced right knee pain following running and lower limb strength training. He denied any history of direct knee trauma associated with these activities. Since onset, the pain has recurred intermittently and has not fully resolved with rest. It remains localized to the right knee, with no involvement of other joints. The pain typically worsens following physical activity. The patient occasionally reports clicking sounds during knee movement without any sensation of joint locking or dislocation. No associated systemic symptoms—such as fever, night sweats, weight loss, or loss of appetite—have been noted.

History of past illness

No prior knee injuries.

Personal and family history

The patient had no significant past medical, surgical, or family history.

Physical examination

Examination revealed tenderness along the medial joint line of the right knee. The joint was painful on palpation, but the range of motion was preserved, with extension measured at -5° and flexion up to 130°. McMurray's test was positive, suggesting a medial meniscus tear.

Laboratory examinations

Laboratory investigations revealed no abnormalities.

Imaging examinations

Plain radiographs of the right knee demonstrated normal bone density and joint spaces, with no evidence of fracture or dislocation (Figure 1). MRI revealed a tear of the medial meniscus with a small joint effusion. The medial meniscus was obliquely ruptured and displaced inward into the medial groove on the anteromedial aspect, located between the medial femoral condyle and the medial joint capsule. A cystic structure approximately 0.5 cm in diameter was observed at the edge of the torn meniscus. The anterior and posterior cruciate ligaments appeared continuous, with normal morphology and alignment. Both the medial and lateral collateral ligaments were intact. The articular surfaces were smooth, and no abnormalities were detected in the surrounding muscles or soft tissues (Figure 2).

Figure 1  No obvious abnormalities were detected on X-ray imaging.

Figure 2  The magnetic resonance T2-weighted image shows a torn and flipped medial meniscus (indicated by the white asterisks) and a meniscus cyst is located on the free-edge of the flipped medial meniscus (indicated by the yellow arrow).

FINAL DIAGNOSIS

Based on the patient's history, physical examination, imaging studies, and findings from diagnostic arthroscopy, a diagnosis of a medial meniscus tear associated with a medial meniscus free-edge cyst was established (Figure 3).

Figure 3 Arthroscopic view through the anterolateral approach of the left knee reveal that the medial meniscus has an oblique tear forming a flipped flap (indicated by the black asterisks), combined with a free-edge cyst (indicated by the blue arrow). mFC: Medial femoral condyle; mTP: Medial tibial plateau.

TREATMENT

Diagnostic arthroscopy revealed an oblique tear in the body of the medial meniscus with anterior medial flipping. A cyst approximately 0.5 cm in diameter was identified at the free edge of the flipped meniscal flap. Partial resection of the oblique tear was subsequently performed (Figure 4), and the resected meniscal tissue, including the cyst, was submitted for pathological examination. The arthroscopic portals were closed using 2/0 prolene sutures following surgical excision.

Figure 4 The resected portion of the meniscus during surgery. The black asterisks indicates an oblique tear of the meniscus, and the blue arrow points to a cyst at the free edge of the meniscus.

On the first postoperative day, the patient commenced static quadriceps strengthening exercises and active range-of-motion training, which were continued for six weeks. Sutures from the arthroscopic portals were removed after two weeks.

OUTCOME AND FOLLOW-UP

Pathological examination confirmed the presence of a cyst at the free edge of the meniscus (Figure 5). At nearly two months postoperatively, the patient demonstrated marked improvement in knee function, with ongoing recovery anticipated. The clinical course remained favorable, with no recurrence of the meniscal cyst, pain, or limitations in joint mobility.

Figure 5  Pathological hematoxylin and eosin staining of the cyst tissue section reveals a typical cyst wall.

DISCUSSION

The notable finding in this case was the identification of a meniscal cyst located at the free edge of the meniscus—a phenomenon not previously documented in the literature. This observation aligns with recent studies on lateral meniscal cysts, which suggest that such cysts are commonly associated with meniscal injuries and are amenable to arthroscopic treatment. Existing literature indicates that meniscal cysts predominantly affect the lateral meniscus, with reported prevalence ratios ranging from 5:1 to 10:1 compared to the medial meniscus. These cysts are frequently linked to traumatic events. Passler et al[6] reported a 1.4% detection rate of meniscal cysts during knee arthroscopy, with a distribution of 87.5% in the lateral meniscus and 12.5% in the medial meniscus. Similarly, Seger and Woods[7] reported a lateral-to-medial meniscus cyst ratio of 10:1.

However, advances in MRI technology have led to evolving insights. In contrast to earlier findings based solely on arthroscopic or surgical data, recent imaging-based studies suggest that medial meniscal cysts are more common than previously believed[4,8]. Campbell et al[4] reported a revised ratio, indicating that medial meniscal cysts occur approximately twice as frequently as lateral cysts.

Meniscal cysts are generally categorized by their anatomical location as either para-meniscal or intra-meniscal. Para-meniscal cysts are typically located on the side of the meniscus adjacent to the joint capsule[9]. The cyst presented in this case was situated at the free edge of the meniscus, a location where para-meniscal cysts have not been previously described. This position might lead to it being mistaken for an intra-meniscal cyst while the cyst may appear intra-meniscal radiologically, however, its surgical behavior and histology support a para-meniscal classification, and the structural integrity of the surrounding meniscal tissue remained intact. To date, no cases involving a para-meniscal cyst at the free edge of the meniscus have been reported. This case broadens the spectrum of known morphological presentations of para-meniscal cysts and contributes to a more comprehensive understanding of this pathology.

Meniscal cysts are relatively rare entities encountered in meniscal surgery and are typically associated with underlying meniscal tears. The prevailing hypothesis regarding the pathogenesis of para-meniscal cysts suggests the extrusion of synovial fluid from adjacent meniscal tears[1,2,4,10]. A history of trauma is a known risk factor, with studies showing that 37.5% of patients diagnosed with meniscal cysts have a documented history of knee injury. Nevertheless, many cases may result from chronic degenerative processes, particularly among older individuals[11].

In the present case, although an oblique tear was observed in the medial meniscus, there was no apparent tear or degenerative change in the region surrounding the cyst. This raises the possibility that the cyst may have developed spontaneously. Alternatively, repetitive microtrauma or biomechanical alterations leading to mechanical irritation of the free edge may have contributed to its formation. However, as this is a single case report, further investigation is required to validate these hypotheses and confirm the authenticity of this presentation.

MRI remains the imaging modality with the highest diagnostic efficacy for meniscal cysts. Chen et al[12] reported that computed tomography can be useful in evaluating cyst size, location, internal content, and anatomical relationships with the meniscus and surrounding structures. Rutten et al[13] indicated that high-resolution ultrasonography may serve as a viable alternative to conventional MRI. Although these alternative imaging techniques can be utilized when MRI is unavailable, they are not commonly employed in routine clinical practice.

In the present case, although the meniscal cyst was visualized on MRI, its small size made detection challenging. This highlights the importance of a comprehensive MRI evaluation of the meniscus that includes both the capsular and free-edge regions to ensure accurate identification and assessment of meniscal cysts. For future cases involving free edge meniscal cysts, specific MRI protocols may enhance diagnostic sensitivity and specificity. These could include reduced slice thickness or the use of targeted signal enhancement techniques to improve visualization of small cystic structures—representing a promising direction for future research.

Regarding treatment, as most meniscal cysts are associated with underlying meniscal tears, the primary therapeutic approaches include arthroscopic meniscus repair combined with cyst excision or total meniscectomy. Cowden and Barber[14] recommend total meniscectomy for symptomatic meniscal tears accompanied by cysts. However, Kumar et al[15] reported that during midterm follow-up, patients undergoing total meniscectomy exhibited significant declines in Lysholm scores compared to preoperative levels, primarily due to postoperative joint degeneration. Pedowitz et al[16] advocated for partial meniscectomy, which targets the underlying pathology while preserving meniscal structure to prevent further degeneration.

With advancements in arthroscopic repair techniques and evolving surgical concepts, Thor et al[17] suggest that the most common horizontal meniscal tears—frequently associated with cyst formation—can now be effectively addressed through meniscal repair. Therefore, horizontal meniscal tears with concurrent cysts may be managed simultaneously using arthroscopic repair and cyst excision. However, it remains uncertain whether meniscus repair improves outcomes or reduces the recurrence rate of meniscal cysts[17]. While arthroscopy enables concurrent meniscus repair and cyst excision, the efficacy of repair alone in preventing cyst recurrence requires further investigation.

In the present case, an oblique tear of the medial meniscus had flipped into the anterior medial gutter, with a cyst located at the free edge of the torn segment. Arthroscopic partial meniscectomy was sufficient for definitive management. Notably, the flipped tear exposed the free-edge cyst, facilitating its identification and excision. In cases involving other types of meniscal tears, such cysts may remain obscured by the medial femoral condyle, potentially complicating preoperative assessment and intraoperative visualization. When MRI strongly suggests the presence of a medial meniscus free-edge cyst, modification of the standard arthroscopic approach may be warranted. Specifically, the anterolateral portal can be positioned more distally to optimize visualization of the medial meniscus. If necessary, a pie-crusting release of the medial collateral ligament may be performed to widen the medial joint space and improve access for both diagnostic and therapeutic procedures.

CONCLUSION

To the best of our knowledge, this is the first documented case report describing a rare variant of meniscal cyst located at the free edge of the meniscus. This finding expands the known morphological spectrum of meniscal cysts and highlights the importance of meticulous evaluation of the meniscal free edge on MRI to avoid overlooking this uncommon presentation. Surgeons should maintain a high index of suspicion for free-edge meniscal cysts in cases of anterior medial flipping and inconclusive imaging findings.