Abnormalities of the meniscus are rare in the general population and tend to occur in the lateral meniscus ^[1]. They can cause a range of symptoms such as pain, swelling, and locking due to changes in the structure of the knee joint. Several types of meniscal abnormalities can present in the knee joint, including the most common form, discoid meniscus, as well as less common forms such as ring-shaped meniscus, Wrisberg meniscus, and double-layered meniscus [1, 2]. A review of the literature has shown that only a small number of cases of double-layered meniscus have been documented ^[3]. In this report, we present a new type of double-layered meniscus that has not been described in the literature before. The patient and her family were informed that data concerning their case would be submitted for publication and they provided consent.

A 12-year-old girl complained of knee pain and clicking that had persisted for 3 months without trauma. Physical examination of the patient revealed limited extension (range of motion is 20°–135°) and a positive McMurray test on the lateral side. An enlarged lateral joint space with high signal intensity in the region of the lateral meniscus was observed on magnetic resonance imaging (MRI) (Fig. 1). On arthroscopic visualization of the knee joint, the first structure observed was an incomplete discoid meniscus with inflammation and central vascularization (Fig. 2a). When the central part of this structure was excised, an accessory meniscus was observed overlying the normal lateral meniscus (Fig. 2b). While the posterior part of the accessory meniscus was connected to the underlying normal meniscus, there was no connection between these two structures in the periphery (Fig. 3). While the thickness of the upper incomplete discoid meniscus was similar to that of the normal meniscus, its mobility was higher. Initially, the central part of the incomplete discoid meniscus was removed, resulting in the classic appearance of a double-layered meniscus and then the upper accessory meniscus was removed (Fig. 4).

After surgery, the patient’s pain was completely gone by the end of the 1^st month and the full range of motion was achieved at the end of the 6^th week. Our patient performed post-operative physiotherapy compatibly and was allowed to do active sports at the end of the 3^rd month. The annual follow-ups were carried out, and the patient’s 3-year follow-up showed no occurrence of any complications. At the end of the 3^rd year, the International Knee Documentation Committee (IKDC) score was 95.

Since its first description by Bailey and Blundel in 1974, the presence of a double-layered lateral meniscus has only been reported in 15 cases in the literature [3, 4]. An anatomical study of 437 patients to investigate meniscal abnormalities reported a prevalence rate of 0.5% for a double-layered lateral meniscus ^[1]. However, in arthroscopic studies involving a larger number of patients, the prevalence rate ranges from 0.06% to 0.09% [5, 6]. Meniscal abnormalities typically present with symptoms such as knee pain, locking, and restricted range of motion. MRI and diagnostic arthroscopy can be used to make a differential diagnosis ^[7]. Because the MRI appearance of a double-layered meniscus is similar to bucket-handle tears and horizontal meniscal tears, it can be difficult to make a definitive diagnosis based on imaging alone ^[8]. Therefore, in most cases, a definitive diagnosis is made through arthroscopic examination. In our case, there were findings supporting this view. On examination of the MRI images in our case, a preliminary diagnosis of a horizontal tear accompanying the discoid meniscus could be considered. However, the arthroscopic examination revealed that it was a double-layered meniscus, not a tear. This distinction was made thanks to two important findings. First, the upper meniscus was in an incomplete discoid structure, whereas the lower meniscus was in a normal shape. Second, the posterior part of the accessory meniscus was found to be attached to the normal meniscus, whereas peripheral part lacked any attachment. This observation is compatible with the separated type double-layered meniscus pattern previously reported in the literature [8, 9]. Although incomplete discoid meniscus is one of the most commonly observed morphological variations, there is limited information in the literature regarding its association with other meniscal abnormalities. The only information available on the relationship between discoid meniscus and double-layered meniscus is a case report by Saygi et al., in which incidentally observed a structure resembling a discoid meniscus beneath the normal meniscus during examination of the lateral meniscus in a patient who underwent surgery for medial meniscus pathology ^[10]. In our case, the patient presented with lateral meniscus symptoms and an incomplete discoid meniscus was observed lying the normal lateral meniscus.

We recommend arthroscopy for accurate diagnosis and treatment of lateral meniscus pathology when MRI cannot provide a definitive diagnosis. As this case shows, it is important to be aware of the possibility of rare conditions and to perform a thorough examination during arthroscopy to accurately diagnose abnormalities.

The presence of a double-layered meniscus with an incomplete discoid accessory meniscus, reported for the first time in this case, is a notable abnormality that should be considered in cases of suspected meniscal abnormalities.