A 14-year-old boy visited our hospital complaining of worsening left knee pain. He had twisted the left knee while playing basketball 18 months earlier. On physical examination, range of motion in the left knee was 0-140 . The patient showed tenderness on palpation along the lateral aspect of the joint line and a positive result for McMurray's test on the lateral side. X-rays revealed that growth plates on both the femur and tibia were still present. Magnetic resonance imaging (MRI) revealed a complete lateral discoid meniscus with degeneration within the meniscus (Fig. 1). The clinical diagnosis was complete lateral discoid meniscus, and the first arthroscopic surgery was performed. Complete lateral discoid meniscus with a radial tear was confirmed on arthroscopic examination (Fig. 2a), and partial meniscectomy was performed (Fig. 2b). No other abnormalities were noted. Partial to full weight-bearing was allowed as tolerated without immobilization of the knee, and physiotherapy including active motion exercises and thigh muscle strengthening started on postoperative day 1. After arthroscopic surgery, pain in the left knee improved, and the patient returned to playing basketball 5 months after the surgery.
At the 7-month follow-up, the patient revisited our hospital complaining of left knee pain and left knee locking developed during daily activity. No history of recent trauma was elicited. His height had increased 2.4 cm since the first surgery. On physical examination, range of motion in the left knee was 0-130 , with a positive result for McMurray's test of the lateral side. MRI revealed a low-signal cord between the anterior and posterior portions of the lateral meniscus, resembling a bucket handle tear of the lateral meniscus. A second arthroscopic surgery was performed based on these findings. During arthroscopy, we found that the anterior and posterior horns of the lateral meniscus were connected by meniscus-like tissue forming a ring-shaped meniscus (Fig. 2c). The newly formed interhorn meniscal bridge had not been present at the first surgery, and was thus suspected to represent the cause of the new knee pain. As a result, this bridge was resected. The posterior potion of the lateral meniscus was sutured with FAST-FIX360 (Smith & Nephew, Andover, MA) after identification of a longitudinal tear with instability. Full weight-bearing was started immediately, but knee flexion was limited to 90  for the first 3 weeks. Six months after the second arthroscopic surgery, the patient returned to playing basketball without any symptoms.
Hematoxylin and eosin staining of the newly formed interhorn meniscal bridge showed areas of chondrocyte-like cells mixed in collagenous fibers (Fig. 3a), with capillary vessels infiltrating the region (Fig. 3b). The newly formed interhorn bridge could thus have represented a meniscus-like tissue that appeared secondary to tissue repair.
One year after the second arthroscopic surgery, at 16 years old, the patient visited our hospital with right knee pain after stumbling on some stairs. Although the growth plates on both femur and tibia had closed, physical examination and MRI indicated complete discoid lateral meniscus similar to that in the left knee. Arthroscopic partial meniscectomy was performed. Currently, as of two years after the right knee surgery and three years after the second surgery on the left knee, the patient remains asymptomatic. Recent follow-up MRI of both knees has not shown any morphological changes.
Written informed consent was obtained from the patient and his family for publication of this case report and accompanying images.