Introduction Meniscal cysts are more likely to occur in mn between the ages of 20 and 40 years, with an estimated incidence of 1% to 8% in published studies. They particularly develop on the medial anterior side of the knee. Their symptoms include focal knee pain, a palpable mass at the joint line, and a catching sensation. Different theories have been proposed regarding the etiology of meniscal cysts, such as trauma, infection, and tumors. One of the most accepted etiologies is that meniscal cysts are caused by a horizontal tear on the meniscal surface. Owing to a check valve mechanism, the unidirectional flow of synovial fluid is generated from the intra-articular space to the peri-articular soft tissues. The availability of high-resolution ultrasonography has resulted in increased detection of meniscal cysts. Both sensitivity and specificity have been reported to be over 90%. Ultrasound-guided aspiration and steroid injection for meniscal cysts have a simple, safe, and immediate effect. Therefore, it should be considered as an alternative treatment for patients unsuited for surgery or as a possible interim treatment until surgery. Surgical treatment includes curettage of the cysts and meniscal repair or resection. Disrupting the check valve between the meniscus tear and cyst can prevent one-way fluid retention due to adhesions and scarring of the coat. However, it is still important to have adequate curettage. We believe that the use of intraoperative ultrasonography for meniscal cysts will make surgery easier and more reliable. Case Presentation Case 1 The patient was a 52-year-old woman. She consulted a physician regarding pain in her left knee, which she had had for 1 week. Ultrasonography and MRI revealed a medial meniscus tear and a meniscal cyst. Although puncture and steroid injection were performed, the pain did not improve. She was referred to our hospital for treatment. Radiographs showed mild osteoarthritis. Physical examination showed tenderness in the medial joint space. She also had pain during deep flexion of the knee, such as when squatting. Ultrasonography showed a horizontal tear of the medial meniscus and a hypoechoic region deep in the medial collateral ligament. This led to the diagnosis of a meniscal cyst. MRI also showed a horizontal tear of the posterior segment of the medial meniscus and a contiguous, polycystic meniscal cyst, similar to the ultrasonographic findings. First, we performed an ultrasound-guided injection. After local anesthesia with a 27G needle, we aspirated the cysts using an 18G needle and then injected steroids. The patient's pain improved immediately after the injection, and the effect lasted for approximately 1 week. However, she felt pain 2 weeks after the injection and an ultrasonogram showed that the cyst had enlarged again. Therefore, we decided to treat it surgically. The medial meniscus showed a horizontal tear, as shown on the MRI. Here, we tried to treat meniscus tears and meniscal cysts by using ultrasonographic guidance. After putting on a clean probe cover, the assistant applied the probe along the long axis to the site of the meniscal cyst. The surgeon could view both ultrasonographic and arthroscopic monitors. The arthroscope and rasp were inserted through the anteromedial and anterolateral portals, respectively. The meniscus and cysts were rasped while the location of the cyst was confirmed via ultrasonography. For the meniscus repair, three all-inside techniques were performed. Next, an additional small skin incision was made anterior to the medial collateral ligament, and extra-articular curettage of the cyst was also performed with ultrasonography. The surgery was completed after sufficient curettage of the cyst was confirmed. No recurrence was observed up to 19 months after surgery. Case 2 The patient was a 21-year-old man with pain in the left knee and a mass in the anterior part of his knee, which he had experienced for 3 months. He consulted a physician and was diagnosed with a lateral meniscal cyst. His symptoms slightly improved with the injection, but the pain and swelling recurred. He was referred to our hospital for treatment. Extension of the left knee was restricted. There was a hypoechoic palpable mass at the lateral joint. MRI also showed similar findings. The effect of injection from the previous hospital was temporary, so we decided to perform surgery. The assistant placed the ultrasonographic instrument along the longitudinal axis at the site of the cyst. An arthroscope was inserted through the anteromedial portal. An additional portal was placed anterior to the lateral collateral ligament. Curettage was performed while confirming the location of the cyst with a shaver and radiofrequency. We confirmed adequate curettage of the cyst. The lateral meniscus had a transverse tear and a horizontal tear, so we repaired it with all-inside and outside-in techniques. No recurrence was observed up to 8 months after surgery. Case Series Between February 2008 and August 2021, nine patients with meniscal cysts visited our hospital. Their average age was 33.9 years, with a male-to-female ratio of 4:5. The medial side was affected in two-thirds of the total cases. In all cases, the cysts were polycystic and the meniscus had horizontal tears. Eight of nine patients underwent injections, and in all cases, the pain immediately decreased to less than one-half. The average duration of the injection effect was approximately 8 days. Surgery was required in all cases: meniscal repair was performed in seven cases and meniscectomy in two cases. The average postoperative observation period was 19 months, and there was no recurrence in any of the cases. Conclusion As a treatment for meniscal cysts, ultrasound-guided injections can have good short-term outcomes but often eventually require surgery.The use of intraoperative ultrasonography is effective in three ways: in identifying the location of the cyst, in confirming the actual area of curettage, and in evaluating whether the curettage was sufficient.