Popliteal cysts in children are mostly asymptomatic cystic masses behind the knee joint. Compared with adults, popliteal cysts in children have a lower incidence with unknown causes, and the reported prevalence ranges from 2.4–6.3%[11]. However, popliteal cysts are very common in children with juvenile rheumatoid arthritis, and studies have found that they can be as high as 61%, which may be related to repeated fluid accumulation in the joints[12]. Among children, popliteal cysts most often occur between the ages of 4 and 7, most of which are asymptomatic and can be cured by conservative treatment, including braking, NSAIDs, intra-articular injection, etc. However, it has been pointed out in the literature that the self-healing process of some cysts takes more than several years[13]. Some scholars also suggest surgical treatment, especially for patients with continuous enlargement of cysts, peripheral vascular and nerve compression, or even rupture of cysts[14]. They believe that surgery is effective for these children.
Rauschning also gave a detailed description in his study, including the "fissure like" structure formed by the medial head of the gastrocnemius muscle and the semimememial tendon, and the "fissure like" structure was like a valve[15]. During flexion, it opens due to the tension of the semimembrane tendon. During the extension process, the connection is closed due to the intersqueezing of the semimembrane muscle and the medial head of the gastrocnemius muscle. In response to this "valve mechanism" theory, some people [11, 16] tried to close the valve opening with valve suture to reduce the recurrence of cysts. Lindgren et al. measured the pressure in the joint cavity and the gastrocnemius semistemmal sac during normal knee flexion and extension[16]. This repair may not be able to maintain changes in normal pressure, which may account for the high rate of cyst recurrence in patients after valve closure repair.
In adults, with the continuous development of arthroscopy technology, many literatures have confirmed the effectiveness of minimally invasive knee arthroscopy technology in the treatment of popliteal cysts, with a very low recurrence rate[17–19]. They believe that popliteal cysts in adults are communicated with the knee joint (Fig. 3). However, there is still controversy about whether popliteal cysts in children are communicated with joint lumen, but more and more literature studies believe that popliteal cysts in children are also communicated with joint lumen[20, 21], and it is determined that the cause of popliteal cysts in children is the accumulation of fluid in joint lumen into the space of gastrocnemius semi-membrane sac. Our arthroscopic surgical technique is also based on this theory and can provide an effective, minimally invasive treatment for popliteal cysts in children and reduce the risk of recurrence.
In our study, we removed the capsular fold intraoperatively and enlarged the posteromedial valvular junction area by at least 5 mm. This is sufficient to convert the unidirectional flow between the joint cavity and the gastrocnemi-semimembrane sac into normal bidirectional flow. The final follow-up results were satisfactory, with no recurrence and postoperative complications in all patients.
A major shortcoming of this study was the absence of a control group with open surgery and conservative treatment. In addition, the number of cases and the length of follow-up were insufficient. We will conduct further controlled studies with large numbers of cases and long-term follow-up to confirm the feasibility of this technique.