A 62-year-old male patient presented with persistent left L5 radicular pain affecting his mobility for several months. The patient reported mild lumbar pain and no complaints of sphincter abnormalities. However, in the last week he has experienced severe neurological claudication. Physical examination did not reveal any motor deficits. The preoperative lumbar magnetic resonance imaging (MRI) shows an intracanal cyst (21 mm × 14 mm ×12 mm) with a thick capsule and heterogeneous contents (Figure 1). The cyst has a considerable impact on the thecal sac, resulting in a notable narrowing of the central canal. A complete resection was performed using the ipsilateral unilateral biportal endoscopy (UBE) technique under general anaesthesia.

The procedure was conducted from the patient's left side. Once 2 ports have been created (cranial endoscopic portal/caudal working portal; Figure 2), the relevant anatomical references in the L4–5 space have been identified (Supplementary Video Clip 1). The initial step is to drill the inferior edge of the superior lamina until the upper insertion of the ligamentum flavum can be identified. This crucial step serves a dual purpose: it facilitates the detachment of the ligament and allows sufficient space for the endoscopic camera movement during cyst dissection. The second step is to detach the ligamentum flavum at the caudal insertions (upper lamina of the lower vertebra). Subsequently, the ligamentum flavum is dissected in a systematic manner, from cranial to caudal and from medial to lateral, in order to identify any adhesions to the dura and the epidural veins. In view of the dimensions of the cyst, we have elected to open the cyst wall and perform an intracyst resection while maintaining the capsule (debulking). This procedure will reduce the pressure exerted by the cyst wall on the healthy dura mater, thereby facilitating a more accurate dissection. The cyst was successfully removed without any complications. To reduce the risk of recurrence, the edges of the capsule attached to the dura mater were coagulated using a radiofrequency probe set at the minimal effective power settings under continuous saline irrigation. This method aims to shrink the remaining wall of the cyst while ensuring thermal protection for the adjacent neural structures. The patient was discharged from the hospital within 24 hours. A postoperative MRI was performed, confirming the correct decompression and the absence of surgical complications (Figure 3). The patient experienced an immediate resolution of symptoms. One year after the procedure, no instability sign was observed.

Synovial cysts are dilations of synovial sheaths that originate in the zygapophyseal joint capsule of the lumbar spine [1]. The exact cause of facet cysts is not fully understood, but it is believed to be associated with joint destabilization due to degenerative or traumatic changes in the ligamentum flavum and the facet joints. The prevalence ranges from 0.5% to 1.1% in older studies [2]. However, with the improved diagnostic possibilities of MRI, the number of identified cysts has increased to 5.8% in recent decades [3]. They are mostly found in the lumbar spine, particularly at the L4–5 level, and can cause symptoms such as neurogenic claudication, radiculopathy, or even cauda equina syndrome. These cysts can be classified as either a true synovial cyst or a pseudo-cyst [4]. A true synovial cyst is characterized by a visible connection between the joint and the cyst on the MRI, while a pseudo-cyst, like a ganglion, does not show this relationship [5,6]. It is important to note that this cyst often exhibits a strong tendency to adhere to the dura. In some cases, the inflammatory changes observed in the capsule can present a significant challenge in differentiating it from the true dura.

Several surgical options are available for treating cysts, including conservative management, corticosteroid injection, percutaneous cyst rupture aspiration and surgical excision with or without fusion [1,7-10]. However, conservative and percutaneous therapies may not provide long-lasting effects [3,6]. Resection of the cyst and decompression of neural structures is the preferred method of treatment as it can lead to immediate and long-lasting improvement of symptoms.

UBE represents a novel technique employed for the treatment of a range of spinal disorders [7,8]. UBE surgery is particularly efficacious in the decompression of lumbar juxta-synovial cysts [1]. It provides a clear, magnified surgical view from the ipsilateral to the contralateral side, thereby rendering it an optimal choice even for those new to endoscopic surgery. UBE may result in superior clinical outcomes, including reduced muscle damage, diminished postoperative opioid use, and shorter hospital stays in comparison to microscopic decompression [7].

It is crucial to consider a number of factors when determining the most appropriate endoscopic surgical approach [4,9,11]. It is somewhat surprising that the size of the cyst has not been taken into account previously in order to decide the best approach, given its potential impact on the surgical strategy. It is incontestable that the volume of the cyst correlates with the strength of its attachment to the capsule and dura. A greater volume will result in a larger area of attachment between the cyst wall and the dura. While there is no exact definition of the size at which cysts should be considered small or large, some authors have assessed the impact of cysts on the spinal canal. Campbell et al. [5] established a classification system for these cysts, categorising them into three types based on the percentage of canal stenosis: less than 25%, 25%–50%, or more than 50%. In the case of small cysts, the contralateral endoscopic approach is becoming increasingly popular due to its ability to facilitate facet joint preservation and simplify cyst dissection [1]. However, larger cysts have a greater volume adhered with the dura, making them more difficult to dissect. Surgeons should be aware that this requires more experience, planning, and dissection skills. An ipsilateral approach could be an alternative as we show in this case. The ipsilateral approach in large cysts allows for the debulking of the cyst in the early stage, which helps reduce the mass effect on the dura and guide the surgeon in identifying the cyst wall. This may contribute to reducing the tractions during the dissection and the risk of dural tear (Table 1).

The video provided by the authors offers an excellent illustration of the surgical technique for the removal of a large synovial cyst using an ipsilateral endoscopic approach, which avoids dura retraction and the risk of damaging surrounding nervous structures. However, further prospective randomized studies are required to obtain more precise conclusions.