In this case, we successfully ruptured an LFC by percutaneous interlaminar epidural injection, which was a relatively simple procedure. The LFC was located inside the ligamentum flavum away from the facet joint and was not accessible by the conventional approach. Moreover, since there was no communication between the LFC and the facet joint, it was impossible to rupture the LFC through the facet joint. The position of the LFC was checked using MRI images and used to calculate the needle approach trajectory in the interlaminar approach to rupture the cyst. We did not perform a follow-up MRI. But, as contrast media spread well to the epidural space and the patient's symptoms improved, it is estimated that the cyst inside the ligamentum flavum was successfully ruptured. No symptoms recurred even at the outpatient clinic visit one year after the procedure.

The term "juxtafacet cyst" refers to all cysts occurring in the posterior elements of the spinal canal. Ligamentum flavum cysts are a subtype of juxtafacet cysts [4] and are recognized to be the rare cause of cord and nerve compressions [5]. PLL cysts and FJCs are also types of JFCs. JFCs have two histological subtypes. Some authors distinguish synovial cysts and ganglion cysts by the presence or absence of a synovial lining membrane. Synovial cysts communicate with the facet joint and are surrounded by the inner wall of the synovial lining membrane, while a ganglion cyst is a pseudocyst surrounded by a fibrous wall [2]. Almost all cysts of the ligamentum flavum and PLL cysts are pseudocysts, known as ganglion cysts [6].

The incidence of FJCs is thought to be less than 0.5% in the general symptomatic population [5]. FJCs arise from a defect in the synovial membrane and are thought to be caused by a combination of the degeneration of the zygapophyseal joint and destabilizing changes in the spine [4]. However, LFCs are rare and occur frequently in the lower lumbar region. The cause of LFCs has not yet been fully identified, but it has been suggested that persistent stress on the ligamentum flavum may lead to mild chronic trauma such as ligament leukemia and degenerative disc disease in patients [6]. Therefore, the distribution of LFCs correlates with mobility and the incidence of degenerative changes. LFCs in the lumbar spine commonly occur at the L4–5 level, followed by the L5–S1, L3–4, and L2–3 levels. Only a few cases have been reported in the cervical spine [7].

There are no specific clinical symptoms for LFCs. The majority of symptomatic cysts usually present with lower leg pain. Wildi et al. [8] reported that 97% of the patients complained of radicular pain, and 76% of the patients had low back pain. Thus, LFCs can be confused with symptoms caused by herniated nucleus pulposus. Neuroimaging, especially MRI, plays an important role in the diagnosis of JFCs. Generally, the cysts have a variable signal on T1-weighted and hyperintense T2-weighted images. A hyperintense extradural mass with a peripheral hypointense rim on T2-weighted images was consistent with an LFC observed in the histopathologic examination [9]. In our patient, MRI T2-weighted images showed thickened ligamentum flavum containing the cyst with a high-intensity signal and a low-intensity signal on T1-weighted images. LFCs are adjacent to or located within the ligamentum flavum and do not communicate with the facet joint. The differential diagnosis of imaging studies between LFC and FJC is helpful for surgeons because FJCs are more difficult to resect, requiring exploration of the facet joint [10]. The histological features of the LFC, with an absence of a synovial lining membrane and no communication with the facet joint, helped us decide how we should perform the procedures.

Some cases of the spontaneous resolution of cysts have been reported, but most cases require treatment. The treatments reported for LFC range from conservative management to surgical decompression with stabilization. Surgical decompression is the treatment of choice. Surgical decompression may be required for the patient who has significant symptoms or is refractory to conservative care. The goal of surgery in a patient with an LFC is spinal decompression through laminectomy and the resection of the affected ligamentum flavum and the cyst. If complete excision of the cyst is performed, a minimal recurrence rate is assured. Even in the surgical department, endoscopic resection has been attempted due to the risk and complications of open surgery [11]. Conservative treatment can be an option for patients who cannot receive surgical intervention due to old age or systemic diseases, or patients who refuse surgical intervention. Conservative treatment includes medication, physical therapy, needle aspiration, and mechanical cyst rupture. Among the treatment modalities, minimally invasive procedures can be considered for patients with a high risk for surgery. Many percutaneous methods have been reported recently such as computed tomography-guided treatment, and epiduroscopic removal. Bureau et al. [12] reported that percutaneous steroid injections were often effective and could result in complete regression of the cyst. Shuang et al. [13] conducted a meta-analysis including 29 studies and reported that percutaneous cyst resolution procedures had the potential to be an alternative to surgery. The procedure we performed is a much simpler and more accessible method compared to surgical intervention. Not only does it take less time, but it has the advantages of being less expensive, and less invasive. This case is thought to be meaningful because few cases of conservative treatment for an LFC have been reported in the literature.

There were some limitations to this case report. First, we could not confirm whether the FJC was ruptured after two attempts. The LFC seemed to press the nerve root and the FJC was relatively small in the MRI image. Therefore, we thought that the patient’s symptoms were mainly caused by the LFC, not the FJC. But given the possibility that FJC can cause symptoms, it needs to be treated using other methods such as epiduroscopic removal [14]. Second, no histological examination of these cysts was performed. Since the cyst was estimated to be LFC from the histological, anatomical, and radiologic characteristics, this case may further contribute to the treatment modality of LFCs if the cyst is histologically identified. Third, a follow-up MRI was not performed due to the economic issues of the patient. It would have helped to identify how the cysts changed after the procedure. Further studies are needed to evaluate the long-term outcomes and incidence of complications.

In conclusion, LFC can be treated by the interlaminar epidural approach and this procedure could be suggested as a good option for treating an LFC.

Funding: Not applicable

Conflicts of interest: The authors declare no competing interests.