Juxta-facet cysts are a well-recognized pathology commonly found in the lumbar spine. Depending on the location, they are further classified into facet joint cysts, posterior longitudinal ligament cysts, and ligamentum flavum cysts (LFCs)^10,17). While facet cysts are the most common entity, LFCs have rarely been reported, presenting a challenging clinical scenario in initial imaging studies. In general, these LFCs are incidental findings and are an uncommon cause of neurological deficit. Here, we present a rare case of an LFC in the lumbar spine causing right S1 radiculopathy, which was successfully removed through surgery with excellent post-operative results.

An 80-year-old woman came to the outpatient clinic, primarily complaining of lower back pain that radiated to her right buttock and leg and had been ongoing for 2 months. The patient could not walk or carry out daily activities normally due to severe pain expressed as a numerical rating scale of 8. These symptoms were not relieved by medication, physiotherapy, or rest.

On examination, the patient showed no motor weaknesses in the lower limb, with a negative straight leg raising test on both sides. Reflexes in her right lower limb were diminished, and hypoesthesia was noted at the right S1 dermatome. Magnetic resonance (MR) imaging (MRI) of the lumbar spine showed a cystic mass adjacent to the right L5/S1 facet joint, which was hypointense on T1-weighted images (WIs) and hyperintense on T2-WIs (Fig. 1). MR neurography demonstrated the lesion compressing the right S1 exiting root (Fig. 1).

The patient consented and was scheduled for a right partial hemilaminectomy under epidural anesthesia. In the surgical field, the excised mass contained whitish cystic fluid, and was found to originate from the ventral surface of the ligamentum flavum (Fig. 2). The right S1 nerve root was completely decompressed, after removing the mass. Histopathological evaluation of the mass showed a cystic lesion without synovial lining, suggestive of a degenerative cyst, such as an LFC (Fig. 3).

After the surgery, the patient reported improvement in her back and radicular pain, was ambulatory without any assistance, and was discharged on the 4th post-operative day. At 6-month follow-up, she no longer had any pain, and there was no sign of recurrence of the LFC on the follow-up MRI.

LFCs, first described by Moiel et al.²¹⁾ in 1967, are rare causes of radiculopathy frequently found in the lower lumbar area³⁰⁾. Being the most mobile segment, the L4/5 level is the most common lesion where LFCs are found in the lower lumbar region, with L5/S1 level being the second²⁵⁾ (Table 1).

The ligamentum flavum is primarily composed of elastic fibers with a smaller proportion of collagen fibers, distinguishing it from other spinal ligaments. In the spine, collagen fibers impart tensile strength and stiffness, while elastic fibers contribute to compliance and support stress during multiaxial deformation¹⁵⁾. The exact mechanism behind cyst formation in the ligamentum flavum is not well understood, but it is believed that continuous chronic microtrauma to the ligament may lead to cyst development³⁴⁾. These microtraumas can also induce regenerative processes involving the deposition of type-2 collagen, which reduces the ligament's elasticity and promotes further degeneration and myxoid changes³⁷⁾.

LFCs do not have specific clinical symptoms and are often discovered incidentally. Symptomatic LFCs typically present with radiculopathy and can easily be confused with the first impressions of an intervertebral disc herniation¹²⁾. The presentation of LFCs varies depending on the cyst's location, size, and growth rate. Asymmetric compression by the cyst may cause unilateral radicular pain, resembling foraminal stenosis, while a centrally located cyst occluding the spinal canal may lead to symptoms of canal stenosis and claudication-type leg pain. Common symptoms include radicular pain seen in 97% of the patients, combined with sensory changes, motor deficits, a positive Lasègue sign, or abnormal reflexes²⁸⁾. Although rare, surgical emergencies such as cauda equina syndrome can arise from cysts, particularly when complicated by infection or hemorrhage²³⁾. In our case, the patient experienced low back pain and severe radicular pain but did not exhibit motor deficits.

MRI is the gold standard for differentiating LFCs from other intraspinal cystic lesions. The differential diagnoses of LFCs include discal cysts, facet joint cysts, perineural cysts, dermoid cysts, schwannomas, ependymal cysts, or infected cysts. Another modality used for diagnosis is MR neurography, which allows direct imaging demonstration of LFCs and their relation to adjacent structures. The MRI demonstrated in our case showed a synovial cyst that seemed to originate from the facet joint or ligamentum flavum. MR neurography showed an extradural cystic mass located in the dorsal area of the right inferior facet and compressing the right S1 nerve root.

The decision-making process for the operative management of LFCs largely depends on the neurological condition of the patient. Asymptomatic and incidentally diagnosed cases typically do not require surgical intervention. However, symptomatic cases necessitate surgical treatment to alleviate neurological symptoms. Conservative treatments have shown varying degrees of success but are generally ineffective long-term. While percutaneous steroid injections into synovial and juxta-facet cysts may provide short-term relief, percutaneous cyst aspiration has proven ineffective⁸⁾. Surgical excision is the preferred treatment when conservative methods fail. The primary goal of surgery is spinal decompression and complete removal of the cyst along with the affected ligamentum flavum. Most cysts are non-adherent to the dura, allowing for careful dissection and removal. Achieving complete excision significantly reduces the likelihood of recurrence, though adhesions to the dura can lead to incomplete resection and recurrence^4,18). Adequate exposure for visualization and mobilization during surgery is crucial to avoid the risk of excessive traction on the nerve root and thecal sac. Surgical outcomes are typically positive, with significant improvements in pain and neurological function.

LFCs, though rare, should be considered in the differential diagnosis for neurogenic leg pain when imaging studies indicate an epidural cystic lesion. Conservative treatment has not achieved successful long-term results. When considering operative management, adequate exposure for visualization and mobilization is necessary to avoid the risk of excessive traction on the thecal sac and nerve root. The lesion must be excised completely to prevent recurrence.