Lumbar Degenerative Spondylolisthesis and Synovial Facet Cyst With Adjacent Intradural Filum Terminale Lipoma: A Case Report

Abstract

Background

Spinal lipomas of the conus medullaris and filum terminale are the most common forms of occult spinal dysraphism. Clinical presentations vary based on anatomic location and size. Filum terminale lipomas occur in up to 5% of the general population based on cadaveric and magnetic resonance imaging studies. Most filum terminale lipomas are asymptomatic and rarely require treatment.

Case Presentation

This is the first known reported case of a 64-year-old woman presenting with lumbar radiculopathy with cascading spondylolisthesis at L3 to L4 and L4 to L5, synovial facet cyst at L4 to L5, and concurrent large filum terminale lipoma. After nonoperative management strategies failed, an extensive discussion about the source of her symptoms was completed. It was believed that the primary driver of her radicular pain was the synovial cyst and spondylolisthesis at L4 to L5, despite the large size of the lipoma. Surgical treatment entailed an L4 to L5 anterior lumbar interbody fusion, posterior spinal instrumentation and fusion using robotic-assisted navigation, and decompression of the synovial cyst through a transfacet approach. Postoperatively, the patient’s buttock and leg pain was resolved. She returned to work with no restrictions and reported no pain or neurological symptoms at her final follow-up.

Conclusions

Incidental filum terminale lipomas may be safely observed when surgically managing a patient with a concurrent, adjacent, and symptomatic degenerative lumbar spondylolisthesis and synovial facet cyst.

Clinical Relevance

In patients undergoing surgery for symptomatic degenerative lumbar spondylolisthesis with an associated synovial facet cyst, an incidentally identified, adjacent intradural filum terminale lipoma without tethered cord symptoms can generally be managed nonoperatively, avoiding unnecessary intradural exploration and its associated risks.

Level of Evidence

5.

Introduction

Lipomas of the filum terminale occur in up to 5% of the general population in cadaveric and magnetic resonance imaging (MRI) studies^1,2 and represent between 7% and 46% of spinal lipomas treated in several large case series.³ The majority of filum terminale lipomas (FTLs) are asymptomatic and do not require treatment.³ These masses can be truly occult in that they sometimes lack the subcutaneous soft-tissue mass and cutaneous manifestations that accompany most conus medullaris lipomas.⁴ This report presents a rare operative case of a 64-year-old woman with right buttock and posterior thigh pain who was found to have a degenerative L3 to L5 cascading spondylolisthesis and right L4 to L5 synovial facet cyst in the setting of an incidentally found intradural extramedullary lipoma of the filum terminale. The present report is presented in accordance with CARE guidelines to ensure clarity.⁵

Case Presentation

A 64-year-old woman presented with a chief complaint of right buttock and posterior thigh pain radiating to the knee. The pain was exacerbated by standing, walking, and bending. She reported several near falls and complained of her right leg giving out a few times per week. She also reported bilateral numbness in her feet and hands for several months. She denied loss of bowel and bladder function but reported numbness in her right foot that awoke her at night. She reported a vague history of being taken to the hospital at 2 weeks old for something “large on her back.” She stated that there was a pinhole area on her lower back where her mother was able to pull out what seemed to be fiber-like material. She works as a bus driver. Her previous medical treatments included muscle relaxers and anti-inflammatories. An interventional radiologist performed a right L4 to L5 transforaminal epidural steroid injection and attempted a right L4 to L5 facet aspiration 4 months before her initial presentation to our spine clinic. No aspirate was able to be obtained; however, she reported significant pain relief immediately after her epidural steroid injection. A right L4 to L5 transforaminal epidural steroid injection was repeated 3 months before her initial presentation to the clinic. She did not experience any pain relief after the second epidural steroid injection.

On examination, the patient appeared to be well. A midline dimple was present from her previous spina bifida occulta surgery (Figure 1). She ambulated with a normal gait and was able to heel and toe walk without difficulty. In an upright stance, she appeared well balanced in the coronal and sagittal planes. She had no tenderness to direct palpation in the midline of her back or paraspinal musculature. She had full strength and normal sensation to 1-time testing in bilateral upper and lower extremities in all major muscle groups. Reflexes were 2+ at the biceps, brachioradialis, patella, and Achilles. She had a negative Hoffman sign, and no clonus was noted. Babinski’s reflexes were plantar bilaterally.

Figure 1.

Clinical photograph demonstrating a midline skin dimple from previous spina bifida occulta surgery (red arrow) and postoperative paramedian incisions from minimally invasive lumbar fusion surgery (green arrows).

Full-length spine radiographs were obtained, which demonstrated a grade 1 cascading spondylolisthesis of L3 to L4 and L4 to L5 (Figure 2). An MRI was obtained 5 months before her initial presentation, which identified a large intraspinal lipoma at the level of L4 that occupied most of the spinal canal (Figure 3). A right-sided synovial facet cyst and facet effusion were noted at L4 to L5 (Figure 3). A computed tomography (CT) myelogram of the lumbar spine was ordered for further diagnostic workup of the intraspinal mass. This finding further characterized it as an intradural, extramedullary mass centered within the thecal sac at the L4 level. The mass measured 1.7 × 2.0 × 3.6 cm in the anteroposterior, transverse, and craniocaudal dimensions, respectively (Figure 4). The conus medullaris was at the L3 to L4 level, which was compatible with a tethered cord.⁶ An incomplete bony defect in the posterior sacrum at S3 to S4 was noted, compatible with spina bifida occulta.⁷

Figure 2.

Preoperative full-length spine radiographs. (A) Anteroposterior view of the lumbar spine. (B) Lumbar lateral view demonstrating a grade 1 retrolisthesis of L2 on L3 as well as a grade 1 cascading anterolisthesis of L3 to L4 and L4 to L5 (black arrows).

Figure 3.

Magnetic resonance imaging of the lumbar spine. (A) Sagittal T2-weighted image demonstrating intradural lipoma at the level of L4. (B) T2-weighted image showing a large right-sided synovial facet cyst at L4 to L5. (C) Transverse view showing right-sided synovial facet cyst (arrow).

Figure 4.

(A) Sagittal and (B) transverse computed tomography myelography images demonstrate an intradural, extramedullary mass centered within the thecal sac at the L4 to L5 level (blue arrows). The mass measured 1.7 × 2.0 × 3.6 cm in the anteroposterior, craniocaudal, and transverse dimensions, respectively.

The intradural lesion demonstrated imaging features consistent with a large FTL, but its morphology exceeded that of a thin fatty filum and instead represented a well-formed, intradural, extramedullary lipomatous mass centered at L4. Based on its location, composition, and relationship to the low-lying conus, the lesion is most consistent with a type 4 lipoma under the Morota embryological classification, which reflects late-phase secondary neurulation failure. Despite its size and the patient’s low-lying conus, there were no features of tethered cord syndrome, a distinction central to the decision-making in this case.

Given the patient’s clinical presentation and failure of nonoperative management, surgical treatment options were discussed. Several case factors were discussed with the patient in the shared decision-making for surgery. The L3 to L4 spondylolisthesis did not have severe stenosis and appeared stable in slip magnitude from standing to recumbency. There were no facet effusions at L3 to L4 and no other signs of mobility or neurologic compression. L4 to L5 had a mobile spondylolisthesis with facet effusion and a large right-sided (symptomatic side) synovial facet cyst within the canal, causing severe lateral recess stenosis. The FTL appeared to cause severe stenosis at the L4 level. Her clinical lower lumbar cutaneous dimple and history of a surgical procedure performed within the first few weeks of life were also considered. Given the overlapping pathologies and complexity of this case, it was reviewed at a multidisciplinary spine conference as well as shared with spine surgery experts across the nation. There was no treatment plan consensus among expert spinal surgeons.

To address the primary radicular complaint, surgical options discussed included synovial facet cyst excision via laminectomy vs lumbar fusion. The indications for fusion included grade 1 spondylolisthesis with large facet effusions and synovial cyst formation, suggesting continued mobility/instability. It was discussed with the patient that cyst excision via laminectomy may encounter exposure of the dura, lipoma, and previous scar bed, which may increase surgical risks. It was also discussed that cyst recurrence might occur after cyst excision alone. However, cyst recurrence would be significantly less likely when lumbar fusion is performed. After extensive counseling, the patient elected to proceed with an L4 to L5 anterior lumbar interbody fusion (ALIF), minimally invasive posterior spinal instrumentation using robotic-assisted navigation through paramedian incisions, and decompression of the L4 to L5 synovial cyst via a transfacet approach to avoid exposing the dura/lipoma, the cutaneous dimple, and prior surgical scar bed. The facets were bone grafted to complete a posterior spinal fusion.

The patient was taken to the operating room and placed supine on the surgical table. An anterior retroperitoneal exposure was performed by a senior vascular access surgeon. After confirming the surgical level, a standard ALIF was performed. Because the patient had a low-lying conus and large intradural lipoma, we limited interbody distraction during the ALIF to avoid generating excess tension across the filum terminale or neurological structures. Implant sizing was based on restoring physiological disc height, reducing the spondylolisthesis, ensuring appropriate implant-endplate contact, and restoring segmental lordosis. Proper sizing was based on the patient’s anatomy and the press-fit feel intraoperatively. The smallest-sized cage that achieved the surgical goals was selected. No changes in neuromonitoring occurred during cage placement, and the patient exhibited no postoperative symptoms suggestive of traction on tethered elements. Once the disc space was adequately prepared and the appropriately sized implant determined, a 3D-printed titanium interbody cage packed with recombinant human bone morphogenic protein-2 (rh-BMP2; Infuse, Medtronic Corp, Minneapolis, MN, USA) and fresh frozen allograft was inserted for interbody fusion. Accurate positioning of the cage and near-complete reduction of the spondylolisthesis were confirmed with fluoroscopy. Integrated screws were placed through the cage, fixing it to the vertebral bodies.

After completion of the ALIF, the patient was transferred to the prone position on a radiolucent Jackson table. Pedicle screws were placed bilaterally at L4 and L5 using robotic-assisted navigation and a minimally invasive technique via lateral paramedian incisions, avoiding the previous midline cutaneous dimple (Figure 1). Rods were then inserted in a minimally invasive, submuscular fashion. Using the same paramedian incisions and avoiding the midline cutaneous dimple, the right facet was identified and cleared of soft tissue. A partial facetectomy of the L4 inferior articular facet and partial laminectomy were performed to expose the lateral recess. A large sac-like fluid-filled synovial cyst was identified and removed with a micropituitary. The cyst was sent for pathology. The remaining facet was decorticated and packed with bone for fusion. The left facet was also decorticated and bone grafted. The incisions were closed, and sterile dressings were applied. Strength and sensation were unchanged from preoperative baseline in the postanesthesia care unit.

The patient ambulated >400 ft and was discharged home on postoperative day 1. She reported resolution of her preoperative right buttock and leg pain. Her surgical pain improved, and at her 4-week postoperative visit, she denied back, buttock, or leg pains. Strength and sensation remained stable in bilateral lower extremities. At her 3-month postoperative visit, she was released back to lifting weights in the gym and walking a mile and a half regularly. She continued to deny any pain, and her strength and sensation remained stable. She returned to her full-time job as a bus driver without limitations. Radiographs from the 3-month postoperative visit demonstrated an intact anterior lumbar interbody cage and instrumentation with no evidence of subsidence, loosening, or failure (Figure 5). Postoperative radiographs obtained postoperatively at 18 months further demonstrated intact instrumentation and no evidence of failure (Figure 6). Postoperative MRI at 2 years showed complete resolution of the right-sided synovial facet cyst when compared to preoperative imaging (Figure 7).

Figure 5.

(A) Anterior-posterior and (B) lateral postoperative radiographs at 3 mo demonstrated the unchanged position of spinal implants and unchanged alignment of the L4 to L5 vertebrae.

Figure 6.

(A) Anteroposterior and (B) lateral postoperative radiographs at 18 months showing unchanged position of spinal implants and maintained alignment of the L4 to L5 vertebrae.

Figure 7.

(A and B) 2-year postoperative magnetic resonance images demonstrating complete resolution of a right-sided synovial cyst at L4 to L5. (B) Transverse view shows the resolution of the lesion compared to the preoperative images (Figure 3).

Discussion

FTLs are a type of lumbosacral lipoma in which the fat is entirely within the filum terminale and separate from the conus medularis.⁸ With the widespread availability of MRI, FTLs are identified more frequently in asymptomatic individuals.^2,9 Even in the prenatal period, its occurrence can be detected by sonographic examination.¹⁰ Other postnatal options include CT, CT myelogram, and MRI.^2,4,8 When symptomatic, reported findings are urological complaints, back pain, and leg pain.^8,9 Screening strategies for intraspinal anomalies in infants, including FTL, have been proposed, but their real utility is controversial because of the high rate of asymptomatic patients.¹¹

In our case presentation, the patient was an adult with a history of surgery for a mass in the lumbar spine within the first few weeks of life and had a cutaneous dimple on the skin in the lumbosacral region. Dimples in the gluteal fold are common in newborns and are not generally considered to carry a significant risk of intraspinal anomalies.^12,13 However, the risk of FTL is increased when the dimples are deeply excavated or are accompanied by congenital anomalies.¹²

Several theories have been proposed to explain the development and occurrence of FTL. They are believed to occur due to failed primary neurulation. Other authors hypothesized that its occurrence is due to embryogenic changes seen during primary and secondary neurulation.¹⁴ Based on this last theory, a classification of spinal lipomas was developed: type 1 is defined as pure primary neurulation failure; type 2 ranges from primary to secondary neurulation failure; type 3 consists of secondary neurulation failure (early phase); and type 4 is defined as secondary neurulation failure (late phase). The lipoma in the patient described in this case is classified as type 4.¹⁴ The genetic basis of FTL is still unclear, but a recent study found a high probability of FTL among siblings of FTL patients, suggesting that genetic factors may play a role in its development.¹⁵ Our patient did not have siblings, so this association could not be explored.

Several authors have attempted to correlate radiological findings with clinical presentation. In symptomatic patients, the fat within the filum was closer to the conus, or patients had a low-lying conus (below L2) when compared to asymptomatic patients.^8,16 Bulsara et al¹⁶ also reported that fat in the filum within 13 mm of the conus medullaris was most predictive of neurological symptoms. In the present case presentation, the patient had a low-lying conus (L3 to L4), but her lipoma was located near L4, and she reported no neurological symptoms prior to a few months before presentation. Her concomitant spondylolisthesis and synovial facet cyst were believed to be the etiology of her symptoms rather than the lipoma, making this case rather unique given their immediate proximity. While the presence of a lipoma may initially raise concern and influence surgical planning, current best practice supports conservative management in the absence of clinical or radiographic evidence of occult tethered cord symptoms. This logic was the basis for pursuing a surgical plan to primarily address the degenerative pathology with surgical approaches and techniques that would leave the lipoma undisturbed in this case.

The treatment of FTL has generated much controversy. Some physicians advocate surgical treatment for all patients regardless of symptoms, and others propose that surgery be withheld until symptoms develop.^3,17 When symptoms are present, surgical untethering by sectioning the FTL can prevent symptom progression and often results in improvement.¹⁸ Surgical untethering seems to be inherently low-risk microsurgery in experienced hands, even without the use of intraoperative neuromonitoring.¹⁹ In our case presentation, the patient presented with overlapping pathologies, including FTL and spondylolisthesis/synovial facet cyst; however, her symptoms were most consistent with radiculopathy related to the spondylolisthesis/synovial facet cyst. Nonsurgical treatment was unsuccessful, and when analyzing her complex clinical presentation and radiography, a limited surgical treatment targeting the spondylolisthesis/cyst and avoiding the lipoma was offered. This case underscores the importance of careful patient clinical history interviews, scrutinizing preoperative imaging, and thoughtful selection of surgical treatment targets; addressing patient complaints while avoiding incidental radiological findings and potential increased surgical risks. This case report presents a patient with a common degenerative spinal pathology (spondylolisthesis with synovial facet cyst) with an incidentally found asymptomatic intradural lipoma. The workup and treatment philosophy that led to a successful resolution of the patient’s radicular complaints using minimally invasive surgical techniques to avoid potential complications can be used by surgeons who may encounter a similar, albeit rare, clinical situation in the future.

It is important to emphasize that the presence of a large FTL does not inherently mandate surgical untethering. Current evidence supports observation when no clinical symptoms of tethered cord syndrome are present, even in cases with low-lying conus or sizable intradural lipomas. In this patient, careful correlation of history, examination, and imaging supported that her radiculopathy arose from the degenerative L4 to L5 pathology rather than traction-related mechanisms. Thus, treatment of the degenerative pathology alone, rather than untethering, was selected. Creating distraction via interbody fusion across a preexisting FTL may create new neurological symptoms, and surgeons should be mindful to select appropriately sized implants to achieve surgical goals while avoiding overdistraction.

Conclusion

The majority of FTLs are asymptomatic and do not require treatment, even large ones as in this case. Surgeons should be keenly aware of possible symptoms associated with FTLs, but differentiation of symptomatology between other pathologies is also necessary to individualize and determine the best treatment options. Incidental FTLs may be safely observed when surgically managing a patient with a concurrent, adjacent, and symptomatic degenerative lumbar spondylolisthesis and/or synovial facet cyst. This case highlights the importance of individualized surgical decision-making in patients with overlapping degenerative pathology and incidentally discovered FTLs. It provides a useful framework and case example that can guide informed discussions with patients and colleagues.