Discussion

Lumbar synovial cysts are cystic lesions that arise from the synovium of degenerated facet joints. Some of these cysts do not necessarily demonstrate communication with the adjacent facet joint and are sometimes referred to as ganglion cysts. These latter cysts are not lined by synovial cells.1 The term juxta-articular cyst has also been used interchangeably with lumbar synovial cyst. Lumbar synovial cysts are most commonly encountered within the lumbar spine, most frequently at the L4–5 level, but also at other lumbar levels. They occur more frequently in adult women than in adult men. Their incidence is approximately 0.6%.2 When these cysts are in contact with the spinal canal or exit foramen, and when they reach sufficient size, they may become symptomatic, resulting in axial low back pain with or without a radicular component, depending on their location. The radicular pain often corresponds to the compressed nerve root that is seen on the patient's imaging examination.

Because these lesions can cause significant symptoms, they are often identified on MRI studies near the time of the patient's clinical presentation. The cyst is usually associated with a facet joint that demonstrates advanced facet arthropathy. Synovial cysts are round well circumscribed masses that are hypo- to iso-intense on T1 weighted MRI and hyperintense on T2 weighted sequences. In some cases it might be possible to demonstrate signal intensity continuity between the cyst and the adjacent facet joint. This is not common, however, and the definitive way to establish a communication between the cyst and the facet joint is by performing an arthrogram, either via fluoroscopic or CT guidance. The presence of cyst–facet joint communication is important because it is the only way that a cyst can potentially be aspirated or ruptured when using a facet joint injection technique that indirectly accesses the cyst. The cyst wall is moderately thick and shows variable enhancement following intravenous contrast administration. Chronic cysts may contain calcification within their walls, a finding which is better visualized on CT.1 As the cyst expands in size it will impinge on the adjacent neural elements. This results in either focal axial low back pain and/or radiculopathy.

Symptomatic lumbar synovial cysts can be managed with a few different treatment options. An initial attempt at a conservative management is usually initiated by the treating clinician. This often consists of a combination of bed rest, oral analgesics, non-steroidal anti-inflammatory agents, and/or a short course of steroid therapy. Temporary relief, at best, is achieved with conservative management. Surgery is another possible treatment option for symptomatic lumbar synovial cysts. The surgical option is believed to be curative, especially when the offending cyst is resected. The extent of the surgery is variable and may include laminectomy, facetectomy, or possible posterior spinal fusion with bone graft or instrumentation.2 ,3 These open surgical procedures are not without risks. The risks of open lumbar spine surgery include bleeding, infection, dural tear, and CSF leak. More long term complications include spinal de-stabilization in patients who have not been fused, as well as the possible creation of new pain generators within the treated lumbar spine. Some patients may not be good surgical candidates due to their medical comorbidities. The efficacy of surgery for effectively treating symptomatic lumbar synovial cysts and eradicating pain is not 100%. In the largest surgical series that addressed the issue of surgical efficacy, 90% pain relief was observed.4

Fluoroscopy guided percutaneous facet joint injection was developed as a treatment option that is more invasive than conservative management but less aggressive than open surgery. The objective of this procedure is to access the affected facet joint space using a percutaneously placed spinal needle that is advanced under fluoroscopic guidance. In those cases where it is possible to access the facet joint and where the joint communicates with the cyst, it is usually possible to opacify the joint space and the cyst with a radiopaque contrast agent. Attempts can then be made to aspirate the cyst contents by aspirating the joint as well as rupturing the cyst by pressurizing the joint with an injection of contrast, liquid steroid agent, and/or anesthetic agent. The injection is considered to be successful if the cyst is ruptured, as evidenced by disappearance of the cyst and/or the epidural spread of the previously contained contrast agent. Successful cyst rupture is almost always associated with marked improvement in the patients’ clinical symptoms.1 This technique, with minor variations, has been used for the past several decades as the secondline therapy for symptomatic synovial cysts prior to considering surgical interventions.

A review of the literature for percutaneous fluoroscopy guided trans-facet therapy shows moderate to favorable benefits with this treatment. Many of the initial studies consist of small patient series with limited clinical follow-up and short follow-up periods. One study of 12 patients that included imaging follow-up noted that symptomatic relief, seen in 75% of patients, correlated well with either complete or partial reduction of cyst volume, which was also seen in 75% of patients.1 In another series of 32 patients, pain relief was achieved in 72% of patients.5 The cyst recurrence rate in this group of patients was 37.5% (12 patients), with five of these patients undergoing a repeat successful cyst rupture and six of these patients ultimately requiring surgical resection of the cyst. Patients who had a repeat attempt at cyst rupture had a 50% chance of a therapeutic benefit.5 By far the largest study to investigate this treatment option included 101 patients.4 In this study, the authors found that the average duration of pain relief following facet injection was 8.4 months. The treatment benefit was attributed to both mechanical decompression of the cyst and to the anti-inflammatory effect of the epidural steroid extravasation. They also noted that 50% of these patients eventually required an open surgical procedure and that there was no statistically significant difference between patients who received injections only versus patients who underwent surgery with respect to pain scores or Oswestry Disability Index scores.4 The clinical follow-up in this retrospective series was somewhat limited as there was only a 79% response rate. Additionally, the post-procedure evaluation in this study consisted only of a telephone interview, with no direct clinical evaluation of the patients.

The percutaneous image guided trans-facet joint, or indirect cyst rupture technique, has inherent limitations. First, the operator must be able to access the joint space which is not always possible in the setting of severe facet arthrosis. In those cases where it is possible to perform an arthrogram, the joint space may not communicate with the cyst. Thus facet joint access and lack of cyst communication are two potential barriers to this treatment option. In some cases, when it is possible to access the joint and opacify the cyst, it may not be possible to rupture the cyst. This is not an infrequent occurrence and has been noted in prior large studies, with the incidence of fluoroscopy confirmed ‘successful’ cyst rupture ranging from 75% to 81%.1 ,4 There are two major reasons for not achieving cyst rupture: (1) it may not be feasible to adequately pressurize the joint as the contrast agent leaks around the needle entry site and (2) the cyst wall may be thick or calcified and resistant to the pressurization maneuver.

Given the challenges associated with percutaneous image guided trans-facet therapy, we sought to develop alternative and more direct treatment strategies for symptomatic lumbar synovial cysts. The advent of CT fluoroscopy enabled us to consider different access routes and approaches for symptomatic lumbar synovial cysts, regardless of whether or not they communicate with the facet joint. Cross sectional imaging improves the visualization of all key structures and facilitates cyst identification as well as the effects of intervention. Needle guidance, with co-axial techniques, is also improved with CT fluoroscopy. This enables a percutaneous image guided technique with a somewhat more direct approach to accessing and rupturing the cyst. In our series, different approaches, interlaminar, trans-facet, or transforaminal, were used to access each cyst. The location of the cyst and its relation to the dural sac and exiting nerve root were primary determinants in establishing the safest needle trajectories. Cysts that are within the spinal canal might be accessed via the dorsolateral epidural space through an interlaminar approach. Cysts that are located within the neural foramen may require a coaxial technique either with a transforaminal or inferior facet joint approach. With these approaches and with direct visualization with CT, cyst rupture is now more readily achievable than with the previous techniques and technology. The traditional percutaneous image guided facet injection technique did not and would not be effective in those nine facet injections in our series in which the cyst did not communicate with the facet joint. This may account for the variable success rate with indirect cyst rupture via facet injection that has been previously reported in the literature.2 In our study, cyst rupture was objectively demonstrated with CT confirmation in each case. This was associated with at least temporary symptomatic improvement in all patients. In the six patients with cyst recurrence, symptoms developed within 6 months of the initial percutaneous treatment. Cyst communication with the facet joint was observed in four of these six patients with cyst recurrence. No other specific trends in this small group of six patients with respect to cyst size, morphology, or location were identified. The percutaneous procedure was repeated in four of these patients with a successful outcome in terms of cyst rupture and durable symptom relief. Two of the six patients underwent open surgical treatments, including cyst resection and laminectomy in both patients, and posterior fusion in one of the two patients. Despite adequate decompression, their symptoms have persisted. Certainly, while open surgery has been shown to have a high success rate in the treatment of symptomatic lumbar synovial cysts, a systematic review of the literature shows that the overall success rate, in terms of symptomatic relief, is approximately 92%.6 With respect to lumbar synovial cyst eradication or recurrence, decompression surgery alone is associated with a cyst recurrence rate of 1.8%. The lumbar synovial cyst recurrence rate of 0% is seen only after open surgical decompression and lumbar fusion, a more extensive surgical procedure. The latter surgical approach may benefit those patients with lumbar synovial cysts and associated spondylolisthesis.2 ,7 As with any other surgical procedure, complications may and do occur, and include CSF leak, postoperative instability, infection, and bleeding. In a large series of 194 patients that were treated with lumbar spine surgery, the observed complication rate was 6%.3 Thus the management of symptomatic lumbar synovial cysts is challenging, regardless of the treatment option.

Our favorable experience (an overall success rate (90%) that was higher compared with studies that have used the facet injection techniques) with these additional percutaneous techniques and with CT fluoroscopy suggests that these techniques and tools may be of benefit in the treatment of symptomatic synovial cysts. With CT, the immediate demonstration of cyst location and communication or non-communication with the facet joint facilitates the subsequent treatment decisions with respect to procedure and approach. Those cysts that do not communicate or do not rupture with the traditional ‘pressurization’ technique may indeed require a tandem or coaxial approach. Recurrent cysts that have been previously treated with the traditional indirect facet joint injection technique might also be considered for a possible tandem or coaxial approach.

The necessity for clinical follow-up of these patients is reinforced by the potential for cyst and symptom recurrence, particularly during the first year after percutaneous treatment. Our 2 year duration for following up these patients is consistent with the observation periods that have been described in the literature for this specific condition.2 With this follow-up time frame we were able to show a much longer duration of treatment benefit for the direct percutaneous tandem or coaxial techniques compared with the indirect, or facet, injection technique. Overall, 18 out of the 20 patients in our case series (90%) have remained symptom free over the study interval. Moreover, the requirement for subsequent open surgical intervention in this case series (two out of 20 patients) was much less than the 54% rate that has been previously reported with the fluoroscopy guided percutaneous facet injection technique.4

Our study limitations include a relatively small sample size at a single institution, performed by one operator. This is a case series and patients were not randomized into facet only injections versus direct cyst injection techniques. These percutaneous tandem and coaxial procedures do require an element of operator experience and comfort level with spine injection techniques. The potential risks of these procedures include dural puncture, subarachnoid space injection, and vascular or neural injury. The key with respect to procedure safety, however, is to always identify the cyst prior to attempting direct rupture. Nevertheless, the favorable results that have been observed in our series, combined with the increased durability of the treatment effect, suggest that a direct approach may be considered in those patients who have not responded to other non-surgical therapies.