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The diagnosis and management of synovial cysts: Efficacy of surgery versus cyst aspiration

Background: The surgical management of lumbar synovial cysts that have extruded into the spinal canal remains controversial (e.g. decompression with/without fusion). Methods: The neurological presentation, anatomy, pathophysiology, and surgical challenges posed by synovial cysts in the lumbar spine are well known. Neurological complaints typically include unilateral or, more rarely, bilateral radicular complaints, and/or cauda equina syndromes. Anatomically, synovial cysts constitute cystic dilatations of synovial sheaths that directly extrude from facet joints into the spinal canal. Pathophysiologically, these cysts reflect disruption of the facet joints often with accompanying instability, and potentially compromise both the cephalad and caudad nerve roots. Results: Aspiration of lumbar synovial cysts, which are typically gelatinous and non-aspirable, and typically performed by “pain specialists” (e.g. pain management, rehabilitation, radiologists, others) utilizing fluoroscopy or CT-guided aspiration, is associated with 50–100% failure rates. Surgical decompression with/without fusion (as the issue regarding fusion remains unsettled) results in the resolution of back and radicular pain in 91.6–92.5% and 91.1–91.9% of cases, respectively. Conclusions: After a thorough review of the literature, it appears that the treatment with the best outcome for patients with synovial cysts is cyst removal utilizing surgical decompression; the need for attendant fusion remains unsettled. The use of an alternative treatment, percutaneous aspiration of cysts, appears to have a much higher recurrence and failure rate, but may be followed by surgery if warranted.


INTRODUCTION
Synovial cysts that have extruded into the lumbar spinal canal may be managed by "pain specialists" (e.g. pain management, rehabilitation, radiologists, others) utilizing fluoroscopy or computed tomography (CT)-guided aspiration techniques, or by spinal surgeons utilizing decompression with or without fusions. In this review, we explore the pathology, diagnosis, and relative efficacy of the multitude of techniques available for addressing synovial cysts arising in the lumbar spinal canal [ Figures  1-12, Video, and Table 1].

ANATOMY OF SYNOVIAL CYSTS
Lumbar synovial cysts are cystic dilatations of synovial sheaths that directly emanate from facet joints and extrude into the spinal canal. When Tillich et al. correlated magnetic resonance (MR) scans with surgical findings for 18 patients with synovial cysts, the cysts measured an average of 16 mm (ranged 10-28 mm); 3 additionally (17%) contained hemorrhages. [25] Two fluid types are found within these cysts. [11] The most common gelatinous cysts are not easily aspirated, while the less frequently encountered serosanguinous cysts, containing less viscous fluid, may be somewhat more amenable to percutaneous aspiration. Nevertheless, whether or not fluid can be removed from within these cysts, the fibrous capsule that remains continues to contribute to the clinically relevant compression of the thecal sac and nerve root. [1]

PATHOLOGY OF SYNOVIAL CYSTS
In Wilby et al.'s study, 27 consecutive patients with stenosis undergoing laminectomy and en bloc resection of the ligamentum flavum and facet joints yielded 51  The illustration demonstrates an L4-L5 partial laminectomy (coronal hemilaminectomy) with medial facetectomy/ foraminotomy that deliberately spares the pars interarticularis bilaterally, overlying the pedicles (P), to enhance stability. Observe the decompression of the thecal sac (DURA) and bilateral inferiorly exiting L5 nerve roots (R) that are now free in the lateral recesses. For the smaller synovial cysts, such a single-level decompression may suffice Figure 5: The axial non-contrast lumbar CT scan demonstrates marked congenital stenosis accompanied by superior facet hypertrophy. Note that air (hypodense) within the facet joints is indicative of instability. With such severe spinal stenosis, epidural steroid injections would likely fail or result in dural fistulas as there is essentially no epidural compartment. Similarly, if this patient had a synovial cyst, there would have been virtually no space available for "cyst aspiration" Figure 8: A right-sided near-complete L4 hemilaminectomy has been performed, allowing a dental tool to be passed underneath the pars interarticularis extending through the superior L4-L5 foramen overlying the foraminally/far laterally exiting L4 nerve root. In cases where minimal stenosis is present, an L4-L5 synovial cyst that typically extends to the L3-L4 level will require a full L4 hemilaminectomy with medial facetectomy/foraminotomy at the L3-L4 levels in order to adequately visualize the superior L4 nerve root and avoid damaging it during the dissection of the superior portion of the synovial cyst capsule Figure 6: The illustration demonstrates the marked arthrotic changes of the facet joints that occur with degenerative spondylolisthesis at the L4-L5 level that contribute to marked impingement on the superior, foraminally/far laterally exiting L4 nerve root specimens and 28 synovial cysts. [27] Specimens were compared to 47 comparable cadavers. Articular cartilage and bone fragments were embedded in the walls of 89% of cysts, while a bursa-like channel averaging 12 mm in length extended between the medial facet joint and yellow ligament in 75% of cases. Synovial cyst formation was, therefore, attributed to blockage of these channels from osteoarthritic changes consisting of fragments of cartilage, bone, granulation tissue, and scar. [27] When Kusakabe et al. studied 46 juxtafacet cysts, they similarly documented a communication channel between the cyst and the joint in all cases, along with "moderate to severe facet degeneration." [12] En bloc specimens consisting of medial facetectomy and the attached ligamentum flavum and cysts revealed three shapes: small protrusions, semicircular cysts, or round cysts. [12] Additionally, cyst walls contained elastic and collagen fibers that had undergone fibrinoid Following an L3-L5 laminectomy with medial facetectomy and foraminotomy at both the L3-L4 and L4-L5 levels (note: more extended right-sided foraminal dissection was warranted), a rightsided thick, fibrous, and gelatinous filled cyst was decompressed and excised/teased away (dense adhesions) from the underlying dura, L4, and L5 nerve roots Figure 12: The thick, fibrinous, and gelatinous synovial cyst removed in this case measured 2.5 cm. Intraoperative dissection here warranted laminectomy from L3-L5 with extended foraminal dissection on the right for exposure of the synovial cyst extending from L4 to L5 level cephalad. Note, how the Penfield elevator is utilized to dissect the capsule away from the underlying dura. This is feasible only if there is truly a plane present, as in many instances, the capsule is densely adherent to the underlying dura (See Video). The cyst once skeletonized, is ruptured with the Kerrison rongeur. Following decompression of the cyst which contained thick, gelatinous tissue, the margins of the residual capsule may be more readily defined and dissected away from the underlying thecal sac and superiorly/foraminally, and inferiorly/ foraminally exiting nerve roots (See Video) degeneration, or alternatively often there was no synovial lining. Furthermore, cysts often contained fibrinoid and myxoid degeneration. Similar pathologic findings were identified in Pytel's study of 985 surgical specimens with a 1.6% incidence of synovial cysts and another 5.3% of cases with synovial tissue often embedded within disc material. [19] [18] Cysts predominated at the L4-L5 level (61%) and were accompanied by disc degeneration/spondylosis in 54% of cases.

CT AND MR DIAGNOSIS OF SYNOVIAL CYSTS
Liu et al. observed that CT scans could document cysts adjoining facet joint capsules, but noted that MR revealed more specific information [ . [14] On short TR/ TE, MR images revealed slightly hyperintense (3 cases) or isointense (1 case) cysts, while long TR/TE showed hyperintensities (2 cases) or hypointensities (others). [14] Furthermore, hypointense signals surrounding cysts often indicated rim calcification better confirmed on CT.
MR can additionally differentiate between the hyperintense signals characteristic of fluid within synovial cysts and focal hypointense signals typical of hypertrophy and/or ossification of the yellow ligament [ Figures 1-12]. [11] Unfortunately, MR scans cannot readily distinguish between the more frequently encountered gelatinous (non-aspirable) versus the rarer serosanguinous (more readily aspirated) cysts. [11] MR studies further demonstrate the large, thick, fibrous, and often calcified capsules surrounding the fluid within the synovial cysts, which are predominantly responsible for the majority of neural/dural compression and neurological dysfunction [ Figures 1-4]. Moreover, enhanced MR studies document dense adhesions between the overlying cyst capsules and the underlying dura and nerve roots, the cephalad/foraminally exiting and the caudad/lateral recess exiting nerve roots [ Figure 11]. For example, a synovial cyst at the L4-L5 level typically contributes to dural compression along with both cephalad L4 and caudad L5 nerve root compromise.
Some patients with synovial cysts exhibit no active Grade I slip (25% or less of the vertebral body width), while others exhibit instability associated with degenerative spondylolisthesis. In particular, older patients with marked degenerative changes of the facet joints accompanied by bony osteophytic bridging across facet joints and/or vertebrae on dynamic X-rays, and/or CT studies, without active motion, may not warrant fusion.
MR-documented increased fluid in the facet joints is highly correlated with degenerative spondylolisthesis and synovial cyst degeneration, and may indicate instability [ Figure 4]. Alicioglu and Sut noted that facet joint osteoarthritis was present in 30 patients with denegerative spondylolisthesis, and that synovial joint effusions occurred in 24 patients with synovial cysts. [2] Tillich et al. documented instability in 11 (61%) of 18 patients with synovial cysts, 6 of whom also demonstrated degenerative spondylolisthesis. [25]
synovial cysts can spontaneously resolve. Swartz and Murtagh reported a case in which a patient originally presented with radicular pain and an MR documented L5-S1 lumbar synovial cyst. [24] Eighteen months later, utilizing conservative medical management alone, clinical symptoms abated and the follow-up MR showed resolution/full regression of the cyst. Similarly, Ewald and Kalff reported on six cases in which synovial cysts spontaneously resolved. [9]

SURGICAL MANAGEMENT
Utilizing the operating microscope is extremely helpful in avoiding cerebrospinal fluid fistulas during decompression and removal of synovial cysts as these fistulas are more likely to arise secondary to dense adhesions between the cyst capsule and underlying dura/nerve roots. In Epstein's retrospective analysis of 110 predominantly geriatric patients undergoing multilevel laminectomies with non-instrumented fusions, 5 of 10 patients who developed dural tears had attendant synovial cysts. [7] For the 10 patients with dural tears, the average age was 74 years, and they had undergone slightly more extensive laminectomies (average 5.5 levels) and noninstrumented fusions (average 1.8 levels). Notably, all 10 patients exhibited marked OYL that extended to and through the dura in 3 patients. Alternatively, for the 100 patients without dural tears, 8 of whom also had synovial cysts, the average age was a younger 69 years, and they had undergone lesser average 5.0-level laminectomies and 1.6-level non-instrumented fusions. More critically, for these latter patients, 57 had moderate/hypertrophied OYL, while 22 showing marked OYL. [7] Of interest, Short Form (SF)-36 outcomes at one postoperative year were comparable for both groups irrespective of the presence/ absence of dural tears.
Intraoperative somatosensory evoked potential monitoring, electromyographic monitoring, and sphincter monitoring are also useful while resecting synovial cysts. In particular, monitoring these potentials alerts the surgeon to inadvertent, excessive traction that may occur during dissection/manipulation of the cyst away from the underlying thecal sac and/or nerve roots, thereby avoiding permanent injury (e.g. cauda equina syndrome, root and sphincteric deficits).

Surgical pathology for synovial cysts
Patients with synovial cysts are also frequently symptomatic from underlying lumbar stenosis (central, lateral recess, foraminal, far lateral). Rarely, small unilateral cysts compress just the unilateral dura and the inferiorly exiting nerve root in the lateral recess. More typically, larger synovial cysts compress both the cephalad, foraminally and/or far laterally exiting nerve root, and the caudally exiting root, along with the potential for more extensive unilateral or bilateral thecal sac and/or cauda equina compromise [ Figures 6-9].
For the safe excision of typically large synovial cysts, the decompression must start at the level above the cyst. This allows for safe identification of the superiorly/ foraminally exiting nerve root that often only becomes visible once the synovial cyst is dorsally debulked [ Figure 11]. Beginning the dissection/decompression at the immediately supradjacent spinal level will help avoid inadvertently damaging the cephalad root. Alternatively, solely decompressing from the level at which the cyst originates may risk damaging or transecting the cephalad root that is typically markedly compressed and adherent to the ventral aspect of the cyst capsule. An example of the resection technique follows. When removing an L4-L5 synovial cyst, the initial decompression should begin at the L3-L4 level with clear identification of the L4 root as it traverses the L3-L4 interspace. When the decompression is extended inferiorly to the L4-L5 level, this allows for full lateral/foraminal decompression of the cephalad root, along with the caudad root [ Figures 6-9, 12, Video].

MINIMALLY INVASIVE SURGERY FOR SYNOVIAL CYSTS
One study cited the efficacy of minimally invasive surgical (MIS) techniques utilized to excise synovial cysts. In Landi et al.'s series, microsurgical excision of synovial cysts utilizing hemilaminectomy with partial arthrectomy without fusion achieved uniform clinical success with postoperative MR confirmation of adequate decompression. [13] In Pirotte et al.'s study, immediate resolution of symptoms occurred in 46 patients with lumbar synovial cysts following microsurgical excision that included bipolar coagulation of the synovial membrane, while maintaining the integrity of the facet joint [ Table 1]. [18] Of interest, within the first postoperative year, 5 (10.9%) patients developed contralateral synovial cysts also requiring microsurgical management.

DECOMPRESSIONS WITHOUT FUSIONS
Many patients with synovial cysts have been successfully managed with decompression without fusions. When Métellus et al. evaluated (1992)(1993)(1994)(1995)(1996)(1997)(1998)) 77 patients with symptomatic lumbar synovial cysts undergoing decompressive surgery without fusion, patients averaging 63 years of age were divided into those with radicular pain (n = 51 patients) and those with bilateral neurogenic claudication (n = 26 patients) [ Table 1]. [16] Degenerative disc disease and facet joint osteoarthritis was accompanied by a 48% incidence of spondylolisthesis. Patients were followed up on an average of 45 months during which time a synovial cyst recurred in just one patient. Excellent/good outcomes were seen in 97.4% of patients; 89% of motor deficits resolved. [16] Of interest, only 1 of 77 patients later required a fusion to address symptomatic spondylolisthesis.
Deinsberger et al. treated 31 patients with lumbar synovial cysts (14 with degenerative spondylolisthesis) with microsurgical resection (27 patients) and with laminectomy for stenosis (4 patients) [ Table 1]. [4] Within 12-30 postoperative months, 80.7% achieved excellent/ good results. Persistent low back pain was more typical for those with preoperative olisthesis and was attributed to instability. [4] Epstein evaluated both surgeon-and patient-based (SF-36) outcomes to assess the results of laminectomies performed to excise synovial cysts with coexistent lumbar spinal stenosis (45 patients) or stenosis with degenerative spondylolisthesis (35 patients) [ Table 1]. [5] Of interest, surgery required average 3.8-and 3.5-level laminectomies, respectively, for the two patient groups. When Epstein evaluated SF-36 outcomes following 45 decompressive laminectomies for the excision of synovial cysts with stenosis, 58% of all patients exhibited good/excellent outcomes. [5] Although five patients developed postoperative olisthesis, only the two who were symptomatic required secondary fusions. [5] Other second operations included neurolysis/duralysis of scar (1 patient), excision of a recurrent synovial cyst (1 patient), and herniated discs (3 patients). Nearly comparably, the 35 patients with synovial cysts, lumbar stenosis, and degenerative spondylolisthesis exhibited a 63% incidence of good/excellent postoperative results. [5] Here, however, although olisthesis increased from Grade I to Grade II in 11 patients, only 2 were sufficiently symptomatic to warrant secondary fusions. Second operations for the remaining three individuals included the resection of postoperative scar tissue (2 patients) and removal of a recurrent synovial cyst (1 patient). When outcomes were assessed 2 years postoperatively, SF-36 improvement on the Physical Function Scale also increased by +44 and +38 points, respectively. Notably, the more moderate quality of postoperative outcomes could not solely be attributed to the necessity for secondary fusion, but rather was variously attributed to postoperative scar, recurrent synovial cyst, and recurrent disc herniations. [5] When Epstein reviewed synovial cysts, up to 40% of patients had accompanying spinal stenosis with degenerative spondylolisthesis. [6] Decompressions included unilateral or bilateral laminotomies, hemilaminectomies, or laminectomies alone or in combination with in situ or instrumented fusion. Adequate outcomes spanning 1-2 postoperative years were documented on SF-36 outcome questionnaires.

DECOMPRESSION WITH NON-INSTRUMENTED FUSIONS
Another group of Epstein's older patients whose dynamic X-rays documented mild/moderate motion only, were sufficiently managed with non-instrumented one-level or two-level posterolateral fusions (depending on the number of contiguous levels of olisthy). Fusions utilizing local autologous laminectomy bone combined with a bone graft expander [e.g. Vitoss-beta tricalcium phosphate (B-TCP); DePuy Orthopedics, Warsaw, IN, USA] [ Table 1]. [8] Out of 60 patients averaging 70 years of age, fusion rates and outcomes were assessed following average 5.4-level lumbar laminectomies and 1-to 2-level non-instrumented fusions performed with B-TCP (Vitoss) and laminectomy autograft. [8] Laminectomies addressed multilevel stenosis (60 patients), with varying combinations of OYL (46 patients), disc herniations (20 patients), and/or synovial cysts (8 patients). Fusions addressed degenerative spondylolisthesis (48 patients), spondylolisthesis/lysis (2 patients), or degenerative scoliosis (10 patients). Fusion was documented independently by two neuroradiologists blinded to the study design, and utilized dynamic X-rays and thin-cut 2D-CT scans which were obtained up to 2 years following surgery. Utilizing these stringent criteria, although pseudarthrosis was documented in 9 (15%) patients, only two were sufficiently symptomatic to warrant secondary fusion. Odom's criteria revealed 28 excellent, 23 good, 5 fair, and 4 poor results, while SF-36 improvement occurred on six of eight Health Scales in all patients. [8] Bydon  Table  1]. [3] Following decompressions (84%) or fusions (16.0%), 92.5% and 91.1% of patients, respectively, experienced complete resolution of their back or leg pain. [3] Within an average of 25.4 postoperative months, 6.2% required secondary surgery, with the majority (n = 47) undergoing fusion for mechanical back pain/instability. Same-level recurrence of synovial cysts was noted in 1.8% of patients undergoing decompressions alone, but none occurred following fusions.
Weiner et al. studied the long-term (average 9.7 years, range 5-22 years) postoperative outcomes of microdecompression for lumbar synovial cysts with and without arthrodesis [ Table 1]. [26] Of the 46 patients, 23 had decompressions with arthrodesis whereas 23 had decompression alone. Overall, 88% reported relief of the preoperative pain/symptoms. Fifteen percent of patients went on to need additional surgery (three requiring fusion, and four developing adjacent segment stenosis requiring a secondary decompression and fusion at the adjacent level). [26] In Xu et al.'s study, 167 consecutive patients with 195 symptomatic synovial cysts underwent various surgical procedures over a 19-year period [ Table 1]. [28] These included unilateral hemilaminectomy (n = 51), bilateral laminectomy (n = 39), facetectomy with in situ fusion (n = 18), and facetectomy with instrumented fusion (n = 56). Prior to surgery, the complaints included 97.5% radiculopathy, 82.5% back pain, 20% neurogenic claudication, and 3.2% bladder complaints. Postoperatively, although back and radicular pain improved in 91.6% and 91.9% of patients, respectively, those undergoing instrumented fusions exhibited better outcomes compared with patients having decompressions alone. [28]

DECOMPRESSIONS WITH INSTRUMENTED FUSIONS
Younger patients or those who demonstrate active olisthesis/translational instability on dynamic X-rays and no significant degenerative changes on preoperative CT studies may warrant instrumented fusions. These fusions may include laminectomy with pedicle screw instrumentation and posterolateral fusion. Other fusion alternatives such as transforaminal lumbar interbody fusion (TLIF) or posterior lumbar interbody fusion (PLIF) may also be utilized.

FREQUENCY OF EPIDURAL/FACET STEROID INJECTIONS
As spine injection frequencies have escalated from 18 to 90%, there is increasing concern regarding the clinical efficacy of these procedures. [10] Utilizing Medicare Physician Part B claims from 1994 to 2001, Friedly et al. found an increase of frequency for epidural (271%) and facet injections (231%) for patients over the age of 65 years. [10] The total reimbursement jumped from $24 million to $175 million, with costs for each injection doubling from $115 to $227. In Friedly et al.'s study, epidural injections addressed low back pain in 36%, spinal stenosis in 23%, and a combination of sciatica/ radiculopathy/discs in 41% of patients. The authors concluded thus: "Less than half were performed for sciatica or radiculopathy, where the greatest evidence of benefit is available. These findings suggest a lack of consensus regarding the indications for epidural steroid injections and are cause for concern given the large expenditures for these procedures." [10]

FREQUENCY OF PERCUTANEOUS SYNOVIAL CYST ASPIRATION
Predominantly "pain specialists", and occasionally "spinal surgeons" have utilized CT-guided and/or fluoroscopic techniques to aspirate synovial cysts with/without accompanying epidural/transforaminal steroid injections [ Table 1]. [11,20,21,23] However, a review of multiple studies utilizing these techniques reveals often short-term benefits, with high 50-100% long-term failure rates. [11,20,21,23] Notably, the majority of cysts are gelatinous and nonaspirable, leaving the large residual cyst capsules to continue to compress the neural/dural structures, and contribute to ongoing neurological dysfunction.
In one series, 9 (90%) of 10 patients undergoing fluoroscopic-guided cyst aspiration with local or transforaminal epidural steroid injections performed by physiatrists did poorly. [23] In a second study, 2 (100%) of 2 patients exhibited failure following attempts by orthopedists at aspiration of gelatinous cysts. [11] In a third study, 9 (50%) of 18 fluoroscopically guided, contrast-enhanced cyst aspirations performed by physiatrists exhibited significant long-term benefits defined by only 9.9 postoperative months. Nevertheless, the authors concluded that this was a "safe and effective" alternative to the open surgical removal of synovial cysts. [21] Interestingly, they interpreted this as a 50% success rate rather than a 50% failure rate. In a fourth single-case study, pain management specialists (radiologists, anesthesiologists) claimed they were the first to "successfully" perform lumbar "CT assisted (fluoroscopically guided) aspiration and cyst rupture," in the pain medicine literature. [20] Schultz et al. performed a comparative prospective, non-randomized analysis of 45 patients with sciatica/ claudication, undergoing microsurgical resection (25 patients) versus percutaneous CT-guided destruction (20 patients) of lumbar synovial cysts [ Table 1]. [22] All treated microsurgically were asymptomatic at an average of 27 postoperative months, while only 8 (40%) out of 20 patients treated percutaneously improved at 24 months; 12 later required microsurgery. The authors concluded thus: "Minimally invasive cyst destruction is not a viable alternative to microsurgical resection considering the long-term outcome, neither in this study nor in the reviewed literature." [22] Allen et al. evaluated 32 patients with symptomatic lumbar synovial cysts, who underwent fluoroscopically guided, contrast-enhanced, percutaneous facet cyst distention and rupture; this was followed by an intraarticular facet joint injection of 1 ml Kenalog and 1 ml of 1% Lidocaine [ Table 1]. [1] Additionally, 17 of 32 patients received transforaminal epidural steroid injections just prior to rupturing the cyst. Although the authors concluded that long-term results were excellent (with pain relief in 72%; average follow-up 1 year), 37.5% of patients exhibited synovial cyst recurrence. Of the 11 of 12 who chose to undergo cyst rupture again, 5 improved, but 6 required surgery.
In Martha et al.'s series, 100 patients with lumbar facet joint synovial cysts underwent fluoroscopically guided corticosteroid facet joint injections and attempted cyst rupture. [15] Although cyst rupture was documented fluoroscopically in 81% of cases, 54% required subsequent surgery over an average of 8.4 months [ Table 1]. Furthermore, the authors observed thus: "Successful cyst rupture does not appear to have added benefit, and it was associated with increased disability 3 years post injection." Parlier-Cuau et al. assessed the clinical results of facet steroid injections in the treatment of symptomatic lumbar facet joint synovial cysts after 1 month, 6 months, and long term (mean 26 months) [ Table 1]. [17] After 6 months, 18 of 30 patients (60%) had fair or poor outcomes and 14 of the 18 required surgery.

CONCLUSION
After a thorough review of the literature on the treatment of symptomatic synovial cysts and a study of the anatomy of these cysts, it appears that the treatment with the best outcome for these patients is surgical decompression and removal of the cyst. The question of fusion for all patients after this surgery and the manner in which it is done is still unsettled from the literature that was reviewed. Percutaneous aspiration of the cysts appears to be a treatment alternative that has a much higher recurrence and failure rate than surgery, but may be followed by surgery to correct the problem. An economic analysis of the comparison of cyst aspiration versus surgery does not appear in the literature.