ABSTRACT

We report two cases of synovial cyst about the knee which caused peroneal nerve palsy. Both had a preoperative MRI which showed the lesion to contain fluid and defined the anatomic location, which aided the surgeon in his dissection. Preoperative considerations in both cases were that of solid neoplasms, particularly neurofibromas. One case of synovial cyst was totally contained within the nerve sheath and the other originated from the tibiofibular joint and compressed the peroneal nerve. In the literature such cysts are referred to as ganglia. The pathologic finding of synovial lining cells in both of these cases and review of other cases in the literature allows us to postulate that some of these are synovial cysts and originate from bursae, extensions from the joint cavity, or by synovial metaplasia. Surgical treatment involves excision or drainage of cyst and obliteration of any joint space connection if recurrence is to be avoided.

CASE REPORTS

1. A 34 year old male presented with left peroneal nerve paralysis and palpable mass lateral to fibula head. There was a remote history of trauma and occasional symptoms of snapping on knee flexion in the area of the mass. MRI demonstrated a tubular and multilobular cystic lesion lateral to fibular head (Fig. 1a). At surgery this was found to be an intraneural synovial cyst which extended for a length of about 12 cm. Partial resection and drainage were accomplished taking care to preserve nerve integrity. No joint space communication was discovered.

Pathologically, the specimen consisted of a multicystic cavity lined by attenuated cuboidal cells, which stained positively with low molecular weight keratin by the peroxidase antiperoxidase technique (Fig. 1b). On initial follow-up there was return of nerve function.

2. A 24 year old male presented with a painful lump behind his left knee and peroneal nerve palsy. There was no history of trauma. MRI demonstrated a multiloculated cystic mass contiguous with posterior horn lateral meniscus and tibiofibular joint which extended lateral to fibular head (Fig. 2).

At surgery this cystic mass measured 6 cm by 2.5 cm by 2 cm. and had a pedicle that communicated with tibiofibular joint. The common peroneal nerve was flattened over the dome of the cyst but was not invaded. Complete excision was accomplished. Pathology was similar to case 1.

Six weeks after surgery the patient was doing well with near complete resolution of his paresthesias and motor deficits. He continued to have paralysis of the extensor hallucus longus muscle and decreased sensation over the deep peroneal nerve distribution.

DISCUSSION

Ganglia and synovial cysts are often grouped together because they are both benign cystic masses which are difficult to distinguish clinically (1). Pathologically, they are differentiated by the finding of lining synovial cells in synovial cysts and their absence in ganglia. Synovial cysts tend to occur around joints, and often communicate with the joint cavity. Bursae are synovial cysts that result from irritative forces which induces synovial metaplasia of fibrous tissue. Bursae do not communicate with the joint. The fluid contained in synovial cysts and bursae is of low viscosity. Ganglia result from myxoid degeneration of fibrous tissue with accumulation of excessive amounts of mucopolysaccharides in intercellular spaces. They are devoid of lining cells and are surrounded by myxoid fibrous tissue. Ganglia more commonly arise in relation to tendon sheaths and tend to have higher viscosity fluid.

The most common locations for cystic lesions which result in compressive nerve symptoms are the wrist, elbow, knee, and ankle (2). The peroneal nerve is most frequently compressed at the knee where it courses laterally around the fibular head. The peroneal nerve is prone to entrapment here even in patients without mass lesions as it negotiates a fibrous tunnel near the origin of peroneus longus muscle (3). After the peroneal nerve gives off its small articular branch which communicates with the tibiofibular joint, it divides into superficial and deep branches (Fig. 3). The deep peroneal nerve follows the course of the anterior tibial artery to the dorsum of the foot, innervating anterior compartment leg muscles and sensation. The superficial peroneal nerve remains lateral and supplies the lateral compartment muscles (peroneus longus and brevis) and lateral leg sensation.

Intraneural benign cystic lesions are a distinct entity, and most commonly found in relation to the peroneal nerve, with approximately 60 cases being reported to date (4). Ulnar, median, radial, and tibial nerves have also had cysts within the nerve sheath described. Most cases described in the literature have been described as ganglia, although more recent reports with more extensive pathologic examinations are describing synovial cysts more frequently (4). Our case one is typical for the classic description in the literature (i.e., a 34 year old male with symptoms over a year).

Approximately 40% of cases described to date have had demonstrated at surgery a peduncle connecting the intraneural cyst to the tibiofibular articulation, although our case did not (4). Extirpation of this peduncle has been emphasized as an essential component of therapy, if recurrence of the lesion is to be avoided (5). The articular branch of the peroneal nerve is felt to provide a pathway for communication between the peroneal nerve and the tibiofibular joint and may need to be sacrificed in dissection (Fig. 3). Case one had no communication with the joint space and the cyst can therefore be postulated to result from synovial metaplasia, perhaps related to abnormal movement of the nerve around the fibula (history of snapping).

The larger cyst which we describe in case two was extraneural, but caused extrinsic pressure on the peroneal nerve. The location we observed was distinctly different from the typical Bakers cyst which is more medially situated between medial gastrocnemius and semimembranosus muscles. Laterally, bursal spaces have been described deep to the biceps brachii tendon, deep to the fibular collateral ligament, and deep to the iliotibial tract. Any of these could potentially enlarge and compress the peroneal nerve.

The cysts which have been described in the literature causing extrinsic pressure have arisen either from menisci or the tibiofibular joint (6). Our case two had MRI findings showing continuity with both of these structures. However, meniscal cysts (usually ganglia) are invariably associated with meniscal tears which our patient did not have. The pathological finding of synovial cells supports the origin of the cyst from the tibiofibular joint.

Pre-operative imaging in both these cases was with MRI, which is the currently preferred modality for imaging soft tissue pathology. There has been only one case report in the literature describing MRI for imaging a cyst causing peroneal palsy and the images in that case did not demonstrate the surgically discovered stalk communicating with the tibio-fibular joint (7). Therefore the absence of this finding on imaging should not deter the surgeon from a careful exploration. Although plain films, angiography, CT, and arthrography have also been utilized in cases described in the literature, their utility compared to MRI in demonstrating tissue features and anatomic relations is less. Not used in our cases or described in the literature is ultrasound which is the preferred modality for distinguishing cystic versus solid lesions. Finally EMG is a useful method of verifying nerve involvement when the clinical exam is equivocal.

REFERENCES

1. Jenson DR. Ganglia and synovial cysts: their pathogenesis and treatment. Ann Surg 1937;105:592-602.

2. Brooks DM. Nerve compression by simple ganglia: a review of thirteen collected cases. J Bone Joint Surg 1952;34B:391-400.

3. Leach RE, Purnell MB, Saito A. Peroneal nerve entrapment in runners. Am J Sports Med 1989;17:287-291.

4. Nucci F, Artico A, Santoro A, et al. Intraneural synovial cyst of the peroneal nerve: report of two cases and review of the literature. Neurosurgery 1990;26:339-344.

5. Parkes A. Intraneural ganglion of the lateral popliteal nerve. J Bone Joint Surg 1961;43B:784-790.

6. Stener B. Unusual ganglion cysts in the neighborhood of the knee joint: a report of six cases- three with involvement of the peroneal nerve. Acta Orthop Scandinav 1969;40:392-401.

7. Leon J, Marano G. MRI of peroneal nerve entrapment due to a ganglion cyst. Mag Res Imag 1987;5:307-309.

FIGURES

1A. Coronal T2-weighted image shows cyst lateral to fibula head. Peroneal nerve is not seen.

1B. Pathologic specimen (hematoxylin and eosin, 750X) shows fibrous wall lined by one to three layers of polyhedral and cuboidal cells, consistent with synovial cells.

2. Sagittal T2- weighted image demonstrates multiloculated cystic lesion posterior to lateral meniscus and in continuity with the tibio-fibular joint (arrow).

3. Lateral schematic view of common peroneal nerve and its three major branches; articular (a), superficial peroneal (b), and deep peroneal (c).