A Posttraumatic, Joint-connected Sural Intraneural Cyst—With a New Mechanism of Intraneural Recurrence: A Case Report

Neal M. Blitz, DPM, FACFAS,1 James Prestridge, DPM,2 Kimberly K. Amrami, MD,3 and Robert J. Spinner, MD4

Intraneural ganglion cysts are rare in occurrence and most commonly involve the peroneal at the fibular neck. We present a case of a traumatically induced intraneural ganglion cyst of the sural nerve that developed after a nondisplaced posterior ankle fracture. The intraneural ganglion cyst was connected to the subtalar joint by its articular branch and ascended several centimeters into the distal fourth of the leg. It was resected from the sural nerve proper and the posterior branch of the lateral calcaneal nerve, and the articular trunk was ligated. The patient developed subclinical intraneural recurrence, which was detected on a postoperative magnetic resonance imaging (MRI). Retrospective reinterpretation of the preoperative and postoperative MRIs revealed that ligation of the articular trunk proximal to a major branch (ie, the anterior branch of the lateral calcaneal nerve) led to increased intraneural cyst propagation distally: within the blind stump of the articular trunk and within several anterior branches of the lateral calcaneal nerve but not within the parent sural nerve or its continuation, the lateral dorsal cutaneous nerve. This mode of intraneural, but extraparental nerve recurrence can be easily understood by considering the altered fluid dynamics, particularly the increased resistance. This case report provides further evidence not only supporting the articular theory of intraneural ganglion formation but also highlighting the importance of searching for, identifying, and treating the pathologic articular branch connection near its joint connection in all cases. Level of Clinical Evidence: 4. (The Journal of Foot & Ankle Surgery 47(3):199–205, 2008)

Key words: ankle fracture, intraneural ganglion, sural nerve

fibular neck is the most common site (1). Reports of intraneural Ganglion cysts occurring within the epineurium of a periph- ganglia exist in the foot and ankle region, mostly affecting the eral nerve are described as intraneural ganglia. They may result within the tarsal tunnel (2–16). in pain, motor dysfunction, and sensory loss. Approximately The pathogenesis of intraneural ganglia remains contro- 300 cases of intraneural ganglion cysts have been reported in a versial; however, increasing amounts of evidence suggest that multitude of locations. The at the intraneural ganglia arise from synovial joints (1, 17–21). This unifying articular (synovial) theory has demonstrated joint connections at the most common site, the peroneal nerve at Address correspondence to: Neal M. Blitz, DPM, FACFAS, Kaiser North Bay Consortium Residency Program, Department of Orthopedics the fibular neck to the superior tibiofibular joint; at rare and Foot & Ankle Surgery, Kaiser Permanente Medical Center, 3925 Old locations; and even at remote sites from joints in which Redwood Highway, Suite 242, Santa Rosa, CA 95403. E-mail: cases,nealblitz@ joint connections had been previously refuted (3, 13, yahoo.com. 1Attending Podiatric Surgeon, Research Director, Kaiser North Bay 14, 19–23). We believe that a capsular perforation in prox- Consortium Residency Program, Department of Orthopedics and Foot & imity to the articular nerve branch allows cyst fluid to egress Ankle Surgery, Kaiser Permanente Medical Center, Santa Rosa, CA. from the joint (20, 24). Trauma may be an inciting event 2Resident Podiatric Surgeon, PGY-3, Department of Podiatry, Kaiser Foundation Hospital, Vallejo, CA. that initiates this flow of joint fluid (1, 25). The articular 3Associate Professor, Department of Radiology, Mayo Clinic, Roches- branch serves as a conduit for the fluid subsequently to ter, MN. dissect into the parent nerve and propagate along a path of 4Professor, Departments of , Orthopedics and Anatomy, Mayo Clinic, Rochester, MN. least resistance (1, 19). Financial Disclosure: None reported. We present a case of a traumatically induced sural intra- Conflicts of Interest: None reported. neural ganglion with a new pattern of intraneural recurrence Copyright © 2008 by the American College of Foot and Ankle Surgeons 1067-2516/08/4703-0006$34.00/0 demonstrating the effect of increased resistance on cyst doi:10.1053/j.jfas.2008.01.002 propagation.

VOLUME 47, NUMBER 3, MAY/JUNE 2008 199 nerve—an intraneural ganglion. The intraneural ganglion extended in a tubular fashion within an articular branch from the posterior aspect of the subtalar joint (Figure 2, A). The intraneural ganglion cyst extended proximally above the subtalar joint to the distal fourth of the leg. Its maximal dimension was 1.2 ϫ 1.4 cm at a point 1.5 cm above the subtalar joint (Figure 2, B). The appearance of the tapered joint connection with proximal expansion of the cyst was consistent with a “balloon sign” (14). The top of the balloon was 2.8 cm above the articular branch but a linear streak extended an additional 2.5 cm above that (total ϭ 5.3 cm above the subtalar joint). A “signet ring” sign was present with eccentric displacement of the nerve fascicles at this level (Figure 2, C). Distinct intraneural cyst could be seen within the sural and the lateral calcaneal branches (Figure 2, D). Subtle evidence of descent within the anterior and posterior branches of the lateral calcaneal nerve and the sural nerve distal to the level of the articular branch (ie, the lateral dorsal cutaneous branch) (approximately 1.5 cm) was present (Figure 2, A and B). Capsular vessels between the intraneural ganglion and the ankle and subtalar joints were identified (Figure 2, E). In addition, the posterior FIGURE 1 Weightbearing lateral radiograph of the right ankle. A posterior malleolar fracture is evident (arrow). A metallic marker was talofibular ligament was disruputed and there was grade 2–3 placed directly over the soft tissue mass, highlighting the oval soft chondromalacia of the tibiotalar joint. tissue shadow, which corresponds to the mass lesion seen on MRI Treatment of the posterior malleolus fracture involved and at operation. weightbearing as tolerated in a removable cam walker. Surgery of the mass was performed 7 weeks after the injury Case Report (Figure 3, A). A curvilinear incision was made over the mass. The lesser saphenous vein was reflected anteriorly. A A 38-year-old man presented to the primary author sural intraneural ganglion cyst was evident and the nerve (N.M.B.) complaining of an enlarging soft tissue mass that fascicles of the sural nerve were displaced anteriorly (Figure developed 3 weeks following a right ankle inversion injury. 3, B). Using standard microsurgical techniques, cyst was His symptoms included shocking sensations, numbness, and resected from the sural nerve proper and the posterior pain to the lateral aspect of the right foot. On physical branch of the lateral calcaneal branch (Figure 3, C). The examination, a 2.5 ϫ 1.5-cm soft tissue mass was palpable articular trunk was identified and was deeply ligated with posterior to the lateral malleolus at the level of the ankle 3–0 absorbable suture and then transected. However, the joint. Numbness and allodynia were present in the lateral articular branch was not resected from its joint of origin at foot and percussion tenderness over the distal aspect of the the subtalar joint. The histological and immunohistochem- sural nerve at the level of the ankle and just proximal to it ical features of the intraneural ganglion cyst were confirmed produced radiating dysesthesias into the lateral portion of (Figure 4, A and B). foot. The ankle was without effusion and pain free through The patient was kept non-weightbearing postoperatively range of motion. until the first follow-up appointment at 2 weeks. At the Weightbearing radiographs of the right ankle demon- initial postoperative visit, the patient reported the return of strated a small, nondisplaced posterior malleolar fracture sensation to the distal lateral aspect of the foot. Examination involving less than 10% of the tibial plafond (Figure 1). The revealed a deficit in light touch remained in the distribution mortise was anatomic and without medial clear space wid- of the lateral calcaneal nerve in addition to region of the ening and proximal leg films did not demonstrate a fibular incision. Over the postoperative course, some sensation shaft or Maisonneuve fracture. A soft tissue mass was was restored along the dorsolateral aspect of the foot and evident that projected posterior to the distal fibula and in the vicinity of the incision. Plain films and MRI anterior to the just inferior to the ankle documented incomplete union of the posterior malleolar joint. fracture. At 2-year follow-up examination the patient was A magnetic resonance image (MRI) scan demonstrated asymptomatic. an increased T2 signal intensity mass in the posterolateral Postoperative MRI at that time demonstrated intraneu- aspect of the ankle intimately associated with the sural ral, but extraparental nerve, cyst recurrence. The subtalar

200 THE JOURNAL OF FOOT & ANKLE SURGERY FIGURE 2 Preoperative right ankle MRIs. (A) Maximum intensity projection (MIP) image from a sagittal short-tau inversion recovery (STIR) data set shows the sural intraneural cyst (asterisk). The joint connection to the subtalar joint (arrow) is seen with dissection along the articular branch (small arrowhead) to the parent nerve. Note the predominant proximal propagation of the cyst with proximal “streaking” of cyst within the parent nerve (better seen in Figure 2, B) and subtle distal descent of cyst into the anterior (large arrowhead) and posterior (thin arrow) branches of the lateral calcaneal nerve. The connection of the articular branch to the sural nerve occurs near the origin of the anterior branch of the lateral calcaneal nerve. (B) Sagittal oblique MIP from a fast spin echo (FSE) T2 data set with fat suppression shows the large cyst (asterisk) extending along the articular branch (small arrowhead). Subtle evidence of cyst descent is seen within the distal sural nerve (large arrow) as it passes deep to the articular branch; distal cyst extension can be seen in the anterior (arrowheads) and posterior (thin arrow) branches of the lateral calcaneal nerve. (C) Axial T2-weighted image with fat suppression demonstrates intraneural cyst (asterisk) with eccentrically displaced nerve fascicles laterally and posteriorly (arrows), the “signet ring” sign. The cutaneous marker corresponds to the palpable mass. (D) Axial T2-weighted fast recovery FSE image with fat suppression just distal to Figure 2, C, shows cyst within the main sural nerve (asterisk) as well as anterior (arrowhead) and posterior (arrow) branches of the lateral calcaneal nerve. There is a cutaneous marker in place marking the palpable abnormality. (E) Sagittal STIR image demonstrates the sural intaneural ganglion as an oval hyperintense mass (asterisk). Capsular vessels can be misconstrued as articular branches of the sural nerve to the posterior ankle (short arrow) and subtalar (long arrow) joint capsule. Vessels are not as bright as cyst fluid on this sequence allowing confident differentiation. joint connection persisted, increasing in prominence below the subtalar joint. In contrast to the preoperative compared to the preoperative images (Figure 5, A–C). MRI, there was no cyst apparent proximal to the trunca- Cyst extended now only 1.1 cm above the subtalar joint, tion point, either within the sural nerve proper or the being truncated at the level of the ankle joint (Figure 5, posterior branch of the lateral calcaneal nerve. Distal cyst A). The proximally directed balloon sign was not present enlarged and became more prominent. The intraneural on the postoperative MRI. Instead, the recurrent cyst cysts were seen within the articular trunk blind stump extended from the subtalar joint along the articular (approximately 1 cm) as well as 3 anterior branches of branch, slightly superiorly into the articular trunk blind the lateral calcaneal nerve resulting in 4 adjacent, distally stump and then inferiorly 2.7 cm below the subtalar joint, directed inverted balloon signs (Figure 5, B). A signet reaching its maximal size (1.8 ϫ 1.1 cm) at a point 1 cm ring sign could also be seen within the anterior branches

VOLUME 47, NUMBER 3, MAY/JUNE 2008 201 FIGURE 3 Operative findings. (A) The soft tissue mass (arrows)is readily visualized on the posterolat- eral aspect of the right ankle. (B)At operation, the sural intraneural ganglion (asterisk) is seen in rela- tion to the anteriorly displaced sural nerve (arrowheads). (C) Post resection of the intraneural gan- glion, the sural nerve proper (large arrows) and the posterior branch of the lateral calcaneal nerve (small arrows) have been maintained.

FIGURE 4 . (A) The find- ing of ganglion cyst within the epineurium of nerve is consistent with the diagnosis of an intraneural cyst (hematoxlyn and eosin, ϫ20). (B) S-100 protein positivity in the ar- ticular trunk neural pedicle is noted (adjacent to the cyst) (hematoxlyn and eosin, ϫ100). of the lateral calcaneal nerve demonstrating eccentric the articular trunk (proximal to the take-off of the anterior displacement of the individual by the cysts (Figure branch of the lateral calcaneal nerve), cyst was present in 5, D). the articular trunk blind stump and the posterior branches of the lateral calcaneal nerve, but not within the sural nerve Discussion itself (Figure 6). According to the unifying articular theory, intraneural cyst in the sural nerve would not be anticipated, This case strengthens the argument for the articular the- and in fact would be prevented by such ligation (this ab- ory. First, it provides clear evidence of a joint connection in sence of intraneural cyst in this location would provide an unusual location. Of the 4 previous cases of sural intra- additional evidence against de novo formation). Instead, a neural ganglia (6, 26–28), this is the second case in which new pattern of intraneural recurrence was observed, one that a joint connection between the intraneural ganglion and a could be easily understood based on the altered fluid dy- joint was identified. Pringle et al (26) also found a connec- namics. Because the joint connection to the subtalar joint is tion to the calcaneocuboid joint. As the sural nerve partially maintained (and the intraarticular pressures are presumably innervates both the subtalar and calcaneocuboid joints (as elevated), cyst propagation continues along the articular well as several other joints in the foot and ankle region) branch and articular trunk to the point of ligation; because (29), it is logical that intraneural ganglia can originate from of the increased resistance, the intraneural cyst then de- these joints. scends down the anterior branches of the lateral calcaneal Second, as has been shown previously, intraneural recur- nerve. The typical balloon-shaped proximal propagation rence occurred because a neural pathway (ie, a patent artic- pattern seen preoperatively is reversed postoperatively lead- ular branch–joint connection) persisted after surgery (1).In ing to an inverted balloon sign, where cyst becomes bigger previously reported cases where decompression by itself distally because of the increased forces (“back pressures”) was performed without articular branch disconnection, in- experienced due to the pathologic block proximally. traneural recurrence occurred predominantly in a proximal While it is possible that the ankle fracture and the subtalar direction with more limited dissection distally. In this case, joint-related intraneural cyst were detected coincidentally in since ligation was performed within the proximal portion of this patient, we believe that trauma was a causative factor

202 THE JOURNAL OF FOOT & ANKLE SURGERY FIGURE 5 Postoperative right ankle MRIs. (A) Sagittal T2-weighted image with fat suppression shows recurrent intraneural cyst (asterisk) now extending from the subtalar joint (large arrow) along the articular branch (small arrowhead) to its blind stump (ϩ) in the articular trunk and then distally. When compared to the preoperative imaging there is a distinct truncation point (large arrowhead) with increased extent and prominence of cyst descent. The lesser saphenous vein (thin arrow) is seen due to flow-related enhancement on this sequence. (B) Sagittal T2-weighted image with fat suppression immediately lateral to Figure 5, A, shows ballooning cyst within the blind stump of the articular trunk (ϩ) and the anterior branches of the lateral calcaneal nerve (asterisks) creating an “inverted balloon” sign due to increased proximal resistance related to the ligation of the nerve at initial surgery (arrow, lesser saphenous vein). (C) 3D MIP of a T2-weighted fast recovery FSE acquisition with fat suppression showing the truncation point proximal to the take-off of the articular branch (arrowhead) and anterior branch of the lateral calcaneal nerve (asterisk). The cyst within the blind stump of the articular trunk (ϩ) and the individual anterior branches (asterisks) of the lateral calcaneal nerve is better demonstrated in Figure 4, B. The saphenous vein (arrow) is also seen. (D) Axial T1-weighted image showing intraneural cyst (signet ring sign) within the anterior branches of the lateral calcaneal branches with eccentric displacement of the individual nerve fascicles (arrows).

(1, 25) by 2 potential explanations: (1) there was concurrent about, searching for, identifying, and treating the pathologic injury to the 2 neighboring joints; and (2) a communication articular branch near its joint connection in all cases of between the 2 joints, which is known to occur normally and intraneural ganglia. While resection of the sural nerve can pathologically as seen on arthrography (30, 31), facilitated be performed with acceptable morbidity, we currently the passage of fluid under increased intraarticular pressures would recommend decompression of the intraneural cyst to pass from the ankle joint to the subtalar joint and prop- through a small longitudinal epineurotomy (away from agate through the latter’s articular branch. nerve fascicles) combined with disconnection (transection This case report emphasizes the importance of knowing and ligation) of the articular branch at the level of the joint

VOLUME 47, NUMBER 3, MAY/JUNE 2008 203 FIGURE 6 Drawing of the normal anatomy, pathoanatomy of the intra- neural ganglion cyst initially and at follow-up. The proposed mechanism for the intraneural extraparental re- currence within the articular trunk blind stump and the anterior branches of the lateral calcaneal nerve is illustrated. (With permission of the Mayo Foundation for Medical Education and Research. All rights reserved.) of origin. Based on this unified theory, we contend that 11. Fujita I, Matsumoto K, Minami T, Kizaki T, Akisue T, Yamamoto T. disconnection of the articular branch near the joint of origin Tarsal tunnel syndrome caused by epineural ganglion of the posterior can eliminate intraneural recurrence rates. This case illus- tibial nerve: report of 2 cases and review of the literature. J Foot Ankle Surg 43:185–190, 2004. trates that ligature within the articular trunk protects intra- 12. Chow HT, Lui TG. 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