By 2 weeks postoperatively, the numbness of the patient’s left foot had completely resolved, as did his pain. By 3 months after surgery, his extensor hallucis longus strength was 5/5, and ankle dorsiflexion was 4-/5. At 6 months, his ankle dorsiflexion strength was 5/5, and he was completely asymptomatic. At 2 years postoperatively, he remained completely asymptomatic. A follow-up MRI of the left knee showed a ganglion cyst present at the proximal tibiofibular joint with resolution of the intraneural ganglion cyst within the CPN (Figure 6).
Discussion
Intraneural ganglia of peripheral nerves are relatively rare, most commonly occurring in the CPN.6,8,9 A literature search reveals that this condition is only sparsely reported in orthopedic journals. This report, therefore, describes this rare, yet curable, condition. As noted, without appropriate intervention, the condition has a high likelihood of recurrence with only a brief interruption of symptoms.6,8,9,12
The operative technique delineated in this report relies heavily on research demonstrating that peroneal intraneural ganglia develop from the superior tibiofibular joint and gain access to the CPN via the recurrent articular branch.8,13 Research indicates that such ganglia preferentially proceed proximally along the deep portion of the CPN, within the epineurium.6 This hypothesis was corroborated in our case by the swollen appearance of the CPN proximal to its bifurcation.
Currently, there is no consensus on treatment of intraneural ganglion cysts of the CPN. However, evidence suggests that disconnection of the recurrent branch of the CPN may be important in successfully treating the condition.6,9,14 This unified articular theory was initially proposed by Spinner and colleagues12 in 2003 and recommends that surgical treatment focus on the articular branch as the source of cyst fluid.6,9,12,14 This theory by Spinner and coauthors12,14 was substantiated in our case: Once the articular branch was disconnected, cyst fluid was easily expressed via antegrade massage through the disconnected end. Pathologic analysis of a portion of the detached articular branch is also recommended to rule out other cystic lesions, such as cystic shwannomas.14
The history of the unified articular theory began in the mid-1990s, when Dr. Robert Spinner, board certified in both orthopedic and neurologic surgery, began researching causes of intraneural ganglion cysts. At the time, such ganglia were often treated by radical resection of the nerve and the cyst. Based on his review of literature, and his own cases, Spinner15 developed the theory that, just as with extraneural ganglia, these cysts are fed by fluid from the joint. According to Spinner,9 the sources of such connections were very small articular nerve branches that connect the nerve to the joint. His research led him to the original citation of such an intraneural ganglion of the ulnar nerve, first described by Dr. M. Beauchene, a French physician, in 1810.16 Spinner also discovered that Beauchene’s original dissection specimen had been preserved and was displayed in a medical museum in Paris. When Spinner went to France to view the specimen, he indeed found an intraneural ganglion of the ulnar nerve. On closer inspection, Spinner also discovered a small articular nerve branch containing a “hollow lumen” that would have been capable of allowing the passage of fluid into the nerve and leading to the development of a cyst.16
In our case, in the first operation, a simple incisional decompression of the CPN was performed. Unfortunately, the ganglion cyst quickly recurred, as did the patient’s symptoms. In the second surgical procedure, the articular branch connecting the peroneal nerve to the proximal tibiofibular joint was incised and disconnected from the nerve. This allowed the nerve to be decompressed and prevented a recurrence of the ganglion cyst within the nerve with complete resolution of the patient’s symptoms. This difference alone most likely accounts for the rapid recurrence of symptoms after the initial operation, since the fluid was simply drained, but the source was not detached, allowing the ganglion to recur.6,12,14 This is similar in theory to excising the attachment of a ganglion cyst at the wrist from the underlying joint capsule rather than performing a needle aspiration or puncturing of the cyst.12
Regarding the imaging techniques used to identify intraneural ganglia, it is essential that the surgeon be aware of the unified articular theory and the likely presence of an articular branch. Such branches are extremely small and may be easily missed on imaging and intraoperatively.17,18 MRI is the best method to image these cysts because of its superior ability to visualize soft-tissue lesions.18,19 Intraneural ganglion cysts typically appear as homogenous, lobulated, well-circumscribed masses that are hyperintense on T2-weighted MRI.3,19 Gadolinium may also offer diagnostic utility, because these masses do not enhance with its use on T1-weighted MRI.3,17,19 By employing these techniques, one may easily view most of the ganglion cyst. To image the small articular branch, Spinner and colleagues17 recommend thin-section images with high–spatial resolution T2-imaging. They also advocate obtaining multiple image views and planes to increase the likelihood of successful imaging.17