Answer: Neuroma of the tibial nerve

Discussion

On magnetic resonance imaging, a Baker cyst appears as a well circumscribed mass in the posterior aspect of the knee. As the mass is fluid-filled, it should produce an intermediate signal on a proton-density-weighted image and a high signal intensity on a T2-weighted image. A STIR (short-T1-inversion-recovery) magnetic resonance imaging scan, which suppresses fat signals and increases the signal of fluid, can be helpful in distinguishing a cystic structure from a mass lesion. However, it did not aid in the diagnosis for our patient. An independent musculoskeletal radiologist reviewed the magnetic resonance imaging scans and believed that there were specific findings with which to dispute the radiographic diagnosis of a Baker cyst. First, the proton-density-weighted image showed a low signal intensity, which did not indicate a Baker cyst. Second, the mass was located adjacent to, not medial to, the lateral head of the gastrocnemius. Because of these two findings, the differential diagnosis for this mass should have included a sheath schwannoma, a leiomyoma, or a ganglion cyst of the lateral head of the gastrocnemius.

Neuromas-in-continuity are neuromas in a nerve that has not been completely severed. Sunderland(10) described two types: those in a nerve with an intact perineurial sheath and those in a partially divided nerve. Spindle neuromas are swellings or enlargements in an intact nerve secondary to chronic irritation, friction, or pressure. With repeated continuous trauma, fibrous tissue proliferates, constricting nerve fibers and interfering with their nutrition. Eventually, the swelling becomes a large, collagenized mass with fibrotic replacement of the nerve fibers and vessels. Examples of this type of spindle neuroma are those occurring in Morton metatarsalgia, the ulnar digital nerve of the thumb in bowler's thumb, the greater occipital nerve where it pierces the trapezius fascia, the lateral femoral cutaneous nerve where it passes beneath the inguinal ligament in meralgia paresthetica, the posterior interosseus nerve on the back of the wrist, the branch of the axillary nerve to the teres minor, and the lateral branch of the deep peroneal nerve where it crosses the tarsal navicular bone(2,6,7,9). A lateral neuroma is a type of neuroma that occurs when part of the nerve with its perineurium has been injured, and it is commonly seen by surgeons dealing with trauma. A spindle-cell neuroma of the tibial nerve was the cause of the pain in the patient in our report.

The etiology of the formation of the spindle neuroma in the tibial nerve of our patient may have been due to chronic irritation from the degenerative changes in the knee or from pressure of the Baker cyst. The decision to operate was dictated by the worsening symptoms. This case report has described a rare but treatable cause of posterior pain in the knee.

Reference:

DeLuca, P. F; and Bartolozzi, A.R.: Tibial neuroma presenting as a Baker cyst. A case. J. Bone and Joint Surg., 81-A: 856-8, June 1999.