Pain located in the posterior aspect of the knee is a common symptom in patients who have degenerative changes in the knee. The etiology of the pain can be a meniscal tear in the posterior horn, a large effusion, a ruptured Baker cyst, or, least commonly, a peripheral nerve entrapment. To our knowledge, this case report is the first to document a neuroma-in-continuity of the tibial nerve as a cause of posterior pain in the knee.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
†3B Orthopaedics, 1800 Lombard Street, First Floor, Philadelphia, Pennsylvania 19146.
A sixty-seven-year-old woman was seen in November 1991 because of pain in the right knee that had been present for two years. The patient described the pain as being posterior, prone to aggravation with certain activities, and diminished with rest. She did not have pain while in bed at night, but the posterior aspect of the knee was tender when touched. In the past, she had had low-back pain that radiated into the right calf and foot. A diagnosis of spinal stenosis had been made, and a laminectomy had been performed between the fourth and fifth lumbar levels. This operation seemed to relieve the low-back pain, but the patient had persistent sensitivity and pain in the right lower limb. A venogram and a Doppler study had been performed to rule out a deep venous thrombosis, and the results were normal. The history revealed that the patient had had excision of a sebaceous cyst from the scalp and fibrocystic disease of the breast, but no other soft-tissue tumors, birthmarks, or café-au-lait spots.
On physical examination, the patient had a full range of motion of the knees. There was moderate tenderness in the posterolateral aspect of the right knee. There was no large mass in the popliteal space, but there was a central area of tenderness. There was no effusion of the joint, pathological laxity, tenderness at the joint line, or patello femoral signs. A magnetic resonance imaging scan of the right knee was interpreted by a radiologist as demonstrating a 2.5-centimeter-diameter Baker cyst (Figs. 1 and 2). This cyst was aspirated, and five milliliters of gelatinous fluid was obtained.
In November 1991, an arthroscopy of the right knee revealed a torn lateral meniscus. A partial lateral meniscectomy was then carried out. Because the presumed Baker cyst was painful, the patient was placed in the prone position and an open exploration of the popliteal space was done. A 2.5-centimeter-diameter mass in continuity with the tibial nerve was identified (Fig. 3). A biopsy was performed, and it demonstrated a cystic component, which explained the gelatinous aspirate. The wound was then closed.
One month after the arthroscopy, the patient noted mild pain behind the right knee and down the leg. Five months after the arthroscopy, she had increased pain in the posterior aspect of the right knee as well as pain in the right leg. The patient was referred to a neurosurgeon for excision of the neuroma.
Initial evaluation by the neurosurgeon revealed a positive Tinel's sign over the mass in the popliteal fossa. There was a decrease in the deep tendon reflex of the right ankle. Sensation was preserved over the sole and the dorsum of the foot. The patient had a full active range of motion of the foot and ankle. Electromyography revealed normal findings. On June 10, 1992, a resection of a neuroma-in-continuity of the tibial nerve was performed with use of intraoperative monitoring. A spindle-cell neuroma was found. No fibers of the nerve were cut.
The patient was evaluated six years after the excision of the neuroma. At this time, she had no pain in the right knee or leg. There was some reduction in toe flexion, but there was excellent plantar flexion and inversion strength. Sensation on the plantar aspect of the foot was normal.
The cause of posterior pain in the knee is multi-factorial. Usually, this symptom is found in patients who have degenerative changes in the knee. The source of pain is commonly a degenerative meniscal tear with an associated Baker cyst. A neuroma-in-continuity as a cause of pain in the knee has not been reported in the English-language literature to our knowledge. Moreover, the symptoms and signs in our patient were suggestive of a large popliteal cyst that may have been compressing the tibial nerve. Such a cyst, described by Baker1 in 1877, may be produced either by herniation of the synovial membrane through the posterior part of the capsule of the knee or by escape of fluid through the normal communication of a bursa (either the semimembranosus or the medial gastrocnemius bursa) with the knee5. In children, the cyst infrequently communicates with the joint and intra-articular abnormality is rare. These cysts often resolve spontaneously in children3. However, in adults, intra-articular abnormality is common and the cyst may recur if the intra-articular abnormality is not corrected. A number of authors have reported giant synovial cysts of the calf in patients who had rheumatoid arthritis8. Some have described a syndrome suggestive of acute thrombophlebitis caused by a dissecting popliteal cyst4. The cyst must be distinguished from a lipoma, xanthoma, fibrosarcoma, as well as vascular and other tumors; rarely is an aneurysm confusing. A magnetic resonance imaging scan of the knee can aid in the diagnosis of a Baker cyst. Appropriate sequences aid in the differentiation of cystic lesions from mass lesions.
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. Sunderland10 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 bone2,6,7,9. 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.
NOTE: The authors thank Mark Schweitzer, M.D., for his review of the magnetic resonance imaging scans.