Tibial Nerve Entrapment and Heel Pain Caused by a Baker's Cyst

To the Editor:

Following is a report of a rare case of heel pain due to tibial nerve entrapment secondary to a Baker’s cyst. Although discussed in the literature, entrapment of the tibial nerve in the popliteal fossa by a Baker’s cyst is not recognized by many clinicians, and the frequency of the condition is unknown [1].

The knee joint has the body’s largest synovial membrane and is the most common site for cyst formation [2]. The knee-joint cavity is connected to four sliding bursae: the suprapatellar, the popliteal muscle, the medial sural triceps, and the gastrocnemiussemimembranosus [3]. Baker’s cysts are formed when the gastrocnemius-semimembranosus bursa communicates with the knee joint and an effusion occurs. Baker’s cysts usually occur in the middle third of the popliteal fossa, emerging in the cleft between the gastrocnemius and semimembranosus muscles. They become firm with full extension of the knee and soft when the joint is flexed [4].

In relation to the popliteal fossa, the tibial nerve runs directly vertical to the knee joint. Distally in the fossa, the nerve plunges deep to the lateral head of the gastrocnemius. In the foot it supplies the proximal one-third of the sole of the foot and medial heel innervated by the medial calcaneal nerve.

Entrapment of the tibial nerve usually occurs in the tarsal tunnel and can result in heel pain, which is one of the most common complaints in podiatric practice. However, this phenomenon is frequently overlooked. Even when it is recognized, the level of entrapment is often not correctly determined.

Case Report

A 61-year-old woman presented with pain, “pins and needles,” and numbness in her left heel of approximately 1 year’s duration, with insidious onset. She also reported occasional burning on the plantar aspect of her left foot. The symptoms were aggravated by walking or standing. She denied having a history of back pain. She had been previously treated with injections, strapping, and arch supports, which did not alleviate the symptoms.

The patient’s medical history was significant for bilateral carpal tunnel syndrome, hypertension, hyperthyroidism, and osteoarthritis of both knees.

The physical examination revealed sensation to be inconsistent. The Achilles tendon reflex was 1+ bilaterally. The patellar reflex was 2+ bilaterally. Manual muscle testing was 5/5 bilaterally. There was no evidence of a positive Tinel’s sign in the left foot or ankle.

The patient was scheduled for an electromyography and nerve conduction study to rule out tarsal tunnel syndrome. The study revealed that there was no evidence of entrapment of the tibial nerve at the left ankle. However, there was a prolongation of the tibial F-wave, which is a measure of proximal conduction along the course of the nerve. This suggests the presence of entrapment along the course of the nerve proximal to the site of stimulation at the ankle. It was also noted during the study that the patient’s left popliteal fossa was somewhat fuller than the right one.

The patient was then scheduled for an ultrasound of the left popliteal fossa. The ultrasound revealed a cystic mass medially in the left popliteal fossa with thick, irregular septa and low to medium echo levels. The findings were consistent with the diagnosis of a Baker’s cyst.

After aspiration of the cyst by her rheumatologist, the patient experienced immediate relief of the symptoms pertaining to the plantar aspect of her left foot.

Discussion

This case is unusual in that the entrapment of the tibial nerve was proximal rather than distal: Heel pain secondary to proximal nerve entrapment has not been previously described in the literature. Moreover, this patient’s cyst was located medially in the popliteal fossa, rather than in the middle one-third as in the typical presentation. This suggests that the location of the cyst caused compression of the axons that largely contribute to the medial calcaneal nerve.

Entrapment of the tibial nerve at any level, including the S1 nerve root, can cause paresthesia on the plantar aspect of the foot. This is because of ischemia secondary to compression and subsequent ionic and axonal transport. Subsequently, there is increased endoneurial fluid pressure and intraneural edema. When the compressive force is removed or decreased to the point at which the nerve is revascularized, spontaneous discharges are generated along the axons. When these ectopic impulses arise anywhere along the length of the larger myelinated fibers, such as the tibial nerve, they are subjectively interpreted as paresthesia [5].

Conclusion

Heel pain is one of the most common disorders of the foot, and plantar fasciitis is the most common diagnosis. However, there are other etiologies that are overlooked by most clinicians, such as S1 radiculopathy, seronegative arthritis, tendinitis, bursitis, thrombophlebitis, and entrapment of the tibial nerve or its branches. In elderly patients, vascular insufficiency may produce burning in the heel during activity [6,7]. The diagnosis of nerve entrapment can easily be substantiated by nerve conduction studies and electromyography.