Exercise induced neuropathic lower leg pain due to a tibial bone exostosis

ABSTRACT Objectives: The differential diagnosis of chronic exercise induced lower leg pain in sporters includes compartment syndrome and medial tibial stress syndrome. However, severe discomfort may also be caused by nerve entrapment. Methods: Here we present a marathon runner who reports pain day and night in the lower leg. Deep palpation suggested the presence of a bony tumor deep in the calf musculature, and digital pressure on the soleal sling was painful and elicited paresthesias in the foot. A swab test indicated a hypo-esthetic sole of the foot. Imaging revealed the presence of a tibial exostosis that was hypothesized to narrow the soleal tunnel and irritate the tibial nerve. Results: Via a medial infragenual approach, the soleal tunnel was opened. A bony prominence was found in direct contact to the tibial nerve. Resection of the exostosis with tibial nerve neurolysis completely abolished all of his symptoms. Conclusion: An awkward lower leg discomfort that is present at night and worsens during exercise combined with altered foot sole skin sensation in the presence of a tibial bone exostosis may suggest tibial nerve neuropathy. If conservative therapies fail, resection and neurolysis is advised.


Introduction
The differential diagnosis of exercise induced lower leg pain (ELP) in runners is extensive and includes tendinopathies, chronic exertional compartment syndrome (CECS), medial tibial stress syndrome (MTSS) and stress fractures [1]. The standard work up of ELP includes an X-ray, or a CT or an MRI scan of the lower leg and foot. However, even extensive imaging and functional studies may not always lead to a correct diagnosis. The present patient is a marathon runner who reported leg pain and tingling sensations toward the foot, during the day as well as at night. Physical examination revealed diminished skin sensation (hypo-esthesia) of the foot sole. Imaging revealed a calcified lesion in continuity to the proximal tibial bone. He underwent surgical removal of the lesion and neurolysis of the tibial nerve. Histology demonstrated an osteocartilaginous exostosis. The aim of this contribution is to increase the awareness of sports physicians regarding ELP of a neuropathic origin.

Case report
A 61 year-old male amateur runner presented to our department of sports medicine with left-sided ELP that had started 5 months previously during a run. Since then, he experienced a continuous pain that also prevented him from having a good night's sleep. Running was impossible. Previously, he ran approximately 50 km a week and regularly cycled. His earlier medical history was uneventful. There was no history of smoking. He was treated under the diagnosis calf strain by a physiotherapist but to no avail. His general practitioner ordered a lower leg ultrasound that suggested 'bone overgrowth'.
At presentation, he also reported tingling sensations down toward the foot that were also present at rest. Palpation of the tibial or fibular bone was not painful. Deep palpation of the calf area did not elicit the pain. An X-ray demonstrated a localized peel-shaped calcification on the posterior aspect of the proximal tibia diaphysis that was also visible on a CT scan ( Figure 1).
Because it was unclear whether this lesion was originating from bone or its periosteal layer or from muscle, an additional MRI was performed. The radiologist was convinced that it was a benign lesion, possibly calcification of ruptured muscle ( Figure 2).
As the lesion was in close contact with the neurovascular bundle, the patient was initially hesitant regarding invasive therapy. However, his pain and tingling sensations worsened in the following months. He also developed annoying fasciculations on the medial side of the foot and numbness of the hallux. He agreed to undergo an evaluation of a vascular surgeon who was skilled in diagnosing and treating neuropathic lower leg pain syndromes.
During his physical examination, a painful tumor was palpated deep in the calf musculature. Pressure on the soleal sling was painful and elicited paresthesias in the foot. The sole of the foot was hypo-esthetic compared to the other side. Foot flexion was slightly diminished, judged as 4 on the 1-5 scale. Pulsations of the posterior tibial artery were deemed absent. However, a CT angiography demonstrated no arterial occlusions. He was tentatively diagnosed as having a bony abnormality partially obstructing the soleal tunnel leading to neuropathy of the tibial nerve. The differential diagnosis included osteochondroma, traumatically calcified muscle, or osteochondrosarcoma.
The patient provided written and oral consent to undergo a decompression of the soleal sling and tibial nerve neurolysis. The soleal tunnel was identified via an infrapopliteal medial incision. The soleus muscle was in an ongoing tetanic contraction in reaction to an exostosis that was in continuity with the tibial bone and was partially obstructing the soleal tunnel ( Figure 3). The exostosis was removed.
His postoperative recovery was uneventful, and three weeks postoperatively sensibility of the sole of the foot had normalized with a gradual return of motor function. Six weeks later he could again run without pain. Histopathology revealed an osteocartilaginous exostosis.

Discussion
Weightbearing athletes often present with exercise induced leg pain (ELP) that is of diverse origin and occasionally difficult to diagnose. Symptoms may be ambiguous and nonspecific. A thorough history and physical examination are required as well as appropriate imaging and diagnostic tests once the diagnosis is definite [2]. Common causes of leg pain in athletes are chronic exertional compartment syndrome (CECS) and medial tibial stress syndrome (MTSS). CECS is characterized by pain and tightness that occur after a predictable period of exercise. MTSS is diagnosed as pain that is elicited following palpation of the distal portion of the tibial bone, just proximal to the medial malleolar bone. Less prevalent are tibial stress fractures that are characterized by a local pain and abnormal imaging. By contrast, popliteal artery entrapment syndrome (PAES) is suspected by a severe pain in the calf musculature following exertion that will disappear within 5 minutes of standing. Nerve entrapments may be present if skin sensation is different, for instance dorsal or plantar portions of the foot in case of peroneal nerve or tibial nerve involvement, respectively [2,3]. These conditions often have overlapping clinical presentations. In addition, symptoms may vary within a given diagnostic entity. In the present patient, not imaging but a combination of history (continuous tingling sensations) and physical examination (foot sole hypo-esthesia) provided important clues for the diagnosis.
Electrodiagnostic studies may be helpful in identifying the cause of lower leg neurological symptoms. However, sensitive abnormalities in case of entrapment neuropathies are often lacking. Nerve compression causes little to no axonal degeneration with absent denervation changes on electromyography [4]. Based on the authors' experience with these tests, a neurologist was not consulted in the present patient. Magnetic resonance imaging of the calf may be helpful in identifying potential lesions causing compression of the tibial nerve such as cysts and ganglia [5]. However, primary nerve entrapment in the soleal tunnel may still be present even though MRI shows no space occupying lesion [6]. A thickening of the soleal sling may be a sign of tibial neuropathy, but the associated mechanism is unclear [5].
Therefore, imaging and diagnostic tests should always be judged in the light of the differential diagnosis based on history and physical examination.
An osteochondroma, also known as an osteocartilaginous exostosis, is an osseous stalk that is continuous with the underlying bone via a cartilaginous cap. It is a common benign bone tumor, and most of the time occurring as a solitary lesion [7,8]. Because most of the osteochondromas are asymptomatic, the exact incidence is unknown [7]. They usually develop on the metaphyseal part of long bones of the limbs, and are often turned away from the joint. The distal femur, proximal humerus and the proximal tibia are most frequently involved [7,9]. Soft tissue surrounding an exostosis may be able to move without inducing pain due to a bursa that may develop around well-developed tumors [9]. An osteochondroma may become symptomatic due to a growing volume or after trauma causing a fracture [7,9,10]. Occasionally, symptoms may arise from pressuring muscles, tendons or vascular structures [8]. Compression of a nerve is extremely rare, occurring in <1% [10]. We are not aware of other cases of tibial nerve entrapment in the soleal tunnel due to an osteochondroma. Malignant transformation occurs in approximately 1% in clinically recognized osteochondromas [7,10].
Surgery is only indicated in a symptomatic osteochondroma. Indications for operation are pain, disability, increased risk of fracture, cosmetic or radiographic features of malignancy. Total surgical excision of the exostosis is almost always curative. Neurological recovery with timely excision of the exostosis and surgical decompression is often complete. However, delays in diagnosis and treatment may result in chronic pressure damage to the nerves with irreversible neurological impairments [10,11].

Conclusion
An osteocartilaginous exostosis is a rare cause of pain due to peripheral nerve compression as in our patient. Symptoms may have been elicited by repetitive trauma resulting from marathon running. A variety of imaging studies supported a bony cause of the pain. However, the diagnosis nerve compression was suspected on the basis of the patient's history and sensitive clues during physical examination. A timely excision of the exostosis and neurolysis is beneficial.

Declaration of interest
The authors declare that they have no conflict of interest.

Consent to participate
The subject provided oral and written informed consent for the present case study.