Proximal tibial neuropathy is extremely rare but has been reported in the literature.  The differential diagnosis includes pathology of the popliteal structures including Baker cysts, popliteal artery aneurysms, posterior compartment syndrome, intraneural ganglia or neoplastic invasion of the sciatic and tibial nerves, soleus tendon entrapment, popliteus muscle hemorrhage, or trauma.  Gastrocnemius mediated entrapment via hypertrophy and anomalous fibers have been reported as well [5].  However, the most common presentation is caused by both intact and ruptured Baker cysts and the related sequelae such as posterior compartment syndrome [1-3].  Sanchez et al conducted an extensive literature review of 30 publications including 73 cases of compression syndromes due to Baker cysts and the detailed variants.  In contrast to our case, the review demonstrated Baker cyst pathology usually causes denervation to the gastrocnemius and soleus muscles [3].  Sparing of these muscles is an even rarer finding and would indicate pathology distal to innervation of these muscles.

Elucidating the etiology of nerve injury involves localizing the site of the lesion.  Electrodiagnostic studies and advanced imaging such as MRI and ultrasound are typically useful for localization.  The tibial nerve is a branch of the sciatic nerve that innervates the posterior compartment of the distal leg.  It supplies branches to the medial and lateral heads of the gastrocnemius as it descends through the popliteal space.  Just distal to the tibiofemoral joint it continues inferiorly lying just posterior to the popliteus muscle supplying innervation via a branch called the nerve to the popliteus.  From here it innervates the soleus and dives anteriorly traversing the tendinous arch of the soleus.  As it continues in the posterior compartment it supplies three branches to the tibialis posterior, flexor digitorum longus, and flexor hallucis longus.  These structures enter the foot beneath the flexor retinaculum entering the sole where the tibial nerve terminates into medial and lateral plantar nerves and innervates the muscles of the foot.  The retinaculum is the most common site of entrapment causing tibial neuropathy and is commonly known as tarsal tunnel syndrome.  In this case it is likely that the nerve was entrapped distal to the soleus innervation.  A review of the literature resulted in cases with similar presentation where popliteus muscle hemorrhage and rupture caused proximal tibial neuropathy with sparing of the gastrocnemius and soleus [4].  An additional case with similar findings was caused by entrapment at the tendinous arch of the soleus with formation of pseudoneuroma [5].

The nerve injury in this case was never clearly elucidated but it was thought to be secondary to the knee injury that resulted in increased joint and lower extremity swelling.  It is plausible the swelling was a ruptured popliteal cyst as there was a small one found on MRI.  Other explanations could be traction injury, popliteus swelling due to intramuscular hemorrhage or soleus tendinous arch entrapment.  This case demonstrates how electrodiagnostic testing and imaging studies including magnetic resonance imaging and ultrasound can be utilized in the differential diagnosis of a mononeuropathy of the lower extremity.