PES is classified into six types based on the embryologic origin with type VI now considered functional PES. Type I occurs most commonly, in approximately 50 % of cases, and involves deviation of the popliteal artery medially to the medial head of gastrocnemius. The adult artery either forms later or the medial head of the gastrocnemius migrates early to its normal position on the medial femoral epicondyle. Type I accounts for approximately 50 % of cases of PES. Type II PES occurs when the medial head of gastrocnemius abnormally migrates to a lateral position, compressing the popliteal artery and it accounts for 25 % of all cases of PES. Type III PES involves an accessory slip of gastrocnemius muscle that compresses a normally situated popliteal artery. This likely represents embryologic remnants that mature from the migrating muscle and accounts for 6 % of cases. Type IV PES represents persistence of the fetal axial artery and lies below the popliteus muscle, causing compression of the artery. Type V includes any of type I-IV in addition to the popliteal vein (Fig. 18.2) [3].

The 6th type of PES is also known as functional popliteal entrapment, was first described by Rignault, et al. in 1985 [6]. Arterial occlusion occurs when the popliteal artery runs in its proper anatomic course, as does the medial head of the gastrocnemius, but the artery is compressed between the heads of a hypertrophied gastrocnemius/soleus muscle complex, especially during exercise. This type is commonly seen in professional athletes and military personnel.

The clinical classification of PES is based on symptoms. Class 0 are patients with anatomic PES diagnosed incidentally who are asymptomatic. Class 1 patients complain of pain, paresthesia, and cold feet after physical training or vigorous physical labor. Class 2 patients experience claudication while walking more than 100 m, and class 3 patients experience claudication in less than 100 m. Class 4 patients have rest pain, and class 5 patients have evidence of pedal necrosis [2].