Which Muscles Form the Pes Anserinus?
The tendons of three muscles contribute to this structure:
- Sartorius muscle (m. sartorius) — the longest muscle in the body. It originates from the anterior superior iliac spine, runs obliquely downward and medially, and crosses anteriorly to the medial aspect of the knee.
- Gracilis muscle (m. gracilis) — originates from the inferior ramus of the pubis and the pubic symphysis.
- Semitendinosus muscle (m. semitendinosus) — a long, slender muscle on the posterior aspect of the thigh, originating from the ischial tuberosity (tuber ischiadicum). In the middle third of the thigh, its muscle belly transitions into a long tendon that passes posterior to the medial femoral condyle to join the pes anserinus.
Topography and Insertion Site
The tendons of the sartorius, gracilis, and semitendinosus insert together on the medial surface of the tibia, distal to the tibial condyle and near the tibial tuberosity (tuberositas tibiae). Some superficial fibers of the semitendinosus tendon blend with the deep fascia of the leg (crural fascia).
Adjacent to the insertion site lies the anserine bursa (bursa anserina). Additional subtendinous bursae are frequently associated with the sartorius tendon in this region.
Blood Supply and Innervation
The muscles of the pes anserinus have distinct nerve supplies and vascular profiles:
- Sartorius: Innervated by the femoral nerve (n. femoralis, LII–LIII). Supplied by branches of the femoral artery, including the lateral circumflex femoral artery (a. circumflexa femoris lateralis) and descending genicular artery (a. genus suprema).
- Gracilis: Innervated by the anterior branch of the obturator nerve (n. obturatorius, LII–LIII). Blood supply is provided by the external pudendal artery (a. pudenda externa), obturator artery (a. obturatoria), and profunda femoris artery (a. profunda femoris).
- Semitendinosus: Innervated by the tibial division of the sciatic nerve (n. tibialis, LIV–LV, SI). Supplied by perforating branches (aa. perforantes) of the profunda femoris artery.
Functions and Clinical Significance
The muscles forming the pes anserinus perform the following actions:
- Flex the leg at the knee joint.
- Medially rotate the tibia when the knee is flexed.
- Stabilize the medial side of the knee against valgus stress and rotational forces.
From a clinical perspective, this region is prone to inflammation known as pes anserine bursitis, which involves the bursa anserina and is a frequent cause of medial knee pain in athletes and older adults.
Other 'Pes Anserinus' Structures in Anatomy
In topographical anatomy of the head, similarly named structures exist. The lesser pes anserinus (pes anserinus minor) is formed by the upper labial branches of the infraorbital nerve. Additionally, the term pes anserinus is used to describe the fan-like branching of the facial nerve motor branches as they emerge from the parotid gland anteriorly and inferiorly to the ear canal.