Pes anserinus

The pes anserinus is an anatomical term that is used to describe a broad conjoined tendon of insertion of three muscles - sartorius, gracilis, and semitendinosus. It inserts onto the anteromedial surface of the proximal tibia, just below the knee joint, approximately 4 - 5 centimeters (1.57 - 1.97 inches) distal to the tibial plateau. The pes anserinus lies superficial to the tibial (medial) collateral ligament and may partially fuse with it. It is stabilized by surrounding fascia.

Within the pes anserinus, the tendon of the sartorius is the most anterior, the tendon of the gracilis is in the middle, while the tendon of the semitendinosus is located posteriorly. Interestingly, each muscle forming the pes anserinus arises from a different origin and belongs to a different compartment of the thigh, merging only at the insertion site. The sartorius lies in the anterior (extensor) compartment, the gracilis in the medial (adductor) compartment, and the semitendinosus in the posterior (flexor) compartment.

The term pes anserinus comes from Latin, meaning “goose’s foot.” It has this name because of its broad, sheet-like aponeurotic appearance, which resembles a webbed three-toed foot of a goose. Functionally, the pes anserinus contributes to knee flexion, internal rotation, and stabilization of the medial knee, helping resist valgus and rotational stresses.

The pes anserinus is a variable structure, and three major morphological insertion types have been recognized in several anatomical studies:

• Short insertion - a compact and localized type;
• Band-shaped insertion - an elongated and narrow tendinous band-like insertion;
• Fan-shaped insertion - a broad and widely spread insertion that resembles a fan; it may be up to twice the width of the tendon.

Overall, the sartorius usually has a short tendon insertion, the gracilis most often has a band-shaped insertion, and the semitendinosus typically has a fan-shaped insertion. Additionally, the pes anserinus serves as a harvest site for tendon grafts in ligament reconstructions, such as anterior cruciate ligament (ACL) reconstruction.

The muscles forming the pes anserinus may have some accessory tendons and fascial bands emerging from their tendons, with the majority arising from the semitendinosus tendon. They often have independent insertion sites into the bones or soft tissue. The semitendinosus tendon is the most morphologically variable and may present with up to three tendinous insertions, one of which often attaches to the gastrocnemius or popliteus fascia.

The pes anserinus lies in close relation to the anserine bursa, which is located between the tibial collateral ligament (deep) and the pes anserinus (superficial). The anserine bursa reduces friction between these two structures during knee movements. Clinically, it may become inflamed, resulting in pes anserine bursitis, which presents with pain on the inner side of the knee.