The pes anserinus is a tendon insertion point located on the inner side of the knee. This structure acts as an anchor for a trio of muscles that run down the thigh, playing a significant role in leg movement and stability. Understanding which muscles converge at this point is important for comprehending the mechanics of the knee and hip, and recognizing common sources of inner knee pain.
Location and Anatomy of the Tendon
The pes anserinus is a broad, flat structure that attaches to the anteromedial surface of the tibia. This insertion point is found on the upper part of the tibia, approximately 5 centimeters below the knee joint line.
The Latin name “pes anserinus” translates literally to “goose’s foot,” inspired by the three tendons fanning out slightly as they meet the bone. This creates a three-pronged, webbed appearance. The tendon complex lies superficial to the tibial insertion of the medial collateral ligament (MCL), highlighting its role in medial knee support.
Between the bony surface of the tibia and the overlying tendons is a small, fluid-filled sac called the pes anserinus bursa. This bursa reduces friction, allowing the tendons to glide smoothly over the bone during movement. The close proximity of the tendons and the bursa means that inflammation in this area can involve one or both structures.
The Three Attaching Muscles
The pes anserinus is formed by the tendons of three distinct muscles: the Sartorius, the Gracilis, and the Semitendinosus. These muscles are often remembered collectively by the acronym “SGT.” Though they share a common insertion point, each muscle originates from a different section of the pelvis and belongs to a different compartment of the thigh.
The Sartorius muscle is the most superficial and originates from the anterior superior iliac spine on the front of the hip. The Gracilis muscle originates from the ischiopubic ramus, located in the groin and medial compartment of the thigh. The Semitendinosus muscle is one of the hamstrings from the posterior compartment, originating from the ischial tuberosity.
The arrangement of these three tendons at the insertion site is layered. The Sartorius tendon is the most anterior, the Gracilis tendon lies in the middle, and the Semitendinosus tendon is the most posterior. The tendons are supplied by three different major nerves of the lower extremity: the femoral nerve, the obturator nerve, and the sciatic nerve, respectively.
Combined Muscle Actions
The collective action of the Sartorius, Gracilis, and Semitendinosus muscles is important for the integrated function of the hip and knee joints. Their primary combined action at the knee is to initiate flexion. Working together, these muscles also contribute to internal rotation of the lower leg, particularly when the knee is already bent.
Beyond knee movement, the muscle group provides stability to the inner knee. They function as a secondary restraint against valgus forces, which push the knee inward toward the midline.
The muscles also have actions at the hip. The Sartorius acts to flex and laterally rotate the hip, while the Gracilis is a hip adductor, pulling the leg toward the body’s center. The Semitendinosus, as a hamstring muscle, also contributes to hip extension. Their shared insertion point ensures that movements originating from different parts of the hip are coordinated at the knee joint.
Common Conditions Affecting the Area
The pes anserinus region is a common site of inflammation, primarily manifesting as Pes Anserine Bursitis or Tendinitis. Pes Anserine Bursitis involves the inflammation of the bursa sac located directly beneath the tendon complex. Tendinitis, or tendinopathy, refers to irritation and breakdown of the three tendons themselves.
Patients with these conditions report pain and tenderness on the inner side of the knee, usually two to three inches below the joint line. The pain often develops gradually and is aggravated by activities that involve repetitive knee movement, such as climbing stairs, running, or rising from a seated position. The pain may also be noticeable at night.
Several factors predispose individuals to these inflammatory conditions. These risk factors include:
- Tight hamstring muscles, which increase tension on the tendons.
- Obesity.
- The presence of knee osteoarthritis.
- Abnormal knee alignment, such as a valgus or “knock-kneed” posture.
Runners and athletes involved in sports requiring side-to-side movements are also susceptible due to overuse and repetitive stress on the area.