The ligament of the head of the femur (round ligament of the femur, foveal ligament, or Fillmore's ligament) is a weak^[1] ligament located in the hip joint. It is triangular in shape and somewhat flattened. The ligament is implanted by its apex into the anterosuperior part of the fovea capitis femoris and its base is attached by two bands, one into either side of the acetabular notch, and between these bony attachments it blends with the transverse ligament.^[2]

The ligament becomes taught when the thigh is flexed and either adducted or laterally/externally rotated. The ligament is usually too weak to actually function as a ligament^[4] past childhood;^[5] excessive movement at the hip joint is instead primarily limited by the three capsular ligament of the hip joint.^[4] Nevertheless, more recent research suggests the ligament may have a number of functions, including a significant biomechanical role on the basis of cadaveric studies where increases of range of motion were seen after sectioning of the ligament.^[6]

It has been suggested that some animals, such as the orangutan and Indian elephant, lack a ligamentum teres.^[7][8] However, the presence of a ligamentum teres, albeit with a morphology different from the human version, has been found upon dissection in both these animals. In the orangutan, it is believed to play a significant role in preventing dislocation of the femoral head within extreme ranges of motion. In the Indian elephant, it is the primary support of the hip joint when the hind limbs are abducted.^[9]

• A human femur head with some synovium attached at the bottom and the ligament of the head of the femur attached at the top. A blue suture wire is drawn through the ligament. Ruler in centimetres at left side.
• Structures surrounding right hip-joint
• Hip joint. Lateral view. Ligament of head of femur