The hip joint (also known as coxafemoral joint, acetabulofemoral joint, latin: articulatio coxae) is a ball and socket synovial joint, which is formed between the acetabulum and the head of the femur.
The articular surfaces of the hip joint are the following:
- articular surface of the femoral head (latin: facies articularis capitis femoris);
- lunate surface of the acetabulum of the pelvis (latin: facies lunata acetabuli).
The acetabulum is a cup-like depression on the inferolateral aspect of the pelvis, it is completed by a fibrocartilaginous collar - the acetabular lip or acetabular labrum, which joins at the external margin of the acetabulum.
The acetabulum and the head of the femur are covered in a joint capsule, that is fixed to the acetabular lip, anteriorly at the trochanteric line and posteriorly above the intertrochanteric crest.
There are two groups of ligaments that increase the stability of the hip joint: the intracapsular and the extracapsular ligaments.
The intracapsular ligaments are located in the hip joint cavity. There are two intracapsular ligaments of the hip joint:
- The transverse acetabular ligament (latin: lig. transversum acetabuli) joins the ends of the lunate surface of the acetabulum and passes over the acetabular notch.
- The ligament of the head of the femur (latin: lig. capitis femoris) is a small structure that runs from the acetabular fossa to the head of the femur.
The extracapsular ligaments are continuous with the outer surface of the hip joint capsule. There are four strong extracapsular ligaments:
- The iliofemoral ligament (latin: lig. iliofemorale) is a “Y” shaped ligament between the anterior inferior iliac spine and the intertrochanteric line of the femur. It is the strongest ligament in the human body that prevents external rotation and hyperextension of the hip joint.
- The pubofemoral ligament (latin: lig. pubofemorale) is a triangular shaped ligament between the superior pubic rami and the intertrochanteric line of the femur. This ligament prevents the excessive abduction and inner rotation of the thigh at the hip joint.
- The ischiofemoral ligament (latin: lig. ischiofemorale) extends from the body of the ischium to the trochanteric fossa. It prevents the thigh from excessive inner rotation and adduction at the hip joint.
- The capsular ligament (latin: lig. capsulare) extends from the annular joint, also known as the zona orbicularis, to the anterior inferior iliac spine. The zona orbicularis is a ligament on the neck of the femur which is formed by the circular fibers of the capsule of the hip joint.
The permitted movements at the hip joint are the following: flexion and extension, abduction and adduction, external or lateral rotation and internal or medial rotation of the thigh.