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The hip joint is a ball and socket joint, formed by the head of the Femur (thigh bone) and the acetabulum of the pelvis. The dome-shaped head of the femur forms the ball, which fits snuggly into the concave socket of the acetabulum. The hip joint is a very sturdy joint, due to the tight fitting of the bones and the strong surrounding ligaments and muscles.
Bones of the hip joint
The femur is the longest bone in the body which forms the thigh. The part which articulates with the pelvis to form the hip joint is known as the head of the femur. This is a round, dome shaped protrusion. Close to the top of the femur are two other protrusions, known as the greater and lesser trochanters. The main function of the trochanters is for muscle attachment.
The pelvis is actually two large bones which connect at the front by the pubis symphesis (a cartilage disc) and at the back by the Sacrum. The Sacrum is part of the spine and consists of 4 fused vertebrae which do not move independently of one another. The joints formed by either side of the Sacrum and the two pelvic bones are the Sacroiliac joints (SIJ).
The surfaces of both the head of the femur and the acetabulum are covered with a thin layer of hyaline cartilage which acts to allow smooth movement of the joint.
The hip joint capsule
The joint capsule is a thick ligamentous structure with circular and longitudinal fibres that surround the entire joint. Inside this capsule is a specialised membrane known as the synovial membrane which provides nourishment to all the surrounding structures.
Ligaments of the hip joint
The stability of the hip owes greatly to the presence of its ligaments.
- Iliofemoral ligament: This is a strong ligament which connects the pelvis to the femur at the front of the joint. It resembles a Y in shape and stabilises the hip by limiting hyperextension
- Pubofemoral ligament: The pubofemoral ligament attaches the part of the pelvis known as the pubis (most forward part, either side of the pubic symphesis) to the femur
- Ischiofemoral ligament: This is a ligament which reinforces the posterior aspect of the capsule, attaching to the ischium and between the two trochanters of the femur.
Labrum of the hip joint
Just like the ball and socket joint of the shoulder, the hip joint has a labrum. This is a circular layer of cartilage which surrounds the outer part of the acetabulum effectively making the socket deeper and so helping provide more stability. Labrum tears are a common injury to the hip joint.
Muscle Groups surrounding the hip joint
There are numerous muscles which attach to or cover the hip joint:
- Gluteals: Gluteus Maximus, Gluteus Minimus and Gluteus Medius are the three muscles referred to as the gluteals. They all attach to the posterior surface of the large flat area of the pelvis (Ilium) and travel laterally to insert into the greater trochanter of the femur. Medius and Minimus are responsible for abducting and medially rotating the hip joint, as well as stabilising the pelvis. Gluteus maximus extends and laterally rotates the hip joint.
- Quadriceps: The four Quadricep muscles (Vastus lateralis, medialis, intermedius and Rectus femoris) all attach inferiorly to the tibial tuberosity of the shin. Rectus femoris originates at the Anterior Inferior Iliac Spine (AIIS - protrusion at the front of the ilium) and acts to flex the hip. The 3 other Quad muscles do not cross the hip joint, and attach around the greater trochanter and just below it.
- Iliopsoas: The is the primary hip flexor muscle which consists of 3 parts. Together they attach superiorly to the lower part of the spine and the inside of the ilium (flat upper part of the pelvis). They then cross the hip joint and insert to the lesser trochanter of the femur.
- Hamstrings: The hamstrings are three muscles which form the back of the thigh. They all attach superiorly to the ischial tuberosity (lowest part of the pelvis, sometimes referred to as the sitting bone!) and cause hip extension.
- Groin muscles: There are three main groin muscles, which are anatomically termed the adductor muscles. They all attach superiorly to the pubis and travel down the inside of the thigh. Their action is hip adduction.