The hip joint (Articulatio coxae) is a synovial ball-and-socket joint connecting the head of the femur, the thigh bone, to the acetabulum, the socket of the hip bone. It lies deep where the thigh meets the pelvis. A synovial joint has a fluid-containing cavity that allows its surfaces to move smoothly.
Where is the hip joint located?
The hip joint lies below the lateral part of the pelvis, with its socket directed outwards, forwards and downwards. The femoral head fits into this socket, while the femoral neck connects the head to the shaft of the thigh bone.
The joint is deep to the groin at the front and the buttock muscles behind. The iliopsoas muscle passes across its front; gluteus minimus lies above and to its outer side. Short muscles that rotate the thigh outwards lie behind it.
The femoral nerve and major femoral blood vessels pass in front of the joint. The sciatic nerve passes behind it, separated from the capsule by intervening muscles. These relationships help distinguish the joint itself from the broader surface region called the hip.
Which surfaces form the hip joint?
The rounded femoral head meets the lunate surface of the acetabulum, a horseshoe-shaped region that bears weight. The lunate surface and the femoral head, except for its fovea, are covered by articular cartilage, a smooth layer of hyaline cartilage that reduces friction and distributes load.
The central acetabular fossa, a depression within the socket, does not form a cartilage-covered bearing surface. It contains fat and accommodates the ligament connecting the femoral head to the acetabulum; this ligament attaches to the margins of the acetabular notch and the transverse acetabular ligament.
The acetabular labrum is a rim of fibrocartilage, a tough cartilage containing abundant collagen fibres. It attaches around the acetabular margin, deepens the socket and helps maintain a seal around the femoral head.
At the lower margin, the transverse acetabular ligament bridges the acetabular notch, the gap in the bony rim. It continues the ring formed by the labrum across this gap.
What surrounds the hip joint capsule?
The articular capsule of hip joint is the sleeve enclosing the joint. Its outer fibrous layer provides mechanical support, while its inner synovial membrane produces synovial fluid, which lubricates the moving surfaces.
The capsule attaches around the acetabular margin and transverse acetabular ligament. On the femur, it reaches the intertrochanteric line anteriorly, a ridge between the upper femoral projections, but attaches to the femoral neck posteriorly, short of the intertrochanteric crest.
The zona orbicularis consists of circular fibres within the capsule that surround the femoral neck. These fibres help resist separation of the femoral head from the socket. The capsule becomes tighter during extension and more relaxed during flexion.
Which ligaments stabilise the hip joint?
A ligament is a strong band of fibrous connective tissue. The main capsular ligaments reinforce the capsule and limit excessive movement, particularly when the hip straightens during standing.
The iliofemoral ligament lies at the front of the joint and has a transverse part of iliofemoral ligament and a descending part of iliofemoral ligament. Together, these form its characteristic Y-shaped arrangement.
| Ligament | Position and attachments | Main anatomical role |
|---|---|---|
| Iliofemoral ligament | From the anterior inferior iliac spine and adjacent acetabular margin to the intertrochanteric line | Limits excessive extension and supports upright standing |
| Pubofemoral ligament | From the pubic region of the acetabular margin into the lower capsule | Limits excessive movement away from the midline and extension |
| Ischiofemoral ligament | From the posterior acetabular margin, spiralling around the femoral neck | Reinforces the posterior capsule and limits extension and inward rotation |
| Ligament of head of femur | From the acetabular notch region to a small pit on the femoral head | May carry a small arterial branch to the femoral head |
The ligament of head of femur lies within the fibrous capsule but is covered by synovial membrane. It is not a principal weight-bearing restraint; the socket, labrum, capsule and surrounding muscles provide the main stability.
What movements does the hip joint allow?
The ball-and-socket shape allows movement in several directions. Its depth and strong supporting tissues balance mobility with the stability needed to carry body weight.
- Flexion: moving the thigh forwards towards the trunk.
- Extension: moving the thigh backwards, or straightening it from a flexed position.
- Abduction: moving the thigh away from the body's midline.
- Adduction: moving the thigh towards the midline.
- Medial rotation: turning the thigh inwards.
- Lateral rotation: turning the thigh outwards.
- Circumduction: combining flexion, abduction, extension and adduction so that the thigh traces a cone-shaped path.
During standing and walking, the hip transfers weight between the pelvis and femur. Muscles surrounding the joint also control pelvic position, particularly when one lower limb supports the body.
What supplies the hip joint with blood and nerves?
Branches of the medial and lateral circumflex femoral arteries supply the joint. The medial circumflex femoral artery is particularly important for the femoral head. Retinacular vessels, small vessels running along the femoral neck beneath synovial folds, carry much of this supply.
Branches of the superior and inferior gluteal arteries also contribute. A small branch of the obturator artery may reach the femoral head through its ligament.
The capsule receives sensory branches from the femoral nerve, obturator nerve, superior gluteal nerve and nerve to quadratus femoris. The accessory obturator nerve, when present, may also contribute. These branches convey sensation from the joint, including information about its position.
How can you study the hip joint in the 3D model?
In the 3D model beside the article, rotate and zoom to explore different viewing angles, then select a part. Switch layers, hide or fade parts, or isolate a selection to change the view. You can also pull parts apart or cut a section along the sagittal, coronal or transverse plane, then rotate and zoom to compare the resulting views.