Anatomy of the hip joint

If you have ever wanted to know about the new ‘hip joint’ in town, look no further than the hip joint!

The hip joint is a large, strong joint connecting the pelvis to the lower limb. Let's take a closer look!

The hip joint is a synovial ball and socket joint, where the head of the femur forms approximately two thirds of a sphere, and it articulates with the cup-like acetabulum of the hip bone.

The femoral head is not entirely round, as it has depression on top of it which is called the fovea for the ligament of the head of the femur.

Except for the fovea, the femoral head is also covered entirely in articular cartilage which facilitates smooth movement and prevents bone erosion as it slides within the acetabulum.

The acetabulum, on the other hand, is a bowl like structure on the lateral aspect of the hip bone, and you might remember it is formed by the fusion of the ilium, ischium, and pubis.

The acetabulum is surrounded on the outside by a margin that’s incomplete inferiorly, where the acetabular notch is situated; this makes it look like a bowl with a broken rim.

On the outside of the acetabulum margin, there’s the acetabular labrum, where labrum is a fancy word for lip, which continues over the acetabular notch with the transverse acetabular ligament.

The labrum increases the surface area of the acetabulum to allow more than half of the femoral head to fit within the acetabulum for stability.

There’s also a rough depression in the floor of the acetabulum - the acetabular fossa, right above the notch.

These two structures, the acetabular notch and fossa, are surrounded by the thick and smooth lunate surface, the articular surface on which the head of the femur slides.

Just like most joints, the hip joint is enclosed within a strong joint capsule formed by an external fibrous layer, called the fibrous capsule, and an internal layer, called the synovial membrane.

The fibrous layer attaches proximally to the acetabulum peripheral to the acetabular margin, as well as to the transverse acetabular ligament.

Distally, it attaches to the femoral neck, anteriorly at the intertrochanteric line and root of the greater trochanter; and, posteriorly, the fibrous layer heads towards but doesn't attach to the intertrochanteric crest.

Instead, most of its fibers take a spiral course and attach to the intertrochanteric line of the femur, whereas some deep fibers pass circularly around the neck, forming the orbicular zone which may play a role in stability.

The fibrous layer is actually thicker in some places, where it forms the three intrinsic ligaments of the hip joint which pass in a spiral from the pelvis to the femur, a bit like curled pasta made of fibrous tissue.

Anteriorly and superiorly is the strong, Y-shaped iliofemoral ligament, which attaches to the anterior inferior iliac spine and the acetabular rim proximally, and the intertrochanteric line distally.

Considered the body’s strongest ligament, the iliofemoral ligament tightens and brings the femoral head tightly into the acetabulum when standing upright, and this prevents hyperextension of the hip joint.

Anteriorly and inferiorly, there’s the pubofemoral ligament, which arises from the obturator crest of the pubic bone and passes laterally and inferiorly to merge with the fibrous layer of the joint capsule and blends with the medial part of the iliofemoral ligament.

The pubofemoral ligament tightens during extension and abduction of the hip joint, preventing over-abduction.

And finally, posteriorly there’s the ischiofemoral ligament which goes from the acetabular margin to the neck of the femur.

This ligament spirals superolaterally to the femoral neck, medial to the base of the greater trochanter and it limits internal rotation of the hip.

The synovial layer, the other component of the joint capsule, lines the internal surfaces of the fibrous layer, as well as any intracapsular bony surface not lined with articular cartilage.

More specifically, it covers the neck of the femur between the attachment of the fibrous capsule and the edge of the articular cartilage of the head, as well as the nonarticular area of the acetabulum.

Speaking of which, the synovial membrane covering the femoral neck presents longitudinal synovial folds through which course the subsynovial retinacular arteries, which branch brom the medial and lateral circumflex femoral arteries, that supply the femoral head and neck.

The synovial layer also gives off the ligament of the head of the femur, also called the ligamentum teres, which houses the artery to the head of the femur, a branch of the obturator artery.

The ligament of the head of the femur attaches to the margins of the acetabular notch along with the transverse acetabular ligament, and to the fovea on the femoral head

The ligament lies within the acetabular fossa, cushioned by a fat pad that fills the part of the fossa not occupied by the ligament of the head of the femur. Quite cozy!

Ok, now that we know our hip joint players, let’s look at the hip joint movements, and identify the main muscles that produce them