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 membr,ane 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.
For example, flexion is produced by the iliopsoas, sartorius, rectus femoris, and pectineus.
Extension is produced by gluteus maximus and the hamstrings, and it’s limited by the
tightening of the iliofemoral ligament.
Abduction is carried out by gluteus minimus and medius, tensor fascia lata, and sartorius;
adduction is a job for the adductor longus, brevis, and magnus and for the pectineus and
gracilis muscles; medial rotation is achieved with a little help from gluteus minimus and
medius and the tensor fascia lata; and finally, lateral rotation is partly produced by the
short lateral rotator group - piriformis, obturator externus, obturator internus (and
associated gemelli), and quadratus femoris - but primarily by the gluteus maximus.
The hip joint is also capable of circumduction - which is a combination of all of the movements
mentioned earlier.
Now, all these movements wouldn’t be possible without arteries nourishing the hip joint,
of course!
The large arteries in this area are the medial and lateral circumflex femoral arteries.
The medial circumflex femoral artery is a branch of the profunda femoris artery.
It passes posteriorly and gives off the transverse and ascending branches.
The transverse branch takes part in the formation of cruciate anastomosis which is situated
at the upper part of the back of the femur at the level of the lesser trochanter, whereas
the ascending branch passes to the trochanteric fossa, where it takes part in the formation
of the trochanteric anastomosis.
It also gives off posterior retinacular branches, which supply the head and neck of the femur.
The lateral circumflex femoral artery is the largest branch of the profunda femoris artery.
It runs laterally and divides into ascending, transverse, and descending branches.
The ascending branch contributes to the trochanteric anastomosis and the transverse branch wraps
around the femur posteriorly to take part in the cruciate anastomosis.
The descending branch runs down along the anterior border of the vastus lateralis, which
it supplies and perforates to anastomose with a branch of the popliteal artery.
The trochanteric anastomosis is also joined by branches from the inferior gluteal artery
and superior gluteal artery, while the cruciate anastomosis is also joined by a branch from
the first perforating artery and by a branch of the inferior gluteal artery.
Now let’s look at the retinacular arteries in more detail.
Generally speaking, these arteries run along the neck of the femur and through the retinaculum
of the capsule, hence their name.
Retinacular arteries arising from the medial circumflex femoral artery are more numerous
and supply most of the head and neck of the femur because they are able to pass beneath
the unattached posterior border of the joint capsule.
Retinacular arteries arising from the lateral circumflex femoral penetrate the thick iliofemoral
ligament, there’s fewer of them, and they are smaller.
And one last important blood supply is the artery to the head of the femur, which is
a branch of the obturator artery, varies in size, and it traverses the ligament of the
head of the femur.
When it comes to nerve supply, the hip abides by the Hilton’s law, which states that the
nerves supplying the muscles crossing a joint also innervate that said joint, usually through
direct articular branches.
So if you know the muscles that cross a specific joint, and the nerves that supply those muscles,
you can deduce the nerve supply of the joint.
For example, the hip joint is crossed anteriorly by the flexors of the hip, which are innervated
by the femoral nerve, so the anterior aspect of the hip joint is innervated by the femoral
nerve.
The rest of the hip joint is innervated by the sciatic, obturator, and superior gluteal
nerve.
That is what we call information overload!
Because we don’t want you to strain any brain muscle, let’s take a break and see
if you can identify some of the most important elements of the hip joint!
All right, as a quick recap… the hip joint is a ball and socket type of synovial joint
which is enclosed within a joint capsule formed by a fibrous and a synovial layer.
The fibrous layer of the joint capsule gives off three intrinsic ligaments of the hip joint:
the iliofemoral ligament, the pubofemoral ligament, and the ischiofemoral ligament.
The synovial layer gives off the ligament of the head of the femur, which conducts the
artery to the head of the femur.
Hip joint movements include flexion, extension, adduction, abduction, internal and external
rotation.