Region The hip joint, also known as a ball and socket joint, is formed by the acetabulum of the pelvis and the femoral head, which is the top portion of the thigh bone (femur). It allows for a wide range of movement and stability in the lower body. The proximal femur is largely covered by muscles and, as a consequence, the
greater trochanter is often the only palpable bony structure in the hip region.
Articulation The
hip joint or
coxofemoral joint is a ball and socket
synovial joint formed by the articulation of the rounded
head of the femur and the cup-like
acetabulum of the pelvis. The socket of the acetabulum is pointing downwards and anterolaterally. The socket is also turned such that the outer edge of its roof is more lateral than outer edge of the floor. The Y-shaped growth plate that separates them, the
triradiate cartilage, is fused definitively at ages 14–16. It is a special type of spheroidal or
ball and socket joint where the roughly spherical femoral head is largely contained within the acetabulum and has an average radius of curvature of 2.5 cm. The acetabulum grasps almost half the femoral ball, a grip deepened by a ring-shaped fibrocartilaginous lip, the
acetabular labrum, which extends the joint beyond the equator. The head of the femur is attached to the shaft by a thin neck region that is often prone to fracture in the elderly, which is mainly due to the degenerative effects of
osteoporosis. The acetabulum is oriented inferiorly, laterally and anteriorly, while the femoral neck is directed superiorly, medially, and slightly anteriorly.
Articular angles Acetabular angle (or Sharp's angle) is the angle between the horizontal line passing through the inferior aspects of
triradiate cartilages (
Hilgenreiner's line) and another line passing through the inferior angle of triradiate cartilage to superior acetabular rim. The angle measures 35 degrees at birth, 25 degrees at one year of age, and less than 10 degrees by 15 years of age. In adults the angle can vary from 33 to 38 degrees. The
sagittal angle of the acetabular inlet is an angle between a line passing from the anterior to the posterior acetabular rim and the sagittal plane. It measures 7° at birth and increases to 17° in adults. as seen on an
anteroposterior radiograph. The
vertical-centre-anterior margin angle (VCA) is an angle formed from a vertical line (V) and a line from the centre of the femoral head (C) and the anterior (A) edge of the dense shadow of the subchondral bone slightly posterior to the anterior edge of the acetabulum, with the radiograph being taken from the
false angle, that is, a lateral view rotated 25 degrees towards becoming frontal. or Hilgenreiner angle) is an angle formed parallel to the weight bearing dome, that is, the acetabular
sourcil or "roof", and the horizontal plane, In normal hips in children aged between 11 and 24 months, it has been estimated to be on average 20°, ranging between 18° and 25°. It becomes progressively lower with age. Suggested
cutoff values to classify the angle as abnormally increased include: :*30° up to 4 months of age. :*25° up to 2 years of age. An abnormally small angle is known as
coxa vara and an abnormally large angle as
coxa valga. Because changes in shape of the femur naturally affects the knee,
coxa valga is often combined with
genu varum (bow-leggedness), while
coxa vara leads to
genu valgum (knock-knees). On the lateral side of the hip joint the
fascia lata is strengthened to form the
iliotibial tract which functions as a tension band and reduces the bending loads on the proximal part of the femur. Iliofemoral ligament is a thickening of the anterior capsule extending from
anterior inferior iliac spine to
intertrochanteric line. The
zona orbicularis, which lies like a collar around the most narrow part of the
femoral neck, is covered by the other ligaments which partly radiate into it. The zona orbicularis acts like a buttonhole on the femoral head and assists in maintaining the contact in the joint. The
intracapsular ligament, the
ligamentum teres, is attached to a depression in the acetabulum (the acetabular notch) and a depression on the femoral head (the fovea of the head). It is only stretched when the hip is dislocated, and may then prevent further displacement. This artery is not present in everyone but can become the only blood supply to the bone in the head of the femur when the neck of the femur is fractured or disrupted by injury in childhood.
Blood supply The hip joint is supplied with blood from the
medial circumflex femoral and
lateral circumflex femoral arteries, which are both usually branches of the
deep artery of the thigh (profunda femoris), but there are numerous variations and one or both may also arise directly from the
femoral artery. There is also a small contribution from the foveal artery, a small vessel in the ligament of the head of the femur which is a branch of the
posterior division of the obturator artery, which becomes important to avoid
avascular necrosis of the
head of the femur when the blood supply from the medial and lateral circumflex arteries are disrupted (e.g. through fracture of the neck of the femur along their course).
Muscles and movements The hip muscles act on three mutually perpendicular main axes, all of which pass through the center of the
femoral head, resulting in three
degrees of freedom and three pair of principal directions:
Flexion and
extension around a transverse axis (left-right);
lateral rotation and
medial rotation around a longitudinal axis (along the thigh); and
abduction and
adduction around a sagittal axis (forward-backward); and a combination of these movements (i.e.
circumduction, a compound movement in which the leg describes the surface of an irregular cone). The movements of the hip joint is thus performed by a series of muscles which are here presented in order of importance with the range of motion from the neutral zero-degree position indicated: •
Lateral or
external rotation (30° with the hip extended, 50° with the hip flexed):
gluteus maximus;
quadratus femoris;
obturator internus; dorsal fibers of
gluteus medius and
minimus;
iliopsoas (including
psoas major from the vertebral column);
obturator externus;
adductor magnus,
longus,
brevis, and
minimus;
piriformis; and
sartorius. The
iliofemoral ligament inhibits lateral rotation and extension, this is why the hip can rotate laterally to a greater degree when it is flexed. •
Medial or
internal rotation (40°): anterior fibers of
gluteus medius and
minimus;
tensor fasciae latae; the part of
adductor magnus inserted into the
adductor tubercle; and, with the leg abducted also the
pectineus. •
Extension or
retroversion (20°):
gluteus maximus (if put out of action, active standing from a sitting position is not possible, but standing and walking on a flat surface is); dorsal fibers of
gluteus medius and
minimus;
adductor magnus; and
piriformis. Additionally, the following thigh muscles extend the hip:
semimembranosus,
semitendinosus, and long head of
biceps femoris. Maximal extension is inhibited by the
iliofemoral ligament. •
Flexion or
anteversion (140°): the
hip flexors:
iliopsoas (with psoas major from vertebral column);
tensor fasciae latae,
pectineus,
adductor longus,
adductor brevis, and
gracilis. Thigh muscles acting as hip flexors:
rectus femoris and
sartorius. Maximal flexion is inhibited by the thigh coming in contact with the chest. •
Abduction (50° with hip extended, 80° with hip flexed):
gluteus medius;
tensor fasciae latae;
gluteus maximus with its attachment at the
fascia lata;
gluteus minimus;
piriformis; and
obturator internus. Maximal abduction is inhibited by the neck of the femur coming into contact with the lateral pelvis. When the hips are flexed, this delays the impingement until a greater angle. •
Adduction (30° with hip extended, 20° with hip flexed):
adductor magnus with
adductor minimus;
adductor longus,
adductor brevis,
gluteus maximus with its attachment at the
gluteal tuberosity;
gracilis (extends to the tibia);
pectineus,
quadratus femoris; and
obturator externus. Of the thigh muscles,
semitendinosus is especially involved in hip adduction. Maximal adduction is impeded by the thighs coming into contact with one another. This can be avoided by abducting the opposite leg, or having the legs alternately flexed/extended at the hip so they travel in different planes and do not intersect. ==Clinical significance==