The hip joint (acetabulofemoral joint) is the joint between the femur and acetabulum of the pelvis, and its primary function is to support the body's weight in static (eg, standing) and dynamic (eg, walking or running) postures. Together, the right and left hip joints are crucial in retaining balance. The pelvic inclination angle, which is the single most important element of human body posture, is adjusted at the hips.
Proximally, the femur is largely covered by muscles and, as a consequence, the greater trochanter is often the only palpable bony structure. Distally on the femur, other palpable bony structures are the condyles.
The hip joint is a synovial joint formed by the articulation of the rounded head of the femur and the cup-like acetabulum of the pelvis. It forms the primary connection between the bones of the lower limb and the axial skeleton of the trunk and pelvis. Both joint surfaces are covered with a strong but lubricated layer called articular hyaline cartilage.
The cup-like acetabulum forms at the union of three pelvic bones — the ilium, the pubis, and the ischium. 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 augmented 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 anteriorly.
- The transverse angle of the acetabular inlet (also called Sharp's angle and is generally the angle referred to by acetabular angle without further specification) can be determined by measuring the angle between a line passing from the superior to the inferior acetabular rim and the horizontal plane. This angle, which normally measures 51 degrees at birth and 40 degrees in adults, affects the acetabular lateral coverage of the femoral head and several other parameters.
- 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 degrees at birth and increases to 17 degrees in adults.
- Wiberg's center-edge angle (CE angle) is an angle between a vertical line and a line from the center of the femoral head to the most lateral part of the acetabulum, as seen on an anteroposterior radiograph.
- The vertical-center-anterior margin angle (VCA) is an angle formed from a vertical line (V) and a line from the center 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.
- The articular cartilage angle (AC angle, also called Hilgenreiner angle) is an angle formed parallel to the weight bearing dome (the acetabular sourcil) and the horizontal plane, or a line connecting the corner of the triangular cartilage and the lateral acetabular rim.
Femoral Neck Angle
The angle between the longitudinal axes of the femoral neck and shaft, called the caput-collum-diaphyseal angle (CCD angle), normally measures approximately 150 degrees in newborns and 126 degrees in adults (coxa norma). 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).
Changes in CCD angle are the result of changes in the stress patterns applied to the hip joint. Such changes, caused for example by a dislocation, change the trabecular patterns inside the bones. Two continuous trabecular systems emerging on auricular surface of the sacroiliac joint meander and criss-cross each other down through the hip bone, the femoral head, neck, and shaft.
- In the hip bone, one system arises on the upper part of auricular surface to converge onto the posterior surface of the greater sciatic notch, from where its trabeculae are reflected to the inferior part of the acetabulum. The other system emerges on the lower part of the auricular surface, converges at the level of the superior gluteal line, and is reflected laterally onto the upper part of the acetabulum.
- In the femur, the first system lines up with a system arising from the lateral part of the femoral shaft to stretch to the inferior portion of the femoral neck and head. The other system lines up with a system in the femur stretching from the medial part of the femoral shaft to the superior part of the femoral head.
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 reduce the bending loads on the proximal part of the femur.
The capsule attaches to the hip bone outside the acetabular lip which thus projects into the capsular space. On the femoral side, the distance between the head's cartilaginous rim and the capsular attachment at the base of the neck is constant, which leaves a wider extracapsular part of the neck at the back than at the front. The strong but loose fibrous capsule of the hip joint permits the hip joint to have the second largest range of movement (second only to the shoulder) and yet support the weight of the body, arms and head.
The capsule has two sets of fibers: longitudinal and circular.
- The circular fibers form a collar around the femoral neck and are called the zona orbicularis.
- The longitudinal retinacular fibers travel along the neck and carry blood vessels.
The hip joint is reinforced by five ligaments, of which four are extracapsular and one intracapsular.
Extracapsular ligaments – Anterior (left) and posterior (right) aspects of right hip
The extracapsular ligaments are the iliofemoral, ischiofemoral, and pubofemoral ligaments attached to the bones of the pelvis (the ilium, ischium, and pubis respectively). All three strengthen the capsule and prevent an excessive range of movement in the joint.
- The Y-shaped and twisted iliofemoral ligament is the strongest ligament in the human body. In the upright position, it prevents the trunk from falling backward without the need for muscular activity. In the sitting position, it becomes relaxed, thus permitting the pelvis to tilt backward into its sitting position. The iliofemoral ligament prevents excessive adduction and internal rotation of the hip.
- The ischiofemoral ligament prevents medial (internal) rotation, while the pubofemoral ligament restricts abduction and internal rotation of the hip joint.
- The zona orbicularis, which lies like a collar around the most narrow part of the femoral neck, is covered by the other extracapsular 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.
Intracapsular ligament – Left hip joint from within pelvis with acetabular floor removed (left); right hip joint with capsule removed, anterior aspect (right)
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. It is not that important as a ligament but can often be vitally important as a conduit of a small artery to the head of the femur. This arterial branch 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.
The hip joint is supplied with blood from the medial circumflex femoral and lateral circumflex femoral arteries, which are 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 some contribution from a small artery in the ligament of the head of the femur, which is a branch of the posterior division of the obturator artery. It becomes important in avoiding avascular necrosis of the head of the femur when the blood supply from the medial and lateral circumflex arteries are disrupted (eg, through fracture of the neck of the femur along their course).
The hip has two anatomically important anastomoses, the cruciate and the trochanteric anastomoses, the latter of which provides most of the blood to the head of the femur. These anastomoses exist between the femoral artery or profunda femoris and the gluteal vessels.
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)
- Abduction and adduction around a sagittal axis (forward-backward)
It should also be noted that:
- Some hip muscles also act on either the vertebral joints or the knee joint
- With their extensive areas of origin and/or insertion, different part of individual muscles participate in very different movements
- The range of movement varies with the position of the hip joint
Additionally, the inferior and superior gemelli may be termed triceps coxae together with the obturator internus, and their function simply is to assist the latter muscle.
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 degrees with the hip extended, 50 degrees 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, which is why the hip can rotate laterally to a greater degree when it is flexed.
- Medial or internal rotation (40 degrees) - Anterior fibers of gluteus medius and minimus, tensor fascia latae, the part of adductor magnus inserted into the adductor tubercle, and, with the leg abducted, the pectineus.
- Extension or retroversion (20 degrees) - 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 degrees) - Hip flexors: Iliopsoas (with psoas major from vertebral column) and tensor fascia 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 degrees with hip extended, 80 degrees with hip flexed) - Gluteus medius, tensor fascia latae, gluteus maximus with it 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 degrees with hip extended, 20 degrees 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.
Note: Top image of hip bones by Stephen Woods