Hip

The hip, medically called the coxa, is both a region and a joint on the outer side of the pelvis. This region sits to the side and in front of the buttocks, below the iliac crest, and beside the obturator foramen, with muscles and soft tissue covering the upper part of the thigh bone. In adults, the three pelvic bones—ilium, ischium, and pubis—have fused into a single hip bone that forms the deep, upper-inner wall of this area.

The hip joint, known scientifically as the acetabulofemoral joint, is a ball-and-socket connection between the pelvic acetabulum and the head of the femur. Its main job is to carry the weight of the upper body whether standing still or moving, and it is essential for keeping balance and controlling the tilt of the pelvis. Hip pain can come from many causes, including nerve problems, osteoarthritis, infection, injury, and genetic conditions.

The joint is formed where the cup-like acetabulum—made from the ilium, pubis, and ischium—meets the rounded head of the femur. The Y-shaped growth plate between these bones, called the triradiate cartilage, fuses completely between ages 14 and 16. This is a special type of ball-and-socket joint where the roughly spherical femoral head, with an average radius of curvature of 2.5 cm, sits mostly inside the acetabulum. The acetabulum holds nearly half of the femoral ball, and its grip is deepened by a ring of fibrocartilage called the acetabular labrum, which extends the joint past the equator. The center of the acetabulum, the fovea, does not touch the femur; instead, it is lined with a fat pad and attached to the ligamentum teres. The labrum is horseshoe-shaped, and its lower notch is bridged by the transverse acetabular ligament. The space between the femoral head and the upper acetabulum is normally 2 to 7 mm.

The head of the femur connects to the shaft via a thin neck, which is prone to fracture in older adults due to osteoporosis. The acetabulum faces downward, outward, and forward, while the femoral neck points upward, inward, and slightly forward.

Several angles describe the joint’s geometry. The acetabular angle (Sharp’s angle) is measured between a horizontal line through the lower points of the triradiate cartilages (Hilgenreiner’s line) and a line from the lower angle of that cartilage to the upper rim of the acetabulum. It is 35 degrees at birth, 25 degrees at one year, and under 10 degrees by age 15, ranging from 33 to 38 degrees in adults. The sagittal angle of the acetabular inlet is the angle between a line from the front to the back rim of the acetabulum and the sagittal plane; it is 7 degrees at birth and 17 degrees in adults. Wiberg’s center-edge (CE) angle is measured on an anteroposterior X-ray between a vertical line and a line from the center of the femoral head to the outermost part of the acetabulum. The vertical-center-anterior margin (VCA) angle is formed by a vertical line and a line from the femoral head’s center to the front edge of the dense subchondral bone just behind the acetabulum’s front edge, seen on a false-angle X-ray (a lateral view rotated 25 degrees toward the front). The articular cartilage (AC) angle, also called the acetabular index or Hilgenreiner angle, is the angle between the weight-bearing roof of the acetabulum and the horizontal plane, or between a line connecting the corner of the triangular cartilage and the lateral acetabular rim. In normal hips of children aged 11 to 24 months, it averages 20 degrees (range 18–25 degrees) and decreases with age. Cutoffs for an abnormally high angle are 30 degrees up to 4 months and 25 degrees up to 2 years.

The angle between the long axes of the femoral neck and shaft, called the caput-collum-diaphyseal (CCD) angle, is about 150 degrees in newborns and 126 degrees in adults (coxa norma). A smaller-than-normal angle is coxa vara, and a larger one is coxa valga. Because changes in the femur’s shape affect the knee, coxa valga often goes with genu varum (bow-leggedness), while coxa vara leads to genu valgum (knock-knees). Changes in the CCD angle come from altered stress patterns on the hip, such as from a dislocation, which change the trabecular patterns inside the bones. Two continuous trabecular systems emerge on the auricular surface of the pelvis.