The innominate (hip) bone

The irregularly shaped innominate consists of three bones fused together: the ilium, the pubis and the ischium.

 

The ilium

The ilium is the upper broad blade for the attachment of ligaments and large muscles. It forms the pelvic brim between the hip joint and the articulation with the sacrum. The anterior two-thirds of the projecting ilium forms the iliac fossa medially, which is part of the lateral and posterior abdominal wall, and the gluteal surface laterally, for attachment of the gluteal muscles. The posterior one-third of the medial surface, which is thicker, carries the auricular surface for the articulation with the sacrum; behind this is a prolonged rough part, the iliac tuberosity, for the attachment of the strong sacroiliac ligaments which bear the body weight. The upper border of the ilium is the iliac crest which is convex superiorly as well as being curved anteroposteriorly with the anterior part curved outwards. The iliac crest ends anteriorly as the anterior superior iliac spine and posteriorly as the posterior superior iliac spine. Both spines and the whole of the crest can be palpated. Behind the anterior spine on the outer border is the prominent tubercle of the crest. Below the anterior superior spine is the anterior inferior iliac spine separated by a shallow notch. Similarly, below the posterior superior spine is the posterior inferior iliac spine, again separated by a shallow notch, the two posterior spines being closer together than the two anterior spines. Below the posterior inferior spine is the deep greater sciatic notch. The iliac fossa is separated from the sacropelvic surface of the ilium by the arcuate line which forms part of the pelvic brim. The anterior part of this line has an elevation, the iliopubic eminence, marking the junction of the ilium with the pubis. The part of the ilium which participates in the formation of the acetabulum is the body of the ilium.

The outer gluteal surface of the ilium follows the curvature of the iliac crest. It shows three curved gluteal lines which demarcate the attachments of the gluteal muscles. The most obvious of these lines is the posterior gluteal line, which passes down from the iliac crest to the front of the posterior inferior spine. The anterior gluteal line is a series of low tubercles coming from the iliac crest curving upwards and backwards below the iliac tubercle and then down towards the greater sciatic notch. The inferior gluteal line is less prominent, curving from below the anterior superior iliac spine towards the apex of the greater sciatic notch. Below the inferior gluteal line is an area of multiple vascular foramina. The fusion of the ilium and the ischium is marked by a rounded elevation between the acetabulum and the greater sciatic notch. Above this the ilium forms the major part of the notch. The gluteal surface is succeeded inferiorly by the acetabular portion of the ilium.

The iliac fossa is the smooth internal concavity of the ala of the ilium. It becomes narrower inferiorly ending at the roughened iliopubic eminence, the line of junction between the ilium and the pubis. Its deepest part, high in the fossa, is composed of paperthin translucent bone. The pelvic brim, marked by the arcuate line of the ilium, is the posterior – inferior limit of the iliac fossa. Behind and below the iliac fossa and the arcuate line is the sacropelvic surface of the ilium. Posterior to the iliac fossa, this region exhibits the auricular surface for articulation with the first two segments of the sacrum and, behind and above it, the tuberosity. The roughened tuberosity provides attachment for the short posterior sacroiliac ligaments and for fibres of erector spinae and multifidus muscles. The auricular area extends from the pelvic brim to the posterior inferior iliac spine. Its surface is gently undulating, being convex above to concave below, and roughened by numerous tubercles and depressions. The surface is covered with hyaline cartilage forming a synovial joint, which is immobile, with the ala of the sacrum. In later years, fibrous bands often joint the articular surfaces within the joint space.

The pubis

The pubis is an angulated bone. The body of the pubis projects laterally and superiorly as the superior ramus to join the ilium and ischium at the acetabulum, forming one fifth of the acetabulum. A thin and flattened inferior ramus extends inferiorly and posterolaterally from the body to fuse with the ischium below the obturator foramen. The body is quadrilateral in shape, with the symphyseal surface being oval; it is crossed by several transverse ridges to which the fibrocartilage of the symphysis pubis is attached, and coated with hyaline cartilage for the secondary cartilaginous joint that constitutes the symphysis pubis. The upper border of the body is the pubic crest, being marked laterally by the pubic tubercle. From this tubercle two ridges diverage laterally into the superior ramus. The upper of these two ridges is the pectineal line, which is continuous with the arcuate line of the ilium, and forms part of the pelvic brim. The lower, rounded ridge is the obturator crest, passing downwards into the anterior margin of the acetabular notch. Between these two ridges is the iliopubic eminence. Below the obturator crest on the superior pubic ramus is the deep obturator groove.

The ischium

The ischium is the posterior inferior part of the innominate, being angulated and in the same plane as the pubis. The apex of the angulation is blunt and rounded, forming the ischial tuberosity, divided transversely by a low ridge. A smooth oval above this ridge is further sub-divided by a vertical ridge into lateral and medial areas. In the sitting positon, the weight of the body rests on the two ischial tuberosities. Anteriorly the tuberosity passes upwards as the ischial ramus, being continuous with the inferior pubic ramus, forming the ischiopubic ramus. The body of the ischium forms two-fifths of the acetabulum. The posterior border of the body is continuous above with the ilium, forming the greater sciatic notch. Inferiorly this border ends as the blunt medially projecting ischial spine, below which is the groove forming the lesser sciatic notch. The pelvic surface of the body is continuous with the pelvic surface of the ilium, forming part of the lateral wall of the pelvis.

The acetabulum is formed by the fusion of the three component bones of the innominate; the ilium, ischium and pubis meet at a Y-shaped cartilage which forms their epiphyseal junction. The anterior one-fifth of the acetabulum is formed by the pubis, the superior posterior two-fifths by the body of the ilium, and the inferior posterior two-fifths by the body of the ischium. The acetabulum is a hemispherical hollow on the outer surface of the innominate, facing downwards, forwards and laterally. The prominent rim of the acetabulum is deficient inferiorly forming the acetabular notch. The rim gives attachment to the acetabular labrum of the hip joint, its uneven internal edge provides an attachment for the synovial membrane of the joint. The acetabular labrum continues across the acetabular notch to produce the transverse ligament.  The transverse ligament and margins of the notch give attachment to the ligament of the head of the femur. The heavy wall of the acetabulum consists of a semilunar articular portion covered with hyaline cartilage, open below, and a deep, central non-articular portion, the acetabular fossa. The acetabular fossa is formed mainly from the ischium, and its wall is frequently thin.

The obturator foramen is a large aperture ringed by the sharp margins of the pubis and ischium, those of the pubis overlapping each other in a spiral forming the obturator groove, which runs obliquely forwards and downwards from the pelvis into the thigh, being converted into a canal by a specialization of the obturator fascia. The obturator membrane is attached to the margins of the foramen, except superiorly at the obturator groove.

Ossification

Each innominate bone ossifies from eight centres: three primary centres, one each for the ilium, ischium and pubis; and five secondary centres, one each for the iliac crest, the anterior inferior iliac spine, the ischial tuberosity, the pubic symphysis and the triradiate cartilage at the centre of the acetabulum. The sequence of ossification has functional significance because of the support given to the pelvic organs and its weight transmission role. The primary ossification centres appear during the third, fourth and fifth months of development. At birth the ilium, ischium and pubis are quite separate and the secondary ossification centres have not yet appeared. By the age of thirtee or fourteen, the major parts of the ilium, ischium and pubis are completely bony but are still separated by the Y-shaped triradiate cartilage in the acetabulum. At the age of eight or nine, three major centres of ossification appear in the acetabular cartilage. The largest appears in the anterior wall of the acetabulum and fuses with the pubis. Further centres appear in the iliac acetabular cartilage superiorly which fuses with the ilium, and in the ischial acetabular cartilage posteriorly which fuses with the ischium. Fusion of the three bones in the acetabulum occurs between sixteen and eighteen years. The other secondary ossification centres appear at about puberty and unite with the major bones between twenty and twenty-two.

Palpation

The anterior superior iliac spines can easily be palpated in the living subject, particularly in females where they tend to be further apart. They are situated at the anterior end of the iliac crest, being found in the upper part of the pocket area. They act as an excellent landmark for many of the surface markings in this region. Tracing backwards from these spines, the iliac crest is easily palpable, having a large tuberosity about 5cm from its anterior end. If this bony crest is followed as far back as possible, the rather smaller posterior superior iliac spines can be felt. Each is situated in a dimple in the female subject, whereas in the male each spine appears as a small raised tubercle.

About 10cm below the centre of the iliac crest, the greater trochanter of the femur can be clearly felt. Hip measurements, when buying trousers or a skirt, are calculated erroneously around these two trochanters. In sitting, the body rests on the ischial tuberosity of each innominate bone; if the hands are placed under this area the tuberosites can be easily felt. This part of the tuberosity is covered with a bursa which often becomes painful and swollen when sitting for too long on a hard surface; this is termed a bursitis.

If the hands are drawn down the front of the abdominal wall, about 5cm above the genitalia, a bony ring can be felt. This has a central depression where the pubic symphysis is situated with each pubic tubercle about 1cm above and lateral on either side.