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The coxal (innominate) bone or hip-bone (os coxae, French: os iliaque or os coxal)  is a large, irregularly shaped bone articulated behind 'with the sacrum, and in front with its fellow of the opposite side, the two bones forming the anterior and side walls of the pelvis. The coxal bone consists of three parts, named ilium, ischium, and pubis, which, though separate in early life, are firmly united in the adult. The three parts meet together and form the acetabulum (or cotyloid fossa), a large, cup-like socket situated near the middle of the lateral surface of the bone for articulation with the head of the femur.

The ilium [os ilium] is the upper expanded portion of the bone, and by its inferior extremity forms the upper two-fifths of the acetabulum. It presents for examination three borders and two surfaces.

Borders

When viewed from above, the thick crest [crista iliaca] or superior border of the coxal bone is curved somewhat like the letter S, being concave medially in front and concave laterally behind. Its anterior extremity forms the anterior superior iliac spine, which gives attachment to the inguinal (Poupart's) ligament and the sartorius; the posterior extremity forms the posterior superior iliac spine and affords attachment to the sacro-tuberous (great sacro-sciatic) ligament, the posterior sacro-iliac ligament, and the multifidus. The crest is narrow in the middle, thick at its extremities, and may be divided into an inner lip, an outer lip, and an intermediate line. About two and a half inches from the anterior superior spine is a prominent tubercle on its external lip.

The external lip of the crest gives attachment in front to the tensor fascia lata; along its whole length, to the fascia lata; along its anterior half to the external oblique; and behind this, for about an inch, to the latissimus dorsi. The anterior two-thirds of the intermediate line gives origin to the internal oblique. The internal lip gives origin, by its anterior two-thirds, to the transversus; behind this is a small area for the quadratus lumborum, and the remainder is occupied by the sacro-spinalis (erector spince). The internal lip, in the anterior two-thirds, also serves for the attachment of the iliac fascia.

The anterior border of the ilium extends from the anterior superior iliac spine to the margin of the acetabulum. Below the spine is a prominent notch from which fibers of the sartorius arise, and this is succeeded by the anterior inferior iliac spine, smaller and less prominent than the superior, to which the straight head of the rectus and the ilio-femoral ligament are attached. On the medial side of the anterior inferior spine is a broad, shallow groove for the ilio-psoas as it passes from the abdomen into the thigh, limited below by the ilio-pectineal eminence, which indicates the point of union of the ilium and pubis.

The posterior border of the ilium presents the posterior superior iliac spine, and below this, a shallow notch terminating in the posterior inferior iliac spine which corresponds to the posterior extremity of the auricular surface and gives attachment to a portion of the sacro-tuberous (great sacro-sciatic) ligament. Below the spine, the posterior border of the ilium forms the upper limit of the greater sciatic notch.

Surfaces

The external surface or dorsum is concave behind, convex in front, limited above by the thick superior border or crest, and traversed by three gluteal lines.

The posterior gluteal line commences at the crest about two inches from the posterior superior iliac spine and curves downward to the upper margin of the greater sciatic notch. The space included between this ridge and the crest affords origin at its upper part to the gluteus maximus, and at its lower part, to a few fibres of the piriformis, while the intermediate portion is smooth and free from muscular attachment. The anterior gluteal line begins at the crest, one inch behind its anterior superior iliac spine, and curves across the dorsum to terminate near the lower end of the superior line, at the upper margin of the greater sciatic notch. The surface of bone between this line and the crest is for the origin of the gluteus medius. The inferior gluteal line commences at the notch immediately below the anterior superior iliac spine and terminates posteriorly at the front part of the greater sciatic notch. The space between the anterior and inferior gluteal lines, with the exception of a small area adjacent to the anterior end of the spine for the tensor fascia lata, gives origin to the gluteus minimus. Between the inferior gluteal line and the margin of the acetabulum the surface affords attachment to the capsule of the hip-joint, and on a rough area (sometimes a depression) toward its anterior part, to the reflected tendon of the rectus femoris.

The left coxal or hipbone (lateral view.)

The internal surface presents in front a smooth concave portion termed the iliac fossa, which lodges the iliacus muscle. The fossa is limited below by linea arcuata, the iliac portion of the terminal (ilio-pectineal) line. This is a rounded border separating the fossa from a portion of the internal surface below the line, which gives attachment to the obturator internus and enters into the formation of the minor (true) pelvis. Behind the iliac fossa, the bone is uneven and presents an auricular surface, covered with cartilage in the recent state, for articulation with the lateral aspect of the upper portion of the sacrum; above the auricular surface are some depressions for the posterior sacro-iliac ligaments and a rough area reaching as high as the crest, from which parts of the sacro-spinalis (erector spinae) and multifidus take origin. The rough surface above the auricular facet is known as the tuberosity of the ilium.

The left coxal bone or hip-bane (medial aspect.)

The ischium [os ischii] consists of a body, a tuberosity, and a ramus. The body, which has somewhat the form of a triangular pyramid, enters superiorly into the formation of the acetabulum, to which it contributes a little more than two fifths, and forms the chief part of the non-articular portion or floor. The inner surface forms part of the minor (true) pelvis and gives origin to the obturator internus. It is continuous with the ilium a little below the terminal (ilio-pectineal) line, and with the pubis in front, the line of junction with the latter being frequently indicated in the adult bone by a rough line extending from the ilio-pectineal eminence to the margin of the obturator foramen. The outer surface in eludes the portion of the acetabulum formed by the ischium. The posterior surface is broad and bounded laterally by the margin of the acetabulum and behind by the posterior border. The capsule of the hip-joint is attached to the lateral part and the piriformis, the great sciatic and posterior cutaneous nerves, the inferior gluteal (sciatic) artery, and the nerve to the quadratus femoris lie on the surface as they leave the pelvis. Inferiorly this surface is limited by the obturator groove, which receives the posterior fleshy border of the obturatorus when the thigh is flexed. Of the three borders, the external, forming the prominent rim of the acetabulum, separates the posterior from the external surface and gives attachment to the glenoid lip. The inner border is sharp and forms the lateral boundary of the obturator foramen. The posterior border is continuous with the posterior border of the ilium, with which it joins to complete the margin of the great sciatic notch [incisura ischiatic major]. The notch is converted into a foramen by the sacrospinous (small sacro-sciatic) ligament, and transmits the piriformis muscle, the gluteal vessels, the superior and inferior gluteal nerves, the sciatic and posterior cutaneous nerves, the internal pudic vessels and nerve, and the nerves to the obturator internus and quadratus femoris. Below the notch is the prominent ischial spine, which gives attachment internally to the coccygeus and levator ani, externally to the gemellus superior, and at the tip to the sacrospinous ligament. Below the spine is the small sciatic notch [incisura ischiadica minor], covered in the recent state with cartilage, and converted into a foramen by the sacro-tuberous (great sacro-sciatic) ligament. It transmits the tendon of the obturator internus, its nerve of supply, and the internal pudic vessels and nerve.

The rami form the flattened part of the ischium which runs first downward, then upward, forward and medially from the tuberosity toward the inferior ramus of the pubis, with which it is continuous. The rami together form an L-shaped structure with an upper vertical ramus [ramus superior] and a lower horizontal ramus [ramus inferior]. The outer surface of the rami gives origin to the adductor magyius and obturator externus; the inner surface, forming part of the anterior wall of the pelvis, receives the crus penis (or clitoridis) and the ischiocavernosus and gives origin to a part of the obturator internus. Of the two borders, the upper is thin and sharp, and forms part of the boundary of the obturator foramen; the lower is rough and corresponds to the inferior ramus. It is somewhat everted and gives attachment to the fascia of Colles, and the transversus perinei. To a ridge immediately above the impression for the crus penis (or clitoridis) and the ischiocavernosus , the urogenital trigone (triangular ligament) is attached. The posterior and inferior aspect of the superior ramus is an expanded area forming the tuberosity [tuber ischiadicum].

The tuberosity is that portion of the ischium which supports the body in the sitting posture. It forms a rough, thick eminence continuous with the inferior border of the inferior ramus, and is marked by an oblique line separating two impressions, an upper and lateral for the semimembranosus, and a lower and medial for the common tendon of the biceps and semitendinosus, while the lower part is markedly uneven and gives origin to the adductor magnus. The upper border gives origin to the inferior gemellus; the inner border, sharp and. prominent, receives the sacro-tuberous (great sacro-sciatic) ligament, while the surface of the tuberosity immediately in front is in relation with the internal pudic vessels and nerve. The outer border gives origin to the quadratus femoris.

The pubis [os pubis] consists of a body and two rami superior and inferior. The body is somewhat quadrilateral in shape and presents for examination two surfaces and three borders. The anterior surface looks downward, forward and slightly outward, and gives origin to the adductor longus, the adductor brevis, the obturator externus, and the gracilis. The posterior surface is smooth, looks into the pelvis, and affords origin to the levator ani, the obturator internus, and the puboprostatic ligaments. The upper border or crest of the body is rough and presents laterally a prominent bony point, known as the tubercle [tuberculum pubicum] or spine, for the attachment of the inguinal (Poupart's) ligament. The upper border extends from the pubic tubercle medialward to the upper end of the symphysis, with which it forms the angle of the pubis. The upper border is a short horizontal ridge, which gives attachment to the rectus abdominis and pyramidalis. The medial border is oval in shape, rough, and articular, forming with the bone of the opposite side the symphysis pubis [facies symphyseos]. The lateral border is sharp and forms part of the boundary of the obturator foramen.

The inferior ramus, like the inferior ramus of the ischium, with which it is continuous, is thin and flattened. To its anterior surface are attached the adductor brevis, adductor magnus, and obturator externus. The posterior surface is smooth and gives attachment to the crus penis or clitoridis, the sphincter urethrae (urogenitalis), the obturator internus, and the urogenital trigone (triangular ligament). The lateral border forms part of the circumference of the obturator foramen, and the medial border forms part of the pubic arch and gives attachment to the gracilis.

The superior ramus extends from the body of the pubis to the ilium, forming by its lateral extremity the anterior one-fifth of the articular surface of the acetabulum. It is prismatic in shape and increases in size as it passes laterally. Above it presents a sharp ridge, the pecten or pubic portion of the terminal (ilio-pectineal) line continuous with the iliac portion at the ilio-pectineal eminence, and affording attachment to the conjoined tendon [falx aponeurotica inguinalis], the lacunar (Gimbernat's) ligament, the reflected inguinal ligament (fascia triangularis), and the pubic portion of the fascia lata; the iliac portion of the terminal (iliopectineal) line gives attachment to the psoas minor, the iliac fascia, and the pelvic fascia. Immediately in front of the pubic portion of the line is the pectineal surface; it gives origin at its posterior part to the pectineus, and is limited below by the obturator crest, which extends from the pubic tubercle to the acetabular notch. The inferior surface of the ascending ramus forms the upper boundary of the obturator foramen and presents a deep groove [sulcus obturatorius] for the passage of the obturator vessels and nerve. The posterior surface is smooth, forms part of the anterior wall of the pelvic cavity, and gives attachment to a few fibers of the obturator internus.

Immature coxal (innominate) bone, showing a cotyloid bone.

According to the BNA, the body [corpus ossis pubis] is the portion corresponding to the acetabulum. The remainder of the bone is described as consisting of the ramus superior and the ramus inferior, which meet at the symphysis. Thus the divisions according to the BNA are different from those in the description above given.

The acetabulum is a circular depression in which the head of the femur is lodged and consists of an articular and a non-articular portion. The articular portion is circumferential and semilunar in shape [facies lunata], with the deficiency in the lower segment. One-fifth of the acetabulum is formed by the pubis, two-fifths by the ischium, and the remaining two-fifths are formed by the ilium. In rare instances the pubis may be excluded by a fourth element, the cotyloid bone. The non-articular portion [fossa acetabuli] is formed mainly by the ischium, and is continuous below with the margin of the obturator foramen. The articular portion presents a lateral rim to which the glenoid lip is attached, and a medial margin to which the synovial membrane which excludes the ligamentum teres from the synovial cavity is connected. The opposite extremities of the articular lunate surface which limit the acetabular notch are united by the transverse ligament, and through the acetabular foramen thus formed a nerve and vessels enter the joint.

The pelvis of a foetus at birth, to show the three portions of the coxal bones.

The obturator (thyreoid) foramen is situated between the ischium and pubis. Its margins are thin, and serve for the attachment of the obturator membrane. At the upper and posterior angle, it is deeply grooved for the passage of the obturator vessels and nerve.

The nucleus for the pubis appears bout the end of the fourth month 3 nucleus for the ischium appears Q the third month

Blood-supply

The chief vascular foramina of the coxal bone are found where the bone is thickest. On the inner surface, the ilium receives twigs from the ilio-lumbar, deep circumflex iliac, and obturator arteries, by foramina near the crest, in the iliac fossa, and below the terminal line near the greater sciatic notch. On the outer surface the chief foramina are found below the inferior gluteal line and the nutrient vessels are derived from the gluteal arteries. The ischium receives nutrient vessels from the obturator, internal and external circumflex arteries, and the largest foramina are situated between the acetabulum and the ischial tuberosity. The pubis is supplied by twigs from the obturator, internal and external circumflex arteries, and from the pubic branches of the common femoral artery.

Coxal or hip-bone, showing secondary centers.

Ossification

The cartilaginous representative of the coxal bone or hip-bone consists of three distinct portions, an iliac, an ischiatic, and a pubic portion; the iliac and ischiatic portions first unite and later the pubic portion, so that eventually there is found a single cartilaginous mass. Early in the second month a center of ossification appears above the acetabulum for the ilium. A little later a second nucleus appears below the cavity for the ischium, and this is followed in the fourth month by a deposit in the pubic portion of the cartilage. At birth, the three nuclei are of considerable size, but are surrounded by relatively wide tracts of cartilage; ossification has, however, extended into the margin of the acetabulum. In the eighth year, the rami of the pubis and ischium become united by bone, and in the twelfth year the triradiate cartilage which separates the three segments of the bone in the acetabulum begins to ossify from several centers. Of these, one is more constant than the others and is known as the acetabular nucleus. The triangular piece of bone to which it gives rise is regarded as the representative of the cotyloid or acetabular bone, constantly present in a few mammals. It is situated at the medial part of the acetabulum and is of such a size as to exclude entirely the pubis from the cavity. With this bone, however, it eventually fuses, and afterward becomes joined with the ilium and ischium, so that by the eighteenth or twentieth year the several parts of the acetabulum have become united. In the fifteenth year other centers appear in the cartilage of the crest of the ilium, the anterior inferior iliac spine, the tuberosity of the ischium, and the pubic pecten. The epiphyses fuse with the main bone about the twentieth year. The fibrous tissue connected with the tubercle of the pubis represents the epipubio bones of marsupials.

Coxal of hip-bone (inner surface) at the height year.

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