ILIUM

 

The Ilium or the flank forms the upper extended plate-like part of the hip bone. It has upper and lower parts and three surfaces. The upper end is called the iliac crest which forms the two-fifths of the acetabulum and the lower end which is smaller than upper end is fused with pubis and ischium at the acetabulum. The upper part is much expanded, and has gluteal, sacropelvic and iliac (internal) surfaces. The posterolateral gluteal surface is an extensive rough area; the anteromedial iliac fossa is smooth and concave; and the sacropelvic surface is medial and posteroinferior to the fossa, from which it is separated by the medial border.

Ilium

Ilium

 

Iliac crest

The iliac crest is the superior border of the ilium. It is broad and convex upwards but sinuous from side to side, being internally concave in front and convex behind. Its ends project as anterior and posterior superior iliac spines. The anterior superior iliac spine is palpable at the lateral end of the inguinal fold; the lateral end of the inguinal ligament is attached to the anterior superior iliac spine. The posterior superior iliac spine is not palpable but is often indicated by a dimple, approximately 4 cm lateral to the second sacral spinous process, above the medial gluteal region.

The iliac crest has ventral and dorsal segments. The ventral segment occupies slightly more than the anterior two-thirds of the iliac crest. It has internal and external lips and a rough intermediate zone that is narrowest centrally. The dorsal segment, which occupies approximately the posterior 1/3^rd in humans. It has two sloping surfaces separated by a longitudinal ridge ending at the posterior superior spine. The tubercle of the iliac crest projects outwards from the outer lip approximately 5 cm posterosuperior to the anterior superior spine. The summit of the iliac crest, a little behind its midpoint, is level with the 4^th lumbar vertebral body in adults and with the 5^th lumbar vertebral body in children aged 10 years or less.

Anterior border

The anterior border descends to the acetabulum from the anterior superior spine. Superiorly it is concave forwards. Inferiorly, immediately above the acetabulum, is a rough anterior inferior iliac spine, which is divided indistinctly into an upper area for the straight head of rectus femoris and a lower area extending laterally along the upper acetabular margin to form a triangular impression for the proximal end of the iliofemoral ligament.

Posterior border

The posterior border is irregularly curved and descends from the posterior superior spine, at first forwards, with a posterior concavity forming a small notch. At the lower end of the notch is a wide, low projection known as the posterior inferior iliac spine. Here the border turns almost horizontally forwards for approximately 3 cm, then down and back to join the posterior ischial border. Together these borders form a deep notch, the greater sciatic notch, which is bounded above by the ilium and below by the ilium and ischium. The upper fibres of the sacrotuberous ligament are attached to the upper part of the posterior border. The superior rim of the notch is related to the superior gluteal vessels and nerve. The lower margin of the greater sciatic notch is covered by piriformis and is related to the sciatic nerve.

Medial border

The medial border separates the iliac fossa and the sacropelvic surface. It is indistinct near the crest, rough in its upper part, then sharp where it bounds an articular surface for the sacrum and finally rounded. The latter part is the arcuate line, which inferiorly reaches the posterior part of the iliopubic ramus, marking the union of the ilium and pubis.

• Gluteal surface

Gluteal surface is the outer surface of the ilium, which is convex in front and concave behind, like the iliac crest. It is rough and curved, convex in front, concave behind, and marked by three gluteal lines which divides into four areas. The posterior gluteal line is shortest, descending from the external lip of the crest approximately 5 cm in front of its posterior limit and ending in front of the posterior inferior iliac spine. Above, it is usually distinct, but inferiorly it is poorly defined and frequently absent. The anterior gluteal line, the longest, begins near the midpoint of the superior margin of the greater sciatic notch and ascends forwards into the outer lip of the crest, a little anterior to its tubercle. The inferior gluteal line, seldom well marked, begins posterosuperior to the anterior inferior iliac spine, curving posteroinferiorly to end near the apex of the greater sciatic notch. Between the inferior gluteal line and the acetabular margin is a rough, shallow groove. Behind the acetabulum, the lower gluteal surface is continuous with the posterior ischial surface.

The articular capsule is attached to an area adjoining the acetabular margin, most of which is covered by gluteus minimus. Posteroinferiorly, near the union of the ilium and ischium, the bone is related to piriformis.

• Iliac fossa

The iliac fossa, the internal concavity of the ilium, faces anterosuperiorly. It is limited above by the iliac crest, in front by the anterior border and behind by the medial border, separating it from the sacropelvic surface. It forms the smooth and gently concave posterolateral wall of the greater pelvis. Below it is continuous with a wide shallow groove, bounded laterally by the anterior inferior iliac spine and medially by the iliopubic ramus.

• Sacropelvic surface

The sacropelvic surface, the posteroinferior part of the medial iliac surface, is bounded posteroinferiorly by the posterior border, anterosuperiorly by the medial border, posterosuperiorly by the iliac crest and anteroinferiorly by the line of fusion of the ilium and ischium. It is divided into iliac tuberosity and auricular & pelvic surfaces. The iliac tuberosity, a large, rough area below the dorsal segment of the iliac crest, shows cranial and caudal areas separated by an oblique ridge and connected to the sacrum by the interosseous sacroiliac ligament. The sacropelvic surface gives attachment to the posterior sacroiliac ligaments and, behind the auricular surface, to the interosseous sacroiliac ligament. The iliolumbar ligament is attached to its anterior part. The auricular surface, immediately anteroinferior to the tuberosity, articulates with the lateral sacral mass. Shaped like an ear, its widest part is anterosuperior, and its ‘lobule’ posteroinferior and on the medial aspect of the posterior inferior spine. Its edges are well defined but the surface, though articular, is rough and irregular. It articulates with the sacrum and is reciprocally shaped. The anterior sacroiliac ligament is attached to its sharp anterior and inferior borders. The narrow part of the pelvic surface, between the auricular surface and the upper rim of the greater sciatic notch, often shows a rough pre-auricular sulcus (that is usually better defined in females) for the lower fibres of the anterior sacroiliac ligament. The pelvic surface is anteroinferior to the acutely curved part of the auricular surface, and contributes to the lateral wall of the lesser pelvis. Its upper part, facing down, is between the auricular surface and the upper limb of the greater sciatic notch. Its lower part faces medially and is separated from the iliac fossa by the arcuate line. Anteroinferiorly, it extends to the line of union between the ilium and ischium. Though usually obliterated, it passes from the depth of the acetabulum to approximately the middle of the inferior limb of the greater sciatic notch.

Muscle attachments

The attachment of sartorius extends down the anterior border below the anterior superior iliac spine.

The iliac crest gives attachment to the anterolateral and dorsal abdominal muscles, and to the fasciae and muscles of the lower limb.

The fascia lata and iliotibial tract are attached to the outer lip and tubercle of its ventral segment.

Tensor fasciae latae is attached anterior to the tubercle. The lower fibres of external oblique and, just behind the summit of the crest, the lowest fibres of latissimus dorsi are attached to its anterior two-thirds. A variable interval exists between the most posterior attachment of external oblique and the most anterior attachment of latissimus dorsi, and here the crest forms the base of the lumbar triangle through which herniation of abdominal contents may rarely occur.

Internal oblique is attached to the intermediate area of the crest.

Transversus abdominis is attached to the anterior two-thirds of the inner lip of the crest, and behind this to the thoracolumbar fascia and quadratus lumborum. The highest fibres of gluteus maximus are attached to the dorsal segment of the crest on its lateral slope.

Erector spinae arises from the medial slope of the dorsal segment.

The straight head of rectus femoris is attached to the upper area of the anterior inferior spine.

Some fibres of piriformis are attached in front of the posterior inferior spine on the upper border of the greater sciatic foramen.

The gluteal surface is divided by three gluteal lines into four areas. Behind the posterior line, the upper rough part gives attachment to the upper fibres of gluteus maximus and the lower, smooth region to part of the sacrotuberous ligament and iliac head of piriformis. Gluteus medius is attached between the posterior and anterior lines, below the iliac crest, and gluteus minimus is attached between the anterior and inferior lines.

The fourth area, below the inferior line, contains vascular foramina. The reflected head of rectus femoris attaches to a curved groove above the acetabulum.

Iliacus is attached to the upper two-thirds of the iliac fossa and is related to its lower one-third. The medial part of quadratus lumborum is attached to the anterior part of the sacropelvic surface, above the iliolumbar ligament.

Piriformis is sometimes partly attached lateral to the pre-auricular sulcus, and part of obturator internus is attached to the more extensive remainder of the pelvic surface.

Vascular supply Branches of the iliolumbar artery run between iliacus and the ilium; one or more enter large nutrient foramina lying posteroinferiorly in the iliac fossa. The superior gluteal, obturator and superficial circumflex iliac arteries contribute to the periosteal supply. The obturator artery may supply a nutrient branch. Vascular foramina on the ilium underlying the gluteal muscles may lead into large vascular canals in the bone. Innervation The periosteum is innervated by branches of nerves that supply muscles attached to the bone, the hip joint and the sacroiliac joint.

OSSIFICATION

Ossification is by three primary centers: one each for the ilium, ischium and pubis. The iliac centre appears above the greater sciatic notch prenatally at about the 9^th week and the pubic centre in its superior ramus between the 4^th and 5^th months. The pubis is often not recovered from prenatal remains due to its size and fragility and because it is the last of the hip bones to begin ossification (Scheuer and Black 2004). At birth the whole iliac crest, the acetabular floor and the inferior margin are cartilaginous. Gradual ossification of the three components of the acetabulum results in a triradiate cartilaginous stem extending medially to the pelvic surface as a Y-shaped epiphysial plate between the ilium, ischium and pubis, and including the anterior inferior iliac spine. Cartilage along the inferior margin also covers the ischial tuberosity, forms conjoined ischial and pubic rami and continues to the pubic symphysial surface and along the pubic crest to the pubic tubercle. The ossifying ischium and pubis fuse to form a continuous ischiopubic ramus at the 7^th or 8^th year. 

Secondary centres, other than for the acetabulum, appear at about puberty and fuse between the 15^th and 25^th years. There are usually two for the iliac crest (which fuse early), and single centres for anterior inferior iliac spine (although it may ossify from the triradiate cartilage) and symphysial surface of the pubis (the pubic tubercle and crest may have separate centres). Progression of ossification of the iliac crest in girls is an index of skeletal maturity and is useful in determining the optimal timing of surgery for spinal deformity. Between the ages of 8 and 9 years, 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, the second in the iliac acetabular cartilage superiorly, fusing with the ilium, and the third in the ischial acetabular cartilage posteriorly, fusing with the ischium. At puberty, these epiphyses expand towards the periphery of the acetabulum and contribute to its depth. Fusion between the three bones within the acetabulum occurs between the sixteenth and eighteenth years. Delaere et al have suggested that ossification of the ilium is similar to that of a long bone, possessing three cartilaginous epiphyses and one cartilaginous process, although it tends to undergo osteoclastic resorption comparable with that of cranial bones. During development, the acetabulum increases in breadth at a faster rate than it does in depth. Avulsion fractures of pelvic apophyses may occur from excessive pull on tendons, usually in athletic adolescents. The most frequent examples of such injuries are those to the ischial tuberosity (hamstrings) and anterior inferior iliac spine (rectus femoris).

Complied & written by Dr. Palak Shah.