The os innominatum

Examine the bone and revise the following structures.

The ilium with its iliac crest running between the anterior and posterior superior iliac spines; below each of these are the corresponding inferior spines. Well-defined ridges on its lateral surface are the strong muscle markings of the glutei. Its inner aspect bears the large auricular surface which articulates with the sacrum. The iliopectineal line runs forward from the apex of the auricular surface and demarcates the true from the false pelvis.

The pubis comprises the body and the superior and inferior pubic rami.

Il iac crest

Anterior gluteal line

Posterior gluteal line Posterior superior spine Posterior inferior spine Greater sciatic notch

Ischial spine Lesser sciatic notch Obturator foramen Ischial tuberosity

Gluteal Lines

Anterior superior spine

Inferior gluteal line Anterior inferior spine

Acetabulum

Iliopectineal eminence

Superior ramus Pubic tubercle Body of pubis Inferior ramus Ramus of ischium

Il iac crest

Anterior gluteal line

Anterior superior spine

Posterior gluteal line Posterior superior spine Posterior inferior spine Greater sciatic notch

Inferior gluteal line Anterior inferior spine

Acetabulum

Iliopectineal eminence

Superior ramus Pubic tubercle Body of pubis Inferior ramus Ramus of ischium

Fig. 92 Lateral view of the os innominatum.

Ischial spine Lesser sciatic notch Obturator foramen Ischial tuberosity

The ischium has a vertically disposed body, bearing the ischial spine on its posterior border which demarcates an upper (greater) and lower (lesser), sciatic notch. The inferior pole of the body bears the ischial tuberosity then projects forwards almost at right angles into the ischial ramus to meet the inferior pubic ramus.

The obturator foramen lies bounded by the body and rami of the pubis and the body and ramus of the ischium.

All three bones fuse at the acetabulum which forms the socket for the femoral head, for which it bears a wide crescentic articular surface.

The pelvis is tilted in the erect position so that the plane of its inlet is at an angle 60° to the horizontal. (To place a pelvis into this position, hold it against a wall so that the anterior superior spine and the top of the pubic symphysis both touch it.)

The sacrum (Fig. 93)

The sacrum is made up of five fused vertebrae and is roughly triangular. The anterior border of its upper part is termed the sacral promontory and is readily felt at laparotomy.

Its anterior aspect presents a central mass, a row of four anterior sacral foramina on each side (transmitting the upper four sacral anterior primary rami), and, lateral to these, the lateral masses of the sacrum. The superior aspect of the lateral mass on each side forms a fan-shaped surface termed the ala.

Note that the central mass is roughly rectangular—the triangular shape of the sacrum is due to the rapid shrinkage in size of the lateral masses of the sacrum from above down.

Posteriorly lies the sacral canal, continuing the vertebral canal, bounded by short pedicles, strong laminae and diminutive spinous processes. Perforating through from the sacral canal is a row of four posterior sacral foramina on each side. Inferiorly, the canal terminates in the sacral hiatus, which transmits the 5th sacral nerve. On either side of the lower extremity of the hiatus lie the sacral cornua. These can easily be palpated by the finger immediately above the natal cleft.

Innominatum
Fig. 93 The sacrum in: (a) posterior and (b) anterior views.

On its lateral aspect, the sacrum presents an auricular facet for articulation with the corresponding surface of the ilium.

The 5th lumbar vertebra may occasionally fuse with the sacrum in whole or in part; alternatively, the 1st sacral segment may be partially or completely separated from the rest of the sacrum. The posterior arch of the sacrum is occasionally bifid.

Note that the dural sheath terminates distally at the second piece of the sacrum. Beyond this the sacral canal is filled with the fatty tissue of the extradural space, the cauda equina and the filum terminale. (For sacral block, see page 132.)

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