Innominate Bone

INNOMINATE BONE, os innominatum (from Latin in- without, not and nominare - to name), a term introduced by Vesalius to designate the pelvic bone (os coxae), which was unnamed by Galen, resembling an hourglass in shape and consisting of three bones (ilium, ischium, and pubis) that fuse together with the onset of puberty; final ossification is completed between 22 and 25 years. At the junction, in the narrowest part of the bone, on its outer surface is the so-called acetabulum for articulation with the head of the femur (epag-throsis - 'nut-like joint'). The anterior-inner surface of the ilium is concave (iliac fossa), behind it is the auricular surface for articulation with the corresponding surface of the sacrum, and further back is a roughness for ligament attachment. The upper S-shaped curved edge of the bone (crest) in front ends in a well-palpable spine, serving for muscle attachment. In the ischium and pubis, the body and two branches each are distinguished; between them remains an opening, closed by a membrane (foramen obturatum), with an opening at the top. The upper branch of the ischium thickens downward into the ischial tuberosity, serving for attachment of the posterior thigh muscles and as support when sitting. At the junction of the upper and lower parts of the pubis is an oval area for articulation with the paired bone in the pubic symphysis, about two centimeters from which laterally is the pubic tubercle.