AN53.1-4 | Osteology — Glossary

Standard reference position: standing erect, feet together, arms at sides, palms facing forward; all anatomical descriptions are relative to this position

Superior aperture of the true pelvis bounded by the sacral promontory, arcuate line, pectineal line, and pubic symphysis

Smallest AP diameter of the pelvic inlet (10.5 cm); from sacral promontory to nearest point on posterior pubic symphysis; determines fetal head engagement

Distance from sacral promontory to lower pubic symphysis (12.5 cm); measurable on vaginal examination; obstetric conjugate = diagonal minus 1.5 cm

Most common female pelvic type with round inlet; best obstetric prognosis for normal vaginal delivery

Male-type pelvis with heart-shaped inlet, prominent ischial spines, narrow subpubic angle; causes obstructed labour

Anterior projection of the upper border of S1; posterior boundary of the pelvic inlet; landmark for measuring conjugate diameters

Bony projection on the ischium between the greater and lesser sciatic notches; gives origin to the sacrospinous ligament; landmark for pudendal nerve block

Transitional vertebra where L5 is partially or completely fused with the sacrum, giving only 4 mobile lumbar vertebrae

Transitional vertebra where S1 separates from the sacrum, functioning as a 6th lumbar vertebra

Oblique hole in the bone shaft for the nutrient artery; its opening faces away from the growing end of the bone

Cup-shaped socket on the lateral surface of the hip bone formed by the fusion of ilium, ischium, and pubis; articulates with the femoral head

Curved line through the centre of the pelvic canal from inlet to outlet; the path the fetal head follows during labour

Lesser pelvis below the pelvic brim; contains the pelvic viscera and forms the birth canal

Greater pelvis above the pelvic brim; bounded by iliac fossae; part of the abdominal cavity

Strong joint between the sacrum and ilium; partly synovial, partly syndesmosis; transmits body weight to the lower limbs

Angle formed by the ischiopubic rami below the pubic symphysis; >90 degrees in females, <90 degrees in males

Pain at the tip of the coccyx, commonly from direct trauma (falling on buttocks) or childbirth injury

Obstructed labour where the fetal head gets stuck in the transverse position at the level of the ischial spines, unable to rotate to OA

Clinical measurement of pelvic diameters to assess adequacy of the birth canal for vaginal delivery

Classification of four pelvic types based on inlet shape: gynaecoid, android, anthropoid, and platypelloid

Opening at S4/S5 where the sacral laminae fail to fuse; site for caudal epidural block