Bony Pelvis: Anatomy and Structure

Bony Pelvis

• It is formed by 2 hip bones, the sacrum, and the coccyx, held together by 4 joints and strong ligaments
• Anteriorly, the two hip bones articulate at the symphysis pubis
• Posteriorly, each hip bone articulates with the sacrum at the sacroiliac joint
• Inferior and posterior, the sacrum articulates with the coccyx at the sacrococcygeal joint
• Pelvic bones, in the anatomical position (during standing), the anterior superior iliac spine (ASIS) and the symphysis pubis' upper border lie in the same vertical plane
• The pelvic inlet plane is at a 50-60º angle to horizontal

Pelvic Inlet

• This region corresponds to the trunk's perineum region
• The trunk's perineum region is lowered and diamond-shaped
• The pelvic inlet is bounded by the symphysis pubis and pubic crest anteriorly and iliopectineal lines laterally
• Sacrum ala and promontory are located posteriorly

Pelvic Outlet

• Anteriorly, Symphysis pubis lower border is the start of the pelvic outlet
• On each side, the outlet is lateral to the ischiopubic rami, ischial tuberosity, and sacrotuberous ligaments
• Posteriorly is the coccyx tip

Pelvic Cavity

• The pelvic inlet plane divides the pelvic cavity into divisions
• The False (greater) pelvis is above the pelvic inlet and contains abdominal viscera, bounded on each side by the iliac fossa
• The True (lesser) pelvis is below the pelvic inlet, containing pelvic viscera with upper and lower openings (pelvic inlet and outlet)
• The Antero-inferior boundary of the Lesser pelvis is the Symphysis pubis and pubic rami
• The Posterior boundary of the Lesser pelvis is the Sacrum and coccyx
• The Lateral boundary of the Lesser pelvis is the Pelvic aspect of fused ilium and ischium

Pelvic Diameters

Antero-Posterior Diameters

• The inlet is located between the midpoints of the sacral promontory and the symphysis pubis' upper border
• The cavity is located between the midpoints of the S3 vertebra and the symphysis pubis posterior surface
• The outlet is from coccyx apex to midpoint of symphysis pubis' lower border

Transverse Diameters

• The inlet is defined by the maximum distance between similar points on opposite sides of the pelvic brim
• The cavity is the widest transverse distance between cavity side walls
• The outlet is between the medial borders of 2 ischial tuberosities

Oblique Diameters

• The inlet is from the iliopubic (iliopectineal) eminence to the opposite sacroiliac joint
• The cavity is from one sacroiliac joint's lowest point to the contralateral obturator membrane midpoint
• The outlet runs from one side of the sacrotuberous ligament midpoint to the contralateral ischiopubic junction

Lesser Pelvis Dimensions

Antero-Posterior Diameters

• Inlet: 10 cm (male), 11 cm (female)
• Outlet: 8 cm (male), 12.5 cm (female)
• Cavity: 10.5 cm (male), 13 cm (female)

Transverse Diameters

• Inlet: 12.5 cm (male), 13 cm (female)
• Outlet: 8.5 cm (male), 11 cm (female)
• Cavity: 12 cm (male), 12.5 cm (female)

Oblique Diameters

• Inlet: 12 cm (male), 12.5 cm (female)
• Outlet: 10 cm (male), 12cm (female)
• Cavity: 11 cm (male), 13 cm (female)

Sacrum Shape and Parts

• The sacrum is a triangular bone formed by the fusion of 5 sacral vertebrae
• The base is formed of the S1 vertebra body and sacrum alae, articulating with the L5 vertebra
• The apex is the S5 vertebra's lower surface, articulating with the coccyx

Sacrum Surfaces

• The anterior surface is smooth and concave with 4 pairs of anterior sacral foramina for the passage of upper sacral nerve ventral rami and lateral sacral vessels
• The posterior surface is irregular and convex, has 4 pairs of posterior sacral foramina for passage of upper sacral nerve dorsal rami
• The posterior surface features the median sacral crest from the fusion of upper sacral spines, and lateral sacral crest from the fusion of sacral transverse processes
• The posterior surface also contains right and left intermediate sacral crests from the fusion of sacral articular facets
• Its upper part has an auricular surface which articulates with the ilium auricular surface

Sacral Ala and Sacral Promontory

• The sacral ala is related to the lumbo-sacral trunk
• The sacral promontory is the anterior projecting edge of the S1 vertebra

Sacral Canal Contents

• The contents are cauda equina, filum terminale, dura and arachnoid mater, vertebral venous plexus, areolar tissues
• Cauda equina is the collection of sacral and coccygeal nerve roots below the spinal cord end
• The filum terminale is the pia mater prolongation passing through the sacral hiatus and fixed to the coccyx back
• The dura and arachnoid mater both end at the S2 lower border

Sacral Hiatus

• Sacral hiatus is located at the Sacral canal lower end
• The 1-5th pair of sacral nerve roots, coccygeal nerve roots, and filum terminale all pass through here

Sacrum Relations and Attachments

Anterior Surface

• Iliacus, piriformis & coccygeus muscles

Nervous Structures

• Sacral plexus
• Two sympathetic chains

Arterial Structures

• Median sacral artery
• Lateral sacral artery