Anatomy of the Pelvis Study Notes

ANATOMY OF THE PELVIS - PART 1: OSTEOLOGY

Introduction to the Pelvis

  • The pelvis consists of:

    • 2 ossa coxae (hip bones)

    • Sacrum

    • Coccyx

  • The pelvis is stabilized by ligaments of the pelvic girdle, sacrum, and lumbar vertebrae.

Hip Bone (Os Coxae, Innominate Bone)

  • In adults, the hip bone is formed by the fusion of three bones:

    • Ilium

    • Ischium

    • Pubis

  • At birth, these bones are connected by hyaline cartilage known as triradiate cartilage, which is centered in the acetabulum (hip socket).

  • Fusion of the three bones begins around 15-17 years of age and is usually complete by 20-25 years.

  • After this period, there is no detectable trace of the fusion line.

Diagnostic Features of Individual Bones of the Pelvis

  • The bones of the pelvis can be diagnosed based on their anatomical features.

    • Key landmarks include:

    • Iliac crest

    • Anterior superior iliac spine

    • Iliac fossa

    • Ischial spine, tuberosity, and ramus

    • Pubic tubercle and symphysis

  • The pelvic girdle configuration is crucial for functions like bearing body weight and movement stress.

Anatomy of the Ossac Coxae

  • Each os coxae contributes significantly to the body mechanics:

    • Structurally strong due to fused composition of ilium, ischium, and pubis.

    • Articulates with the sacrum at the sacroiliac joint.

The Acetabulum

  • Also referred to as the hip socket or vinegar cup.

  • It is the junction point where the ilium, ischium, and pubis meet.

  • The acetabulum articulates with the head of the femur, forming the hip joint.

Divisions of the Pelvis

  • The pelvis is divided into different anatomical regions:

    • True Pelvis: The space created between the pelvic inlet and the pelvic outlet.

    • False Pelvis: Located above the pelvic brim; contains parts of the abdominal cavity.

  • The pelvic inlet is oriented more anteroposteriorly in males compared to females, where it tends to be transversely oval.

Structural and Functional Differences Between Male and Female Pelvis

  • Male Pelvis Features:

    • Iliac wings are less flared.

    • Transverse diameter of the pelvic inlet is shorter than in females.

    • Pubic arch (subpubic angle) is narrower.

    • Public symphysis is deeper (taller).

  • Female Pelvis Features:

    • Smoother, lighter, and less prominent muscle and ligament attachments.

    • Enlarged pelvic outlet with a broad pubic angle (> 100°) and less curvature of sacrum and coccyx.

    • Wide, circular pelvic inlet and broad, low pelvis with ilia projecting laterally.

Measurements and Boundaries of the Pelvic Inlet

  • The pelvic brim is defined by:

    • Sacral promontory

    • Ala of the sacrum

    • Arcuate line

    • Pectineal line (pecten pubis)

    • Pubis tubercle

    • Pubic symphysis

  • True and false pelvis are essential for different physiological functions including childbirth.

Anatomical Positioning of Hip Bone

  • In the anatomical position:

    • The anterior superior iliac spine (ASIS) and the anterior aspect of the pubis lie in the same vertical plane.

    • The ischial spine and the superior end of the pubic symphysis lie in the same horizontal plane.

    • The symphyseal surface of the pubis is vertical and parallel to the mid-sagittal plane.

    • The obturator foramen lies inferiomedial to the acetabulum.

    • The tip of the coccyx is aligned at the level of the superior half of the pubis.

    • The acetabulum faces inferiolaterally, with the acetabular notch directed inferiorly.

ANATOMY OF THE PELVIS - PART 2: MUSCLES AND FASCIAE OF PELVIS/PERINEUM

Muscles of the Pelvis

  • The muscles within the pelvis may be divided into two main groups:

    1. Obturator Internus and Piriformis:

    • Known as muscles of the lower extremity.

    1. Levator Ani and Coccygeus:

    • Together referred to as the pelvic diaphragm. This diaphragm can be further divided into:

      • Iliococcygeus

      • Pubococcygeus

      • Puborectalis

    • Note that the pelvic diaphragm is not the same as the urogenital diaphragm.

Muscles and Ligaments of the Perineum

  • The perineum lies below the pelvic diaphragm, structured in two portions:

    • Anal Region: Contains the terminal part of the anal canal.

    • Urogenital Region: Contains external urogenital organs.

Deep Boundaries of the Perineum

  • The deep boundaries of the perineum are defined as follows:

    • Anteriorly: Pubic arch and arcuate ligament of pubis.

    • Posteriorly: Tip of the coccyx.

    • Lateral: Inferior rami of the pubis, ischium, and the sacrotuberous ligament.

  • The perineum area is diamond-shaped, bordered by the scrotum in front, buttocks behind, and medial aspect of the thighs on either side.

Composition of Perineal Muscles

  • The muscles of the perineum can be categorized into:

    1. Anal Region Muscles:

    • External anal sphincter

    • Internal anal sphincter

    • Corrugator ani

    1. Urogenital Region Muscles:

    • Superficial transverse perineal

    • Ischiocavernosus

    • Bulbocavernosus

    • Deep transverse perineal

    • Sphincter urethrae

Pelvic Fascia

  • The pelvic fascia can be divided into four types:

    • Fascial sheaths of the Obturator Internus

    • Piriformis fascia

    • Pelvic diaphragm fascia

    • Fascia associated with pelvic viscera

Summary

  • The anatomy of the pelvis plays a vital role in various bodily functions, including support and movement. It displays considerable sexual dimorphism, especially in relation to pregnancy and childbirth considerations. A comprehensive understanding of the components, muscle arrangements, and anatomical positioning is essential for medical applications in gynecology, obstetrics, and urology.