Osteology of Pelvis and Perineum PDF

Module 1: Osteology of Pelvis and Perineum Gross Anatomy of Pelvis Pelvic Bone Pelvic bone on each side is formed from ilium, ischium, pubis fused together at the acetabulum. ▪ Acetabulum: depression in pelvic bone at which femur articulates the pelvic bone. ▪ Ischium and pubis joined infe...

Module 1: Osteology of Pelvis and Perineum Gross Anatomy of Pelvis Pelvic Bone Pelvic bone on each side is formed from ilium, ischium, pubis fused together at the acetabulum. ▪ Acetabulum: depression in pelvic bone at which femur articulates the pelvic bone. ▪ Ischium and pubis joined inferiorly via ischiopubic ramus → forms a ring enclosing obturator foramen. ▪ Pelvic bone on two sides are joined by: - Posterior: sacrum - Anterior: pubic symphysis Osteology of the pelvis Pelvic bone can be treated as a tubular bone with inlet and outlet - Significance: inadequate size of female pelvic inlet or outlet may warrant caesarian section in pregnancy → pre-partum size estimation Pelvic Inlet Hip Bone The left and right hip bones are two irregularly shaped bones that form part of the pelvic girdle. The hip bones have three main articulations: i. Sacroiliac joint – articulation with the sacrum. ii. Pubic symphysis – articulation between the left and right hip bones. iii. Hip joint – articulation with the head of femur. Composition of hip bone The Ilium The ilium is the widest and largest of the three parts of the hip bone, and is located superiorly. The body of the ilium forms the superior part of the acetabulum (acetabular roof). Immediately above the acetabulum, the ilium expands to form the wing (or ala). The wing of the ilium has two surfaces: ▪ Inner surface – has a concave shape, which produces the iliac fossa (site of origin of the iliacus muscle). ▪ External surface (gluteal surface) – has a convex shape and provides attachments to the gluteal muscles. ▪ The superior margin of the wing is thickened, forming the iliac crest. It extends from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS). On the posterior aspect of the ilium there is an indentation known as the greater sciatic notch. Clinical significance The anterior superior iliac spine (ASIS) is an important anatomical landmark The Pubis The pubis is the most anterior portion of the hip bone. It consists of a body, superior ramus and inferior ramus (ramus = branch). ▪ Pubic body – located medially, it articulates with the opposite pubic body at the pubic symphysis. Its superior aspect is marked by a rounded thickening (the pubic crest), which extends laterally as the pubic tubercle. ▪ Superior pubic ramus – extends laterally from the body to form part of the acetabulum. ▪ Inferior pubic ramus – projects towards the ischium. Together, the superior and inferior rami enclose part of the obturator foramen – through which the obturator nerve, artery and vein pass through to reach the lower limb. Clinical significance Pubic rami fracture The Ischium The ischium forms the posteroinferior part of the hip bone. Much like the pubis, it is composed of a body, an inferior ramus and superior ramus. The inferior ischial ramus combines with the inferior pubic ramus forming the ischiopubic ramus, which encloses part of the obturator foramen. The posterorinferior aspect of the ischium forms the ischial tuberosities and when sitting, it is these tuberosities on which our body weight falls. Near the junction of the superior ramus and body is a posteromedial projection of bone; the ischial spine. Two important ligaments attach to the ischium: ▪ Sacrospinous ligament – runs from the ischial spine to the sacrum, thus creating the greater sciatic foramen through which lower limb neurovasculature (including the sciatic nerve) transcends. ▪ Sacrotuberous ligament – runs from the sacrum to the ischial tuberosity, forming the lesser sciatic foramen. Clinical significance Pelvic fracture The Pelvic Girdle The pelvic girdle is a ring-like bony structure, located in the lower part of the trunk. Structure of the Pelvic Girdle The bony pelvis consists of the two hip bones (also known as innominate or pelvic bones), the sacrum and the coccyx. There are four articulations within the pelvis: ▪ Sacroiliac joints (x2) – between the ilium of the hip bones, and the sacrum ▪ Sacrococcygeal symphysis – between the sacrum and the coccyx. ▪ Pubic symphysis – between the pubis bodies of the two hip bones. ▪ Ligaments attach the lateral border of the sacrum to various bony landmarks on the bony pelvis to aid stability. Functions of the Pelvis The strong and rigid pelvis is adapted to serve a number of roles in the human body. The main functions being:  ▪ Transfer of weight from the upper axial skeleton to the lower appendicular components of the skeleton, especially during movement. ▪ Provides attachment for a number of muscles and ligaments used in locomotion. ▪ Contains and protects the abdominopelvic and pelvic viscera. The Greater and Lesser Pelvis The osteology of the pelvic girdle allows the pelvic region to be divided into two: ▪ Greater pelvis (false pelvis) – located superiorly, it provides support of the lower abdominal viscera (such as the ileum and sigmoid colon). It has little obstetric relevance. ▪ Lesser pelvis (true pelvis) – located inferiorly. Within the lesser pelvis reside the pelvic cavity and pelvic viscera. The junction between the greater and lesser pelvis is known as the pelvic inlet. The outer bony edges of the pelvic inlet are called the pelvic brim. a. Pelvic Inlet Pelvic inlet is delineated by the pelvic brim - Separates abdominal cavity from the pelvic cavity or false pelvic cavity from true pelvic cavity Borders: - Posterior: margin of ala (wing) of sacrum - Anterior: pubic symphysis - Lateral: → Arcuate line on ilium → Pectinate line on pubis True (obstetrical) conjugate: distance from tip of S1 to upper boundary of pubic symphysis - True AP diameter of pelvic inlet - ~11.5 cm in Caucasians - Difficult to estimate in vivo Diagonal conjugate: distance from tip of S1 to lower boundary of pubic symphysis - ~13.5 cm in Caucasians - Can be estimated trans-vaginally in vivo Some alternative descriptive terminology can be used in describing the pelvic inlet: ▪ Linea terminalis – the combined pectineal line, arcuate line and sacral promontory. ▪ Iliopectineal line – the combined arcuate and pectineal lines. This represents the lateral border of the pelvic inlet. b) Pelvic Outlet Pelvic outlet formed by: - Anterior: pubic symphysis - Anterolateral: ischiopubic ramus - Lateral: ischial tuberosity - Posterolateral: inferior margin of sacrotuberous ligament - Posterior: coccygeal tip - Sacrotuberous ligament: ligament connecting sacral body and ischial tuberosity - Size estimated by measuring distance between bilateral ischial tuberosities The angle beneath the pubic arch is known as the sub-pubic angle and is of a greater size in women. Pelvic Outlet Clinical significance Assessment of the female bony pelvis The lesser pelvis is the bony canal through which the fetus has to pass during childbirth. It is therefore of great importance to determine the diameter of this canal and therefore the childbearing capacity of the mother. Sacrum Bony Landmark The sacrum is formed by the fusion of the five sacral vertebrae. It has an inverted triangular, concave shape. The bone consists of a base, apex and four surfaces: Base – articulates superiorly with the fifth lumbar vertebra and its associated intervertebral disc. Apex – abuts the coccyx inferiorly. Auricular surfaces – located laterally on the sacrum and shaped like the outer ear – hence the name. Each articulates with the auricular surface of the ilium. Anterior and posterior surfaces – provide attachment to pelvic ligaments and muscles. Internally, the central canal of the vertebral column continues along the core of the sacrum and ends at the 4th sacral foramina, as the sacral hiatus. Surfaces of the Sacrum There are two surfaces of the sacrum – a coarse dorsal surface and a relatively smooth pelvic surface. When the patient is standing, the pelvic surface faces anteroinferiorly while the dorsal surface is in the posterosuperior direction. Surface of sacrum (posterior and anterior surfaces) Clinical significance The sacrum contributes to the sexual dimorphism of the pelvis. Firstly, the sacral promontory is less prominent in females than in males. Coccyx The coccyx (also known as the tailbone) is the terminal part of the vertebral column. It is comprised of four vertebrae, which fuse to produce a triangular shape. The coccyx consists of an apex, base, anterior surface, posterior surface and two lateral surfaces. The base is located most superiorly, and contains a facet for articulation with the sacrum. The apex is situated inferiorly, at the terminus of the vertebral column. The lateral surfaces of the coccyx are marked by a small transverse process, which projects from Co1. The coccyx articulates with the sacrum at a fibrocartilaginous joint called the sacrococcygeal symphysis. The sacrococcygeal symphysis is supported by five ligaments: ▪ Anterior sacrococcygeal ligament ▪ Deep posterior sacrococcygeal ligament ▪ Superficial posterior sacrococcygeal ligament ▪ Lateral sacrococcygeal ligaments ▪ Interarticular ligaments Clinical significance Fracture coccyx Coccydynia Sacrococcygeal Teratoma