Pelvic Bone

The pelvic bone, also known as the pelvis, is a complex bony structure located at the base of the spine, forming the pelvic region's foundation. It serves several crucial functions, including supporting the upper body's weight when sitting or standing, transferring that weight to the lower limbs, and protecting vital organs such as the bladder, intestines, and reproductive organs.

1.0Introduction

• The pelvic (hip) girdle is made up of two hip bones, also known as the coxal bones or os coxa. These hip bones are joined at the front by a joint called the pubic symphysis and connect at the back with the sacrum at the sacroiliac joints. 
• Together with the sacrum, coccyx, and pubic symphysis, the hip bones form a deep, bowl-like structure called the bony pelvis. The plural of pelvis can be pelvis or pelvises.
• The bony pelvis provides strong, stable support for the vertebral column and protects the pelvic and lower abdominal organs. It also plays a crucial role in linking the bones of the lower limbs to the axial skeleton. 
• At birth, each hip bone consists of three separate bones: the ilium (upper part), pubis(front and lower), and ischium (back and lower). These bones are separated by cartilage, but by the age of 23, they fuse into a single structure. 
• Although these bones function as one, anatomists often refer to each hip bone in its original three-part form for clarity.

2.0Bones of Pelvic Girdle 

The pelvic girdle is formed by:

• Two Hip bones
• Sacrum 
• Coccyx
  
• The pelvic cavity lodges and protects the reproductive organs, nerves, and blood vessels of the lower limb, urinary bladder, and terminal part of the large intestine. 
• The combined weight of the trunk, head and upper limbs rests on the pelvis at its articulation with the 5th lumbar vertebra.

• Hip Bones (Os Coxae): It consists of 3 parts-ilium, ischium and pubis, which are 3 separate bones in early life. The line of union of 3 parts is in the acetabular cavity, a deep socket for articulation with the head of the femur to form a hip joint.

3.0Ilium 

• The ilium (IL-ee-um), the largest of the three parts of the hip bone, consists of an upper section called the ala (wing) and a lower body. The body forms part of the acetabulum, which is the socket that holds the head of the femur. The upper edge of the ilium is the iliac crest, which ends in the front at the anterior superior iliac spine. When this area is bruised during physical activities, such as in contact sports, the injury is known as a hip pointer. Just below this is the anterior inferior iliac spine. 
• At the back, the iliac crest ends at the posterior superior iliac spine, with the posterior inferior iliac spine just beneath it. These spines provide attachment points for tendons of muscles in the trunk, hip, and thighs. Below the posterior inferior iliac spine lies the greater sciatic notch, through which the sciatic nerve, the longest nerve in the body, passes along with other nerves and muscles.
• The inner surface of the ilium features the iliac fossa, a concave area where the iliacus muscle attaches. Just behind this fossa is the iliac tuberosity, where the sacroiliac ligament attaches, and the auricular surface, which connects with the sacrum to form the sacroiliac joint. Extending forward and downward from the auricular surface is the arcuate line. 
• The lateral surface of the ilium is marked by three arched lines—the posterior gluteal line, anterior gluteal line, and inferior gluteal line —which serve as attachment points for the gluteal muscles.

4.0Ischium 

• The ischium (IS-kee-um), which forms the lower and rear part of the hip bone, consists of a superior body and an inferior ramus (plural: rami). 
• The ramus is the part of the ischium that connects with the pubis. Notable features of the ischium include the prominent ischial spine, the lesser sciatic notch located just below the spine, and the rough, thickened ischial tuberosity. 
• Since the tuberosity lies just beneath the skin, sitting on a hard surface for an extended period often causes discomfort.
• The ramus of the ischium, along with the pubis, surrounds the obturator foramen, the largest foramen in the skeleton. 
• Despite allowing the passage of blood vessels and nerves, the foramen is mostly covered by the fibrous obturator membrane, which gives the foramen its name ("obturator" meaning "closed up").

5.0Pubis

• The pubis (plural: pubes), also known as the pubic bone, forms the front and lower portion of the hip bone. It consists of three main parts: the superior ramus, inferior ramus, and the body, which lies between the two rami. The pubic crest forms the upper border of the body, and at its outer end is the pubic tubercle, a small projection. 
• This tubercle marks the start of the pectineal line, which extends upward and outward along the superior ramus, eventually merging with the arcuate line of the ilium. These lines serve as important landmarks for differentiating between the upper (false) and lower (true) portions of the bony pelvis.
• The pubic symphysis is the joint that connects the two pubic bones of the pelvis. It is composed of a fibrocartilage disc. Beneath this joint, the inferior rami of the two pubic bones come together to form the pubic arch. During the later stages of pregnancy, the hormone relaxin, produced by the ovaries and placenta, increases the flexibility of the pubic symphysis, making childbirth easier. This loosening of the joint, along with a shifted center of gravity from the growing uterus, also affects the mother's gait during pregnancy.
• The acetabulum, a deep, cup-like depression formed by the union of the ilium, ischium, and pubis, acts as the socket for the rounded head of the femur, creating the hip joint. On the lower part of the acetabulum is the acetabular notch, an indentation that forms an opening (foramen) for the passage of blood vessels and nerves and provides an attachment point for ligaments, such as the ligament of the head of the femur.

6.0Articulated Pelvis

Each hip bone articulates behind the sacrum, forming the Sacro-iliac joint in front of its fellow opposite side at the Pubic symphysis.

Pelvis is divided into 2 parts by the Pelvic brim:

• True pelvis or Pelvis minor: lies below pelvic brim.
• False pelvis or Pelvis major: lies above pelvic brim.

Pelvic brim is formed by :

• Sacral Promontory 
• Arcuate Line of the sacrum and ilium  
• Iliopectineal Line 
• Pubic Crest 
• Upper Portion of the Pubic Symphysis

False and True Pelvis 

• The bony pelvis is divided into two sections, the superior and inferior portions, separated by the pelvic brim. This brim forms the boundary between the pelvic cavity and the abdomen, creating an inlet into the pelvic cavity. 
• The pelvic brim can be traced using various landmarks on the hip bones, forming an oblique plane. Starting from the sacral promontory at the back, the brim extends laterally along the arcuate lines of the ilium, then continues downward along the pectineal lines of the pubis, and finally moves forward along the pubic crest to the upper portion of the pubic symphysis. 
• Together, these points outline an oblique plane that sits higher in the back than the front, marking the circumference of the pelvic brim.
• The part of the pelvis above the pelvic brim is known as the false (greater) pelvis. It is bordered by the lumbar vertebrae at the back, the upper parts of the hip bones on the sides, and the abdominal wall in front. 
• The false pelvis is part of the lower abdomen and contains structures such as the upper portion of the urinary bladder (when full), the lower intestines in both genders, and in females, the uterus, ovaries, and uterine tubes.
• Below the pelvic brim lies the true (lesser) pelvis, which includes a defined inlet, outlet, and cavity. It is bordered by the sacrum and coccyx at the back, the lower parts of the ilium and ischium on the sides, and the pubic bones at the front. 
• The true pelvis surrounds the pelvic cavity and contains the rectum and urinary bladder in both genders. In females, it also holds the vagina and cervix, while in males, it contains the prostate.
• The pelvic inlet, which marks the upper opening of the true pelvis, is outlined by the pelvic brim. The pelvic outlet the lower opening is covered by muscles forming the floor of the pelvis. The pelvic axis is an imaginary curved line running through the true pelvis, from the center of the pelvic inlet to the center of the pelvic outlet.