People often use the term “hip bones” to describe the prominent bony structures around their hips. However, what is commonly called a “hip bone” is not a single bone but a complex anatomical structure. This structure plays a central role in connecting the upper body to the lower limbs and forms part of a larger bony ring. Understanding its components provides insight into how the body supports weight and allows movement.
The Coxal Bone
The primary bone commonly referred to as the “hip bone” is technically known as the coxal bone. This large, irregularly shaped bone is also called the innominate bone or pelvic bone. Humans have two coxal bones, one on each side of the body, which meet in the front.
These bones are constricted in the center and expand both above and below, contributing to the overall structure of the pelvis. The coxal bone connects the axial skeleton, including the spine, to the lower limbs. It forms a socket, called the acetabulum, which articulates with the head of the femur to create the hip joint. This articulation is important for movement and weight bearing.
The Three Fused Components
Each coxal bone is not a singular entity from birth; rather, it is formed from three distinct bones that fuse during development. These three components are the ilium, ischium, and pubis. These individual bones begin to fuse during puberty, completing by the late teenage years or early twenties.
The ilium is the largest and most superior part of the coxal bone, forming the broad, flared upper portion. Its superior margin is thickened and known as the iliac crest, which can be felt along the top of the hip and serves as an attachment point for various muscles.
The ischium forms the posteroinferior part of the hip bone and is often referred to as the “sit bone” because it bears the body’s weight when sitting. It features a prominent roughened area called the ischial tuberosity.
The pubis, or pubic bone, constitutes the anterior and inferior part of the coxal bone and is the smallest of the three components. It curves inward to meet the pubis from the opposite side at the pubic symphysis.
The Pelvic Structure
The two coxal bones, along with the sacrum and coccyx, collectively form the complete pelvic girdle, often simply called the pelvis. This basin-shaped structure is a strong framework that provides stability and support to the body. The pelvis supports the weight of the upper body, transmitting it to the lower limbs, and protects several internal organs located within the pelvic cavity, including parts of the urinary, digestive, and reproductive systems. It also provides numerous attachment points for muscles and ligaments important for movement of the trunk and lower limbs.