The term “hip bone” is frequently used in everyday language to refer to the large, irregular bone that forms the side of the pelvis. This common name is not the formal anatomical designation for the structure. In anatomy, the hip bone is a complex component of the skeletal system known by several precise scientific names. Understanding the correct nomenclature is important because the single hip bone is actually made up of three distinct parts that fuse together as a person matures.
The Anatomical Name of the Hip Bone
The single bone commonly referred to as the hip bone is officially called the Os Coxae (Latin for “hip bone”) or the Innominate bone (“unnamed” bone). Each Os Coxae is a large, irregularly shaped structure that begins as three separate bones in childhood. These three components gradually fuse together, typically completing the process by the late teenage years, to form the single adult hip bone.
The largest and uppermost section is the Ilium, which forms the wide, fan-shaped flare felt just below the waistline. Its uppermost border, the iliac crest, is an attachment point for numerous muscles. The lower, posterior portion is the Ischium, often known colloquially as the “sit bone” because its projection, the ischial tuberosity, bears the body’s weight when seated.
The third component is the Pubis, which forms the anterior section of the Os Coxae. The Pubis articulates with the pubic bone from the opposite side, forming a joint at the midline of the body. These three distinct regions—the Ilium, Ischium, and Pubis—converge on the lateral surface of the bone.
This convergence point forms the Acetabulum, a deep, cup-shaped socket that receives the head of the femur (thigh bone) to create the hip joint. The name Acetabulum is Latin for “vinegar cup,” referring to its shape. This deep socket design provides the joint with stability, connecting the lower limb to the torso.
The Larger Structure: The Pelvic Girdle
The two Os Coxae bones join with other bones to create a complete ring structure known as the Pelvic Girdle, or bony pelvis. This girdle serves as a stable connection point between the axial skeleton and the lower limbs. The Pelvic Girdle encompasses more than just the two hip bones, which is a key difference in scope.
The girdle is completed at the back by the Sacrum, a large, triangular bone formed by the fusion of five vertebrae, which sits between the two Ilium sections. Inferior to the Sacrum is the Coccyx, or tailbone, which is also part of the structure. The joints connecting these components are important for the girdle’s stability and function.
Anteriorly, the two Pubis bones meet at the Pubic Symphysis, a slightly movable joint composed of fibrocartilage. Posteriorly, the Os Coxae articulates with the Sacrum at the Sacroiliac Joints. These joints are reinforced by strong ligaments to limit movement. The bony pelvis shows differences between sexes, with the female pelvis typically wider and shallower, an adaptation related to childbirth.
Essential Roles of the Hip Structure
The hip and pelvic structure performs several functions necessary for human movement and protection. A primary role is the transfer of weight from the upper body, through the spine and the pelvic girdle, and into the lower limbs. This structural design allows the body to maintain an upright posture and effectively distribute forces during standing, walking, and running.
The surfaces of the Os Coxae provide extensive attachment sites for numerous muscles, including the gluteal muscles and the hamstrings. These muscles are responsible for locomotion and stabilizing the trunk over the legs. A network of strong ligaments also secures the joints, ensuring stability while allowing for a wide range of motion at the hip joint.
Beyond its biomechanical function, the bony pelvis creates a protective basin for several internal organs. It shields portions of the intestines, the urinary bladder, and the internal reproductive organs from external forces. The sturdy pelvic ring ensures these soft tissues are safeguarded within the skeletal frame.