The coxal bone, commonly known as the hip bone or innominate bone, is a large, irregularly shaped bone that forms a significant part of the human skeleton. It acts as a foundational component of the pelvic girdle, a basin-shaped structure connecting the trunk to the lower limbs. This bone plays an important role in connecting the axial skeleton (skull, vertebral column, and rib cage) to the appendicular skeleton of the lower extremities.
Anatomical Location
Two coxal bones are positioned on each side of the lower trunk, contributing to the lateral and anterior boundaries of the pelvic cavity. Each coxal bone forms a strong articulation posteriorly with the sacrum, a triangular bone at the base of the spine, creating the sacroiliac joint. This joint provides stability and transfers forces between the spine and lower limbs.
Laterally, the coxal bone features a deep, cup-shaped socket known as the acetabulum. This socket articulates with the head of the femur, the thigh bone, forming the hip joint. The hip joint is a ball-and-socket joint, allowing for a wide range of motion in the lower limb. The coxal bones support the weight of the upper body, distributing it to the legs during activities such as standing, walking, and running. This arrangement also creates a stable base for the trunk, linking it to the movements of the lower body.
Component Bones
The coxal bone is not a single entity but develops from the fusion of three distinct bones: the ilium, ischium, and pubis. These three components are separate in children, connected by cartilage, and typically fuse completely into one solid bone by adulthood, generally between the ages of 15 and 25. The point where these three bones meet and fuse is at the acetabulum.
The ilium is the largest and most superior part of the coxal bone. Its broad, flared upper portion, known as the iliac crest, is easily palpable and serves as a significant anatomical landmark. The ischium forms the posterior and inferior region of the coxal bone and is sometimes referred to as the “sit bone” because it bears the body’s weight when seated.
The pubis is the smallest and most anterior-inferior portion of the coxal bone. It joins with the ischium to form a large opening called the obturator foramen, which is mostly covered by connective tissue. The two pubic bones also meet at the front of the pelvis, forming the pubic symphysis, a cartilaginous joint.
Key Functions
The coxal bone performs several important functions that are fundamental to human movement and bodily support. One primary function is weight bearing and the efficient transfer of weight. It transmits the entire weight of the upper body, including the torso, head, and arms, through the sacrum to the lower limbs, enabling upright posture and locomotion. This structural role allows individuals to stand, walk, and run while maintaining balance.
Another significant function of the coxal bone is providing extensive attachment points for numerous muscles. Muscles of the trunk, hip, and thigh originate from or insert onto various surfaces and projections of the coxal bone. These muscle attachments are fundamental for movements of the hip joint, such as flexion, extension, abduction, and adduction, which are essential for daily activities. Furthermore, the coxal bones, along with the sacrum and coccyx, form the bony pelvis, which acts as a protective basin for internal organs. This includes safeguarding delicate structures such as the urinary bladder, parts of the lower intestine, and reproductive organs within the pelvic cavity.