The coxal bone, a fundamental part of the human skeletal system, connects the upper body to the lower limbs. This complex structure supports body weight and enables various movements. Its unique design provides both stability and mobility, essential for daily activities like walking, running, and maintaining balance.
Understanding the Coxal Bone
The coxal bone, also known as the hip bone or innominate bone, is a large, irregularly shaped bone. Two coxal bones join to create the pelvic girdle, forming the sides and anterior wall of the pelvic cavity. While functioning as a single unit in adulthood, it is composed of three distinct parts that fuse during development. Together with the sacrum and coccyx, the coxal bones complete the bony pelvis, providing a sturdy framework for the lower body.
Key Anatomical Components
Each coxal bone forms from the fusion of three distinct bones: the ilium, ischium, and pubis. These components are distinct during youth but fully fuse by the end of puberty. Their union occurs around the acetabulum, a cup-shaped cavity on the bone’s outer surface. This acetabulum is where the thigh bone (femur) articulates to form the hip joint.
The ilium is the largest, uppermost part of the coxal bone. It forms the broad, flaring portion of the hip and connects posteriorly with the sacrum at the sacroiliac joint. Landmarks include the iliac crest (the curved upper margin), the anterior superior iliac spine (ASIS), and the posterior superior iliac spine (PSIS). The iliac fossa, a shallow depression, is on its internal surface.
The ischium forms the lower, posterior part of the coxal bone, often called the “sit-down bone” because it bears body weight when seated. The ischial tuberosity is a roughened area that supports this weight. Superior to the tuberosity is the ischial spine, and the greater sciatic notch allows passage for nerves and blood vessels. The ischium contributes to the obturator foramen, a large opening within the bone.
The pubis forms the anterior, inferior portion of the coxal bone. It connects with the pubis of the opposite coxal bone at the pubic symphysis, a cartilaginous joint in the midline. The pubis consists of a body and two rami. The pubic crest forms the anterosuperior border of the united pubic bodies.
Functional Significance and Articulations
The coxal bones perform several functions, including supporting upper body weight and transferring it to the lower limbs. They also provide attachment points for muscles and ligaments involved in hip and thigh movements. As part of the pelvic girdle, the coxal bones protect pelvic organs such as the bladder, reproductive organs, and parts of the intestine.
The coxal bone forms three joints, each contributing to stability and mobility. The hip joint, or coxofemoral joint, is a ball-and-socket synovial joint formed by the articulation of the femoral head with the acetabulum of the coxal bone. This joint allows for a wide range of motion, including flexion, extension, abduction, adduction, and rotation, while maintaining stability due to the deep acetabular socket.
The sacroiliac joints connect each ilium to the sacrum. These joints are largely immobile but transmit body weight from the axial skeleton to the lower limbs. The pubic symphysis, a cartilaginous joint, joins the two coxal bones anteriorly. This joint allows limited movement, which can increase during childbirth.
Common Issues and Conditions
The coxal bone and its associated joints can be affected by various conditions, leading to pain and impaired function. Fractures are common, particularly hip and pelvic fractures. Hip fractures, often involving the upper femur near the hip joint, frequently occur in older adults due to falls, especially with osteoporosis. In younger individuals, high-impact events like car accidents or falls can cause hip or pelvic fractures.
Pelvic fractures can range from minor breaks to severe injuries that may involve damage to surrounding organs, nerves, or blood vessels. Stable pelvic fractures often result from low-impact events, such as minor falls or stress from activities like jogging. Unstable pelvic fractures, which involve multiple breaks and displacement, are typically caused by high-energy trauma.
Osteoarthritis, a wear-and-tear condition, can affect the hip joint, causing pain and stiffness. This degenerative joint disease involves the breakdown of cartilage within the joint.
The sacroiliac joints can also become inflamed, a condition known as sacroiliitis, which causes pain in the lower back and buttocks. Sacroiliitis can result from injury, various forms of arthritis, pregnancy, or infection. Pelvic pain can also arise from misalignment, stiffness, or movement problems related to overuse or poor posture.