The posterior skeleton is the framework for the back of the human body, supporting the upper body’s weight and anchoring muscles. It includes the vertebral column, the back of the skull, the posterior rib cage, and the pelvic girdle. Understanding this structure is fundamental to appreciating how the body maintains posture, protects internal systems, and facilitates movement.
Anatomical Breakdown of the Posterior Skeleton
The vertebral column, or spine, is the central pillar of the posterior skeleton, composed of vertebrae that are divided into five regions: seven cervical (neck), twelve thoracic (mid-back), five lumbar (lower back), five sacral, and four coccygeal.
Each vertebra consists of an anterior body for weight-bearing and a posterior vertebral arch that encloses the spinal cord. Between each vertebra lies an intervertebral disc, a cartilaginous cushion that absorbs shock and allows for movement.
The pelvis forms the base of the posterior skeleton, connecting the spine to the lower limbs. Each of the two hip bones is formed from the fusion of three separate bones: the ilium, ischium, and pubis. The ilium is the large, wing-like superior part; the ischium is the posteroinferior part that bears weight when sitting; and the pubis is the anterior part. Posteriorly, the two iliac bones articulate with the sacrum at the sacroiliac joints, forming a strong, weight-bearing ring.
The posterior portion of the rib cage is also part of this framework. All twelve pairs of ribs articulate with the thoracic vertebrae posteriorly. A typical rib connects to two adjacent vertebrae, which creates a protective enclosure for the heart and lungs and provides attachment points for muscles.
At the top of the posterior skeleton is the occipital bone, which forms the back and base of the skull. It features a large opening called the foramen magnum, through which the brainstem connects to the spinal cord. On either side of the foramen magnum are the occipital condyles, smooth surfaces that articulate with the first cervical vertebra (the atlas). This atlanto-occipital joint allows for the nodding motion of the head.
Essential Roles of the Posterior Skeleton
The primary role of the posterior skeleton is to provide structural support and maintain an upright posture. The vertebral column acts as a flexible rod that bears the weight of the head, trunk, and upper limbs, transferring it to the pelvic girdle. The natural curves of the spine help to absorb shock and distribute body weight during activities like walking and running.
Protection of internal structures is another function. The vertebral column houses the spinal cord within its vertebral canal, shielding it from injury. The posterior rib cage and thoracic vertebrae form a protective barrier for the heart, lungs, and major blood vessels, while the pelvic bones safeguard the reproductive organs, bladder, and lower digestive tract.
This bony framework is also for movement and flexibility. The joints between the vertebrae, facilitated by intervertebral discs and facet joints, permit a range of motion, including bending and twisting. The articulation of the skull with the cervical spine allows for head movements, and the pelvic girdle provides a stable base for the lower limbs.
Development and Changes Over a Lifetime
The posterior skeleton begins forming early in fetal development through ossification, where cartilage is replaced by bone. Vertebrae develop from embryonic tissue, with primary ossification centers appearing around the seventh week of gestation. Secondary ossification centers appear after birth and contribute to vertebral growth, continuing until around age 26 when the vertebrae are fully formed.
Similarly, the ilium, ischium, and pubis of the pelvis develop from separate cartilages. They begin to fuse during puberty, a process completed in early adulthood.
The curves of the spine are not fully present at birth. An infant’s spine has a single C-shaped curve, known as a primary curvature, retained in the thoracic and sacral regions. As a baby learns to lift its head, the cervical curve develops, and when they begin to stand and walk, the lumbar curve forms to balance the body’s weight.
Fusion of certain bones is a natural part of skeletal maturation. The five sacral vertebrae begin to fuse during development and become a single bone—the sacrum—by early adulthood. The four small coccygeal vertebrae also fuse to form the coccyx, or tailbone.
Growth in the length of the vertebral column occurs at growth plates located at the top and bottom of each vertebral body. As a person ages, changes in bone density become more common. While bone is constantly remodeled, the rate of bone loss can eventually outpace bone formation, leading to a gradual decrease in density and strength.
Common Health Concerns for the Posterior Skeleton
Postural issues are a frequent concern that can lead to discomfort and structural changes in the vertebral column. Conditions like kyphosis (an exaggerated outward curve of the thoracic spine) and lordosis (an excessive inward curve of the lumbar spine) can result from poor posture or other underlying conditions. Chronic poor posture places uneven stress on the vertebrae and intervertebral discs, contributing to long-term back pain.
Injuries to the posterior skeleton range from minor to severe. A herniated disc occurs when the soft center of an intervertebral disc pushes through a tear in the exterior, potentially pressing on spinal nerves and causing pain, numbness, or weakness. Vertebral fractures can result from significant trauma, like a fall or car accident, or occur with minimal stress in individuals with weakened bones.
Degenerative conditions often affect the posterior skeleton with age. Osteoarthritis, the most common form of arthritis, can cause the cartilage in the facet joints of the spine to wear down, leading to pain and stiffness. Degenerative disc disease is another age-related condition where intervertebral discs lose hydration and height, reducing their ability to cushion the vertebrae and potentially leading to pain.
Certain skeletal disorders can impact the structure and function of the posterior skeleton. Scoliosis is a condition characterized by an abnormal sideways curvature of the spine, which may require monitoring or intervention. Osteoporosis is a disease that causes bones to become weak and brittle, making them highly susceptible to fractures, particularly in the vertebrae of the spine.