The Human Back
The anatomy of the human back is very complex and is composed of various bones, muscles, tendons and ligaments extending from the base of the skull to the pelvis. Knowledge about the anatomy of the back helps in better understanding of the diagnosis and treatments available for back pain.
The major bones of the human back include:
- Shoulder blades (scapula)
- Rib cage (thoracic cage)
- Spine (vertebral column)
Shoulder Blades (Scapulae)
The shoulder blades are flat, triangular bones present in the upper back; connecting upper arm bones to the collar bone or clavicle. The shoulder blades are held in position by the attached muscles, tendons and ligaments. Some of the muscles are attached to the spine of the scapula and help in movement of the shoulder. Different muscles of the scapula are engaged in diverse movements and functions which are as follows:
- Levator scapula muscle raises the shoulder blade
- Pectoralis minor muscle lowers the shoulder blade
- Trapezius muscle raises and rotates the collar bone
- Rhomboideus major muscle rotates the blade inward
- Serratus anterior muscle stabilizes shoulder blade
Rib cage (Thoracic cage)
The rib cage is a semi-rigid structure comprising of bone and cartilage that covers the thoracic cavity. The thoracic cage is composed of 24 ribs, the breast bone or sternum, costal cartilages (placed between the ribs and sternum), and the 12 thoracic vertebrae. The ribs attached to the sternum are called true ribs while those not connected to the sternum but attached to the true ribs are known as false ribs. The last two ribs are connected directly to the spine and are known as floating ribs.
Spine (vertebral column)
The spine or backbone extends from the base of the skull to the pelvis. It is a major component of the human back, responsible for maintaining structure, shape and stability of the back. The human spine has three distinct curves that increase stability and also prevent injury to the spinal cord.
The spine comprises of 33 vertebrae, stacked one above the other. Based on the vertebrae, spine can be divided into four regions:
- The cervical or neck region- It comprise of the first seven vertebrae viz. C1-C7, present in the neck
- The thoracic or upper back region- It comprise of twelve vertebrae viz. T1-T12, present in the thoracic area. The size of the vertebrae increases from the first to the twelfth vertebra
- The lumbar or lower back region- It comprises of five vertebrae viz. L1-L5, and is the common site for back pain. These vertebrae help in forward, backward and sidewise movement of the body
- The sacrum and coccyx region- It comprises of 9 vertebrae (5 in sacrum and 4 in coccyx) fused together; present at the base of the spine.
There are two facet joints at each vertebra that help in the forward, backward and twisting movements of the spine. Two vertebrae are separated by a cushion-like material called intervertebral disc. It acts as a shock absorber and helps in the movement of the spinal column. The structure of the intervertebral disc can be altered by trauma, disease, and the aging process.
The center of the column is hollow, forming a spinal canal, through which passes the spinal cord. The spinal cord extends from the base of the skull to the first lumbar vertebra. The final segment of the spinal cord is known as cauda equina. The spinal cord controls the functions of different regions of the body such as arms, chest, abdomen and back. The spinal nerves arising from the cauda equine supply the lower body parts such as legs.
The spinal nerves collect information from different parts of the body and transmit them to the brain for interpretation. A severe spinal cord injury can interrupt the transmission of information to the brain. This can result in loss of sensations, difficulty or lack of movement of the arms or legs, and even respiratory failure.
Pinched nerve is a common injury to the spinal cord that occurs due to excessive pressure exerted on the nerve by the bone, cartilage, tendon or muscles and is a painful condition. The nerve compression can be secondary to trauma, disease, or inflammation that results in pain, numbness and muscle weakness. Bone spurs due to arthritic degeneration can also infringe on the spinal nerves and can cause pain.
In addition to the discs, other structures such as muscles and ligaments also provide strength and flexibility to the spine. Based on their function, muscles supporting the spine are classified as flexors, extensors or rotators.
Ligaments are bands of fibrous tissues that connect two or more bones and provide stability to a joint, while at rest or in motion. Some ligaments are also involved in resisting movement such as hyperextension or hyper-flexion.
Lamina is a thin, plate like covering of the vertebrae that prevents the displacement of the vertebrae. The displacement of the vertebrae is known as spondylolisthesis and causes back pain due to nerve compression. Based on the cause of displacement, spondylolisthesis may be classified as congenital spondylolisthesis, degenerative spondylolisthesis, isthmic spondylolisthesis, pathological spondylolisthesis, post-surgical spondylolisthesis, and traumatic spondylolisthesis.