The spinous process or processus spinosus in Latin is a landmark structure projecting from each of the vertebra. It acts as a lever for the muscles and ligaments of the spine that control posture and active movements (flexion, extension, lateral flexion and rotation) of the vertebral column. It can be felt through the skin as bony knobs along the vertebral column.
Each of the 33-35 vertebrae (depending on bone fusion) is unique; hence the vertebral spines vary considerably in size, shape, and direction. Most spines project dorsally and caudally from the junction of the laminae which are plates of bone that form the posterior walls of each vertebra. The spinous process lie in the median plane and project posteriorly, though minor deflection can occur.
The C3–C6 vertebrae’s spinous processes are short and bifid in white males, but no so in females and people of African descent. C7 is characterized by a long spinous process that is the most prominent in 70% of individuals. It can be felt by running one’s finger along the posterior aspect of the neck, and is called the vertebra prominens for that reason. C1 is called the atlas, and it is unique for having neither a body nor a spinous process. Vertebra C2 however, is characterized by its large bifid spinous process that can be felt deep in the nuchal groove, the superficial vertical groove at the back of the neck.
T5–T8 demonstrate all the features typical of thoracic vertebrae, their spinous processes are overlapping, which limits their movement. T1–T4 vertebrae share some of the features of the cervical vertebrae. And T1 is considered atypical for its long and almost horizontal spinous process that is nearly as prominent as that of the vertebra prominens (C7), while the T9–T12 vertebrae’s spinous processes resemble that of the lumbar vertebrae.
The lumbar spinous processes are short, sturdy, thick, broad, and hatchet shaped. Those of the upper three or four sacral vertebrae are fused. They form a central ridge called the median sacral crest. S5 however, has no spinous process. The coccyx vertebrae segments all lack spinous processes too, as well as pedicles and laminae.
Spinous process fractures are minor and can occur due to direct trauma to the vertebrae or from excessive pulling on the muscles that attach to it. Symptoms can include back pain, radiating numbness or tingling, crunching sound, and limited movement.
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