The human spine forms a complex and adaptable structure that supports the body, enables movement, and protects the delicate spinal cord. It is typically composed of individual bones called vertebrae, arranged in distinct regions from the neck down to the tailbone. While the spine generally follows a standard anatomical pattern, natural variations can occur in its development.
Understanding Spinal Variations
A typical adult human spine contains 24 movable vertebrae, with additional fused vertebrae forming the sacrum and coccyx at the base. These 24 movable vertebrae are divided into seven cervical (neck), twelve thoracic (upper back), and five lumbar (lower back) segments. An “extra vertebra” usually refers to a variation where one segment of the spine takes on characteristics of an adjacent region, or where an additional bony structure forms.
Two common types of these variations are lumbarization and sacralization, which involve the lower back and pelvis. Lumbarization occurs when the first sacral vertebra (S1), normally fused within the sacrum, appears as a separate, sixth lumbar-like vertebra. Conversely, sacralization happens when the fifth lumbar vertebra (L5) partially or completely fuses with the sacrum or the pelvis. Another distinct variation is a cervical rib, which is an additional rib originating from one of the cervical vertebrae, most commonly the seventh (C7).
How Often Extra Vertebrae Occur
The occurrence of these spinal variations, often collectively referred to as lumbosacral transitional vertebrae (LSTV), is common. Prevalence rates for LSTV in the general population range widely, from approximately 2.6% to 40% across different studies, influenced by diagnostic methods and the specific populations examined. For sacralization, estimates range from 1.7% to 36%. Lumbarization occurs in about 3.4% to 7.2% of individuals.
Cervical ribs are less frequent but still present in a notable portion of the population. Their prevalence is estimated to be between 0.05% and 6.2%, with many studies citing figures around 0.2% to 0.5%, or about 1 in 200 to 500 people. A significant number of individuals with these anatomical differences remain unaware of them throughout their lives because the variations often do not cause any symptoms.
Health Implications and Identification
In some cases, these anatomical differences can contribute to specific health concerns. For example, a sacralized L5 vertebra might alter the biomechanics of the lower back, potentially leading to localized back pain, nerve compression, or accelerated degeneration of adjacent spinal discs. Similarly, lumbarization can result in lower back pain, stiffness, reduced mobility, or sciatica due to altered spinal mechanics or nerve irritation.
A cervical rib can sometimes compress nerves or blood vessels in the neck and shoulder area, a condition known as thoracic outlet syndrome. This compression can lead to symptoms such as pain, weakness, numbness, or tingling in the arm and hand. These spinal variations are identified through medical imaging techniques such as X-rays, computed tomography (CT) scans, or magnetic resonance imaging (MRI), which provide detailed views of the bone and surrounding soft tissues.