The human coccyx is the final portion of the vertebral column, typically comprised of three to five vertebral segments.[1] Its proximal vertebral segment articulates with the sacrum via a fibrocartilaginous joint; however, the fusion of the sacrum and coccyx has been a feature in many cases, with prevalence differing by age and sex.[2][3] Each inter-vertebral segment has also been shown to differ in the incidence of fusion with its superior and inferior counterparts. Distally, the coccyx maintains a concave-shaped curve and anteriorly angles into the pelvis. Though not readily visible, each coccygeal vertebrae does contain a transverse process, and when fused, form what is known as the anterior and posterior transverse grooves of the coccyx. The coccygeal vertebrae, however, do not contain any pedicle, lamina or spinous processes. The coccyx serves as the site of attachment for multiple pelvic ligaments and tendons and contains an extensive plexus of coccygeal nerves. Damage to any one of these components can have implications in several clinical conditions.

Conventional knowledge dictates that the coccyx is a single fused entity; however, magnetic resonance imaging (MRI) and computed tomogram (CT) based evaluations of coccygeal anatomy in different populations have shown that the coccyx contains anywhere from three to five separate bony segments, with the four segments being the most common.[4] Additionally, the fusion that does exist between the coccygeal vertebrae has is predominantly present within the terminal articulation. The most proximal coccygeal vertebrae are typically the largest and contain processes referred to as coccygeal cornua, which articulate with the superiorly placed sacral cornua.[2] Together the cornuae form the sacral foramen, allowing for the traversing of various nerves. As the coccygeal vertebra progress, the length and width of each bone decreases, and together form an overall triangular shape. Given its location, the coccyx plays a significant role in weight-bearing when in a seated position and forms a tripod of support along with the two ischial spines of the pelvis.[2] Via ligamentous connection, the coccyx, also provides a support point for the anal sphincter.[5]

During gastrulation of the human fetus, a trilaminar embryonic disk forms, containing three distinct germ layers (endoderm, mesoderm, ectoderm) along with a notochord, which is adjacent to the neural tube.[6] Between the fourth and fifth weeks of gestation, the mesodermal layer surrounding the notochord differentiates into more specialized paraxial, intermediate, and lateral mesoderm. The paraxial mesoderm then gives rise to about 44 different somites, which will further develop into the future bones of the head, vertebrae, and other bony structures of the human body. By the end of the fifth week, sclerotomes arise from the somites, and under the induction of cellular signals, migrate towards the notochord in a cranial to caudal fashion. Each sclerotome then divides into a cranial area of loosely packed cells and a caudal area of densely packed cells, between which the intervertebral disk forms. This intervertebral disk separates sclerotomes at different levels and forces the cranial cells of one sclerotome to fuse with the caudal cells of the adjacent somite, thus giving rise toa complete vertebral body. Simultaneously, cells surrounding the neural tube will begin forming vertebral arches to form a protective canal for the future spinal cord. These arches create pedicles, laminae, spinous processes, transverse processes, and articular processes, which ultimately fuse with the developing vertebral bodies, giving rise to the bony spine. Typically during the eighth week of gestation, the sacral and coccygeal vertebrae appear.

There are a vast number of vessels that supply the structures of the pelvic region. At the level of L4, the abdominal aorta bifurcates into the internal and external iliac arteries. The internal iliac artery then divides into its anterior and posterior divisions while still located within the pelvis. In contrast, the external iliac continues towards the lower extremities as the femoral arteries.[7] The posterior division of the internal iliac artery includes the iliolumbar artery, lateral sacral artery, and superior gluteal artery. Ultimately, the lateral sacral artery gives off spinal branches which supply both the sacrum and the coccyx. Additionally, directly from the aorta, median sacral arteries arise near the L4 bifurcation site.[8] These arteries develop anastomoses with the lateral sacral artery branches as well and provide more blood supply towards the sacrum and coccyx.

The coccygeal plexus primarily innervate the coccygeal vertebrae, a network of nerves originating from the ventral rami of S4, S5, and Co1 along with the dorsal rami of the sacral sympathetic trunk, converging in the area of the iliococcygeus muscle.[9] This plexus also includes anococcygeal nerves that innervate the subcutaneous tissue surrounding the coccygeal vertebrae. Along with the vertebrae and associated musculature, the coccygeal plexus also innervates the sacrospinous ligament, coccygeal ligaments, and periosteum.