Coccyx

The coccyx is the final portion of the vertebral column, the last few beads on the string of bones that runs from the skull to the pelvis. It lies directly below the sacrum, the large wedge-shaped bone that forms the back wall of the pelvis, and it curves gently forward and downward into the pelvis so that its tip points towards the front of the body. Seen from behind, the whole structure is roughly triangular, broad where it meets the sacrum and tapering to a small apex at its lower end.

In evolutionary and developmental terms the coccyx is the remnant of a tail. It is composed of the coccygeal vertebrae, which are rudimentary in the sense that they have lost most of the features of a typical vertebra. Unlike the vertebrae higher up the spine, they contain no pedicles, no laminae and no spinous processes, and they enclose no part of the spinal canal, since the spinal cord itself ends well above this level. What remains is essentially a series of small vertebral bodies, diminishing in size from top to bottom, together with a pair of upward-pointing projections called the coccygeal cornua on the uppermost segment.

Despite its modest size and simplified structure, the coccyx is not inert. Its position at the very base of the trunk, in the midline behind the pelvic outlet, places it exactly where several mechanical demands converge: the transmission of load in sitting, the suspension of the pelvic floor, and the accommodation of movement during certain bodily functions.

The conventional picture of the coccyx as a single fused bone is only partly accurate. Detailed imaging studies using computed tomography and magnetic resonance imaging have shown that the coccyx contains anywhere from three to five separate bony segments, with four being the most common number. The proximal, or uppermost, segment is the largest, and each segment below it is progressively shorter and narrower, which gives the whole structure its tapering, triangular outline.

Fusion between these segments is variable and tends to be greatest at the lower, terminal joints and least at the upper ones. In many people the first coccygeal segment remains separate from those below it, and it may also remain separate from the sacrum rather than fusing to it. This is why the coccyx is best described as partly fused, or semi-fused, rather than as a solid single bone: it is a short chain of small vertebrae, some joined together and some still articulating.

The uppermost coccygeal segment carries the coccygeal cornua, two small horns that project upward to meet the corresponding sacral cornua above. Together these paired projections help to define the sacral hiatus, the opening at the lower end of the sacral canal through which certain nerves pass. Each coccygeal segment also retains a vestige of a transverse process, and when segments fuse these form shallow ridges known as the anterior and posterior transverse grooves. These features, though small, are the attachment ledges to which muscles and ligaments fix themselves.

Where the base of the coccyx meets the tip of the sacrum lies the sacrococcygeal joint. In most people this is a symphysis, a slightly mobile joint in which the two bones are separated by a thin disc of fibrocartilage, comparable in principle to the discs found between the larger vertebrae above. This fibrocartilaginous disc, reinforced by a set of surrounding ligaments, binds the coccyx to the sacrum while still permitting a small amount of movement.

The movement this joint allows is best described as a gentle nodding. The coccyx can flex, tucking its tip forward and inward towards the pelvis, and extend, swinging its tip backward and outward. This is a modest arc rather than a large hinge, and its magnitude varies considerably from person to person. Studies using dynamic imaging, in which the coccyx is examined both standing and sitting, have documented how the coccygeal angle changes as posture changes, and have described a normal range within which this flexion and extension occurs. Beyond that range the movement is generally regarded as excessive, but the everyday, normal behaviour of the joint is a small, controlled nod.

With advancing age the fibrocartilaginous disc of the sacrococcygeal joint may ossify, turning to bone, so that the coccyx becomes more firmly fused to the sacrum and the small nodding movement is progressively lost. The degree to which this happens differs between individuals and, on average, between the sexes, which is one reason the mobility of the coccyx is so variable across a population.

One of the coccyx's principal mechanical roles is to serve as an anchor. A striking number of muscles and ligaments fix to this small bone, and through these attachments it participates in the support of the pelvic organs and in the shaping of the pelvic outlet.

The most important of these attachments are the muscles of the pelvic floor. The levator ani, a broad, thin sheet of muscle that forms much of the floor of the pelvis, has portions that attach to the coccyx. Its iliococcygeus part runs back to insert onto the lower coccygeal segments, and where the fibres of the two sides meet in the midline they form a raphe that continues as the anococcygeal ligament, a strong band running between the coccyx and the anal canal. Behind the levator ani lies the coccygeus muscle, also called ischiococcygeus, which stretches from the ischial spine on each side to the lateral border of the coccyx and the lowest part of the sacrum. Together the levator ani and coccygeus make up the pelvic diaphragm, the muscular hammock that supports the bladder, bowel and, in women, the uterus, and that maintains continence.

The coccyx also anchors several ligaments, including the sacrococcygeal ligaments that stabilise its own joint and, at its margins, connections related to the sacrospinous and sacrotuberous ligaments that span the back of the pelvis. In addition, part of the gluteus maximus, the large muscle of the buttock, takes origin from the region of the coccyx and the adjacent sacrum. Through all these attachments the coccyx acts as a small but well-connected hub, transmitting the pull of the pelvic floor and buttock muscles and providing a fixed point against which they can act.

The coccyx comes into its own mechanically when a person sits. In the seated position the weight of the upper body is transmitted to the supporting surface not through the whole pelvis but through a small number of bony contact points, and the coccyx is one of them.

The classic description is of a tripod of support. Two of the three points are the ischial tuberosities, the paired bony prominences at the base of the pelvis on which a person effectively sits, one under each buttock. These are commonly and aptly called the sit bones. The third point, at the back and in the midline, is the coccyx. Together these three points form a triangular base over which the seated body is balanced, and the load is shared between them.

How the load is shared depends heavily on posture. When a person sits leaning forward, most of the weight passes through the two ischial tuberosities and the bony bars of the ischium in front of them, and relatively little reaches the coccyx. As the person leans backward, however, the centre of gravity shifts rearward and a greater share of the load is transferred onto the coccyx. Leaning back in a chair therefore loads the tailbone most, whereas leaning forward unloads it. This simple mechanical relationship, between the angle of the trunk and the distribution of weight across the tripod, is central to understanding why the coccyx is a load-bearing structure at all, and why its loading is so posture-dependent.

The small mobility of the sacrococcygeal joint is not merely incidental; it is put to use during several everyday and physiological events. In each case the coccyx moves passively, driven by external load or by the pull of surrounding muscles, rather than by any muscle that moves it directly on its own.

When a person sits down, particularly on a firm surface, the coccyx tends to flex, its tip tucking forward as the seated load presses against it. On standing again it returns towards its resting position. This flex-and-return behaviour is the joint doing its ordinary work, adjusting to the changing distribution of weight as posture shifts between standing and sitting.

During defecation the pelvic floor relaxes and the coccyx moves to allow the passage of stool, again flexing and then recovering. In these functions the coccyx behaves as a mobile back wall to the pelvic outlet, giving a little when it needs to.

The largest demand, in women, comes with childbirth. As the baby's head descends through the birth canal, the pelvic outlet must enlarge to let it pass. The coccyx extends, swinging backward out of the way, so that the front-to-back diameter of the outlet increases and more room is created for the head. This backward movement of the coccyx, together with the widening of the bony pelvis, is part of the way the maternal pelvis accommodates delivery. In each of these scenarios the same small nodding capacity of the sacrococcygeal joint is being exploited, whether to cushion the load of sitting, to permit the bowel to empty, or to open the outlet in labour.

The coccyx is one of the most frequently cited examples of a vestigial structure in the human body, a feature inherited from ancestors in which it was larger and more functional. In the primates from which humans descend, the equivalent bones formed a tail, used for balance, for grasping and for communication while moving through the trees. As the lineage leading to humans adopted upright, two-legged walking, the balancing and signalling roles of a tail were lost, and the tail itself was reduced to the short, curved remnant that is the coccyx.

This evolutionary history is retraced during development. In the early human embryo a genuine tail-like projection appears, containing more vertebral elements than the adult coccyx, and it is at its most prominent around the fifth to sixth week of gestation. Over the following weeks most of this structure regresses and is reabsorbed, leaving behind the small stack of coccygeal segments that persists into adult life. The coccygeal and sacral vertebrae become recognisable around the eighth week.

Calling the coccyx vestigial does not mean it does nothing. It has, in the course of this reduction, been repurposed as an anchor for the pelvic floor and buttock muscles and as a component of seated support. The point of the term is that its present functions are secondary and diminished compared with the balancing and grasping tail from which it descends. The coccyx is thus best understood as a small structure that has been retained and given modest new mechanical jobs rather than one that has been entirely discarded.

Few bones vary between individuals as much as the coccyx does, and this variability is a defining feature of its anatomy. The variation runs along several axes at once: the number of segments, the degree and pattern of fusion, and the overall curvature and angulation.

The number of coccygeal segments ranges from three to five, with four the commonest. The extent of fusion, both between the coccygeal segments themselves and between the coccyx and the sacrum, differs widely and tends to increase with age. Some coccyges are almost entirely fused into a single rigid bone, while others retain several mobile joints.

The shape of the coccyx also varies, and this has been formally classified. A widely used scheme, described by Postacchini and Massobrio, sorts coccyges into four types according to their curvature and angulation. In the first type the coccyx curves gently forward with its tip pointing downward; in the second the forward curvature is more marked; in the third the coccyx angles sharply forward; and in the fourth it is subluxated, meaning partially displaced, at one of its joints. There are also documented differences on average between the sexes in the coccyx's shape, mobility and degree of fusion, which relate in part to its role in childbirth. The practical consequence of all this variation is that no two coccyges are quite alike, and descriptions of its structure and mechanics are necessarily descriptions of a typical arrangement rather than a fixed one.

The coccyx, commonly called the tailbone, is the small, triangular terminal segment of the spinal column, hanging below the sacrum at the very base of the back. It is usually made up of three to five small, rudimentary vertebrae that are partially or wholly fused, four segments being the commonest arrangement. Although it is often dismissed as a functionless leftover, the coccyx is mechanically busy: it forms the rear point of a three-point base of support when a person sits, it serves as the anchor for the muscles and ligaments of the pelvic floor and for part of the gluteus maximus, and it nods through a small range of movement as a person sits, rises, opens the bowels or, in women, gives birth. This page describes, in strictly educational terms, how the coccyx is built and how it works mechanically. It is not medical advice and does not address the diagnosis or treatment of any condition.

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This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.