Coccygeal Ligament Anatomy and Its Role in Tailbone Pain

The coccygeal ligament is a thin, fibrous cord that anchors the bottom of the spinal cord’s protective coverings to the tailbone, serving as one of the final tethering points of the central nervous system. In strict anatomical usage the term refers to the filum terminale externum, but the coccyx is also stabilized by a separate set of ligaments, including the anococcygeal ligament and the sacrococcygeal ligaments, all of which play distinct roles in pelvic stability, continence, and pain. Because the word “coccygeal ligament” floats across several structures in different textbooks and clinical contexts, understanding which structure is being discussed matters for anyone trying to make sense of a diagnosis, an imaging report, or a surgical plan.

The Filum Terminale Externum

The structure most formally called the coccygeal ligament is the filum terminale externum (FTE), the outermost portion of a slender strand of tissue called the filum terminale. The filum terminale as a whole is a three-dimensional, fibro-cellular structure made of connective tissue and glial cells that extends from the tapering end of the spinal cord (the conus medullaris) down to the coccyx. It divides into two segments: an intradural portion called the filum terminale internum, which sits inside the spinal fluid sac, and an extradural portion, the FTE, which exits the sac and attaches to the back of the coccyx.¹ The FTE is usually only a few centimeters long and blends with the periosteum of the tailbone at its lower end. It is sometimes described as the “anchor cable” of the spinal cord, because it limits upward and downward movement of the cord within the spinal canal.

Structurally, the FTE is not the robust, rope-like band most people picture when they hear the word “ligament.” It is delicate, partly elastic, and woven through with tiny blood vessels. Its composition matters clinically because changes in its elasticity can pull on the spinal cord in ways that produce neurological symptoms, a condition discussed further below.

The Anococcygeal Ligament

A completely different structure sharing the coccyx as an attachment point is the anococcygeal ligament (ACL), a fibrous band running between the back of the anal canal and the front of the coccyx. Despite occupying a small physical space, the ACL is surprisingly complex. Cadaveric dissections have shown that it consists of at least two distinct layers. The ventral (front-facing) layer runs from the presacral fascia to the longitudinal muscle layer of the anal canal, while the dorsal (back-facing) layer forms a bundle between the coccyx and the external anal sphincter.² Both layers contain smooth muscle fibers, elastic fibers, and small blood vessels, making the ACL more than a passive strap. It actively participates in the support network that keeps the anal canal positioned correctly.

Further histological work has refined the picture. In elderly male cadavers, the superficial portion of the ACL was found to be made of highly tortuous elastic fibers, with a fine elastic mesh at the coccygeal end, while the deeper portion was composed of straighter collagen and elastic fibers that blended directly into the coccygeal periosteum.³ The tortuous elastic design in the superficial layer suggests it is built to absorb stretch and recoil during activities like sitting down, standing up, and defecation. When those fibers degrade with age, the support they provide weakens, and that has implications for continence.

Why the Anococcygeal Ligament Matters for Continence

The ACL acts as a rear anchor for the pelvic floor’s midline structures. It holds the back wall of the anal canal against the coccyx so that the sphincter muscles can close effectively. When the elastic fibers in the superficial layer lose their recoil or when the collagen fibers in the deep layer thin out, the anal canal can shift position slightly. That shift reduces the mechanical advantage of the sphincter muscles and can contribute to fecal incontinence, particularly in older adults.

The cadaveric findings in elderly men described above are consistent with this idea: the very tortuous elastic fibers that characterized the superficial ACL suggest a structure designed for repeated loading and unloading over a lifetime, and age-related deterioration of those fibers would logically impair that function.⁴ This is one reason surgeons operating in the space between the anus and the coccyx, such as during rectal cancer resections or fistula repairs, take care to identify and preserve the ACL when possible. Disrupting it can worsen post-surgical continence problems.

The Sacrococcygeal Ligaments and Joint

The coccyx is joined to the sacrum above it by a set of sacrococcygeal ligaments, including anterior, posterior, and lateral bands. These ligaments stabilize the sacrococcygeal joint, which in some people is a true synovial joint and in others fuses entirely into a bony bridge. The degree of mobility at this joint varies enormously from person to person and tends to decrease with age as the joint ossifies.

Coccygeal hypermobility, where the tailbone flexes too far when a person sits, is one of the recognized causes of coccydynia, the clinical term for persistent tailbone pain. A coccyx is considered hypermobile when it subluxates more than 25 percent posteriorly during sitting or flexes more than 25 degrees while seated, with subluxation beyond 35 percent considered significant hypermobility.⁵ When the sacrococcygeal ligaments are lax enough to allow this excessive motion, the surrounding soft tissues, including the ACL and the periosteal attachments of the filum terminale, are subjected to repetitive strain. That strain can become a chronic source of pain that is difficult to localize on a standard exam.

Connection to Tethered Cord Syndrome

The filum terminale externum becomes clinically important in tethered cord syndrome (TCS), a condition in which the spinal cord is pulled downward by inelastic tissue at its caudal end. The essential mechanical factor is that any inelastic structure fastening the lower end of the spinal cord produces traction on the lumbosacral cord, and that traction is the key to understanding the disorder.⁶ In a healthy filum terminale, the tissue has enough give to accommodate normal movement, bending, and growth. When the filum becomes thickened, fibrotic, or loaded with fat, it loses that elasticity and begins to act like a taut leash on the spinal cord.

Scanning electron microscopy of filum terminale samples taken from patients with tethered cord syndrome has shown that the three-dimensional architecture of the filum is disrupted. The loss of elasticity resulting from that structural breakdown is directly involved in the tethering.⁷ In children, this can interfere with neurological development and produce symptoms like leg weakness, back pain, bladder dysfunction, and foot deformities. In adults, the symptoms tend to be subtler, often starting as low back pain that worsens with physical activity and may be accompanied by urinary urgency or numbness in the legs.

Treatment for TCS often involves surgically dividing the filum terminale to release the tension on the spinal cord. The procedure is usually performed through a small opening in the lower spine, and outcomes depend on how long the cord has been under tension and how much neurological damage has already occurred. Because the FTE is the portion of the filum that extends beyond the dural sac and attaches to the coccyx, it is the final link in the chain and the one most accessible to the surgeon.

Coccydynia and the Soft Tissues Around the Tailbone

Coccydynia is far more common than tethered cord syndrome and brings the coccygeal ligaments into clinical focus for a different reason. Tailbone pain often develops after a fall or direct trauma, though it also appears without any identifiable injury. Recognized risk factors include obesity and female sex, along with unusual coccyx shape and coccygeal hypermobility.⁸ The pain typically worsens with prolonged sitting, especially on hard surfaces, and improves when standing or walking.

What makes coccydynia frustrating is that standard X-rays taken while the patient is standing often look entirely normal. The problem becomes visible only on dynamic imaging, where lateral radiographs are taken in both standing and seated positions. That comparison can reveal abnormal coccygeal motion that is predictive of pain, as well as subtle cross-sectional findings that might otherwise be overlooked.⁹ Dynamic seated and standing radiographs are considered the preferred initial imaging approach, with CT and MRI reserved for cases where structural, degenerative, or inflammatory pathology needs to be further characterized.¹⁰

The ligaments themselves do not show up well on plain X-rays, which is why the diagnosis is often clinical. When MRI is used, it can reveal inflammation or edema in the soft tissues surrounding the coccyx, including the sacrococcygeal and anococcygeal ligaments, and it can help distinguish ligamentous pain from other causes like coccygeal tumors or infections.

Injection Therapy for Coccyx Pain

When conservative measures like cushioned seating, physical therapy, and anti-inflammatory medications fail to resolve coccydynia, injections of local anesthetic and steroid around the coccyx are a common next step. Interestingly, although the medical literature often describes these as joint injections targeting the sacrococcygeal or intercoccygeal joints, the reality is that many of these injections end up in the soft tissue surrounding the coccyx rather than inside a joint space. The distinction matters because the coccygeal ligaments and periosteum are densely innervated, and reducing inflammation in those tissues may be what actually provides relief.

Study findings on injection success rates are mixed. One prospective study using ultrasound-guided injections reported a success rate of about 60 percent, though it was noted that the injections likely landed in the surrounding soft tissue rather than intra-articularly. Patients whose pain had persisted for less than six months were more likely to experience significant relief from steroid injections.¹¹ This suggests that the earlier the intervention, the better the odds, and that chronic cases may involve tissue changes in the coccygeal ligaments and periosteum that are harder to reverse with a single injection.

Pregnancy, Pelvic Ligaments, and the Coccyx

Pregnant women frequently experience pelvic girdle pain, and it is commonly assumed that the hormone relaxin, which rises during pregnancy and loosens ligaments to prepare for delivery, is responsible. This loosening affects the sacroiliac joints and pubic symphysis, and it is reasonable to wonder whether the sacrococcygeal and anococcygeal ligaments are similarly affected. However, a systematic review of high-quality studies found that the evidence linking pregnancy-related pelvic girdle pain to relaxin levels was actually low, with three of the better-designed studies finding no association at all.¹²

That does not mean pregnancy has no effect on coccygeal structures. The coccyx is pushed backward during vaginal delivery, sometimes dramatically, and the sacrococcygeal ligaments can be stretched or partially torn in the process. Some women develop coccydynia for the first time postpartum, and the mechanism is likely mechanical trauma to the coccyx and its ligaments during delivery rather than a hormonal softening of those same tissues. The practical implication is that persistent tailbone pain after childbirth should be evaluated with dynamic imaging rather than attributed to lingering hormonal effects.

When Terminology Gets Confusing

One reason the topic of coccygeal ligaments can be confusing is that different clinical specialties use the same words to mean different things. A neurosurgeon discussing the “coccygeal ligament” almost certainly means the filum terminale externum. A colorectal surgeon is probably talking about the anococcygeal ligament. An orthopedic specialist or physiatrist dealing with tailbone pain is likely focused on the sacrococcygeal ligaments. And radiologists reporting on MRI scans may use any of these terms depending on the clinical question they were asked to answer.

Adding to the confusion, older anatomy textbooks sometimes use “coccygeal ligament” as a catch-all for any fibrous structure attaching to the coccyx. More recent anatomical literature tends to be precise, but clinical notes and imaging reports do not always follow suit. If you receive a report or diagnosis mentioning a coccygeal ligament, clarifying which structure is meant can save considerable confusion. The filum terminale externum is a spinal structure with neurological implications, the ACL is a pelvic floor structure with implications for continence and rectal surgery, and the sacrococcygeal ligaments are joint-stabilizing structures relevant to pain and mobility. They are neighbors, but they do very different jobs.

How the Coccygeal Ligaments Change with Age

All ligamentous tissues change over a lifetime, but the coccygeal region undergoes some distinctive shifts. The sacrococcygeal joint tends to fuse in many adults, usually starting in the fourth or fifth decade. Once that fusion is complete, the sacrococcygeal ligaments become less clinically relevant because the joint they stabilize no longer moves. Paradoxically, incomplete fusion with ligamentous laxity can be more problematic, producing the hypermobility pattern that is a recognized driver of coccydynia.¹³

The anococcygeal ligament undergoes its own age-related decline. The tortuous elastic fibers in the superficial layer gradually lose their spring, and the collagen in the deep layer can thin and fragment.¹⁴ These changes happen quietly and are rarely diagnosed in isolation. Instead, they contribute to the broader pelvic floor weakening that underlies many cases of late-life fecal incontinence. Strengthening the pelvic floor through targeted exercise can help compensate for some of this ligamentous deterioration, though it cannot reverse the structural changes in the ligament itself.

The filum terminale is somewhat different. In most people it remains stable throughout life and never causes symptoms. When it does become pathological, as in tethered cord syndrome, the problem is usually congenital or develops in childhood rather than appearing for the first time in old age. There are cases of adult-onset tethered cord, but they are comparatively rare and are often triggered by a specific event like spinal surgery, infection, or trauma rather than by normal aging of the filum.