2. The Mechanical Causes of Coccydynia

1. Mechanical causes: where does it really get stuck?

When the coccyx becomes painful, it is tempting to look for a single cause: a fracture, a dislocation, a “displaced bone.”

The coccyx is a terminal point.
It sits at the end of a long, heavily loaded mechanical chain: the spine and the pelvis.
It therefore receives part of the stress generated by:

• body weight,
• prolonged sitting,
• micro-trauma,
• pelvic mobilityPelvic mobilityPelvic mobility is the pelvis's ability to change orientation relative to the spine and the femurs: forward or backward (anterior and posterior pelvic tilt), sideways (lateral tilt) and in rotation.Read the full entry : Pelvic mobility,
• the condition of the soft tissues,
• musculoligamentous attachments.

From this perspective, “mechanical” coccydyniaCoccydyniaPain located in the tailbone (coccyx) region, often worse when sitting.Read the full entry : Coccydynia does not refer only to a bony lesion, but to a failure of the coccygeal segment to adapt to load and movement. Clinical series and dynamic imaging show that mobility abnormalities can be present even when standard imaging looks “normal” [1] (Maigne et al., 2000).

2. Falls, impacts, and forgotten trauma

In general, the most classic cause of coccydynia is direct trauma: a fall onto the buttocks, an impact (sports, a slip, an accident), or a blow to the coccyx.

This trauma can:

• cause a fracture (less common),
• lead to a dislocation of the coccyx,
• subtly alter its mobility without any obvious lesion.

An important point is that the initial trauma may be old, sometimes forgotten, and that the pain may appear or worsen later. This delayed onset, frequently reported, is explained by the gradual accumulation of stress (sitting, posterior pelvic tiltPosterior pelvic tiltPosterior pelvic tilt refers to a backward tilting of the pelvis, which tends to reduce the lumbar curve. It may correspond to a person’s natural pelvic orientation or to a movement adopted depending on the position.Read the full entry : Posterior pelvic tilt, micro-trauma) until a threshold of functional tolerance is exceeded [1] (Maigne et al., 2000).

The coccyx can therefore remain “off-center” or unstable long after the triggering event, and become painful when the cumulative mechanical stress exceeds a tolerance threshold.

3. Childbirth: a specific mechanical stress

In women, childbirth is a particular mechanical context. To allow the baby to pass, the coccyx must move backward.

This mechanism can induce:

• a dislocation,
• residual hypermobility,
• or persistent pain related to a loss of local stability [1] (Maigne et al., 2000).

The nuance is essential: childbirth does not systematically cause a dislocation, but it can represent a maximal stress on a sacrococcygeal joint that is already vulnerable, with imbalanced mobility and secondary tissue irritability [1] (Maigne et al., 2000).

This is one of the reasons why coccydynia is markedly more common in women.

4. Prolonged sitting: the silent micro-trauma

However, not all cases of coccydynia begin with a clear-cut impact. In some people, the pain sets in gradually.

Sitting for a long time on a hard or poorly suited surface concentrates pressure:

• on the ischial tuberosities,
• but also on the tip of the coccyx, especially when the pelvis tilts backward.

Through repetition, these micro-stresses can:

• irritate the soft tissues,
• disturb coccygeal mobility,
• promote the onset of chronic painChronic painPain is called chronic when it persists or recurs for more than three months. It may accompany an identified disease or be a condition in its own right.Read the full entry : Chronic pain [2] (Lirette et al., 2014).

It is the mechanical equivalent of a door that never slams outright, but forces a little… every time it opens.

“Beyond the duration of sitting, it is the very shape of the contact between the seat and the coccyx that determines the areas of stress.”

Imaging research has refined our understanding of risk according to the morphology and dynamics of the coccyx.

Changes in our lifestyles have also introduced the “screen posture”: a posteriorly tilted pelvis, a rounded spine, and increased pressure on the tip of the coccyx.

5. Mechanical abnormalities

Instability, hypermobility, spicule: when the mechanics go awry

Reference studies show that coccydynia is frequently associated with objectifiable mechanical abnormalities… provided they are looked for properly [1,3] (Maigne et al., 2000; Nathan et al., 2010).

• Anterior or posterior dislocation of the coccyx
  An abnormal displacement relative to the sacrum, typically revealed by dynamic sitting/standing radiographs [1] (Maigne et al., 2000).
• Coccygeal hypermobility
  Excessive mobility that makes sitting unstable and irritating — a configuration described in clinical series [1] (Maigne et al., 2000).
• Coccygeal spur (bony spicule)
  A terminal outgrowth likely to irritate the soft tissues, particularly during prolonged weight-bearing [3] (Nathan et al., 2010).

These abnormalities are often invisible on a standard radiograph. They sometimes require a dynamic sitting/standing radiograph, or even more targeted examinations depending on the context [1,2] (Maigne et al., 2000; Lirette et al., 2014).

“Everything is normal on the X-rays.”
In reality, this statement does not rule out a mechanical cause: it frequently indicates that the examination did not explore functional mobility. The pain may stem from dynamic instability, localized overload, and a loss of adaptabilityAdaptabilityAn integrative Blue Portance concept referring to the body’s capacity to adjust its postural organization and its tissue responses to stress, while preserving balance, mobility and load distribution.Read the full entry : Adaptability that only show up under weight-bearing [1,3] (Maigne et al., 2000; Nathan et al., 2010).

6. Factors that perpetuate and aggravate the pain

Even when a mechanical abnormality exists, certain factors act as amplifiers.

One of the best documented is a high body mass index. Maigne et al. showed a clear association between coccydynia and excess weight [1] (Maigne et al., 2000).

While weight is an obvious load factor, the very nature of the contact surface and the geometry of the seat can turn moderate stress into focused stress.

Obesity alters pelvic kinematics during sitting: in people with a high BMI, posterior pelvic tilt is often more pronounced, which increases pressure on the tip of the coccyx [1] (Maigne et al., 2000).

In addition, recent reviews emphasize that prolonged sitting acts not only through pressure, but also through local tissue irritation and the potentiation of chronicity mechanisms [2] (Lirette et al., 2014).

Added to this are:

• postural imbalances of the pelvis,
• a loss of overall mobility,
• chronic muscle tension in the pelvic floorPelvic floorThe pelvic floor is a deep group of muscles and connective tissues located in the lower part of the pelvis. It contributes to the support of the pelvic organs, to sphincter function and to the mechanical coordination of the abdominopelvic complex.Read the full entry : Pelvic floor.

The coccyx then becomes the weak link in a system that no longer distributes stress correctly.

Figure — Mechanical causes of coccydynia

Doctrinal explanation: the figure does not propose a “single cause,” but a sequence. The same case of coccydynia may combine an initial stress (a fall or childbirth), a change in mobility (dislocation or hypermobility), and then repeated overload while sitting. The clinical consequence is not only localized pain: depending on the individual and the tissues involved, it may be accompanied by radiating pain and protective hypertoniaHypertoniaHypertonia refers to an increase in muscle tone or in resistance to passive movement. The term does not describe a single cause: depending on the context, it may reflect a neurological disorder, increased muscle activity, difficulty relaxing, tissue stiffness or a protective strategy related to pain.Read the full entry : Hypertonia of the pelvic floor, contributing to chronicity [2] (Lirette et al., 2014).

This diagram is consistent with the central idea highlighted in dynamic studies: what standard imaging sometimes calls “normal” may correspond to a dysfunction under load, visible only when the coccyx is studied in a functional situation [1] (Maigne et al., 2000).