6. Coccyx morphology and mobility: multifactorial mechanical vulnerability

Coccyx morphology and mobility define how the tailbone adapts to sitting, movement, and mechanical stress. Variations in shape and dynamic behavior influence mechanical vulnerability in coccydynia (tailbone pain).

Coccyx morphology is neither uniform nor neutral across the general population. Anatomical and radiological data show substantial inter-individual variability, which shapes how mechanical loads from sitting, movement, and trauma are dissipated—or, conversely, concentrated.

This morphological diversity defines a baseline mechanical vulnerability: not pathological in itself and not systematically painful, but influential in how the system responds to environmental constraints.

1. The reference morphological classification

Foundational work by Postacchini and Massobrio (1983) proposed a morphological classification of the coccyx into four main types, based on curvature and orientation of coccygeal segments (Postacchini & Massobrio, 1983).

• Type I: coccyx slightly curved forward, harmoniously extending the sacrum.
• Type II: more marked ventral curvature, tip directed toward the pelvic cavity.
• Type III: abrupt anterior angulation between two segments.
• Type IV: subluxation or inter-segmental instability.

Identification of these types relies in particular on dynamic sitting/standing radiographs, as standardized by Maigne et al. (1996) (Maigne et al., 1996).

This approach showed a correlation between certain morphologies (II, III, IV) and a higher incidence of coccydynia, including in the absence of visible lesions on static imaging (Maigne et al., 1996 ; Nathan et al., 2010).

2. Anatomical predisposition and response to trauma

With comparable trauma—similar fall, similar childbirth, identical sitting constraints— some individuals develop persistent chronic pain, while others recover spontaneously.

Pain then appears as the encounter between:

• a specific bony architecture (morphological baseline), and
• a mechanical environmental constraint (sitting, repetition, impact),
• that the system can no longer absorb (adaptive dysfunction).

3. Toward an expanded morphological reading

Clinical data expanded the morphological reading toward six profiles, integrating retroverted or lateralized forms, and configurations associated with spicules (Nathan et al., 2010 ; Garg & Ahuja, 2021).

These morphotypes are not isolated causes. They should be read as mechanical vulnerability factors, whose expression depends on dynamic behavior.

These profiles are neither a diagnosis nor a fatality. They represent baseline mechanical vulnerability factors, whose clinical expression depends on coccyx morphology and mobility, soft-tissue status, and repeated mechanical constraints.

4. Segmental biomechanics: from morphotype to mechanical behavior

Talking about the coccyx “overall shape” helps with classification, but it is insufficient to understand mechanics.

The coccyx is a terminal hinge composed of 3 to 5 segments, linked by inter-coccygeal joints, and articulated with the sacrum (sacrococcygeal joint). Mechanics are therefore not monobloc.

Concretely, each morphotype influences:

• the location of the center of rotation dynamically,
• the existence of a segmental angulation point,
• the presence of a load concentration zone in sitting,
• and how forces are dissipated… or focused.

Key example: a type III is not simply “more curved.” It is a segmental angulation that can become a localized mechanical conflict point. A type IV adds a dimension of dynamic instability, generally better documented with functional imaging (sitting/standing) (Maigne et al., 1996).

5. Physiological mobility: hypermobility vs hypomobility

The coccyx is not only a shape: it is a dynamic segment. Dynamic sitting/standing radiographs have shown physiological mobility, often described within a range of about 20 to 30 degrees depending on context, highlighting its role as a terminal shock absorber (Maigne et al., 1996).

Two opposite deviations can generate pain:

• Hypermobility: excessive angulation in sitting, segmental instability, repeated micro-trauma, local nociceptive activation — frequently described in patterns associated with types III–IV when unstable (Maigne et al., 1996 ; Nathan et al., 2010).
• Hypomobility: adaptive blocking, loss of terminal damping, more direct load transmission. Often less visible on static imaging, but clinically relevant (post-traumatic / protective stiffening).

Doctrinal consequence: the same morphology can be asymptomatic if it remains mobile, painful if it stiffens, and painful if it becomes unstable. Pain depends on the interaction: Morphology × Mobility × Environmental constraints.

Simple examples:

• Type II mobile → preserved damping; Type II stiff → load concentration.
• Type III stable → adaptation possible; Type III unstable → repeated micro-trauma.
• Types V–VI: retroversion / asymmetry → mechanical vectors that may increase tissue conflict or referred pain depending on context.

6. Interaction with soft tissues: the clinical “so what”

The coccyx is not an isolated bone. It sits at a crossroads of insertion and tension: pelvic floor, sacrotuberous ligament, gluteus maximus, posterior fascial convergence. Any variation in morphology or mobility changes tissue tensions.

In practice:

• If hypermobile: repeated ligament traction, irritation of muscular insertions, local micro-inflammation, peripheral sensitization.
• If hypomobile: loss of damping, compression of posterior tissues, loss of fascial glide, defensive pelvic floor hypertonicity.

In both cases, pain is not “purely bony”: it becomes myofascial and neuro-mechanical. Morphology does not directly cause pain; it changes how forces are transmitted to soft tissues, which can trigger local inflammation, reflex contraction, and a vicious pain cycle (Nathan et al., 2010 ; Garg & Ahuja, 2021).

Doctrinal conclusion