coccyx
Enable an AI agent to recognise a coccyx, record its anatomical and functional state, and identify appropriate assessment or care pathways without treating uncertain observations as diagnoses.
Research draft, second pass
A second pass drafted this model: the structure a model of this thing needs, and what is known about it in the world. The line under this one says how the second half was obtained - researched against sources, or recalled without web access, in which case nothing here was read anywhere and every claim is a lead to verify. Unreviewed either way.
Researched by: Codex + Grok
Purpose and description
Enable an AI agent to recognise a coccyx, record its anatomical and functional state, and identify appropriate assessment or care pathways without treating uncertain observations as diagnoses.
The coccyx is the terminal, typically fused caudal segment of the human vertebral column, formed from three to five rudimentary caudal vertebrae that articulate cranially with the apex of the sacrum and provide attachment for pelvic floor and gluteal musculature.
It can be Identify and annotate the coccyx and its visible segments on an anatomical record or image.; Compare shape, alignment and integrity across dated observations while accounting for posture and method.; Link local symptoms and functional limitations to supporting or conflicting coccygeal evidence.; Identify missing evidence or findings that require qualified clinical interpretation.; Record clinician-directed pressure-management measures or interventions and their observed outcomes.; Distinguish a structural observation, a suspected pain source and a confirmed clinical conclusion..
Distinguishing features
Identify the terminal midline skeletal structure immediately inferior to the sacrum; exclude the larger sacral structure above the sacrococcygeal boundary.
Establish identity from anatomical position and coccygeal morphology rather than requiring one fixed segment count or complete fusion.
Separate the coccygeal bone and articulations from adjacent soft tissue; a tender natal-cleft lesion alone does not establish a coccygeal abnormality.
Distinguish the coccyx from the paired ischial tuberosities by its midline terminal spinal position.
Treat pain described as 'tailbone pain' as a symptom location until evidence establishes whether the coccyx is involved.
Scope
+ Identification, boundaries and constituent coccygeal segments
+ Coccygeal shape, orientation, alignment and fusion pattern
+ Sacrococcygeal and intercoccygeal articulation and movement
+ Local attachment relationships and structural integrity
+ Evidence connecting the coccyx to pain, pressure sensitivity and functional limitation
+ Assessment, intervention history and changes in the coccyx over time
- The sacrum and lumbar spine as complete anatomical structures
- Whole-pelvis geometry and pelvic-ring stability
- Pelvic-floor muscles, ligaments and their disorders as independently modelled structures
- Rectal, anal, reproductive and urinary organ conditions
- Overlying skin and natal-cleft conditions, including pilonidal disease
- Whole-person pain management, medication prescribing and surgical protocols
Characteristics
- Anatomical identity and completeness
- Present and apparently complete | partly absent | absent | indeterminate; evidence and reason for absence when known Distinguishes an intact coccyx from a remnant, prior resection or incomplete observation.
- Observable segment count
- Integer count with method and confidence; indeterminate when fusion or image quality prevents counting Supports description and localisation without assuming that variation is pathological.
- Segment fusion pattern
- Fused | unfused | indeterminate, recorded at each identifiable junction Locates potential articulations and prevents fusion from being confused with missing segments.
- Shape and orientation
- Described curvature, angulation, lateral deviation and focal prominence; named classification only when its source is specified Supports recognition, comparison and investigation of local pressure relationships.
- Junction alignment
- Alignment description for the sacrococcygeal and visible intercoccygeal junctions, with posture and assessment method Localises an observed relationship without automatically labelling it an injury.
- Position-dependent movement
- Angular change in degrees or translation in millimetres where measurable, with compared positions and protocol; otherwise qualitative or unknown Allows movement to be assessed separately from appearance in a single position.
- Structural integrity
- No abnormality identified | suspected abnormality | confirmed abnormality | indeterminate, with location, evidence and interpretation Separates observations from diagnostic conclusions and identifies findings needing review.
- Local attachment relationships
- Named attaching structure, attachment region and observed or inferred relationship Connects coccygeal findings to neighbouring tissues while preserving their separate ownership.
- Symptom concordance
- Reported symptom location and provoking activity linked to examination or imaging findings; concordant | discordant | unresolved Prevents anatomical variation or incidental imaging findings from being assumed to explain pain.
- Functional tolerance
- Reported sitting duration in minutes, task-specific limitation and symptom score using a named scale, with conditions and date Makes changes in daily impact comparable across assessments.
Where this came from
wikidata · CC0 1.0
Drafted structure
Bundle to layer to finding to question, as the second pass will find it: 5 bundles · 9 layers · 16 findings · 26 questions.
Coccygeal identity and boundaries Establish which terminal skeletal structure is represented and what portions can be identified.
Coccygeal segmentation, fusion and incomplete visibility can make identity and boundaries uncertain.
Terminal spinal location
Locate the coccyx relative to the sacrum and adjacent pelvic landmarks.
Sacrum-coccyx boundary
Record the evidence identifying the proximal coccygeal boundary and distal extent.
- What anatomical landmarks establish that the observed structure is the coccyx? definition
- Where is its boundary with the sacrum, and is the distal tip included in the observation? boundary
Segments and completeness
Describe identifiable segments, fusion and any absent portion.
Coccygeal segment inventory
Record visible segmentation without interpreting an uncertain count as an anatomical absence.
- How many segments and junctions can be distinguished, and which appear fused? measurement
- If a portion is not identified, what evidence distinguishes limited visibility from anatomical absence or prior removal? provenance
Coccygeal form and articulation Describe shape, junction alignment and observed movement.
A coccyx's appearance and movement must be recorded separately before their significance can be judged.
Curvature and prominence
Describe the coccygeal profile and local projections in an explicit anatomical frame.
Coccygeal profile
Capture curvature, angulation, deviation or focal prominence as observations.
- What curvature, angulation, lateral deviation or focal bony prominence is observed, and at which segment? measurement
- Which view, posture and reference landmarks support that description? provenance
Junction alignment and motion
Assess relationships at the sacrococcygeal and intercoccygeal junctions.
Position-dependent junction state
Record alignment and any demonstrated movement without importing unsupported normality thresholds.
- What alignment or movement is demonstrated at each assessable junction, and between which positions? measurement
- If movement is labelled restricted or excessive, which validated protocol and reference criteria support that label? provenance
Coccygeal integrity and tissue interfaces Locate structural abnormalities and relationships with attached or adjacent tissues.
Local symptoms may involve bone, a junction, an attachment or neighbouring tissue, each requiring distinct evidence.
Bone and junction condition
Describe evidence of injury or other structural change.
Localised structural abnormality
Keep the observed abnormality, its interpretation and its certainty distinguishable.
- What abnormality is observed in the coccygeal bone or junctions, and where exactly is it located? measurement
- What evidence supports its proposed cause or timing, including any connection to reported trauma? provenance
Attachments and adjacent tissues
Connect coccygeal attachment regions and nearby tissue findings to their own anatomical records.
Coccygeal tissue interface
Identify whether a local finding concerns the coccyx, an attachment or a neighbouring structure.
- Which ligament, muscle or other soft-tissue relationship is implicated, and is that relationship directly observed or inferred? provenance
- Which component of the finding belongs to the coccyx and which belongs to a neighbouring tissue model? boundary
Coccygeal symptoms and care decisions Relate local findings to reported symptoms, functional impact and qualified care decisions.
Tailbone-region pain does not by itself establish the coccyx as its source or justify a particular intervention.
Symptom and function concordance
Assess the agreement between symptom location, provoking activities and anatomical evidence.
Coccygeal symptom attribution
Record the strength and limitations of evidence that the coccyx contributes to symptoms.
- Where are symptoms reported, and how do sitting, rising from sitting or other specified activities affect them? measurement
- What examination or imaging evidence supports or challenges a coccygeal source? provenance
- What evidence suggests that an adjacent structure or referred pain requires separate assessment? boundary
Care history and reassessment
Record coccyx-directed care, observed outcomes and unresolved decisions.
Coccyx-directed care status
Link each care action to its clinical rationale and subsequent structural or functional observations.
- What coccyx-directed measures or procedures have been undertaken, when, and under whose clinical direction? provenance
- What changed in sitting tolerance, pain or anatomical state after each action, and how was change assessed? measurement
- What unresolved finding requires qualified review before a further coccyx-directed action can be selected? action
Evidence and external alignment What the world already says about this thing, gathered so the model can be checked against it.
A model that cannot be lined up against existing standards, identifiers and practice cannot be adopted by anyone who already uses them.
Reported evidence
Findings from the breadth pass, kept separate from the structural claims.
Kinds and varieties
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Typical four-segment coccyx (Co1-Co4)
- Three-segment coccyx
- Five-segment coccyx
- Sacrococcygeal synostosis (fused sacrococcygeal joint)
- Type I (slightly curved, apex pointing caudally)
- Type II (more markedly curved, apex pointing anteriorly)
- Type III (sharply angulated anteriorly)
- Type IV (subluxated / retroverted)
- Which of these kinds and varieties hold for the sense of coccyx this model covers, and on what evidence? provenance
Identifiers and schemes
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Wikidata - Q16349 - Item 'coccyx' (also tailbone, os coccygis).
- FMA - FMA:20229 - Foundational Model of Anatomy identifier for the coccyx.
- UBERON - UBERON:0001350 - Uberon anatomical ontology class 'coccyx'.
- MeSH - D003050 - Medical Subject Headings descriptor Coccyx.
- Terminologia Anatomica - TA98 A02.2.06.001 / TA2 1092 - Official anatomical term os coccygis / coccyx.
- ICD-10-CM - S32.2- (fracture of coccyx); M53.3 (sacrococcygeal disorders, not elsewhere classified) - Clinical coding of coccygeal injury and coccydynia, not of the organ itself.
- Which of these identifiers and schemes hold for the sense of coccyx this model covers, and on what evidence? provenance
Standards and regulation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Terminologia Anatomica (FIPAT / IFAA) - official Latin and English anatomical nomenclature for os coccygis.
- ICD-10 / ICD-10-CM (WHO / national adaptations) - codes for coccygeal fracture and sacrococcygeal disorders used in clinical and billing records.
- SNOMED CT (SNOMED International) - concept modelling of the coccyx as a body structure and of coccydynia / coccygectomy as clinical findings and procedures.
- Which of these standards and regulation hold for the sense of coccyx this model covers, and on what evidence? provenance
Real-world use
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Palpated and imaged as a landmark at the caudal end of the spine in trauma, obstetrics, and pelvic-floor assessment.
- Weight-bearing contact surface in sitting; a common site of post-fall or postpartum pain (coccydynia).
- Surgical target of coccygectomy when conservative treatment of refractory coccydynia fails.
- Radiographic and CT landmark for counting caudal vertebral segments and for distinguishing sacralisation versus coccygeal variation.
- Attachment site for the levator ani, coccygeus, gluteus maximus, and the anococcygeal ligament / raphe, relevant to continence and pelvic support.
- Which of these real-world use hold for the sense of coccyx this model covers, and on what evidence? provenance
Typical measurements
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Number of coccygeal segments - 3-5 (mode 4) - count
- Coccygeal length (tip of Co1 to caudal tip) - about 2-4 - cm
- Intercoccygeal / sacrococcygeal angle (morphological type) - near-straight (type I) to >90° anterior angulation (type III) - degree
- Which of these typical measurements hold for the sense of coccyx this model covers, and on what evidence? provenance
Failure modes and hazards
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Coccydynia (chronic tailbone pain) after fall onto the buttocks, prolonged sitting, or vaginal childbirth.
- Coccygeal fracture or dislocation, often from axial trauma in the seated position.
- Hypermobility or posterior subluxation of the sacrococcygeal or intercoccygeal joints on dynamic sitting radiographs.
- Iatrogenic injury or persistent pain after coccygectomy, including wound infection given the proximity to the anus.
- Rare tumours (e.g. sacrococcygeal teratoma, chordoma) presenting at or adjacent to the coccyx.
- Which of these failure modes and hazards hold for the sense of coccyx this model covers, and on what evidence? provenance
Regional variation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- English common name 'tailbone' versus anatomical 'coccyx' / Latin os coccygis; many European languages keep a coccyx-derived form (French coccyx, German Steißbein for the common name).
- Segment number and sacrococcygeal fusion rates vary among individuals and are reported with modest sex and population differences in morphometric series; fusion increases with age.
- Obstetric and pelvic-floor practice treats the coccyx more as a pain generator in some regions (dynamic radiography, injection, coccygectomy pathways) than in others where it is rarely imaged unless fractured.
- Which of these regional variation hold for the sense of coccyx this model covers, and on what evidence? provenance
Neighbouring kinds and how to tell them apart
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- sacrum (especially S5 / sacral apex) - The sacrum is a large triangular fusion of typically five sacral vertebrae with sacral foramina and a sacral canal; the coccyx lacks foramina and a spinal canal and begins distal to the last sacral segment at the sacrococcygeal joint.
- caudal (embryonic tail) vertebrae in other mammals - Non-human caudal vertebrae remain separate, numerous, and often mobile; the human coccyx is a vestigial, usually partly fused remnant of three to five caudal segments.
- anococcygeal ligament / raphe and pelvic floor (soft tissue, not bone) - These are fibromuscular attachments onto the coccyx; they are distinguished from the osseous coccyx on MRI and by remaining mobile and non-ossified on radiographs.
- sacral cornua / coccygeal cornua (paired processes) - Cornua are small articular processes at the sacrococcygeal junction, not the coccygeal body itself; they may form a rudimentary zygapophyseal joint with the sacral cornua.
- Which of these neighbouring kinds and how to tell them apart hold for the sense of coccyx this model covers, and on what evidence? provenance
Sources
- Coccyx - Gray's Anatomy of the Human Body - Classic anatomical description of the coccyx as the terminal caudal vertebrae, segment count, and ligamentous attachments.
- Coccyx | Radiology Reference Article - Segment number variation, sacrococcygeal joint, Postacchini and Massobrio morphological types, and imaging appearance.
- Coccydynia - StatPearls - Clinical presentation of coccygeal pain, aetiology (trauma, childbirth, idiopathic), conservative and surgical management.
- coccyx (Q16349) - Stable identifier, aliases (tailbone, os coccygis), and links to anatomical ontologies (FMA, UBERON, MeSH, TA).
What the second pass must settle
- Does registry entry vr.tr.coccyx cover human anatomy only, as this draft assumes, or must it support other species?
- Which morphological classifications and measurement conventions have sufficient reproducibility for use across coccygeal assessments?
- Which position-dependent imaging protocols and population-specific reference ranges can support interpretation of coccygeal movement?
- What evidence is sufficient to attribute symptoms to the coccyx rather than incidental variation, neighbouring tissue or referred pain?
- Which clinical guidelines should govern escalation criteria and the prerequisites for coccyx-directed interventions?