There is a language used at the craniocervical junction — a language of angles, distances, and ratios — that can tell a skilled reader whether a brainstem is being compressed, whether a skull is settling onto a spine, whether the joint holding the skull to the axis vertebra is moving within safe limits or far outside them. This language is not obscure or experimental. The measurements that constitute it have been described in peer-reviewed literature since the 1930s and refined continuously since. They are not new. They are not controversial. They are simply — in the vast majority of radiological encounters — not used.
That gap between what is measurable and what is measured is the gap in which CCI patients spend years of their lives. They have imaging performed. The images contain the evidence. Nobody makes the measurements that would reveal it. The report says unremarkable. The patient is sent home.
Understanding these measurements — what they are, what they quantify, where they go wrong — is therefore not merely academic. For patients navigating CCI diagnosis, it is survival knowledge. It is the difference between knowing what to ask for and being handed a reassuring result that hides a structural emergency.
What You Are Measuring, and Why It Matters
Before any measurement makes sense, the anatomy must be clear. The craniocervical junction comprises three bony structures — the occiput (base of the skull), the atlas (C1), and the axis (C2) — and the ligamentous complex that holds them in alignment.
Fig. 1 — Schematic of key bony landmarks at the craniocervical junction used in morphometric measurement. Basion (Ba), opisthion, odontoid tip, atlas (C1), and axis (C2) are the primary reference points for the measurements covered in this series. The brainstem/spinal cord trajectory (schematic, not anatomically precise) illustrates what these structures protect.
The critical structures at this junction include: the basion (the most anterior inferior point of the foramen magnum — the hole in the skull through which the brainstem passes), the opisthion (its posterior counterpart), the odontoid process (the upward peg of C2 that projects into the ring of C1), and the posterior axial line (the posterior cortical surface of C2's body and odontoid). Every major craniocervical measurement uses some combination of these landmarks. Their precise identification on imaging is non-negotiable — errors in landmark placement propagate directly into measurement error, and in a space where a few millimetres determines whether compression is occurring, precision is everything.
The Complete Reference Table
All measurements covered in this series, with their normal ranges, abnormal thresholds, imaging modality, and clinical significance. Values represent adult norms; paediatric ranges differ. This table is a reference — the deep-dive articles cover the nuance, pitfalls, and clinical context each measurement demands.
| Measurement | What it assesses | Normal | Abnormal (concern) | Modality | Article |
|---|---|---|---|---|---|
| Grabb-Oakes (pB-C2) | Ventral brainstem compression by odontoid/pannus | ≤ 9 mm | > 9 mm → surgical consideration | MRI (sagittal) | Article 1 |
| CXA — Bony (bony clivoaxial angle) | Angular alignment of brainstem at CV junction | ≥ 135° | < 135° moderate; < 125° severe | MRI / CT (sagittal) | Article 1 |
| CXA — Soft Tissue | Soft-tissue-corrected brainstem angulation | ≥ 135° | < 135°; can differ significantly from bony CXA | MRI (sagittal) | Article 1 |
| ADI (Atlantodental Interval) | Anterior atlantoaxial stability; transverse ligament integrity | Adults ≤ 3 mm; children ≤ 5 mm | > 3 mm adult (subluxation); > 5 mm = ligament rupture | X-ray / CT (lateral) | Article 2 |
| PADI (Posterior ADI / Space Available for Cord) | Space for spinal cord at C1 level | ≥ 14 mm (Steel's rule of thirds) | < 14 mm → cord compression risk | CT / X-ray (lateral) | Article 2 |
| BDI (Basion-Dens Interval) | Craniocervical dissociation; occiput-C2 integrity | ≤ 12 mm | > 12 mm → craniocervical dissociation | CT / X-ray (lateral) | Article 2 |
| BAI (Basion-Axial Interval) | Anterior displacement of skull on C2 | −4 mm to +12 mm | Outside range → instability/dissociation | CT / X-ray (lateral) | Article 2 |
| tBAI (Traynelis BDI+BAI rule) | Combined craniocervical dissociation screening | BDI + BAI both normal | Either abnormal → high specificity for CCD | CT / X-ray | Article 2 |
| Chamberlain Line | Basilar invagination / cranial settling | Odontoid ≤ 3 mm above line | > 3 mm above → basilar invagination | X-ray / CT / MRI (sagittal) | Article 3 |
| McRae Line | Foramen magnum diameter; minimum acceptable cord space | Odontoid below line | Odontoid above McRae line = definite invagination | X-ray / CT / MRI | Article 3 |
| McGregor Line | Basilar invagination (radiographic gold standard) | Odontoid ≤ 4.5 mm (♂) / ≤ 7 mm (♀) above | Above threshold → invagination; sex-specific | X-ray / CT (sagittal) | Article 3 |
| Powers Ratio | Atlanto-occipital dislocation (anterior) | ≤ 1.0 | > 1.0 → anterior AOD; insensitive for posterior/vertical | X-ray / CT (lateral) | Article 4 |
| AOI / OC1 (Atlanto-Occipital Interval) | Occipital condyle–C1 joint space; AOD in EDS/CCI | ≤ 2 mm (CT); varies by method | > 2 mm or asymmetric → AOJ instability | CT (coronal), upright MRI | Article 4 |
| C1–C2 Overhang (Lateral AAI) | Lateral atlantoaxial translation / rotatory instability | Total overhang < 7 mm (Fielding) | ≥ 7 mm → transverse ligament rupture | CT (open-mouth AP), upright CT | Article 4 |
| Harris Measurement (Harris Lines) | Craniocervical dissociation, sum-of-distances method | ≤ 12 mm each component | Sum > 12 mm → CCD (high sensitivity) | CT (sagittal) | Article 3 |
| Wackenheim Clivus Baseline | Odontoid-clivus relationship; cranial settling | Odontoid tangent to or below clivus line | Odontoid above line → basilar invagination | X-ray / CT / MRI (sagittal) | Article 3 |
| Ranawat Index (C1–C2 height) | Cranial settling in rheumatoid arthritis / hEDS | ♂ ≥ 15 mm; ♀ ≥ 13 mm | Below threshold → cranial settling | X-ray / CT (lateral) | Article 3 |
Every number in that table represents a structural reality that can be measured, documented, and acted on — and that routine radiological reporting almost never documents at all.
The Deep-Dive Articles
Each article in this series takes one measurement or closely related measurement group and examines it fully: the anatomy it probes, the technique required to make it correctly, the pitfalls that produce false negatives, the clinical consequences of abnormality, and what patients should know when requesting or reviewing imaging. Click any article to read it in full.
The Clinical Stakes of Each Measurement
These measurements are not performed in a vacuum. Each one interrogates a specific failure mode at the craniocervical junction — and each failure mode produces a recognisable pattern of clinical consequences. Understanding the link between measurement and consequence is what allows a patient to connect their imaging to their experience, and what allows a clinician to understand why these numbers matter beyond the radiological report.
When the odontoid or retro-odontoid pannus encroaches on the brainstem from the front, the consequences are: myelopathy, spasticity, hyperreflexia, proprioceptive loss, bowel/bladder dysfunction — and, in severe cases, respiratory compromise. The CXA quantifies the angulation that produces this compression; a kinking of the brainstem at an acute angle is as dangerous as a direct bony impingement.
→ Deep-dive: Article 1The atlantodental interval is a direct proxy for the integrity of the transverse ligament — the primary restraint against the atlas sliding forward on the axis and crushing the cord. When the ADI widens beyond 3 mm in an adult, the transverse ligament is no longer doing its job. Beyond 5 mm, it has almost certainly ruptured. The PADI tells you what's left for the cord to live in.
→ Deep-dive: Article 2Craniocervical dissociation (CCD) — the skull separating from the spine — is life-threatening in its traumatic form and insidious in its ligamentous form. In hEDS and CCI, partial CCD can exist as a chronic, progressive condition producing the full spectrum of brainstem symptoms while standard imaging misses it because nobody is measuring the basion-dens distance. BDI and BAI together cover anterior, posterior, and superior displacement.
→ Deep-dive: Article 2Basilar invagination — the upward migration of the odontoid into the foramen magnum — is among the most dangerous CCI manifestations. The three classic lines (Chamberlain, McGregor, McRae) each approach this from a different reference frame, with different sensitivity and pitfalls. In EDS-related cranial settling, the progression can be gradual and initially sub-threshold — making repeat measurement over time essential.
→ Deep-dive: Article 3The atlanto-occipital joint (occiput on C1) is the most superior level of the craniocervical junction and the one most frequently overlooked in CCI evaluation. Powers ratio catches anterior atlanto-occipital dislocation but misses posterior and vertical displacement — a critical limitation. The AOI measurement, particularly on upright or traction imaging, captures the joint space changes that reflect ligamentous laxity at this level in hEDS patients.
→ Deep-dive: Article 4Lateral translation of C1 on C2 — the atlas sliding sideways relative to the axis — is one of the most commonly missed forms of atlantoaxial instability because it requires an open-mouth AP or coronal CT view that is rarely included in standard cervical protocols. When the combined lateral overhang exceeds 7 mm, the transverse ligament has effectively failed. In rotatory AAI, this becomes dynamic — visible on motion CT and easily missed on static studies.
→ Deep-dive: Article 4Why These Measurements Are Not Made
The measurements in this series require: familiarity with specific anatomical landmarks, knowledge of the measurement technique and its variants, awareness of the imaging position required (many cannot be made on supine studies), and experience with the normal ranges — which in several cases differ between landmark-identification methods, between static and dynamic imaging, and between adult and paediatric populations.
The result of this knowledge gap is not merely academic. When a patient with CCI presents for imaging, the standard protocol produces a standard report. The standard report does not make craniocervical measurements. The report says unremarkable. The patient's clinician, seeing an unremarkable MRI, concludes there is no structural basis for the patient's symptoms. The patient is referred onward — to psychiatry, to pain management, to another year of diagnostic wandering — while the specific millimetric evidence of what is wrong with their brainstem sits uncalculated in the imaging data.
The evidence is in the scan. The scan is on the server. Nobody is making the measurements. This is not a gap in technology. It is a gap in knowledge, training, and clinical will — and patients pay for it with years of their lives.
1. Supine-only imaging: Many CCI findings are position-dependent and absent on supine studies. 2. Incorrect landmark identification: Small errors in basion or opisthion placement produce large measurement errors. 3. Using wrong variant: CXA-bony and CXA-soft-tissue give different values; confusing them alters interpretation. 4. Applying adult norms to children (and vice versa): ADI normal range differs significantly. 5. Isolated measurement without clinical context: A "borderline" value in a symptomatic hEDS patient may be highly significant; the same value in an asymptomatic adult is likely not. 6. Failure to measure dynamically: Static normal ≠ dynamic normal. The whole point of CCI is that things move when they shouldn't — static imaging, by design, misses movement.
What to Ask For — and How to Ask
If you have CCI or are pursuing a CCI diagnosis, the single most important thing you can do before any imaging appointment is ensure that the right imaging is ordered and that the reporting radiologist knows what measurements to make. This requires explicit, specific requests — because "MRI cervical spine" will not produce the measurements you need.
For MRI: Request sagittal sequences through the craniocervical junction with specific notation to measure Grabb-Oakes (pB-C2), clivoaxial angle (bony and soft-tissue), and any Chiari-related herniation. If possible, request upright or seated MRI — most standard MRI scanners are supine only; upright-capable scanners are available at specialist centres.
For CT: Request CT with craniocervical protocol including sagittal and coronal reformats; ask for Chamberlain, McGregor, and McRae line assessments; BDI and BAI measurement; and open-mouth (AP) view for lateral C1-C2 overhang assessment.
For X-ray (if used): Lateral flexion-extension views are essential for dynamic ADI assessment. Open-mouth odontoid view for C1-C2 lateral overhang. Lateral view for Chamberlain and Powers ratio.
Most importantly: The request for these measurements should ideally come from a referring clinician who understands CCI — a neurosurgeon specialising in the craniocervical junction, or a neurologist familiar with the condition. If you are self-advocating, bring the specific measurement names in writing to the imaging facility and ask that they be included in the radiologist's report.
One measurement.
Every detail.
Each article.
This hub gives you the map. The six articles that follow give you the territory — each measurement examined fully, with technique, pitfalls, clinical consequences, and what abnormality actually means for a living, suffering human being. Start with Article 1: Grabb-Oakes and the Clivoaxial Angle — the measurements most directly tied to brainstem compression and surgical decision-making.
If you are a patient, bring these articles to consultations. If you are a clinician encountering CCI for the first time, this series is written to be medically precise without being inaccessible. The measurements are not difficult. They are simply not being made. That is the problem this series is designed to address.
The craniocervical junction is not forgiving of imprecision. The structures it contains are the oldest, most essential parts of the human nervous system. When it fails, the consequences are not peripheral — they are central to everything that makes a body functional. The measurements in this series are the tools that can document that failure. Using them is not optional, not exotic, and not beyond the capacity of any radiologist or clinician who chooses to learn them. The series that follows makes that learning possible — for patients and clinicians alike.