Magnus Teleradiology · MSK Reference

CMC Joint Congruity
Diagnostic Logic Framework

Normal · Subluxation · Dislocation · Fracture-Dislocation — Systematic Differentiation
Diagnostic Logic
Grading Scale
Joint-by-Joint
Decision Tree
Interactive Tool
Core Diagnostic Framework
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Step 1 — Joint Space Width

The primary quantitative criterion. Measure the narrowest perpendicular distance between articular cortices on the PA view.

Normal: 1–2 mm, uniform throughout the joint
Suspicious: >2 mm or asymmetric vs contralateral
Abnormal: Marked widening, obliteration, or complete loss
Compare ALWAYS with the contralateral CMC joints — individual variation is significant, especially at the 4th CMC.
2
Step 2 — Joint Parallelism (PA)

On the PA view, the articular surfaces of adjacent MC bases and carpal bones must be parallel. Loss of parallelism precedes frank dislocation.

Normal: Parallel articular cortices with uniform joint cleft
Subluxation: Non-parallel cortices; one margin wider than the other
Dislocation: Complete loss of parallelism; overlap or gap
The MC2–4 joints are tightly congruent. Any non-parallelism here is strongly abnormal.
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Step 3 — Dorsal Step-Off (Lateral)

The lateral view is the key diagnostic view for displacement. A continuous dorsal cortical line should be maintained from carpal row to metacarpal base.

Normal: Smooth continuous dorsal cortex, no step-off
Subluxation: Subtle step-off (<50% articular surface contact maintained)
Dislocation: Frank dorsal displacement; <25% or no articular contact
95% of CMC dislocations are DORSAL. Lateral view must always be obtained — PA alone is insufficient.
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Step 4 — Articular Surface Contact

The percentage of articular surface remaining in contact determines the grade between subluxation and dislocation. Assess on the lateral view.

Normal congruity: >75% articular contact
Subluxation: 25–75% articular contact maintained
Dislocation: <25% contact or complete loss
True dislocation = zero articular contact. Partial contact remaining = subluxation by definition.
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Step 5 — Fracture Association

CMC dislocations are frequently fracture-dislocations. The presence of a fracture fragment changes both diagnosis and management.

MC base fragment: Indicates intraarticular fracture; assess size relative to joint surface
Hamate fracture: Invariably associated with 4th–5th CMC dislocation
Bennett fragment: Volar-ulnar MC1 base — small but surgically critical
A tiny avulsion fragment at any CMC base should trigger suspicion for associated subluxation even if the joint appears reduced.
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Step 6 — Stability Inference

Radiographic appearance informs likely stability, which drives clinical management. A radiographically reduced joint may still be unstable.

Stable: Normal alignment, no fragment, intact joint space
Likely unstable: Widened space + fragment; reduced but suspicious
Unstable: Dislocation, significant comminution, >30% articular fragment
If clinical instability suspected but films equivocal — CT is mandatory. Do not discharge on negative plain films alone in high-energy mechanism.
CMC Congruity Grading — Radiographic Classification
Grade Classification Definition Measurements Lateral Finding PA Finding Action
GRADE 0 Normal Full articular congruity, symmetric joint spaces, no displacement Joint gap 1–2mm

>75% contact 		Continuous dorsal cortex. No step-off. Parallel joint margins. Uniform space. Report as normal. No further imaging.
GRADE 1 Sprain / Partial Tear Ligamentous injury without displacement. Joint appears reduced but may be symptomatic. Gap normal or ≤2mm

>75% contact 		No step-off. May show subtle soft tissue swelling. Symmetric spaces. Possible asymmetric widening vs contralateral. Correlate clinically. Stress views if stable.
GRADE 2 Subluxation Partial displacement. Articular surfaces still partially in contact. <50% loss of congruity. Gap 2–4mm

25–75% contact 		Subtle dorsal step-off. MC base partially displaced. Cortical discontinuity. Non-parallel joint margins. Uneven joint space. Possible overlap. CT if equivocal. Orthopedic referral.
GRADE 3 Dislocation Complete displacement. No articular contact. Joint surfaces completely dissociated. Gap >4mm or absent

<25% or 0% contact 		Frank dorsal displacement. MC base sitting dorsal to carpal articular surface. Complete loss of joint alignment. Overlap or complete gap. Urgent orthopedic. Closed reduction ± K-wire.
GRADE 4 Fracture-Dislocation Dislocation with intraarticular fracture. Includes Bennett, Rolando, hamate body fractures. Dislocation +

Fragment present 		Dislocation signs + fracture fragment. May be comminuted. Fracture line at MC base or carpal bone. Joint disruption. CT mandatory. ORIF likely required.
Diagnostic Precision Rules
SUBLUXATION vs DISLOCATION
The critical differentiator is articular surface contact remaining on the lateral view. Any contact remaining = subluxation. Zero contact = dislocation. This distinction drives management.
REDUCED but UNSTABLE
A Grade 1–2 appearance may represent spontaneous partial reduction. Avulsion fragment + clinical tenderness = CT regardless of apparent alignment. Do not falsely reassure on plain films alone.
Joint-Specific Diagnostic Criteria
1st CMC — Thumb
2nd CMC — Index
3rd CMC — Middle
4th CMC — Ring
5th CMC — Little
Normal Criteria — 1st CMC (Trapezio-Metacarpal)
Joint space1–2 mm, uniform
Articular shapeSaddle joint — concave/convex reciprocal curves
MC1 base positionAligned with trapezium; no radial shift
Volar-ulnar cortexIntact; no avulsion fragment
APL alignmentMC1 shaft collinear with trapezium axis
Best viewRoberts PA Lateral
Key pitfallSubtle Bennett fragment easily missed on PA
Injury Patterns — 1st CMC
Bennett Fracture-Dislocation GRADE 4
Oblique intraarticular fracture at the volar-ulnar base. Small fragment remains in socket via anterior oblique ligament. MC1 shaft displaced proximally and radially by APL. Fragment ≥20% of articular surface is significant.
Rolando Fracture GRADE 4
Comminuted intraarticular fracture — Y or T pattern. Three or more fragments at MC1 base. Shaft displaced. Worse prognosis than Bennett. CT mandatory for surgical planning.
Pure CMC Dislocation GRADE 3
Dislocation without fracture — uncommon at 1st CMC. When seen, always scrutinize carefully for a small undetected volar fragment before calling "pure."
Normal Criteria — 2nd CMC (Trapezoid-MC2)
Joint space1–2 mm; tight, congruent
Articular surfaceFlat/slightly curved; interlocked with trapezoid
MC2 base positionStable deep saddle articulation; very little mobility
Ligament stabilityInterosseous and dorsal ligaments — strongest of CMC group
MobilityLeast mobile CMC joint — 0–2° flexion
Injury thresholdRequires high energy; fracture almost always accompanies dislocation
Injury Patterns — 2nd CMC
Dorsal Dislocation GRADE 3
Pure dislocation uncommon due to strong ligamentous restraints. When it occurs, it follows high-energy axial loading. Dorsal displacement visible on lateral. Look for trapezoid fracture simultaneously.
Volar Dislocation RARE
Extremely rare — 2nd CMC is the most reported site for volar CMC dislocation. Volar displacement on lateral view. May be missed on PA alone.
Fracture-Dislocation
MC2 base fracture with dorsal dislocation. Capitate/trapezoid involvement possible. CT mandatory for full characterization and surgical planning.
Normal Criteria — 3rd CMC (Capitate-MC3)
Joint space1–2 mm; tight congruence
MC3 styloidRadial styloid process of MC3 base — normal variant; must not be confused with avulsion
Capitate articulationDistal capitate articular surface flat; MC3 base interlocks snugly
MobilityMinimal — 3rd CMC is the longitudinal axis of wrist motion
Key featureMC3 styloid: normal bony prominence — NOT an avulsion unless displaced from cortex
Injury Patterns — 3rd CMC
MC3 Styloid Avulsion
Avulsion of the radial styloid process at the MC3 base, at the site of the extensor carpi radialis brevis insertion. Indicates significant dorsal ligamentous injury. Key indicator of CMC instability even if joint appears reduced.
Dorsal Fracture-Dislocation GRADE 4
Dorsal displacement of MC3 on capitate, usually with MC3 base comminution. Often associated with 2nd and 4th CMC injuries — multiligamentous disruption. High-energy mechanism.
Normal Criteria — 4th CMC (Hamate-MC4)
Joint space1–2 mm; slightly wider than 2nd–3rd CMC
PA appearanceHamate facet orientation causes natural apparent irregularity on PA
MobilityModerate — 10–15° flexion (more than 2nd–3rd)
Oblique viewBest visualization of 4th CMC joint space
⚠ PitfallNormal variant irregularity mimics subluxation on PA — ALWAYS compare contralateral
Injury Patterns — 4th CMC
Dorsal Dislocation GRADE 3
4th and 5th CMC dislocations almost always occur together due to the shared hamate articulation. Isolated 4th CMC dislocation is rare. Oblique view mandatory. Look for hamate body fracture.
Hamate Body Fracture + Dislocation
Fracture of the hamate body with 4th–5th CMC dislocation. The fracture propagates through the hamate articular surface. CT is essential — often underappreciated on plain films.
Normal Criteria — 5th CMC (Hamate-MC5)
Joint space1–2 mm
MobilityMost mobile CMC joint (2nd–5th) — 20–25° flexion
MC5 base positionArticulates with ulnar facet of hamate; slight ulnar inclination normal
Hamate hookSeparate structure — visible on Carpal Tunnel view / CT only
Most commonMost frequently dislocated CMC joint after 1st CMC
Best viewOblique (pronated)
Injury Patterns — 5th CMC
Reverse Bennett (Boxer's) GRADE 4
Intraarticular fracture at the MC5 base — mirror of Bennett fracture. Volar-ulnar fragment remains anchored. Shaft displaced proximally and ulnarly by ECU tendon. Best seen on PA + oblique. CT for surgical planning.
5th CMC Dislocation GRADE 3
Pure dorsal dislocation. Often combined with 4th CMC. PA shows gap or overlap at hamate-MC5 interface. Oblique view shows dorsal displacement. Common in fight injuries.
Hamate Hook Fracture
NOT directly a CMC dislocation but causes deep ulnar-sided pain near 5th CMC. Completely invisible on PA/lateral/oblique — requires dedicated Carpal Tunnel view or CT. Must be actively excluded.
Radiographic Decision Tree — CMC Congruity Assessment
START: CMC RADIOGRAPH PA + Lateral + Oblique obtained Q1: JOINT SPACE WIDTH (PA) Is CMC joint space 1–2mm and uniform? Compare with contralateral YES NO (>2mm or asymmetric) Q2: PARALLELISM Articular margins parallel? No step-off on lateral? Q3: LATERAL VIEW Dorsal step-off on lateral? MC base displaced? YES NO GRADE 0 NORMAL Full congruity. No injury. Still check for occult avulsion fragment (may be only sign) GRADE 1–2 SUBLUXATION Partial displacement NO Q4: ARTICULAR CONTACT How much surface contact remains on lateral? 25–75% GRADE 2 SUBLUXATION Partial articular contact <25% GRADE 3 DISLOCATION No articular contact Q5: FRACTURE PRESENT? Intraarticular fragment at MC base or carpal bone? YES GRADE 4 FRACTURE-DISLOCATION Intraarticular fracture present NO Pure subluxation/dislocation No fracture component CT HAND If equivocal / high-energy / hamate hook suspected Normal pathway Subluxation pathway Dislocation pathway Fracture-dislocation pathway Decision node Dashed = concurrent assessment
Interactive Grading Tool
Input Radiographic Findings
1–2mm (Normal)
2–4mm (Widened)
Obliterated / Overlap
Parallel
Non-parallel / Step-off
Complete loss
None
Subtle step-off
Frank dorsal displacement
>75%
25–75%
<25%
None / 0%
None
Small (<20% articular)
Large (≥20% articular)
Comminuted
⬡
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