Classification
- Type: Synovial saddle (sellar)
- Degrees of freedom: 2 (flexion/extension, abduction/adduction), plus functional opposition through combined movements
- Region: Hand/wrist complex
Articular Surfaces
- Trapezium (reciprocally saddle-shaped): Concave in the sagittal plane (allowing flexion/extension) and convex in the frontal plane (allowing abduction/adduction). This reciprocal shape is the hallmark of a saddle joint.
- Base of the first metacarpal (reciprocally saddle-shaped): Convex where the trapezium is concave, and concave where the trapezium is convex. This interlocking saddle configuration provides both mobility and inherent stability.
- Joint capsule: Relatively loose to accommodate the wide range of thumb motion. Reinforced by ligaments, particularly the anterior oblique (beak) ligament.
Movements and ROM
| Movement |
Normal ROM |
Plane |
Muscles Producing |
| Flexion |
45–50° |
Frontal (parallel to the palm) |
anatomy/muscles/flexor-pollicis-brevis, anatomy/muscles/opponens-pollicis |
| Extension |
0–20° |
Frontal |
anatomy/muscles/extensor-pollicis-longus, anatomy/muscles/extensor-pollicis-brevis, anatomy/muscles/abductor-pollicis-longus |
| Abduction |
60–70° |
Sagittal (perpendicular to the palm) |
anatomy/muscles/abductor-pollicis-longus, anatomy/muscles/abductor-pollicis-brevis |
| Adduction |
0° (to palm) |
Sagittal |
anatomy/muscles/adductor-pollicis |
| Opposition |
Composite |
Multiplanar |
anatomy/muscles/opponens-pollicis (primary), with flexor pollicis brevis and abductor pollicis brevis |
Opposition is the defining human movement. It combines CMC flexion, abduction, and medial rotation to bring the thumb pad to meet the fingertips. Loss of opposition (from OA, nerve injury, or joint restriction) profoundly impairs hand function — grip strength, precision pinch, and most activities of daily living depend on it.
Capsular Pattern
Abduction > Extension
When the first CMC capsule is restricted, abduction is the most limited movement, followed by extension. Flexion is relatively preserved. This pattern is seen in CMC OA and post-immobilization stiffness.
Resting Position
- Midway between abduction/adduction and flexion/extension (neutral thumb position)
- Capsule most relaxed
Close-Packed Position
- Full opposition
- Maximum ligament tension, maximum bony congruence
End-Feels
| Movement |
Normal End-Feel |
Type |
| Flexion |
Capsular (firm) |
Dorsal capsule and extensor tendons |
| Extension |
Capsular (firm) |
Palmar capsule and thenar muscles |
| Abduction |
Capsular (firm) |
Dorsal capsule, adductor pollicis, first dorsal interosseous fascia |
| Adduction |
Tissue approximation / capsular |
First metacarpal contacts the second metacarpal or thenar soft tissue |
Ligaments
Anterior Oblique Ligament (Beak Ligament)
- Attachments: Palmar tubercle of the trapezium → palmar beak of the first metacarpal base
- Function: The primary stabilizer of the first CMC joint — resists dorsal subluxation during pinch and grip. This ligament is the first to attenuate in early CMC OA, leading to dorsal subluxation of the metacarpal base on the trapezium.
- Injury mechanism: Repetitive pinch and grip activities over years; the ligament gradually attenuates, allowing dorsoradial subluxation. This subluxation is the hallmark radiographic finding of CMC OA.
- Condition link: conditions/osteoarthritis (first CMC OA / "basal joint arthritis")
Dorsoradial Ligament
- Attachments: Dorsal trapezium → dorsal first metacarpal base
- Function: Resists palmar subluxation; contributes to stability during power grip
- Injury mechanism: Traumatic hyperflexion of the thumb
Intermetacarpal Ligament
- Attachments: First metacarpal base → second metacarpal base
- Function: Resists excessive radial abduction; limits the space between the first and second metacarpals
Mobilization Techniques
Hands-on instruction is required. The descriptions below provide clinical reference detail for understanding and supervised practice. They are not a substitute for instructor-led technique training. Correct hand placement, force dosage, and tissue response interpretation require hands-on coaching and feedback.
Convex-Concave Rule at the First CMC Joint
The first CMC is a saddle joint with reciprocal concavities and convexities. For each movement, the convex-concave rule depends on which surface is convex in that plane of motion:
- Flexion/extension: The metacarpal base is convex in the sagittal plane → glide is opposite to the restricted movement
- Abduction/adduction: The metacarpal base is concave in the frontal plane → glide is in the same direction as the restriction
| Restricted Movement |
Glide Direction |
Reasoning |
| Flexion |
Dorsal glide |
Convex metacarpal surface → opposite direction |
| Extension |
Palmar glide |
Convex metacarpal surface → opposite direction |
| Abduction |
Lateral (radial) glide |
Concave metacarpal surface → same direction |
| Adduction |
Medial (ulnar) glide |
Concave metacarpal surface → same direction |
General Contraindications
- Absolute: Acute fracture (Bennett's or Rolando's fracture of the first metacarpal base), acute dislocation, joint infection, acute inflammatory arthritis (RA flare)
- Relative: Advanced CMC OA with bony end-feel (Grade I–II only — do not force into osteophytes), joint replacement (follow surgeon protocol)
Distraction of the First CMC Joint
Purpose: General pain modulation and capsular stretch. Opens the joint space, decompresses arthritic surfaces. The first technique used in CMC OA rehabilitation.
Patient position:
- Seated or supine
- Hand resting on the table, forearm in neutral or slight pronation
Hand placement:
- Stabilizing hand: Thumb and index finger grip the trapezium firmly, stabilizing it against the other carpal bones
- Mobilizing hand: Thumb and index finger grip the proximal first metacarpal (as close to the joint line as possible). Force directed distally along the long axis of the first metacarpal — pulling the metacarpal away from the trapezium.
Technique execution:
- Apply a slow, sustained or oscillatory traction force directed distally along the metacarpal shaft
- Grade I–II: Gentle oscillatory traction for pain modulation — appropriate for acute OA flares
- Grade III: Sustained traction at end-range for capsular stretch
- Duration: 30 seconds per set, 3–5 sets
Indications:
- CMC OA with joint space narrowing and pain during pinch/grip
- Post-immobilization stiffness (thumb spica cast removal)
- Joint play assessment reveals decreased distraction
Technique notes:
- Common error: Gripping the metacarpal shaft too distally — this produces MCP motion, not CMC distraction.
- Reassessment: Reassess pinch pain and grip strength after mobilization. Decreased pain with pinch confirms CMC involvement.
Ulnar Glide (for Abduction)
Purpose: Restores abduction — the most restricted movement in the CMC capsular pattern. Stretches the dorsal capsule and adductor-related structures. Restoring abduction is the priority in CMC OA rehabilitation because abduction loss is the earliest and most functionally disabling restriction (limits web space opening for grasp).
Patient position:
- Seated, hand resting on the table
- Forearm in neutral to slight pronation
Hand placement:
- Stabilizing hand: Grips the trapezium between thumb and index finger
- Mobilizing hand: Grips the base of the first metacarpal. For ulnar glide (to restore abduction using the concave-on-convex rule in the frontal plane — concave metacarpal surface moves in the same direction as the restriction): force directed ulnarly and slightly palmarly (toward the palm).
Technique execution:
- Apply a slow, sustained or oscillatory force directed ulnarly on the first metacarpal base
- Grade I–II: Gentle oscillations for pain modulation in acute OA
- Grade III: Oscillations into the end-range resistance for capsular stretch
- Duration: 30 seconds per set, 3–5 sets
- Reassess thumb abduction (web space opening) between sets
Indications:
- Decreased thumb abduction with capsular (firm) end-feel
- Narrowed web space limiting grasp
- CMC OA with early restriction (abduction is first to go)
Technique notes:
- Common error: Mobilizing the MCP joint instead of the CMC — ensure the contact is on the metacarpal base, not the proximal phalanx.
- Functional test: After mobilization, have the patient pick up a jar or bottle — improved web space opening confirms functional gain.
- Integration: Combine with thenar muscle release (opponens pollicis, adductor pollicis) before mobilization.
Muscles Crossing This Joint
Thenar Muscles
- anatomy/muscles/opponens-pollicis — opposition (flexion + medial rotation at CMC); the primary muscle for bringing the thumb to meet the fingers
- anatomy/muscles/flexor-pollicis-brevis — CMC and MCP flexion
- anatomy/muscles/abductor-pollicis-brevis — CMC abduction; thenar eminence muscle innervated by the median nerve
Extrinsic Thumb Muscles
- anatomy/muscles/abductor-pollicis-longus — abduction and extension at CMC; its tendon forms the lateral border of the anatomical snuffbox
- anatomy/muscles/extensor-pollicis-brevis — extension at CMC; tendon adjacent to APL in the first dorsal compartment
- anatomy/muscles/extensor-pollicis-longus — extension at CMC and IP joints; tendon forms the medial border of the anatomical snuffbox
- anatomy/muscles/adductor-pollicis — powerful adduction; key muscle for pinch strength; contracture narrows the first web space
Conditions Affecting This Joint
- conditions/osteoarthritis — first CMC OA ("basal joint arthritis") is the most common hand arthritis; affects ~33% of postmenopausal women; dorsoradial metacarpal subluxation produces the "shoulder sign" (squared appearance of the thumb base); pain with pinch and grip; grind test positive
- Bennett's fracture — intra-articular fracture-dislocation of the first metacarpal base; the adductor pollicis pulls the metacarpal shaft proximally and radially while the anterior oblique ligament holds a palmar fragment in place
- De Quervain's tenosynovitis — stenosing tenosynovitis of the first dorsal compartment (APL and EPB tendons); Finkelstein's test positive; often confused with CMC OA (both produce radial-sided thumb pain)
- Gamekeeper's/skier's thumb — ulnar collateral ligament injury of the thumb MCP joint; not a CMC condition but enters the differential for thumb pain
Clinical Notes
- CMC OA is the most common hand arthritis and is frequently underdiagnosed. The grind test (axial compression + rotation of the first metacarpal on the trapezium — positive: pain and crepitus) is the key differentiating test. Distinguish from de Quervain's (Finkelstein's test, first dorsal compartment tenderness) and MCP pathology.
- Web space narrowing is the earliest functional deficit. As the anterior oblique ligament attenuates and the metacarpal subluxes dorsoradially, the first web space narrows. Patients cannot open the hand wide enough to grasp large objects (jars, bottles). Restoring abduction is the first mobilization priority.
- The "Z-deformity" of advanced CMC OA. As the metacarpal base subluxes dorsally, the MCP joint hyperextends compensatorily, and the IP joint flexes — producing a zigzag (Z) deformity. This is a late finding indicating significant joint destruction.
- Splinting and mobilization work together. A thumb spica splint reduces pain by limiting CMC motion during daily activities. Mobilization sessions maintain and restore range. The combination of activity modification (splinting) and targeted mobilization is more effective than either alone.
Key Takeaways
- Capsular pattern abduction > extension — abduction loss is the earliest and most functionally disabling restriction in CMC OA.
- The anterior oblique (beak) ligament is the primary CMC stabilizer — its attenuation allows the dorsoradial subluxation that defines CMC OA.
- Grind test differentiates CMC OA from de Quervain's tenosynovitis — both produce radial-sided thumb pain.
- Distraction and ulnar glide (for abduction) are the priority mobilization techniques for CMC OA rehabilitation.
