Metacarpophalangeal Joints: Anatomy, Function, Imaging

Topic

Metacarpophalangeal joints

The metacarpophalangeal (MCP) joints are the articulations between the metacarpal heads and the bases of the proximal phalanges of the fingers. These are condyloid synovial joints permitting flexion, extension, abduction, adduction, and limited circumduction, providing both mobility and stability for fine hand function.

Each MCP joint is stabilized by a fibrous capsule, collateral ligaments, a volar (palmar) plate, and extensor tendons, forming a complex yet highly coordinated structure. The MCP joints are essential for gripping, precision handling, and maintaining finger alignment. The thumb MCP joint is more restricted and functions as a hinge-type joint.

Synonyms

Knuckle joint
Metacarpal-phalangeal articulation
MCP articulation

Location and Structure

Articulation: Between the rounded head of each metacarpal and the shallow concave base of the proximal phalanx.
Type: Ellipsoid (condyloid) synovial joint.
Joint capsule: Encloses the joint, thin dorsally, thickened laterally, reinforced by ligaments.
Articular surfaces: Covered by smooth hyaline cartilage, allowing gliding motion.
Joint cavity: Contains synovial fluid to reduce friction.
Range of motion:
- Flexion: 90–110° (less in index, more in little finger)
- Extension: 30–45°
- Abduction/adduction: 20–30° (when extended)

Relations

Dorsally: Extensor tendons of corresponding digits and sagittal bands
Ventrally (palmar): Palmar plate, flexor tendons, and lumbrical insertions
Laterally: Collateral ligaments and interosseous muscles
Proximally: Metacarpal heads
Distally: Proximal phalangeal bases

Ligamentous Attachments

Collateral ligaments: Strong cords extending from metacarpal heads to proximal phalanx bases; taut in flexion, lax in extension.
Accessory collateral ligaments: Connect to the volar plate and stabilize against hyperextension.
Volar (palmar) plate: Fibrocartilaginous thickening of the palmar capsule; prevents hyperextension and deepens the articular surface.
Deep transverse metacarpal ligament: Connects volar plates of adjacent MCP joints, maintaining finger alignment.
Sagittal bands: Stabilize extensor tendons over the dorsal aspect.

Function

Flexion and extension: Primary movement for gripping and releasing.
Abduction and adduction: Occur when fingers are extended, allowing spread and approximation.
Circumduction: Small combined movement allowing rotational flexibility.
Stabilization: Maintains alignment during flexor/extensor activity.
Fine motor control: Enables precision grasping and coordinated finger motion.

Clinical Significance

Arthritis: MCP joints commonly affected in rheumatoid arthritis and psoriatic arthritis—characterized by synovial thickening, pannus formation, and joint deformity.
Ligament injuries: Collateral or volar plate tears cause instability and pain.
Dislocation: Usually dorsal, due to hyperextension trauma.
Fracture-dislocation: Common with metacarpal head fractures.
Trigger finger: Stenosing tenosynovitis affecting flexor tendon sheath near MCP level.
Infection: Septic arthritis may follow penetrating injury.
Post-traumatic stiffness: Seen after immobilization or capsular fibrosis.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright fatty signal within metacarpal heads and proximal phalanges.
- Cartilage: Smooth, thin intermediate-to-low signal over articular surfaces.
- Joint capsule and ligaments: Low-signal linear bands.
- Pathology: Synovial thickening or pannus—intermediate signal; fracture—linear low-signal defect.
T2-weighted images:
- Cortex: Dark.
- Marrow: Bright signal due to fat, slightly less than T1.
- Cartilage: Intermediate-to-bright, highlights surface irregularity.
- Joint fluid: Hyperintense, outlining articular contours.
- Pathology: Synovitis and effusion appear bright; erosions show hyperintense signal with cortical break.
STIR:
- Normal marrow: Intermediate-to-dark signal.
- Pathologic changes: Bright hyperintensity with indistinct trabecular pattern indicating bone marrow edema, inflammation, or osteitis.
- Excellent for early inflammatory or traumatic pathology (rheumatoid pannus, early osteomyelitis).
Proton Density Fat-Saturated (PD FS):
- Normal cartilage and capsule: Intermediate-to-dark homogeneous signal.
- Pathologic areas: Bright hyperintense regions in bone marrow, cartilage, or soft tissue (synovitis, capsulitis, effusion).
- Very sensitive for small effusions, ligament tears, and erosive arthritis.
T1 Fat-Sat Post-Contrast:
- Normal joint: Mild homogeneous synovial enhancement.
- Active synovitis or pannus: Intense synovial enhancement.
- Chronic arthritis: Peripheral enhancement with central low-signal fibrous pannus.
- Infection: Irregular enhancement of capsule and adjacent soft tissues.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, smooth contour around metacarpal head and phalangeal base.
Trabecular bone: Fine, regular internal pattern.
Joint space: Uniformly narrow, cartilage appears as thin hypodense line.
Pathology:
- Detects subtle fractures, erosions, and osteophytes.
- Evaluates metacarpal head contour and dislocations.
- Demonstrates calcifications or chronic arthropathy changes.

Post-Contrast CT (standard):

Joint capsule: Enhances in inflammation or infection.
Synovium: Hyperdense enhancement in active arthritis or pannus formation.
Soft tissue: Enhancing edema and pericapsular thickening seen in rheumatoid or septic arthritis.
Useful for evaluating erosive changes, osteomyelitis, or postoperative healing.

MRI images

CT images

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