Phalanx Bone of the Hand: Anatomy, Structure, Function, Imaging

Topic

Phalanx bone (hand)

The phalanges of the hand are the small tubular bones forming the fingers (digits). Each finger contains three phalanges—proximal, middle, and distal—except for the thumb, which has only two (proximal and distal). These bones articulate with one another to form the interphalangeal joints, enabling fine digital movement crucial for gripping and manipulation.

The phalanges are arranged in rows:

Proximal phalanges articulate proximally with the metacarpal heads at the metacarpophalangeal (MCP) joints.
Middle phalanges articulate with both proximal and distal phalanges (except in the thumb).
Distal phalanges bear the tuft, supporting the nail bed and fingertip pad.

Each phalanx consists of a base, shaft, and head, with smooth articular surfaces covered by hyaline cartilage. The structure provides balance between strength, mobility, and fine motor control.

Synonyms

Phalanges (plural)
Finger bones
Digital bones

Location and Structure

Number: 14 in total—2 in the thumb, 3 in each of the other four fingers.
Shape: Small, tubular, slightly curved bones; broad proximally and tapered distally.
Parts:
- Base: Proximal end, expanded and concave for articulation with adjacent bone.
- Shaft (body): Cylindrical, tapering distally with smooth anterior and rough posterior surfaces.
- Head: Distal, pulley-shaped (condylar) for interphalangeal articulation.
Articular surfaces: Covered with hyaline cartilage, allowing frictionless flexion and extension.
Composition: Compact cortical bone peripherally with cancellous bone and fatty marrow centrally.

Relations

Dorsally: Extensor tendons, dorsal aponeurosis, and skin of fingers.
Ventrally (palmar): Flexor digitorum profundus and superficialis tendons.
Laterally: Collateral ligaments of the MCP and interphalangeal joints.
Distally: Nail bed (for distal phalanx) and pulp tissue.

Attachments

Capsular attachments: At metacarpophalangeal and interphalangeal joints.
Ligamentous attachments: Collateral ligaments and volar plates attach along sides and palmar surfaces.
Tendinous attachments:
- Dorsal: Extensor digitorum, extensor indicis, and extensor digiti minimi.
- Palmar: Flexor digitorum profundus and superficialis insert on middle and distal phalanges.
Distal phalanges: Flattened tufts provide attachment for nail bed, skin, and fibrous septa of fingertip pad.

Arterial Supply

Digital branches of the proper palmar digital arteries from the superficial and deep palmar arches.
Dorsal digital arteries from the dorsal metacarpal arteries supply the dorsal aspects.

Function

Flexion and extension: Primary motion at interphalangeal and MCP joints.
Fine motor control: Facilitates grasp, precision pinch, and manipulation.
Load transmission: Transfers compressive forces from fingertips through the metacarpals.
Protection: Distal phalanges support the nail bed and protect digital neurovascular bundles.
Anchorage: Serve as attachment points for tendons, ligaments, and joint capsules.

Clinical Significance

Fractures: Common due to crush, avulsion, or hyperextension injuries; classified as shaft, head, or tuft fractures.
Dislocations: At MCP or interphalangeal joints; often associated with ligament injury.
Osteoarthritis: Affects DIP joints (Heberden’s nodes) and PIP joints (Bouchard’s nodes).
Rheumatoid arthritis: Causes erosions, deformity, and joint space narrowing.
Osteomyelitis: Infection secondary to trauma or puncture injuries.
Congenital variants: Short or absent phalanges (brachydactyly, symphalangism).
Imaging role: MRI for soft-tissue and marrow evaluation; CT for detailed cortical assessment.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Bone marrow: Bright (fatty marrow signal in adults).
- Cartilage: Smooth, thin, intermediate-to-low signal over articular surfaces.
- Tendons: Low-signal linear structures along dorsal and palmar surfaces.
- Fractures: Linear low-signal lines through cortex or subchondral bone.
T2-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright, slightly less than T1 but distinctly hyperintense to muscle.
- Cartilage: Intermediate-to-bright; shows chondral irregularities.
- Joint fluid: Hyperintense signal outlining articulations.
- Pathology: Bone contusions and marrow edema appear bright; osteoarthritis shows irregular cartilage thinning.
STIR:
- Normal marrow: Intermediate-to-dark signal.
- Pathologic marrow: Bright hyperintense signal indicating edema, fracture, or infection.
- Very sensitive for early inflammatory or traumatic bone lesions.
Proton Density Fat-Saturated (PD FS):
- Normal: Intermediate-to-dark bone marrow signal.
- Pathologic: Bright hyperintense areas with ill-defined margins in edema or osteitis.
- Ideal for detecting subtle fractures, erosions, or tenosynovitis near phalanges.
T1 Fat-Sat Post-Contrast:
- Normal bone: Mild homogeneous enhancement.
- Osteomyelitis or infection: Patchy or rim enhancement with cortical breach.
- Synovitis or arthritis: Enhancing synovium and pericapsular soft tissue.
- Neoplasm: Heterogeneous enhancement depending on vascularity.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation (bright white) with sharp definition.
Trabecular bone: Fine internal architecture visible.
Articular surfaces: Smooth, regular contours covered by low-density cartilage.
Pathology:
- Acute fractures—cortical discontinuity or displaced fragments.
- Chronic degenerative changes—osteophytes, sclerosis, and cystic changes.
- Tuft injuries—fragmentation or resorption.
- Excellent for evaluating fractures, erosions, and cortical irregularities.