Proximal Phalanx of the Hand: Anatomy, Structure, Imaging

Topic

Proximal phalanx of hand

The proximal phalanx is the first and longest phalangeal bone of each finger and thumb. It forms the skeletal link between the metacarpal bones and the middle or distal phalanges, contributing to the overall length, strength, and mobility of the digits.

In the four fingers (index to little finger), each proximal phalanx articulates proximally with its corresponding metacarpal head and distally with the middle phalanx. In the thumb, the proximal phalanx articulates distally with the distal phalanx (since the thumb lacks a middle phalanx).

The proximal phalanx provides attachment sites for flexor and extensor tendons, collateral ligaments, and joint capsules, enabling coordinated flexion, extension, and grasping movements essential for hand function.

Synonyms

First phalanx of the finger
Basal phalanx
Proximal segment of digit

Location and Structure

Shape: Elongated bone with a concave base, a slender shaft, and a convex head.
Base: Expanded and concave for articulation with the metacarpal head, forming the metacarpophalangeal (MCP) joint.
Shaft: Slightly curved, concave on the palmar surface, convex dorsally.
Head: Rounded, forming the proximal part of the proximal interphalangeal (PIP) joint (or interphalangeal joint in thumb).
Composition: Outer cortical bone with internal cancellous trabeculae and fatty bone marrow.

Relations

Dorsally: Extensor expansion (extensor hood) and skin of the dorsum of the hand.
Palmar surface: Flexor tendons (flexor digitorum superficialis and profundus) with synovial sheaths.
Laterally: Collateral ligaments of MCP and PIP joints.
Proximally: Metacarpal head and joint capsule.
Distally: Middle phalanx or distal phalanx (thumb).

Attachments

Proximal base: Receives joint capsule and collateral ligament attachments of MCP joint.
Distal head: Provides attachment for the volar plate and collateral ligaments of PIP joint.
Palmar surface: Fibrous digital sheath enclosing flexor tendons attaches along the margins.
Dorsal surface: Extensor hood expansion from extensor digitorum and intrinsic hand muscles.

Nerve Supply

Periosteal innervation: Digital branches from median and ulnar nerves provide sensory fibers to periosteum and surrounding soft tissues.

Arterial Supply

Digital arteries from the superficial and deep palmar arches.
Nutrient arteries enter the bone near the midshaft to supply medullary and cortical regions.

Function

Digit movement: Serves as the lever for flexion and extension at MCP and PIP joints.
Grip and precision: Essential for grasping, pinching, and fine motor control.
Structural support: Provides strength and alignment for soft-tissue attachments of the digit.
Force transmission: Distributes axial load during grip and impact.

Clinical Significance

Fractures: Common due to falls, crush injuries, or sports trauma; may be transverse, spiral, or comminuted.
Dislocations: MCP or PIP dislocations may accompany capsular or ligamentous injury.
Arthritis: Degenerative or post-traumatic changes affect MCP and PIP articulations.
Osteomyelitis: Infection secondary to open wounds or hematogenous spread.
Tendon avulsions: Extensor or flexor tendon injuries can detach from phalangeal base or head.
Surgical relevance: Frequent site for fixation in hand fractures, tendon repairs, and reconstructive surgery.

MRI Appearance

T1-weighted images:
- Cortex: Low signal (dark).
- Marrow: Bright, reflecting fatty content.
- Articular cartilage: Smooth intermediate-to-low signal.
- Soft tissue planes: Bright fat surrounding tendons and ligaments.
- Fracture or bone edema: Linear or focal low signal crossing cortex and marrow.
T2-weighted images:
- Cortex: Low signal (black line).
- Marrow: Bright, slightly less than T1 but higher than muscle.
- Cartilage: Intermediate-to-bright signal over articular surfaces.
- Pathology: Marrow edema, effusion, or synovitis appear as bright hyperintense areas.
STIR:
- Normal bone marrow: Intermediate-to-dark signal.
- Pathologic regions: Bright hyperintense signal indicating edema, fracture, infection, or inflammation.
- Highly sensitive for early stress changes and osteitis.
Proton Density Fat-Saturated (PD FS):
- Normal marrow: Intermediate-to-dark, homogeneous appearance.
- Abnormal: Bright hyperintense signals in bone or soft tissues due to edema or tendon injury.
- Excellent for evaluating ligament tears, periosteal reactions, and soft-tissue inflammation.
T1 Fat-Sat Post-Contrast:
- Normal bone: Mild homogeneous enhancement.
- Inflammation/infection: Focal or diffuse marrow enhancement.
- Osteomyelitis: Patchy enhancement with cortical irregularity.
- Synovitis or arthritis: Enhancing joint capsule and pericapsular tissues.

CT Appearance

Non-Contrast CT:

Cortex: High attenuation, sharply defined.
Trabecular bone: Fine honeycomb structure.
Articular surface: Smooth, congruent contours with overlying cartilage line.
Pathology: Demonstrates fractures, erosions, sclerosis, cortical thickening, and dislocations with excellent detail.
Utility: Preferred for fracture mapping, preoperative planning, and bone union assessment.