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Proximal interphalangeal joints of hand

The proximal interphalangeal (PIP) joints are the hinge synovial joints between the heads of the proximal phalanges and the bases of the middle phalanges of the fingers (index to little finger). The thumb lacks a PIP joint, having only one interphalangeal joint.

The PIP joints allow flexion and extension, essential for gripping, pinching, and fine motor control. They are stabilized by a robust capsule, collateral ligaments, and a volar plate that prevents hyperextension. The articular surfaces are covered by hyaline cartilage, permitting smooth movement and load distribution.

Synonyms

  • PIP joint of the finger

  • Middle finger joint

  • Proximal phalangeal articulation

Location and Structure

  • Articulation: Between the head of the proximal phalanx and the base of the middle phalanx.

  • Joint type: Synovial hinge joint (ginglymus).

  • Articular surfaces:

    • Head of proximal phalanx: Trochlear-shaped, convex surface.

    • Base of middle phalanx: Concave surface with shallow groove.

  • Joint capsule: Fibrous, thin dorsally, reinforced laterally by collateral ligaments and volarly by a fibrocartilaginous plate.

  • Synovial membrane: Lines the capsule and produces synovial fluid for lubrication.

Relations

  • Dorsally: Extensor expansion and central slip of the extensor digitorum tendon

  • Volar (palmar) surface: Flexor digitorum superficialis tendon and volar plate

  • Laterally: Collateral ligaments

  • Proximally: Proximal phalanx

  • Distally: Middle phalanx

Ligamentous Attachments

  • Collateral ligaments: Strong cords on each side of the joint; tighten during flexion to prevent side movement.

  • Accessory collateral ligaments: Connect the collateral ligaments to the volar plate.

  • Volar plate: Thick fibrocartilaginous reinforcement of the palmar capsule that limits hyperextension and provides articular stability.

  • Extensor mechanism: The central slip of the extensor tendon inserts on the base of the middle phalanx, controlling extension.

Nerve Supply

  • Digital branches of the median and ulnar nerves (palmar aspect).

  • Dorsal digital branches of the radial nerve (dorsal aspect).

Function

  • Flexion and extension: Primary movements, allowing finger bending during grip and extension during release.

  • Stability: Provided by collateral ligaments and volar plate during dynamic hand activity.

  • Load transfer: Distributes compressive forces through phalanges during grasping.

  • Dexterity: Enables precise digital control for fine motor tasks.

Clinical Significance

  • Arthritis: The PIP joints are commonly affected by osteoarthritis and rheumatoid arthritis, causing deformity and loss of motion.

  • Boutonnière deformity: Due to rupture of the central slip of the extensor tendon, leading to PIP flexion and DIP hyperextension.

  • Sprains and dislocations: Result from hyperextension or axial loading injuries.

  • Fracture-dislocations: Involve volar plate avulsion or intra-articular fracture.

  • Capsulitis or synovitis: From repetitive strain or inflammatory disease.

  • Imaging relevance: MRI and CT crucial for evaluating ligament, cartilage, and bony pathologies.

MRI Appearance

  • T1-weighted images:

    • Bone cortex: Low signal (dark).

    • Marrow: Bright fatty signal in normal adults.

    • Articular cartilage: Smooth intermediate-to-low signal over articular surfaces.

    • Ligaments and volar plate: Low-signal linear structures on both sides of the joint.

    • Pathology: Fractures appear as low-signal lines; bone marrow edema appears intermediate-to-bright.

  • T2-weighted images:

    • Cortex: Dark.

    • Bone marrow: Bright, slightly less than on T1.

    • Cartilage: Intermediate-to-bright, highlighting irregularities or thinning.

    • Joint fluid: Hyperintense, outlining articular contours.

    • Pathology:

      • Effusions and synovitis: bright signal in joint cavity.

      • Cartilage defects: focal hyperintense discontinuity.

      • Ligament tears: discontinuity or irregular hyperintensity in collateral ligaments.

  • STIR:

    • Normal marrow: Intermediate-to-dark signal.

    • Pathologic: Bright hyperintensity in edema, inflammation, or osteitis.

    • Highlights soft-tissue swelling and joint effusion.

  • Proton Density Fat-Saturated (PD FS):

    • Normal marrow and capsule: Intermediate-to-dark signal.

    • Pathology: Bright signal indicating edema, capsulitis, or partial ligament tears.

    • Excellent for detecting subtle cartilage loss, synovial thickening, and effusions.

  • T1 Fat-Sat Post-Contrast:

    • Normal joint: Mild homogeneous enhancement of synovium.

    • Rheumatoid arthritis: Intense synovial enhancement with pannus formation.

    • Infection: Patchy marrow enhancement and pericapsular soft-tissue enhancement.

    • Chronic degeneration: Minimal peripheral enhancement.

CT Appearance

Non-Contrast CT:

  • Cortex: High attenuation, smooth and continuous.

  • Trabecular bone: Fine spongy pattern.

  • Joint space: Uniformly maintained in healthy joints.

  • Pathology:

    • Detects fractures, osteophytes, erosions, and joint space narrowing.

    • Assesses subchondral cysts, sclerosis, and periarticular calcifications.

    • Useful for evaluating post-traumatic alignment and degenerative joint disease.

Post-Contrast CT (standard):

  • Synovium and capsule: Enhancing in inflammatory arthritis or infection.

  • Pericapsular soft tissues: Show enhancement in cellulitis or synovitis.

  • Cartilage defects or erosions: Visible as surface irregularities with adjacent enhancement.

MRI images

Proximal interphalangeal joints coronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT images

Proximal interphalangeal joints coronal ct image

CT images

Proximal interphalangeal joints ct sagittal image