upper extremity

Anatomy of the Hand

The human hand consists of 27 bones, which are categorized into three main groups: phalanges, metacarpals, and carpals. This structure is essential for understanding the functional anatomy of the hand.

Phalanges

The phalanges form the fingers and are found at the distal end of the hand.

  • Each hand has 14 phalanges in total.
  • There are three phalanges per finger (proximal, middle, distal), except for the thumb, which has two (proximal and distal).
    • Definitions:
    • Distal: The phalanx that is furthest away from the palm (topmost section of the finger).
    • Proximal: The phalanx nearest to the palm.
  • Digits: Each finger is referred to as a digit, with the numbering starting at the thumb as digit one and leading to the little finger as digit five.

Metacarpals

The metacarpals are the five bones that form the middle part of the hand, making up the palm's structure.

  • Each metacarpal bone consists of a head (distal end), neck (just below the head), body (mid-section), and base (proximal end).
  • Fracture Location: The neck of the metacarpal is particularly prone to fractures, a common injury known as "boxer's fracture."

Carpals

The carpals are the eight wrist bones arranged in two rows: proximal and distal. Each wrist contains:

  • Proximal Row (near the forearm): Scaphoid, Lunate, Pisiform, Triquetrum.
  • Distal Row: Trapezium, Trapezoid, Capitate, Hamate.
  • Memorization Mnemonics:
    1. Proximal: "Some Lovers Try Positions That They Can't Handle"
    2. Distal: "People Try To Catch High-Altitude Flyers"
Characteristics of Carpal Bones
  • The carpals are considered short bones.
  • Each wrist's carpal bones can differ in shape and position among individuals, which is important for interpreting X-rays.

Joints of the Hand

Interphalangeal Joints

The phalanges articulate at the interphalangeal joints, which are categorized as:

  • DIP (Distal Interphalangeal Joint): The joint closest to the fingertip.
  • PIP (Proximal Interphalangeal Joint): The joint closest to the palm, between the distal and proximal phalanges.

Metacarpophalangeal (MCP) Joints

  • The MCP joints are located between the heads of the metacarpals and the proximal phalanges, commonly referred to as the knuckles.
  • They allow for flexion and extension of the fingers.

X-ray Imaging of the Hand

Projections for Radiographic Evaluation

  1. PA Hand: Central ray at the third MCP joint; the hand is extended flat on the imaging plate.
  2. Lateral Hand: Positioning requires the lateral surface of the affected digit in contact with the plate to minimize magnification and distortion.
  3. PA Oblique Hand: The hand is rotated 45 degrees, with the central ray still at the third MCP joint.

Special Views

  • Ulnar Deviation: Utilized to better visualize the scaphoid bone, angling the X-ray to reveal any fractures clearly.
  • Gaynor-Hart Method: This tangential view focuses on the carpal tunnel, angling the X-ray tube between 25 to 30 degrees toward the palm, entering one inch distal to the base of the third metacarpal.

Anatomy of the Thumb

The thumb has distinct anatomical features:

  • Joints: The thumb only has two joints — the interphalangeal (IP) and the metacarpophalangeal (MCP) joint.
  • The thumb's base is near the trapezium bone, one of the carpal bones closest to the thumb.

Important Considerations for Hand Imaging

  • Positioning: Patients should have their hands flat and all digits extended and separated for ideal imaging and to reduce overlap.
  • Radiation Protection: Collimate closely to the area of interest to limit exposure and improve clarity of imaging.

Discussion Points

  • Fractures are common at specific sites in the hand, notably the neck of the metacarpals and the scaphoid in the wrist.
  • Understanding the anatomy and correct positioning of the hand during imaging is crucial for accurate diagnosis of potential injuries.

Summary

The hand’s anatomy, consisting of 27 bones categorized into phalanges, metacarpals, and carpals, is essential for its function. Knowledge of its detailed anatomy, joint structure, and the radiographic approaches for assessment is critical for anyone studying hand anatomy or radiography.