Lecture 15 – Forearm Flexor Aspect & Wrist

The Forearm as a Rotational Link

The forearm is the segment of the upper limb between the elbow and wrist joints. Its unique function is to rotate the hand through pronation and supination. This rotational capability, combined with movements at the shoulder, allows the hand to be positioned almost 360° around the long axis of the limb, which is essential for reaching and grasping objects in all direction.


The Bones of the Forearm: Radius & Ulna

unlike the arm, which has a single humerus, the forearm contains two long bones that work together:

radius

  • located on the lateral side

  • it’s the primary bone involved in rotation

  • during pronation, the radius crosses over the ulna

ulna

  • located on the medial side

  • it’s the stable bone

    • forms the main articulation at the elbow with the humerus


These two bones are connected along their length by the interosseous membrane, a strong fibrous sheet attached to the medial border of the radius and the lateral border of the ulna. This membrane:

  • acts as a hinge during rotation

  • provides an attachment surface for muscles

  • transmits forces form the hand to the arm


The Radio-Ulnar Joints: The Mechanism of Pronation and Supination

pronation and supination occur through the coordinated action of two pivot joints and the interosseous membrane:

summary: in supination, the radius and ulna are parallel, with the palm facing forward. In pronation, the radius crosses diagonally over the ulna, turning the palm to face backward


Compartment Organisation of the Forearm

the forearm is divided into two compartments by the interosseous membrane and medial/lateral intermuscular septa:

  • anterior (flexor) compartment

    • contains the flexor and pronator muscles

  • posterior (extensor) compartment

    • contains the extensor and supinator muscles


the bulk of the forearm muscles consists of muscle bellies whose long tendons cross the wrist to act on the hand and fingers


Naming the Forearm Muscles: A System for Understanding

There are ~20 muscles in the forearm. Their names often describe their function, location, or shape. Understanding these naming conventions makes memorisation easier


The Anterior Compartment: Muscles of Flexion and Pronation

the muscles of the anterior compartment are organised into three groups: superficial, intermediate, and deep

  • common flexor origin

    • five muscles share a common origin form the common flexor tendon

      • which attaches to the anterior aspect of the medial epicondyle of the humerus

    • this is the anchor point for most wrist and finger flexors


  • superficial group (4 muscles)

these muscles primarily act on the wrist and thumb


  • intermediate group (1 muscle)


  • deep group (3 muscles)


The Wrist: Structure & Function

the wrist is a complex region composed of eight carpal bones arranged in two rows, forming three distinct joints

  • carpal bones


The 3 Joints of the Wrist Complex

  • radio-carpal joint (wrist joint proper)

    • articulation between the distal radius (and the articular disc/triangular ligament) and the proximal row of carpal bones (scaphoid, lunate, triquetral)

    • **The ulna DOESN’T articulate directly with the carpal bones; it’s separated by the articular disc

    • this joint allows flexion, extension, abduction, and adduction


  • midcarpal joint (transverse intercarpal joint)

    • articulation between the proximal and distal rows of carpal bones

    • allows some flexion and extension, contributing to overall wrist movement


  • carpometacarpal joints

    • articulations between the distal row of carpal bones and the bases of the metacarpals

    • fingers: very little movement; provides a stable base

    • thumb: a highly mobile saddle joint between the trapezium and the 1st metacarpal. This allows for opposition, flexion, extension, abduction, and adduction of the thumb


The Carpal Tunnel and Retinacula

  • flexor retinaculum

    • a thick band of deep fascia that spans the wrist

    • it attaches medially to the pisiform and hook of hamate, and laterally to the tubercles of the scaphoid and trapezium

  • carpal tunnel

    • the passageway formed by the carpal bones and the flexor retinaculum

    • it contains:

      • the long flexor tendons

      • the median nerve

  • synovial sheaths

    • lubricated coverings that allow tendons to glide smoothly

    • a common synovial sheath houses the FDS and FDP tendons, while FPL often has a separate sheath

  • palmar aponeurosis

    • a triangular, fan-shaped fibrous sheet that extends from the distal edge of the flexor retinaculum to the fingers

    • it’s continuous with the tendon of palmaris longus (if present) and helps protect underlying structures