Chapter 12

Chapter 12: Wrist Joint

1. Joint Structure

The wrist joint consists of two main joints:

Radiocarpal Joint:

  • Formed by the distal end of the radius and the radioulnar disk proximally.

  • Articulates with the scaphoid, lunate, and triquetrum distally.

  • The ulna is not considered a part of this joint due to the presence of the articular disk, which separates it from the wrist joint.

  • The pisiform, a small carpal bone, does not articulate with the disk as it is anterior to the triquetrum.

Classifications:

  • Synovial Joint and Condyloid Joint: Consists of a concave surface of the radius and articular disk interacting with convex surfaces of the carpals.

  • Biaxial Joint: Allows for movements in two planes—flexion/extension and radial/ulnar deviation.

  • Circumduction: This encompasses a circular movement combining the four actions—flexion, extension, ulnar deviation, and radial deviation—without rotation.

Midcarpal Joint:

  • Located between the proximal and distal rows of carpal bones.

  • Classified as plane joints, permitting gliding motions between carpal bones, facilitating complex wrist movements.

2. Joint Motions

Flexion and Extension:

  • Occurs in the sagittal plane around a frontal axis.

  • Approximate range: 90 degrees of flexion and 70 degrees of extension.

  • Vital for activities requiring wrist mobility, such as typing and sports.

Radial and Ulnar Deviation:

  • Occurs in the frontal plane around a sagittal axis.

  • Approximate range: 25 degrees of radial deviation (moving the wrist away from the body) and 35 degrees of ulnar deviation (moving the wrist towards the body).

  • Movements exhibit a soft tissue stretch end feel, except for radial deviation which presents a bony end feel due to the osseous structures.

3. Bones and Landmarks

Two Rows of Carpal Bones:

  • Proximal Row: Scaphoid, Lunate, Triquetrum, Pisiform.

  • Distal Row: Trapezium, Trapezoid, Capitate, Hamate.

Bony Landmarks:

  • Styloid Processes: Projections on the radius and ulna that provide critical attachments for ligaments, contributing to wrist stability.

  • Hook of the Hamate: Serves as an attachment site for the transverse carpal ligament, playing a crucial role in the integrity of the carpal tunnel.

  • Medial/Lateral Epicondyles: Sites for the attachment of flexor and extensor tendons respectively, influencing forearm and hand motion.

4. Ligaments and Other Structures

Key Ligaments Supporting the Wrist's Stability:

  • Radial Collateral Ligament: Provides medial support; attaches to the radius and specific carpal bones, helping stabilize lateral wrist motions.

  • Ulnar Collateral Ligament: Offers lateral support; connects the ulna to adjacent carpal bones, assisting in the stability of the medial wrist structure.

  • Palmar Radiocarpal Ligament:

    • A thick band limiting wrist extension.

    • Vital for overall wrist function, providing support during activities that require wrist extension, such as pushing or lifting.

  • Dorsal Radiocarpal Ligament:

    • Connects from the posterior distal radius to carpal bones; this ligament is crucial in limiting wrist flexion, maintaining joint integrity.

  • Articular Disk:

    • A fibrocartilaginous structure that functions as a shock absorber located on the distal ulna; articulates with the triquetrum and lunate, allowing for smoother motion and load distribution in the wrist.

5. Muscles of the Wrist

Flexor Group:

  • Flexor Carpi Ulnaris: Connects from the medial epicondyle to the pisiform and fifth metacarpal, involved in wrist flexion and ulnar deviation.

  • Flexor Carpi Radialis: Attaches from the medial epicondyle to the second and third metacarpals, facilitating wrist flexion and radial deviation.

  • Palmaris Longus: Characterized by its unique carpal attachment; it assists in wrist flexion and tightens the palmar aponeurosis, enhancing grip strength.

Extensor Group:

  • Extensor Carpi Radialis Longus and Brevis: Each originates from the lateral epicondyle and extends to the second and third metacarpals; they are key players in wrist extension, especially in activities like throwing.

  • Extensor Carpi Ulnaris: Extends from the lateral epicondyle to the fifth metacarpal; it assists in both extension and ulnar deviation of the wrist, crucial for many hand movements.

6. Anatomical Relationships

Proximal Muscle Attachments:

  • The flexor group attaches on the medial epicondyle, while the extensor group attaches on the lateral epicondyle, highlighting their distinct functional roles in wrist movements.

Innervation:

  • Radial Nerve: Responsible for innervating posterior muscles, allowing extension of the wrist and fingers.

  • Median Nerve: Innervates most anterior muscles, critical for thumb movement and wrist flexion.

  • Ulnar Nerve: Primarily innervates the muscles on the ulnar side, contributing to fine motor movements of the little finger and wrist actions.

7. Summary of Muscle Action and Innervation

Table of Muscle Actions:

  • Flexion: Flexor Carpi Radialis, Flexor Carpi Ulnaris.

  • Extension: Extensor Carpi Radialis Longus/Brevis, Extensor Carpi Ulnaris.

  • Radial Deviation: Flexor Carpi Radialis, Extensor Carpi Radialis Longus.

  • Ulnar Deviation: Flexor Carpi Ulnaris, Extensor Carpi Ulnaris.

Innervation Overview:

  • Radial Nerve: C6, C7.

  • Ulnar Nerve: C8, T1.

  • Median Nerve: C6, C7.

8. Points to Remember

  • Mnemonic for wrist bones: "Sam Likes To Push The Toy Car Hard" (Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate).

  • Understanding contraction types:

    • Isometric: Muscle tension without a change in muscle length.

    • Concentric: Muscle shortens while generating force.

    • Eccentric: Muscle lengthens while under tension.

  • Joint motions should consider the anatomical position and the specific positioning of the wrist in daily activities, emphasizing the importance of wrist biomechanics in both functional and athletic settings.