Carpals

Introduction

Introduction to Carpals Osteology

The human wrist, or carpus, is a complex anatomical region consisting of eight small, intricately shaped short bones called carpal bones. Arranged neatly into two distinct rows—a proximal row and a distal row—these bones form the skeletal framework of the wrist joint and the palm base. Functionally, the carpals serve as a crucial transitional hub between the forearm and the hand, providing both the structural stability needed for heavy gripping tasks and the multi-axial flexibility required for complex manual manipulations.

Study

Anatomy of individual Carpal Bones

The eight carpal bones are categorized into a proximal row and a distal row, ordered from lateral (radial/thumb side) to medial (ulnar/pinky side). 1. Proximal Row: • Scaphoid: The largest bone in the proximal row, characterized by its distinct boat-like shape. It features a prominent tubercle on its palmar surface. It bridges the proximal and distal rows and is highly susceptible to fractures. • Lunate: A crescent or moon-shaped bone sitting precisely in the center of the proximal row. It plays a pivotal role in the radiocarpal joint and is the most frequently dislocated carpal bone. • Triquetrum: A three-cornered, pyramidal bone located on the medial aspect of the wrist, providing structural articulation for the pisiform. • Pisiform: A small, pea-shaped sesamoid bone that develops within the tendon of the flexor carpi ulnaris muscle. It sits entirely on the anterior (palmar) surface of the triquetrum. 2. Distal Row: • Trapezium: A four-sided bone situated on the lateral aspect. It features a prominent tubercle and groove anteriorly, and its distal surface forms a classic saddle joint with the base of the first metacarpal, enabling thumb opposability. • Trapezoid: A small, wedge-shaped bone tucked between the trapezium and the capitate, articulating distally with the second metacarpal. • Capitate: The largest of all the carpal bones. It occupies a central position in the wrist, featuring a rounded head that fits into the concavity formed by the scaphoid and lunate. • Hamate: A wedge-shaped bone on the medial side characterized by a prominent, hook-like projection on its palmar surface called the hook of the hamate (hamulus).

The Carpal Tunnel and Structural Arrangements

The carpal bones do not lie in a perfectly flat plane; instead, they are arranged to form a deep palmar concavity. This bony gutter is converted into a fibro-osseous osteofascial conduit known as the carpal tunnel by a thick, tough band of deep fascia called the flexor retinaculum (transverse carpal ligament). This ligament attaches laterally to the tubercles of the scaphoid and trapezium, and medially to the pisiform and the hook of the hamate. Running directly through this tight tunnel are ten crucial structures: the four tendons of the flexor digitorum superficialis, the four tendons of the flexor digitorum profundus, the single tendon of the flexor pollicis longus, and the median nerve. Any inflammation of the tendon sheaths or narrow compression within this space results in Carpal Tunnel Syndrome.

Articulations and Wrist Mechanics

The carpals articulate extensively to provide structural movement: • Radiocarpal (Wrist) Joint: Formed proximally by the distal end of the radius and the articular disc of the distal radioulnar joint, and distally by the scaphoid, lunate, and triquetrum. Note that the ulna is excluded from direct articulation with the carpals by the triangular fibrocartilage complex (TFCC). • Midcarpal Joint: The functional joint space separating the proximal and distal rows of carpal bones. • Carpometacarpal (CMC) Joints: Articulations between the distal carpal row and the bases of the metacarpals. The most famous is the first CMC joint (trapezium and 1st metacarpal base), which is a highly mobile saddle joint.

Clinical Correlations and Vascular Dynamics

• Scaphoid Fracture: The scaphoid is the most commonly fractured carpal bone, typically resulting from a fall on an outstretched hand (FOOSH). Tenderness is localized in the anatomical snuffbox. Crucially, the blood supply to the scaphoid enters from its distal pole and travels retrogradely (backward) to supply the proximal pole. A fracture across the waist of the scaphoid can cut off blood to the proximal pole, leading to avascular necrosis (AVN) and non-union. • Lunate Dislocation: The lunate is the most frequently dislocated carpal bone, often displacing anteriorly into the carpal tunnel. This displacement can acutely compress the median nerve, mimicking acute carpal tunnel syndrome, and can compromise its own blood supply leading to Kienböck's disease (avascular necrosis of the lunate). • Hook of the Hamate Fracture: Often caused by a direct blow to the palm, classically seen in golfers, baseball batters, or tennis players holding a club or racquet. Because the ulnar nerve passes close to the hook of the hamate through Guyon's canal, a fracture here can result in ulnar nerve injury, manifesting as paresthesia in the 4th and 5th digits and weakness in intrinsic hand muscles.

Summary

Syllabus Summary: Carpals Osteology

The wrist contains 8 carpal bones arranged into a proximal row (Scaphoid, Lunate, Triquetrum, Pisiform) and a distal row (Trapezium, Trapezoid, Capitate, Hamate). Remember the lateral-to-medial mnemonic: 'She Looks Too Pretty, Try To Catch Her'. The scaphoid and lunate articulate with the radius to form the radiocarpal joint, while the ulna is excluded by the TFCC. The carpal arch is enclosed anteriorly by the flexor retinaculum to form the carpal tunnel, housing the median nerve and 9 flexor tendons. Clinically, scaphoid fractures risk retrograde avascular necrosis, lunate dislocations can cause acute median nerve compression, and hamate hook fractures risk damaging the adjacent ulnar nerve.