Anatomical Structures of the Wrist, Hand, and Fingers
Functional Roles of the Upper Extremity Links
The shoulder is structurally equipped for mobility and the gross placement of the arm in space.
The elbow is equipped for stability.
The wrist, hand, and fingers serve as the final links and are equipped for both strength and precision.
Injury to the wrist, hand, or fingers includes impairment of both gross and fine motor movements.
Clinical Implications of Hand and Wrist Injuries
The extent of an individual's disability following an injury depends on the nature of the tasks they must perform and whether the dominant extremity is involved.
In football, a hand injury may have little consequence for a lineman but could be completely disabling for a quarterback or wide receiver.
In basketball, an injury to the non-shooting hand impacts an athlete's ability less than an injury to the shooting hand.
Skeletal Anatomy of the Wrist, Hand, and Fingers
The skeletal structure is comprised of the distal portions of the radius and ulna, the carpal bones, metacarpals, and phalanges.
Distal Radius: This bone broadens to form a small ulnar notch on its medial surface to accept the ulnar head. The radial styloid process projects from its anterolateral border.
Ulnar Head: This structure is circular, with the ulnar styloid process arising from its medial surface.
Carpal Bones: These bones have unusual shapes and irregular surfaces and are aligned in two rows from the radial to the ulnar sides.
Proximal Row: Includes the scaphoid, lunate, triquetrum, and pisiform.
Distal Row: Formed by the trapezium, trapezoid, capitate, and hamate.
Carpal and Metacarpal Alignments:
The trapezium aligns with the first metacarpal.
The trapezoid aligns with the second metacarpal.
The capitate aligns with the third metacarpal.
The hamate aligns with both the fourth and fifth metacarpals.
Pisiform: This bone floats on the triquetrum and acts as a sesamoid bone to improve the mechanical efficiency of the flexor carpi ulnaris muscle.
Clinical Frequency of Injury: The scaphoid is the most commonly fractured carpal bone, while the lunate is the most commonly dislocated.
Metacarpals: Much of the hand's length is formed by the five metacarpals, numbered through from the thumb to the little finger.
The proximal articulating surfaces are concave to accept the convex surfaces of the carpals.
The distal surfaces are convex to accept the concave surfaces of the proximal phalanx in each finger.
Phalanges: Each finger contains a proximal, middle, and distal phalanx, with the exception of the thumb, which has only a proximal and distal phalanx.
The proximal aspect of these bones is the base.
The distal aspect is the head.
Thumb Sesamoid Bones: Two small sesamoid bones are located over the palmar aspect of the distal end of the first metacarpal. These mobile bones improve the mechanical line of pull for the flexor pollicis brevis, abductor pollicis brevis, and adductor pollicis muscles.
Arthrokinematics of the Radioulnar and Radiocarpal Joints
The motion produced by the wrist, hand, and fingers occurs through the interaction of several joints, facilitated by force couples among associated muscles.
Distal Radioulnar Articulation: Formed by the ulnar head and the ulnar notch of the radius. It allows degree of freedom: pronation and supination.
The distal and proximal radioulnar joints work together; restriction at either joint limits the pronation and supination of the entire forearm.
During motion, the radius glides around the ulna. The ulna moves slightly palmarly and medially during supination and posteriorly and laterally during pronation.
Radiocarpal Articulation: An ellipsoidal joint providing degrees of freedom: flexion/extension and radial/ulnar deviation.
Formed by the distal end of the radius articulating with the scaphoid and lunate.
The triangular fibrocartilaginous complex (TFCC) functionally extends this joint to incorporate the triquetrum.
Intercarpal Articulations: These are gliding type joints between the eight carpal bones. They produce arthrokinematic glide and slide motions in coordination with the larger osteokinematic movements of the radiocarpal and metacarpophalangeal joints.
Ligamentous Structures of the Wrist
A complex web of ligaments on both dorsal and palmar surfaces stabilizes the radiocarpal and intercarpal relationships.
Dorsal Superficial Wrist Ligaments:
Ulnar Collateral Ligament: Arises from the ulnar styloid process and attaches to the medial aspect of the triquetrum (dorsally) and the pisiform (palmarly). It checks radial deviation (valgus force) and becomes taut at end ranges of flexion and extension.
Dorsal Radiocarpal Ligament: The primary ligament on the dorsal surface. It arises from the posterior surface of the distal radius and radial styloid process, attaching to the lunate and triquetrum to limit wrist flexion.
Radial Collateral Ligament: Originates from the styloid process of the radius and inserts into the scaphoid and trapezium. It limits ulnar deviation (varus forces) and becomes taut at extreme ranges of flexion and extension.
Secondary Dorsal Stabilizers: Dorsal intercarpal ligaments, primarily the triquetroscaphoid and triquetrotrapezial structures, which assist the dorsal radiocarpal ligament in limiting flexion.
Dorsal Carpal Metacarpal Ligaments: A web-like connection that stabilizes the carpometacarpal relationships.
Dorsal Metacarpal Ligamentous Complex: Connects the proximal bases of each of the five metacarpals to neighboring metacarpals.
Deep Dorsal Structures:
Triangular Fibrocartilage Complex (TFCC): A meniscus homolog that dissipates stresses imposed on the forearm during loading and stabilizes the distal radioulnar joint. It extends the radiocarpal articulation by stabilizing ulnar side carpal bones.
Palmar Superficial Ligaments:
Palmar Radiocarpal Ligament: Arguably the most important for wrist stability. It originates from the anterior surface of the distal radius and splits into three segments named for their attachments: the radiocapitate, radiotriquetral, and radioscaphoid ligaments. As a unit, they maintain alignment and limit wrist hyperextension.
Palmar Intercarpal Ligaments: A web-like complex stabilizing intercarpal articulations, consisting of superficial and deep layers.
Palmar Carpal Metacarpal Ligaments: Stabilize the proximal ends of the metacarpals to the distal carpal row; also consists of superficial and deep layers.
Carpometacarpal and Metacarpophalangeal Joint Mechanics
Carpometacarpal (CMC) Joints:
Metacarpal I articulates with the trapezium.
Metacarpal II articulates with the trapezoid, trapezium, and capitate.
Metacarpal III articulates with the capitate.
Metacarpals IV and V articulate with the hamate.
First CMC (Thumb): A saddle joint with a synovial cavity separate from the other four. It has degrees of freedom: flexion/extension and abduction/adduction. It also allows for opposition (combined movement to touch all fingers) and reposition.
CMC Joints II-IV: Simple gliding joints with minimal motion.
Fifth CMC: Has degrees of movement (flexion/extension, abduction/adduction). The second and third are immobile, while the fourth and fifth allow minimal mobility for strong gripping.
Metacarpophalangeal (MCP) Joints:
Represent the union of the concave articular surface of the proximal phalanx and the convex surface of the associated metacarpal.
First MCP: Classified as a synovial hinge joint with degree of freedom (flexion/extension).
Lateral Four MCP Joints: Ellipsoidal joints with degrees of freedom (flexion/extension, abduction/adduction).
Range of Motion: Maximum abduction and adduction in the lateral four fingers is only possible in full extension. Abduction/adduction of the thumb occurs at the CMC joint.
Interphalangeal (IP) Joint Dynamics and Stability
Classified as synovial hinge joints with degree of freedom.
Thumb: Contains a single simple IP joint.
Fingers: Contain the Proximal Interphalangeal (PIP) joint and the Distal Interphalangeal (DIP) joint.
Stabilizing Structures:
Volar (Palmar) Plate: A thick fibrocartilaginous structure that restricts extension.
Collateral Ligaments: Running obliquely from the dorsal aspect of the carpal side to the palmar aspect of the phalanx to support against valgus and varus forces. These tighten during finger flexion, reducing the need for dynamic stability during gripping.
Extensor Hood: Reinforces the dorsal aspect of the lateral four MCP joints.
Deep Transverse Metacarpal Ligament: Strong bands that limit abduction/adduction and reinforce palmar ligaments.
The instability of IP joints results from their shallow bony configuration, necessitating heavy reliance on these soft tissue structures.
Neurovascular Supply of the Wrist and Hand
Innervation is provided by three peripheral nerves:
Ulnar Nerve: Travels superficially through the palmar wrist, medial to the carpal tunnel, and enters the tunnel of galleon (formed by the hamate and pisiform).
The superficial branch provides sensation to the little finger and the medial one-half of the ring finger.
The deep branch innervates the hypothenar eminence, medial two lumbricals, interossei, and adductor pollicis.
Median Nerve: Follows the flexor digitorum superficialis and enters the carpal tunnel laterally.
Motor branches supply the thenar eminence.
Palmar digital branches provide sensation to the palmar thumb, index finger, middle finger, and lateral one-half of the ring finger, and innervate the lateral two lumbricals.
Radial Nerve: Divides into motor and sensory branches in the proximal forearm.
Posterior Interosseous Nerve (motor): Innervates the wrist and finger extensors.
Superficial Radial Nerve (sensory): Supplies the dorsal aspect of the hand.
Vascularity:
Supplied by the radial and ulnar arteries (branches of the brachial artery).
These arteries combine at the palmar and dorsal surfaces to form the palmar arch and dorsal arch, providing redundant blood supply to both sides of the wrist and hand.