Anatomy of the Wrist and Hand: Bones, Tendons, and Grip

The hand is one of the most extraordinary instruments of the human body: an anatomical structure of surprising complexity that allows for movements ranging from the most vigorous power grip to the finest and most delicate manipulation. This versatility is possible thanks to the perfect interaction between 27 bones, dozens of joints, a dense network of tendons and muscles, and a highly specialized nervous system.

The wrist represents the functional junction between the forearm and hand, acting as a pivot that allows for flexion, extension, radial, and ulnar deviation movements. Together, the wrist and hand constitute a unique biomechanical complex, whose integrity is fundamental for autonomy in daily activities, work, and sports.

Understanding the anatomy of the wrist and hand is essential to comprehend common pathologies — from carpal tunnel syndrome to tendinitis, from fractures to osteoarthritis — and to set up an effective rehabilitation pathway.

The 27 Bones of the Hand

The 27 hand bones comprise carpal, metacarpal, and phalangeal bones in the wrist and hand, providing structural support and enabling fine motor control and grip function. The skeleton of the hand is divided into three main groups: the carpal bones (wrist), the metacarpal bones (palm), and the phalanges (fingers). In total, there are 27 bones in each hand.

Carpal Bones (8 bones)

The carpal bones are eight small bones arranged in two rows, forming the structure of the wrist.

Proximal row (from radial to ulnar side):

• Scaphoid: the largest of the proximal row, it is the most frequently fractured carpal bone. It articulates with the radius and has precarious vascularization which can delay healing.
• Lunate: articulates with the radius. Lunate dislocations are among the most severe carpal injuries.
• Triquetrum: articulates with the articular disc of the distal radio-ulnar joint.
• Pisiform: a small sesamoid bone located on the palmar surface of the triquetrum, palpable on the ulnar side of the wrist.

Distal row (from radial to ulnar side):

• Trapezium: articulates with the first metacarpal, forming the trapeziometacarpal joint (saddle joint) which is fundamental for thumb opposition.
• Trapezoid: the smallest of the distal row.
• Capitate: the largest of all carpal bones, located in the center of the carpus.
• Hamate: characterized by a hook-like process on its palmar surface, through which Guyon’s canal passes.

Metacarpal Bones (5 bones)

The five metacarpals form the skeleton of the palm of the hand. They are numbered from I to V, starting from the thumb. Each metacarpal has:

• A proximal base, which articulates with the carpal bones.
• A slightly curved body (shaft).
• A distal head (the “knuckles” visible on the back of the hand), which articulates with the proximal phalanx.

The first metacarpal (of the thumb) is the shortest and most mobile, and its articulation with the trapezium allows for the opposition movements that distinguish the human hand.

Phalanges (14 bones)

The phalanges form the skeleton of the fingers. Fingers II to V each have three phalanges (proximal, intermediate, distal), while the thumb has only two (proximal and distal), for a total of 14 phalanges.

The Joints of the Wrist and Hand

Radiocarpal Joint

This is the main joint of the wrist, formed by the articular surface of the radius and the proximal row of the carpus (scaphoid, lunate, triquetrum). It is a condyloid joint that allows for:

• Flexion: approximately 80°
• Extension: approximately 70°
• Radial deviation: approximately 20°
• Ulnar deviation: approximately 35°
• Circumduction: a combination of the preceding movements

Midcarpal Joint

It is located between the proximal and distal rows of the carpus. It contributes significantly to wrist movements, especially flexion and radial deviation.

Carpometacarpal Joints

These connect the distal row carpal bones with the base of the metacarpals. The thumb’s (trapeziometacarpal) is a saddle joint with two degrees of freedom, allowing flexion-extension, abduction-adduction, and opposition.

Metacarpophalangeal (MCP) Joints

The “knuckle” joints are condyloid and allow for flexion (approximately 90°), extension, abduction, and adduction of the fingers.

Interphalangeal (IP) Joints

These are hinge (ginglymus) joints that allow exclusively for flexion and extension. Fingers II to V have a proximal interphalangeal (PIP) and a distal interphalangeal (DIP) joint; the thumb has only one interphalangeal joint.

The Carpal Tunnel and the Median Nerve

The carpal tunnel is an osteofibrous canal located on the palmar surface of the wrist. It is delimited:

• Posteriorly and laterally: by the carpal bones, which form a concavity (carpal groove).
• Anteriorly: by the flexor retinaculum (transverse carpal ligament), a robust fibrous band that closes the canal.

Inside the carpal tunnel pass:

• The median nerve: the main sensory nerve of the hand, innervating the thumb, index, middle, and radial half of the ring finger.
• Nine flexor tendons: 4 tendons of the flexor digitorum superficialis + 4 tendons of the flexor digitorum profundus + the tendon of the flexor pollicis longus.

Carpal Tunnel Syndrome

Carpal tunnel syndrome is the most frequent compressive neuropathy of the upper limb. It occurs when the median nerve is compressed within the carpal tunnel, due to:

• Increased content of the tunnel (flexor tenosynovitis, edema).
• Reduced available space (fractures, osteoarthritis).
• Systemic factors: pregnancy, diabetes, hypothyroidism, rheumatoid arthritis.

Typical symptoms are: tingling and numbness in the first three fingers and half of the fourth finger, nocturnal pain, weakness in grip, and in advanced cases, thenar eminence atrophy.

Flexor Tendons

The flexor tendons run along the palmar surface of the forearm and hand. They are responsible for finger closure and grip.

Flexor Digitorum Superficialis (FDS)

• Origin: medial epicondyle of the humerus, coronoid process of the ulna, radius.
• Insertion: palmar surface of the intermediate phalanx of fingers II-V (with a characteristic division into two slips).
• Function: flexion of the proximal interphalangeal (PIP) joint and MCP joints.

Flexor Digitorum Profundus (FDP)

• Origin: anterior surface of the ulna and interosseous membrane.
• Insertion: palmar surface of the base of the distal phalanx of fingers II-V.
• Function: flexion of the distal interphalangeal (DIP) joint, also contributes to PIP and MCP flexion.

Flexor Pollicis Longus (FPL)

• Origin: anterior surface of the radius and interosseous membrane.
• Insertion: base of the distal phalanx of the thumb.
• Function: flexion of the interphalangeal joint of the thumb.

Synovial Sheaths and Pulleys

The flexor tendons of the fingers glide within synovial sheaths and are held close to the skeleton by a system of fibrous pulleys (A1-A5 and C1-C3). Pulleys A2 and A4 are the most important biomechanically: they prevent “bowstringing” (the tendon lifting like a bowstring).

Inflammation of the synovial sheath at the A1 pulley can cause trigger finger (stenosi (restringimento del canale vertebrale o vascolare)ng tenosynovitis), a condition where the finger locks in flexion and unlocks with a painful snap.

Extensor Tendons

The extensor tendons run along the dorsal surface of the wrist and hand. Their anatomy is complex, especially at the fingers, where they form the extensor mechanism.

Main Extensor Muscles

• Extensor digitorum communis: extends fingers II to V.
• Extensor indicis proprius: independent extension of the index finger.
• Extensor digiti minimi: independent extension of the little finger.
• Extensor pollicis longus: extends the distal phalanx of the thumb.
• Extensor pollicis brevis: extends the proximal phalanx of the thumb.
• Abductor pollicis longus: abducts and extends the first metacarpal.

Extensor Retinaculum

On the dorsal surface of the wrist, the extensor tendons pass under the extensor retinaculum, divided into six distinct compartments. Inflammation of the first compartment (abductor pollicis longus and extensor pollicis brevis) causes De Quervain’s tenosynovitis, characterized by pain on the radial side of the wrist.

Extensor Mechanism of the Fingers

At each finger, the extensor tendon expands to form a complex structure called the extensor hood or extensor mechanism, which includes:

• The lateral bands: receive tendons from the interossei and lumbrical muscles.
• The central slip: inserts onto the base of the intermediate phalanx.
• The terminal tendon: formed by the confluence of the lateral bands, inserts onto the base of the distal phalanx.

This architecture allows for coordinated extension of the three phalanges.

Intrinsic Muscles of the Hand

The intrinsic muscles have both their origin and insertion within the hand itself. They are fundamental for fine movements and grip coordination.

Thenar Eminence Muscles

The thenar eminence is the fleshy muscle mass at the base of the thumb. It comprises four muscles:

Thumb opposition is the most characteristic movement of the human hand and depends on the integrity of the thenar muscles and the trapeziometacarpal joint.

Hypothenar Eminence Muscles

The hypothenar eminence is the muscle mass on the ulnar side of the palm, at the base of the little finger. It comprises:

Interossei Muscles

They are located in the intermetacarpal spaces and are divided into:

• Dorsal interossei (4 muscles): abduct the fingers (move them away from the axis of the hand, represented by the III digit). Remembered by the acronym DAB (Dorsal ABduct).
• Palmar interossei (3-4 muscles): adduct the fingers (move them towards the axis of the hand). Acronym PAD (Palmar ADduct).

Both groups contribute to MCP flexion and IP extension, inserting onto the extensor mechanism.

Lumbrical Muscles

The 4 lumbrical muscles originate from the tendons of the flexor digitorum profundus and insert onto the lateral extensor mechanism. Their function is unique and fundamental: they flex the MCP joints and extend the IP joints simultaneously, allowing for the “intrinsic-plus” position (flexed MCPs, extended IPs), essential for many gripping and manipulation activities.

Innervation of the Hand

The hand is innervated by three main nerves:

Understanding innervation is clinically fundamental: each nerve lesion produces a specific pattern of sensory and motor deficit that allows for precise localization of the damage site.

Vascularization of the Hand

The hand receives arterial blood from two main arteries:

• Radial artery: forms the deep palmar arch and contributes to the superficial arch.
• Ulnar artery: forms the superficial palmar arch and contributes to the deep arch.

These two anastomotic arterial arches ensure rich vascularization of the hand, with a certain redundancy that protects perfusion even in case of occlusion of one of the two arteries (verifiable with the Allen’s test).

Biomechanics of Grip

Grip is the primary function of the hand and is classified into two broad categories, each with distinct biomechanical characteristics.

Power Grip

Power grip involves the entire hand and is used when significant force needs to be applied to an object. Characteristics:

• The fingers close around the object, with full flexion of the MCP and IP joints.
• The thumb closes over the fingers, acting as a mechanical “block”.
• The object is in contact with the palm of the hand.
• Force is primarily generated by extrinsic muscles (forearm flexors).
• The grip strength of an adult man is on average 40-50 kg, in women 25-35 kg.

Variants of power grip:

• Cylindrical grip: grasping a hammer, a bottle.
• Spherical grip: grasping a ball.
• Hook grip: carrying a suitcase (fingers flexed, thumb not involved).

Precision Grip

Precision grip involves the fingertips and is used to manipulate small objects with accuracy. Characteristics:

• The object is held between the fingertips and the thumb in opposition.
• There is no contact with the palm.
• Intrinsic muscles (lumbricals, interossei, thenar) are the primary movers.
• High coordination and sensory feedback are required.
• Fingertip sensitivity is the most refined in the body (two-point discrimination threshold: 2-3 mm).

Variants of precision grip:

• Tip pinch: thumb tip against index finger tip (threading a needle).
• Pad-to-pad pinch: fingertip against fingertip (holding a coin).
• Three-jaw chuck pinch: thumb, index, and middle fingers (holding a pen).
• Lateral pinch (key pinch): thumb against the radial side of the index finger (turning a key).

Biomechanical Comparison

A fundamental biomechanical aspect: the wrist in slight extension (20-30°) optimizes grip strength, because it places the finger flexors at their optimal length to generate tension. A flexed wrist reduces grip strength by up to 40%. For this reason, wrist splints are often positioned in slight extension.

Common Wrist and Hand Pathologies

Functional Assessment of the Hand

Hand function assessment includes several parameters:

• Range of motion (ROM): measurement of joint mobility with a goniometer.
• Grip strength: measured with the Jamar dynamometer (power grip) or the pinch meter (pinch strength).
• Sensitivity: two-point discrimination test (Weber), Semmes-Weinstein monofilaments.
• Dexterity: functional tests (Nine-Hole Peg Test, Purdue Pegboard).
• Questionnaires: DASH (Disabilities of the Arm, Shoulder and Hand), Levine questionnaire for carpal tunnel.

Tips for Wrist and Hand Health

• Ergonomics: position keyboard and mouse so that the wrist is in a neutral position during computer work. Use a wrist rest if necessary.
• Active breaks: interrupt repetitive work every 30-45 minutes with stretching exercises for fingers, hand, and wrist.
• Intrinsic muscle strengthening: exercises with elastic bands, stress balls, and therapeutic putty maintain strength and dexterity.
• Flexor stretching: gently extend fingers and wrist to prevent stiffness and compression in the carpal tunnel.
• Protection during sports activities: specific gloves, wraps, and braces for the sport practiced.
• Pay attention to signals: tingling, numbness, weakness in grip, and persistent wrist pain should not be ignored.

Frequently Asked Questions (FAQ)

Frequently Asked Questions

How many bones comprise the human hand and wrist, and what is their primary function?

The human hand contains 27 bones, including 8 carpals in the wrist, 5 metacarpals in the palm, and 14 phalanges in the fingers. These bones form a complex skeletal framework that provides structure, support, and attachment points for muscles and tendons, enabling a wide range of movements.

What is the fundamental difference between a power grip and a precision grip?

A power grip involves the entire hand, with the fingers and thumb wrapping around an object to exert maximum force, such as holding a hammer. In contrast, a precision grip utilizes the fingertips and thumb to manipulate smaller objects with fine control and dexterity, like picking up a coin.

What is the significance of the carpal tunnel in hand anatomy?

The carpal tunnel is a narrow passageway in the wrist formed by carpal bones and the transverse carpal ligament. It houses the median nerve and several flexor tendons, which are crucial for hand sensation and movement. Compression within this tunnel can affect nerve function.

What role does a physical therapist play in maintaining wrist and hand health?

A physical therapist assesses and treats conditions affecting the wrist and hand, focusing on restoring function, reducing pain, and improving mobility. They design individualized exercise programs and provide guidance on injury prevention and ergonomic adjustments to optimize hand health.

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