Chapter 16: The Hand Anatomy, Function, and Pathologies

Skeletal Structure and Joint Anatomy of the Hand

• Distal Phalanges: The most distal bones of the fingers.

• Middle Phalanges: Located between the distal and proximal phalanges (not present in the thumb).

• Proximal Phalanges: Bones at the base of the fingers that articulate with the metacarpals.

• Metacarpals: The five bones of the hand between the carpals and the phalanges.

• Carpals: The eight bones of the wrist.

• Distal Interphalangeal (DIP) Joints: The joints between the middle and distal phalanges.

• Proximal Interphalangeal (PIP) Joints: The joints between the proximal and middle phalanges.

• Interphalangeal (IP) Joint: The single joint between the two phalanges of the thumb ($1^{st}$ digit).

• Metacarpophalangeal (MCP) Joints: Joints between the metacarpals and proximal phalanges.

• Carpometacarpal (CMC) Joints: Joints between the distal row of carpal bones and the metacarpals.

The CMC Joint and Motions of the Thumb

• Thumb CMC Joint Structure: This joint is formed by the first metacarpal and the trapezium. It is characterized as saddle-shaped.

• Available Motions at the $1^{st}$ CMC Joint:
      * Flexion/Extension: Moving the thumb across or away from the palm in the frontal plane.
      * Abduction/Adduction: Moving the thumb away from or toward the palm in the sagittal plane.
      * Opposition/Reposition: Opposition is a combination of flexion and abduction with accessory rotation, allowing the tip of the thumb to touch the other fingertips. Reposition is the return to the anatomical position.

Joints and Kinematics of the Fingers

• Metacarpophalangeal (MCP) Joints:
      * Classification: Biaxial joints.
      * Motions: Flexion and extension; abduction and adduction.

• Proximal Interphalangeal (PIP) Joints:
      * Classification: Uniaxial joints.
      * Motions: Flexion and extension.

• Distal Interphalangeal (DIP) Joints:
      * Classification: Uniaxial joints.
      * Motions: Flexion and extension.

Ligaments and Retinacular Structures of the Wrist

• Flexor Retinaculum: A fibrous band spanning the anterior wrist in a mediolateral direction. It consists of two parts:
      1. Palmar Carpal Ligament: More proximal and superficial; it crosses over the flexor muscles to prevent "bow-stringing" of tendons.
      2. Transverse Carpal Ligament: Deeper and more distal.
          * Medial Attachments: Pisiform and hook of hamate.
          * Lateral Attachments: Scaphoid and trapezium.
          * Function: Arches over the carpal bones to form the carpal tunnel, serving as a passage for the long finger flexor tendons and the median nerve.

• Extensor Retinaculum: A fibrous band traversing the posterior side of the wrist in a mediolateral direction.
      * Medial Attachments: Ulnar styloid, triquetrum, and pisiform.
      * Lateral Attachment: Lateral side of the radius.
      * Function: Holds the extensor tendons close to the wrist.

• Extensor Expansion Ligament (Extensor Hood):
      * Structure: A small triangular-shaped aponeurosis of connective tissue.
      * Dimensions: Wider at the base and narrow distally.
      * Location: Covers the dorsum and sides of the proximal phalanx of the fingers.
      * Distal Insertion: Inserts at the base of the distal phalanx.
      * Attachments: Serves as an attachment point for the extensor tendons, lumbricales, and interossei muscles to the middle and/or distal phalanges.
      * Function: Keeps extensor tendons close to the midline.

Classification and Overview of Hand Muscles

• Extrinsic Muscles:
      * Proximal Attachment: Located proximal to the wrist (forearm).
      * Function: Provide primary movement at the thumb or fingers while assisting in wrist function.

• Intrinsic Muscles:
      * Proximal Attachment: Located at or distal to the carpal bones.
      * Function: Responsible for fine motor control and precision movements of the hand. They function at the thumb or fingers.
      * Subdivisions: Thenar, Hypothenar, and Palm groups.

Extrinsic Muscles: Anterior and Posterior Compartments

• Anterior Extrinsic Muscles:
      * Flexor Digitorum Superficialis (FDS):
          * Origin: Medial epicondyle of humerus (common flexor tendon), coronoid process, and radius.
          * Insertion: Sides of the middle phalanx of fingers $2$ through $5$.
          * Action: Flexes MCP and PIP joints of the fingers; assists in wrist flexion.
          * Nerve: Median ($C7, C8, T1$).
          * Position: Deep to the wrist flexors and palmaris longus.
      * Flexor Digitorum Profundus (FDP):
          * Origin: Upper three-fourths of the ulna.
          * Insertion: Distal phalanx of the four fingers ($2$-$5$).
          * Action: Flexes all three joints of the fingers (MCP, PIP, DIP); assists in wrist flexion.
          * Nerve: Median and Ulnar nerves ($C8, T1$).
          * Position: Deep to the Flexor Digitorum Superficialis.
      * Flexor Pollicis Longus (FPL):
          * Origin: Anterior radius.
          * Insertion: Distal phalanx of the thumb.
          * Action: Flexes all three joints of the thumb (CMC, MCP, IP); assists in wrist flexion.
          * Nerve: Median ($C8, T1$).

• Posterior Extrinsic Muscles:
      * Abductor Pollicis Longus (APL):
          * Origin: Posterior radius, interosseous membrane, and middle ulna.
          * Insertion: Base of the first metacarpal.
          * Action: Thumb abduction at the CMC joint.
          * Nerve: Radial ($C6, C7$).
      * Extensor Pollicis Brevis (EPB):
          * Origin: Posterior distal radius.
          * Insertion: Base of the proximal phalanx of the thumb.
          * Action: Extends CMC and MCP joints of the thumb.
          * Nerve: Radial ($C6, C7$).
      * Extensor Pollicis Longus (EPL):
          * Origin: Middle posterior ulna and interosseous membrane.
          * Insertion: Base of the distal phalanx of the thumb.
          * Action: Extends all three joints of the thumb (CMC, MCP, IP).
          * Nerve: Radial ($C6, C7, C8$).
      * Extensor Digitorum (Extensor Digitorum Communis):
          * Origin: Lateral epicondyle of the humerus.
          * Insertion: Base of the distal phalanx of the $2^{nd}$ through $5^{th}$ fingers.
          * Action: Extends all three joints of the fingers (MCP, PIP, DIP); assists in wrist extension.
          * Nerve: Radial ($C6, C7, C8$).
      * Extensor Indicis (EI):
          * Origin: Distal ulna.
          * Insertion: Base of the distal phalanx of the $2^{nd}$ digit (index finger).
          * Action: Extends all three joints of the index finger (MCP, PIP, DIP); assists in wrist extension.
          * Nerve: Radial ($C6, C7, C8$).
      * Extensor Digiti Minimi (EDM):
          * Origin: Lateral epicondyle of the humerus.
          * Insertion: Base of the distal phalanx of the $5^{th}$ digit.
          * Action: Extends all three joints of the fifth digit (MCP, PIP, DIP); assists in wrist extension.
          * Nerve: Radial ($C6, C7, C8$).

Anatomical Snuffbox

• Definition: A clinical surface anatomy landmark visible with thumb extension.

• Radial Boundary: Formed by the Abductor Pollicis Longus and Extensor Pollicis Brevis.

• Ulnar Boundary: Formed by the Extensor Pollicis Longus.

• Floor: Comprised of the scaphoid bone.

Intrinsic Muscles of the Hand

• Thenar Muscles (Thumb Group):
      * Flexor Pollicis Brevis:
          * Origin: Trapezium and flexor retinaculum.
          * Insertion: Base of the proximal phalanx of the thumb.
          * Action: Flexes CMC and MCP joints of the thumb.
          * Nerve: Median ($C6, C7$).
      * Abductor Pollicis Brevis:
          * Origin: Scaphoid, trapezium, and flexor retinaculum.
          * Insertion: Base of the proximal phalanx of the thumb.
          * Action: Abducts the thumb at the CMC joint.
          * Nerve: Median ($C6, C7$).
      * Opponens Pollicis:
          * Origin: Trapezium and flexor retinaculum.
          * Insertion: First metacarpal.
          * Action: Opposes the thumb at the CMC joint.
          * Nerve: Median ($C6, C7$).

• Deep Palm Muscles:
      * Adductor Pollicis:
          * Origin: Capitate, base of $2^{nd}$ metacarpal, and palmar surface of the $3^{rd}$ metacarpal.
          * Insertion: Base of the proximal phalanx of the thumb.
          * Action: Adducts the thumb at the CMC joint.
          * Nerve: Ulnar ($C8, T1$).
      * Dorsal Interossei:
          * Origin: Two adjacent metacarpals.
          * Insertion: Base of the proximal phalanx.
          * Action: Finger abduction at the MCP joint.
          * Nerve: Ulnar ($C8, T1$).
      * Palmar Interossei:
          * Origin: Palmar surface of $1^{st}, 2^{nd}, 4^{th},$ and $5^{th}$ metacarpals.
          * Insertion: Base of the proximal phalanx of the same finger.
          * Action: Finger adduction at the MCP joint.
          * Nerve: Ulnar ($C8, T1$).
      * Lumbricales:
          * Origin: Tendons of the flexor digitorum profundus.
          * Insertion: Tendon of the extensor digitorum muscle at the proximal phalanx.
          * Action: Flexion of the MCP joint while extending the PIP and DIP joints.
          * Nerve: $1^{st}$ and $2^{nd}$ by the Median nerve; $3^{rd}$ and $4^{th}$ by the Ulnar nerve ($C6, C7, C8$).

• Hypothenar Muscles (Small Finger Group):
      * Flexor Digiti Minimi:
          * Origin: Hook of hamate and flexor retinaculum.
          * Insertion: Base of the proximal phalanx of the $5^{th}$ digit.
          * Action: Flexes CMC and MCP joints of the $5^{th}$ digit.
          * Nerve: Ulnar ($C8, T1$).
      * Abductor Digiti Minimi:
          * Origin: Pisiform and tendon of flexor carpi ulnaris.
          * Insertion: Proximal phalanx of the $5^{th}$ digit.
          * Action: Abducts the MCP joint of the $5^{th}$ digit.
          * Nerve: Ulnar ($C8, T1$).
      * Opponens Digiti Minimi:
          * Origin: Hook of hamate and flexor retinaculum.
          * Insertion: Shaft of the $5^{th}$ metacarpal.
          * Action: Opposes the $5^{th}$ digit at the CMC joint.
          * Nerve: Ulnar ($C8, T1$).

Hand Function and Positioning

• Role of Lower/Upper Limb in Hand Placement:
      * Shoulder and Shoulder Girdle: Position the hand in space.
      * Elbow: Allows the hand to move closer to or farther away from the body.
      * Wrist: Provides stability while the hand manipulates objects.

• Functional Position of the Hand: This is the optimal position for strength and precision.
      * Wrist is slightly extended.
      * MCP and PIP joints of the fingers are slightly flexed.
      * Thumb is in opposition.
      * Maintenance of the thenar web space.

Grasps and Grips

Power Grips

Power grips are used when an object must be held forcefully while being moved by muscles of more proximal joints. These grips are typically isometric between the hand and the object.

• Cylindrical Grip: All fingers are flexed around the object with the thumb and fingers in opposite directions. Examples: holding a hammer, a racquet, or wheelbarrow handles.

• Modified Cylindrical Grip: A variation where the $5^{th}$ finger is flexed the most and the $2^{nd}$ finger is only partly flexed. This allows more forceful and controlled tool use. Example: holding a golf club.

• Spherical Grip: All fingers and the thumb are adducted around an object with fingers spread apart. The palm is often not involved. Examples: holding an apple, a doorknob, or picking up a glass by its top.

• Hook Grip: The $2^{nd}$ through $5^{th}$ fingers are flexed around an object in a hook-like manner; MCP joints are extended, while PIP and DIP joints are flexed. The thumb is generally not involved. This is the only power grip possible for individuals with a median nerve injury. Examples: holding a suitcase, wagon, or bucket handle.

Precision Grips

Precision grips involve holding objects between the tips of the fingers and the thumb, involving both intrinsic and extrinsic muscles. The thumb is usually abducted, and the palm is not involved. These provide fine movement and accuracy for small or fragile objects.

• Pad-to-Pad Grip: Bringing the pads of the finger(s) and thumb together.
      * Three-Jaw Chuck: Involves the thumb and two fingers (usually index and middle). Similar to a power drill's jaws. Example: holding a pen or pencil.
      * Pinch Grip: The thumb and one finger (usually the index) come together; MCP and PIP joints are flexed while distal joints are extended.

• Tip-to-Tip Grip (Pincer Grip): Similar to pad-to-pad, but involves bringing the tip of the thumb against the tip of another digit (usually index) to pick up very small objects.

• Pad-to-Side Grip (Lateral Prehension): The pad of the extended thumb presses an object against the radial side of the index finger. It is a strong grip but less fine than other types. Example: holding a key.

• Side-to-Side Grip: Requires adduction of two fingers (usually index and middle). This is a weak grip with low precision that can be used in the absence of a thumb. Example: holding a cigarette or pencil.

• Lumbrical Grip: MCP joints are flexed while PIP and DIP joints are extended. The thumb opposes the fingers to hold an object horizontally. Example: holding a plate or tray.

Common Wrist and Hand Pathologies

• Colles’ Fracture: A transverse fracture of the distal radius with posterior displacement of the distal fragment; often caused by a Fall On Outstretched Hand (FOOSH) in elderly populations.

• Smith’s Fracture: A distal fragment displacement in the anterior direction, usually caused by falling on the back of the hand.

• Greenstick Fracture: An incomplete fracture occurring in children, typically more proximal.

• Ganglion Cyst: A benign mass usually found on the dorsum of the wrist.

• Carpal Tunnel Syndrome: Compression of the median nerve within the carpal tunnel.

• De Quervain’s Disease: Tenosynovitis characterized by inflammation and thickening of the sheath containing the Extensor Pollicis Brevis (EPB) and Abductor Pollicis Longus (AbPL).

• Dupuytren’s Contracture: Thickening of the palmar aponeurosis, usually in the ulnar area, leading to contracture of the $4^{th}$ and $5^{th}$ fingers.

• Stenosing Contracture (Trigger Finger): Abnormal sliding of a tendon in its sheath, causing the finger to become "locked" in flexion.

• Skier’s Thumb or Gamekeeper’s Thumb: An acute tear of the ulnar collateral ligament of the thumb.

• Swan Neck Deformity: A condition characterized by flexion of the MCP, hyperextension of the PIP, and flexion of the DIP joints.

• Ulnar Drift: Ulnar deviation of the fingers at the MCP joints.

• Mallet Finger: A flexion deformity caused by the disruption of the extensor mechanism at the DIP joint.

• Scaphoid Fracture: Often caused by FOOSH; notable for a high incidence of Avascular Necrosis (AVN).

• Kienböck’s Disease: Necrosis of the lunate bone following trauma.

Questions & Discussion

• What is the only power grip available with a median nerve injury?
      * The Hook Grip is the only one possible because it does not usually involve the thumb, which is heavily reliant on median nerve-innervated muscles.

• What nerve innervates most of the intrinsic muscles of the hand?
      * The Ulnar nerve ($C8, T1$) innervates the hypothenar group, the interossei, the adductor pollicis, and the third and fourth lumbricales.

• What nerve innervates most of the posterior extrinsic muscles?
      * The Radial nerve ($C6, C7, C8$) innervates the Abductor Pollicis Longus, Extensor Pollicis Brevis/Longus, Extensor Digitorum, Extensor Indicis, and Extensor Digiti Minimi.