Functional Anatomy of the Hand

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Introduction

The hand, positioned at the end of the upper limb, is a combination of complex joints whose function is to manipulate, grip and grasp, all made possible by the opposing movement of the thumb.

Key Terms

Axes: lines around which an object rotates. The rotation axis is a line that passes through the centre of mass. There are three axes of rotation: sagittal passing from posterior to anterior, frontal passing from left to right, and vertical passing from inferior to superior. The rotation axes of the foot joints are perpendicular to the cardinal planes. Therefore, motion at these joints results in rotations within three planes. Example: supination involves inversion, internal rotation, and plantarflexion.

Bursae: reduce friction between the moving parts of the joints. A bursa is a fluid-filled sac. There are four types of bursae: adventitious, subcutaneous, synovial, and sub-muscular.

Capsule: one of the characteristics of the synovial joints. It is a fibrous connective tissue which forms a band that seals the joint space, provides passive and active stability and may even form articular surfaces for the joint. The capsular pattern is "the proportional motion restriction in range of motion during passive exercises due to tightness of the joint capsule."

Closed pack position: the position with the most congruency of the joint surfaces. In this position, joint stability increases. For example, the closed pack position for the interphalangeal joints is full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six degrees of freedom, including three translations and three rotations.

Ligament: fibrous connective tissue that holds the bones together.

Open (loose) pack position: position with the least joint congruency where joint stability is reduced.

Planes of movement: describe how the body moves. Up and down movements (flexion/extension) occur in the sagittal plane. Sideway movements (abduction/adduction) occur in the frontal plane. The transverse plane movements are rotational (internal and external rotation).

Hand Structure

The structure of human hand includes 27 bones: eight carpal bones, five metacarpal bones, and fourteen digital bones, including fingers and thumb. The bony segments of the hand are arranged in a series of longitudinal and transverse arches. The bones of the five digital rays are part of the longitudinal arches. There are two transverse arches: the proximal and the distal. The proximal transverse arch is made up of carpal bones. The metacarpal heads of the fingers belong to the distal transverse arch.

The Carpus (Carpal Bones)

Proximal raw:

The scaphoid (navicular): belongs to the proximal raw of the carpal bones. Located the most laterally. Its palmar surface contains the tubercle, the attachment point for the muscles and the floor for the anatomical snuffbox.

The lunate: part of the proximal raw of the carpal bones. It articulates with the scaphoid on the lateral and with the triquetrum on the medial side.

The triquetrum: other names include triquetral, triangular, or cuneiform bone. It is a triangular and pyramidal-shaped bone located in the proximal raw, on the medial side of the wrist. It contains multiple articulation surfaces: lateral for the lunate, anterior for the pisiform, and distal for the hamate.

Distal raw:

The trapezium: located in the distal raw of the carpal bones, on its radial side. It has four articulations for the first metacarpal, second metacarpal, scaphoid and trapezoid bones. This bone is part of the mechanism responsible for the wide mobility of the human hand, where the articulation between the first metacarpal and the trapezium allows for the opposable thumbs function.

The trapezoid: The lesser multangular bone is another name for this bone. This is the smallest bone of the distal raw of the carpal bones. It gives structure to the palm of the hand.

The capitate: the largest and most central carpal bone belonging to the distal raw. It articulates with the bases of the 2nd and 3rd metacarpal bones forming part of the common carpometacarpal joint in the hand. In addition, it articulates with the following: scaphoid and lunate on the proximal, trapezoid on the lateral, and the hamate on the medial surfaces.

The hamate: sits on the medial side of the distal row of carpal bones. The hook of the hamate is the bony process extending from the palmar surface. This carpal bone forms the medial border of the carpal tunnel.

The Metacarpus (Metacarpal Bones)

Five bones- the metacarpals, make up the metacarpus. Each metacarpal bone articulates with one or more carpal bones:

• First metacarpal articulates with the trapezium
• Second metacarpal articulates with the trapezium, trapezoid and capitate
• Third metacarpal articulates with the capitate
• Fourth and fifth metacarpal articulate with the hamate

Additionally the second to fifth metacarpals articulate with each other.

The Phalanges

Long fingers bones are known as phalanges. Apart from the thumb (the pollex) which only has distal and proximal bones, each phalanx has three bones: the distal, middle and proximal phalanx. The middle and proximal phalanges consist of a base, a body, and the head which is the distal part of the phalanx.

The thumb: the typical posiiton of the thumb is pronation and approximately 80 degrees of flexion in relation to the remaining metacarpals of the hand. This position allows opposition of the thumb to the digits.

The index finger: second the most important finger of the hand. It is able to abduct, adduct, flex and extend. The index finger participates in precision pinch and directional grip.

The long finger: is centrally positioned which makes it involved in power grip and precision movements.

The ring finger: weak and only randomly used in precision grip or pinch maneuvers. When lost, it leads to the least amount of impairment in the hand.

The small finger: the weakest of all fingers, but important in grasping while spanning an object manoeuvres due to it's ability to abduct.

Bones, Articulations and Kinematics of the Hand

Bones and Articulations

Bones	Articulations	Characteristics	Key palpation points
Distal row of carpal bones Proximal bases of five metacarpal bones	The carpometacarpal joints (CMC): The first CMC (1CMC) joint or trapeziometacarpal (TMCJ) joint-the CMC of the thumb: between the trapezium and the base of the first metacarpal The second CMC (2CMC) joint: between the trapezoid and the base of the second metacarpal The third CMC (3CMC) joint: between the capitate and the base of the third metacarpal The fourth CMC (4 CMC) joint: between the hamate and the base of the fourth metacarpal The fifth CMC (5 CMC) joint: between the hamate and the base of the fifth metacarpal.	The mobility of the CMC joints increases from radial to ulnar sides of the hand. 1CMC (TMC) is located on the radial aspect of the wrist. Does not contain bony stabilizers. Instead, the stability is provided by five internal ligaments: dorsal radial, posterior oblique, the first intermetacarpal, ulnar collateral, and anterior oblique ligaments. 2CMC and 3CMC: belong to the central column of the hand, relatively rigid. 1CMC, 4CMC, 5 CMC: relative mobility, with 4CMC and 5CMC contributing the most to hand mobility.
Metacarpal bones Proximal phalanges	Metacarpophalangeal (MCP) joints	Consider to be the most important joint for hand function as they contribute 77% of the total arc of finger flexion
Proximal, middle, and distal phalanges	Interphalangeal joints PIP: proximal interphalangeal joint between proximal and middle phalanges DIP: distal interphalangeal joints, between the middle and distal phalanges	The function of the interphalangeal joints of the hand is to permit fine motor movements in the digits. It includes providing flexion towards the palm.

Hand Kinematics

Hand biomechanics consist of seven fundamental manoeuvres needed for basic functions. They involve specific motions which must occur at the wrist, thumb and the fingers:

1. The precision pinch, or the terminal pinch, is used to pick up a small object, like a pen.
   • Motion required: flexion of the interphalangeal (IP) joint of the thumb and the distal IP (DIP) joint of the index finger.
2. The oppositional pinch, or the subterminal pinch with the thumb opposition. The pulp of the index finger connects with the pulp of the thumb needed to hold a piece of paper.
   • Motion required: Extension of the IP and DIP joints.
3. The key pinch requires stability provided by the index finger, optimal length of the digit and a metacarpal phalangeal joint (MCP).
   • Motion required: thumb adduction to the radial aspect of the index finger’s middle phalanx.
4. The chuck grip, or a directional grip is used to perform action that requires an application of rotational and axial forces like when using a screwdriver.
   • Motion required: a combined motion of the index finger, long finger, and thumb.
5. The hook grip allows to hold a briefcase buy its handle. No thumb function is required in this manoeuvre.
   • Motion required: finger flexion at the IP joints and extension at the MCP joints.
6. The power grasp is needed to gripp a club or a bat.
   • Motion required: fingers and thumb flexion, thumb opposition relative to the other digits
7. The span grasp manoeuvre is based on the stability of the thumb, metacarpophalangeal (MCP) and interphalangeal (IP) joints. The span grasp is utilised when grabbing a ball.
   • Motion required: 30 degrees flexion of DIP joints and the proximal IP (PIP) joints, thumb abduction towards the palm

Joint	Type of joint	Plane of movement	Motion	Kinematics	Closed pack position	Open pack position
The carpometacarpal joints (CMC)	1CMC: Synovial, saddle joint. 2CMC/3CMC: Synarthrotic with almost no movement under physiological conditions. 4CMC/5CMC: Synovial, saddle joint.	1CMC: Flexion/extension occurs in the frontal plane as the thumb is rotated at 90 degrees in relation to the plane of the hand Abduction/adduction occurs in the sagittal plane External/internal rotation (axial rotation) where the axis is the length of the thumb Thumb opposition: all thumb movements combined together 2CMC/3CMC: Nonaxial, translational movements. 4CMC/5CMC: Sagittal (flexion/extension) Frontal (abduction/adduction)	1CMC: Flexion/extension Abduction/adduction Axial rotation 2CMC/3CMC: Nonaxial, translational movements only. 4CMC/5CMC: Flexion/extension Abduction/adduction	1CMC: Flexion/extension: total motion of 40-50 degrees Abduction/adduction: total motion of 80 degrees Axial rotation: total motion of 70-110 degrees 2CMC/3CMC: Nonaxial, translational movements only. 4CMC: Flexion/extension: 10/0 degrees Abduction/adduction: 5/0 degrees. 5CMC: Flexion/extension: 20/0 degrees Abduction/adduction: 13/0 degrees	1CMC: maximum opposition 4CMC/5CMC: full flexion	1CMC:slight flexion 4CMC/5CMC: halfway between flexion and extension
Metacarpophalangeal (MCP) joints	Condyloid joint	Sagittal Frontal	Flexion/extension Abduction/ adduction Mild to moderate rotation of the digit.	Thumb MCP: Flexion/extension:60 degrees total Abduction/adduction: 25-30 degrees Second to fifth MCP: Flexion: 90 degrees Extension: 10-30 degrees Abduction/adduction: 25-30 degrees	Thumb MCP: maximum opposition Second to fifth MCP: full flexion	Slight flexion
Interphalangeal joints (IP) PIP DIP	Hinge	Sagittal	Flexion/Extension	Thumb: Flexion: 90 degrees Extension: 10 degrees Second to fifth IP: Flexion: 70-100 degrees (depends on the digit, with digit 5 having the least flexion range of motion) Extension: 2-5 degrees	Full extension	Slight flexion

Ligaments of the Hand

Ligaments Supporting Carpometacarpal Joints

Ligaments Supporting Trapeziometacarpal Joint (TMCJ)

Key ligaments	Origin	Insertion	Action/role	Key palpation points
Anterior oblique ligament (AOL) ("beak ligament")	Palmar tubercle of the trapezium	Volar beak of the first metacarpal base at the palmar and ulnar surface	The primary static stabilizer of TMCJ Prevents dorsoradial subluxation of thumb metacarpal during key pinch Degeneration of AOL frequently leads to the development of osteoarthritis of the first CMC joint.
Posterior oblique ligament (POL)
Ulnar collateral ligament (UCL)
First intermetacarpal ligament (FIMCL)
Dorsoradial ligament (DRL)			The strongest and stiffest of TMCJ ligaments TMCJ stabilizer Proprioceptive function due to rich innervation

Ligaments Supporting Carpometacarpal Joints 2-5 (2CMC-5CMC)

Key ligaments	Origin	Insertion	Action/role	Kay palpation points
Palmar carpometacarpal ligaments
Dorsal carpometacarpal ligaments
Palmar metacarpal ligaments
Dorsal metacarpal ligaments

Clinical Relevance

1. Swan neck deformity, characterised by hyperextension of the PIP joint and flexion in the distal Interphalangeal joint (DIP), is usually caused by a relative over-activity in the extensors of the PIP joint, or a laxity at the volar plate.
2. Boutonniere deformity is a flexion deformity, usually occurring at the attachment of the central slip. It is commonly the result of:

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