Biomechanics of the Wrist and HandThe wrist is the distal joint of the upper limb and allows the hand which is the effect segment, to assume the optimal position for prehension.

Articular complex of the wristThe articular complex of the wrist contain two joints: 1. The radio-carpal joint between the radial head and the proximal row of carpal bones.

2. The mid-carpal joint between the proximal and distal rows of carpal bones. .(figure 1)Radio-carpal joint is an ellipsoidal joint and the carpal aspect presents two convexities transverse convexity and antero-posterior convexity.(figure 2) Figure 1. wrist joint Figure 2. Ellipsoidal jointThe ligaments of the radio-carpal joint are the collateral ligaments (lateral and medial) and the anterior ligaments [attached to the anterior edge of the distal surface of the radius and neck of the capitates] and posterior ligament [forms a strap posteriorly].

Action of the ligaments:

During adduction-abduction the medial and lateral ligaments are active. Starting from the rest position .Adduction ( lateral ligament is stretched, medial slackened

Abduction ( vice versa

During flexion-extension the anterior and posterior ligaments are most active.

Flexion ( posterior ligament stretchedExtension ( anterior ligament stretchedMuscles of the wristMotion

Muscle

Wrist flexion

Flexor Carpi Ulnaris

Wrist flexion

Flexor Carpi Radialis

Assists Wrist flexion

Palmaris Longus

Wrist extension

Extensor Carpi Radialis Longus

Wrist extension

Extensor Carpi Radialis Brevis

Wrist extension

Extensor Carpi Ulnaris

Wrist ulnar deviation

Flexor Carpi Ulnaris

Wrist ulnar deviation

Extensor Carpi Ulnaris

Wrist radial deviation

Flexor Carpi Radialis

Wrist radial deviation

Extensor Carpi Radialis Longus

Synergistic and stabilizing action of the muscle of the wrist:

1- Extensor muscles of the wrist act synergistically with the flexors of the fingers. E.g. during extension of the wrist the fingers are automatically flexes and, to extend the fingers in this position, a voluntary movement is required.

2- Flexor muscles of the wrist act synergistically with the extensors of the fingers. When the wrist is flexed, extension of the proximal phalanx follows automatically.

Kinematics The articular complex of the wrist has basically two degree of freedom. When these compounded with pronation and supination, i. e. rotation of the forearm around its long axis, the hand can be oriented at any angle to grasp or hold an object.Movement of the wrist occur around two axes:

Transverse axis takes place in the sagittal plane with movement:

Flexion the anterior palmar surface of the hand moves towards the anterior aspect of the forearm

Extension the posterior dorsal surface of the hand moves towards the posterior aspect of the forearm. (figure 3) Figure 3.transfarce axis Antero-posterior axis takes place in the frontal plane with movements:Adduction or ulnar deviation: the hand moves toward the axis of the body and its medial (ulnar) border forms an obtuse angle with the medial border of the forearm.

Abduction or radial deviation: the hand moves away from the axis of the body and its lateral (radial) border forms an abtuse angle with the lateral border of the forearm. (figure 4)

Figure 4. Antero-posterior axis.Range of movement of the wrist, range of abduction does not exceed 15( , of adduction is 45(, of flexion is 85(, and range of extension is also 85(.Movement of circumduction [combination of the movement of flexion, extension, adduction and abduction] takes place in two axes of the wrist.The stabilization function of the ligament

Stabilization in the frontal plane

In the frontal plane, the ligaments are essential, because the distal surface of the radius faces distally and medially, so that as a whole it can be represented by a plane running obliquely proximo-distally and medio-laterally. Under the pull of longitudinal muscles, the carpus in the neutral position tends to slip proximally and medially.(Figure 5)

figure 5. In neutral positionWhen the wrist is adducted to approximately 30(, the pull of the muscle now act perpendicular to the plane of the slippage, as a result the carpal bones are pushed back into the cavity and the carpus is stabilized. (figure 6) figure 6. In adductionWhen the wrist is abducted, the pull of the long muscles accentuates the instability and tends to displaced the carpal bones proximally and medially. (Figure 6)

Figure 6. in abductionStabilization in the sagittal planeIn the sagittal plane roughly similar events take place.Because the distal surface of the radius points distally and interiorly (lateral view), the carpal bones tend to slid proximally and interiorly, i.e. in a plane parallel to that of the distal surface of the reduis.(figure 7) figure 7. In neutral

When the wrist is flexed 30( to 40(, the muscular pull tends to displace the carpal bones in a plane perpendicular to that of the distal surface of the reduis, thus repositioning and stabilizing these bones.(figure 8) figure 8. In flexionDuring extension, the tendency for the carpal bones to be displaced proximally and anteriorly is reinforced. (Figure 9) figure 9. In extensionThe full brunt is borne by the two redio-triquetral bands of the anterior and posterior ligaments of the radio-carpal joint. As they run obliquely proximally and laterally, they keep the carpal bone in position and prevent their medial displacement. (Figure 5)Abnormal Movement

Anatomical damage is most often caused by abduction and extension often in combination.Abduction past the locked position caused two types of damage:

Fracture of the distal end of the radius: as the scaphoid is presses against the lateral buttress of the distal reduis Fracture of the scaphoid : surprised in extention and allows its full length to hit the lateral buttress of the distal end of the radius. The radial styloid strikes the lateral surface of the scaphoid, which fractures as a result of shearing force.Extension , when exaggerated, most often causes a Colles fracture. More rarly it causes damage to ligaments with primary rupture of the lunato-capitate ligament and secondarily:

Perilunate dislocation : The lunate maintains its normal articulation with the radius, but the capitate articular surface is dislocated from the lunate, normally dorsally. Anterior dislocation of the lunate: when the head of the capitates moves proximally towards the radius and displaces the lunate anteriorly into the carpal tunnel with compression of the median nerve.Another common injuries [wrist/ hand]Metacarpal (boxers) fracture and mallet or drop finger deformity resulting from injury at distal interphalangeal joints among foot ball receivers and baseball catchers.

Forced abduction of the thumb leading to ulnar collateral ligament injury often results from wresting, football, hocky, and skiing.

De Quervains disease (tendanitis of the extensor pollicis brevis and the abductor pollicis longus), common in golfers [ right handed tends to injury in left wrist]Carpal tunnel syndrome, carpal tunnel is a passage between the carpal bone and the flexor retineculum on the palmar side. Cause is unknown or from repeated forceful wrist flexion , usual onset is swelling caused by acute or chronic trauma leading to compress the median nerve. Symptoms like numbness along the median nerve, clumsiness of the figure function, and eventually weakness and atrophy of the muscles supplied by the median nerve.

The Hand The hand of the man is remarkable instrument, capable of performing countless action, owing to its essential function: prehension. From the functional viewpoint the hand is effector organ of the upper limb, which supports it mechanically and allows it to adopt the optimal position for any given action.Functional position of the hand Wrist

extended 20 degrees

ulnarly deviated 10 degrees Digits 2 through 5

MP joints flexed 45degrees

PIP joints flexed 30-45 degrees

DIP joints flexed 10-20 degrees

Thumb

first CMC joint partially abducted and opposed

MP joint flexed 10 degrees

IP joint flexed 5 degrees. (figure 10)

Figure 10. Functional position

Types of graspTwo types of grasp are differentiated according to the position and mobility of the thumb's CMC and MP joints.

1. POWER grasp (The adductor pollicis stabilizes an object against the palm; the hand's position is static.)

cylindrical grip (fist grasp is a small diameter cylindrical grasp)

spherical grip

hook grip (MP extended with flattening of transverse arch; the person may or may include the thumb in this grasp)

lateral prehension (this can be a power grip if the thumb is adducted, a precision grip if the thumb is abducted). 2. PRECISION (Muscles are active that abduct or oppose the thumb; the hand's position is dynamic.) palmar prehension (pulp to pulp), includes 'chuck' or tripod grips

tip-to-tip (with FDP active to maintain DIP flex)

lateral prehension (pad-to-side; key grip)Muscles of the Hand MuscleAction

abductor digiti minimi (hand)abducts the 5th digit

abductor pollicis brevisabducts thumb

adductor pollicisadducts the thumb

extensor pollicis brevisextends the thumb at the metacarpophalangeal joint

flexor digiti minimi brevis (hand)flexes the carpometacarpal and metacarpophalangeal joints of the 5th digit

flexor pollicis brevisflexes the carpometacarpal and metacarpophalangeal joints of the thumb

flexor pollicis longusflexes the metacarpophalangeal and interphalangeal joints of the thumb

interosseous, dorsal (hand)flex the metacarpophalangeal joint, extend the proximal and distal interphalangeal joints of digits 2-4, abduct digits 2-4 (abduction of digits in the hand is defined as movement away from the midline of the 3rd digit)

interosseous, palmarflexes the metacarpophalangeal, extends proximal and distal interphalangeal joints and adducts digits 1, 2, 4, & 5 (adduction of the digits of the hand is in reference to the midline of the 3rd digit)

lumbrical (hand)flex the metacarpophalangeal joints, extend the proximal and distal interphalangeal joints of digits 2-5

opponens digiti minimiopposes the 5th digit

opponens pollicisopposes the thumb

palmaris brevisdraws the skin of the ulnar side of the hand toward the center of the palm

Mechanism for finger flexion

Range of motion in PIP joints is greater than 90(, in MP 135(, and DP is slightly less than 90(.

Plane of movement of the of flexion of the last four figure worth discussing. The index is flexed in a strictly saggital plane toward the base of the thenar eminence. The axes of the fingers during flexion all converge to a point corresponding to the radial pulse. This can only occur if the other fingers are flexed not in a saggital plane like the index, but increasingly oblique plane.

The little finger shows maximal obliquity of its plane of flexion, the significance of this oblique lies in the fact that it allows the more medial fingers to oppose the thumb like the index. (figure 11) figure 11. Finger flexionMechanism for finger extensionWe can extend the PIP and DIP joints without extending the MP joints. But we can't extend the PIP joint without extending the DIP joint at the same time. Flexing only the DIP joint without also flexing the PIP joint is difficult. Full (active or passive) flexion of the PIP joint prevents active extension of the DIP joint.Tendinous structures comprise the extensor mechanism:

1. The extensor digitorum comunis EDC tendon attaches by a tendinous slip to the proximal phalanx, through which it extends the MP joint.

2. The central tendon (or "slip") proceeds dorsally to attach to base of middle phalanx, where tension can extend the PIP joint.

3. The lateral bands proceed on either side of dorsal midline and rejoin before attaching to the distal phalanx. Tension in the lateral bands extends the DIP joint.

4. The extensor hood surrounds the MP joint laterally, medially, and dorsally, and receives tendinous fibers from the lumbricales and interossei. (figure 11) Figure12. Tendanitous structureAlthough the extensor mechanism's fibres are tendinous, and therefore incapable of producing active force, they still transmit force to their attachments.

Force develops in the extensor mechanism in two ways:

1. Many of the hand's intrinsic muscles attach to the extensor mechanism. Activity in any of these muscles produces force that the extensor mechanism communicates to its distal attachments.

2. The extensor mechanism develops passive tension whenever it is elongated. Hand movements that passively elongate either the extensor mechanism or a structure that attaches to the extensor mechanism produce force in the extensor mechanism itself.

The extensor mechanism's fibers have lines of application that are always dorsal to the lateral axes of the PIP and DIP joints. Therefore,

1. activity in the intrinsic muscles that attach to the extensor mechanism always produces DIP and PIP extension.

2. Passive flexion of the MP joint elongates the extensor mechanism and extends the PIP and DIP joints.

The fibrous lines of application in the hood and lateral bands pass very near the MP joint's lateral axis. Whether these structures move the MP joint in the sagittal plane depends on whether the MP joint is already flexed or extended.

in MP extension:

Action in the extensor digitorum extends the MP joint, and also pulls the extensor mechanism (including the hood) proximally.

In this position, the interosseous muscles' lines of application are very close to the MP joint's lateral axis.

With such small moment arms, these muscles have little effect on MP joint movement in the sagittal plane. However, they still produce MP abduction/adduction when the MP joint is extendedThe Thumb

The thumb plays a unique role in the function of the hand.

Thumb movement include extension (position of reference), flexion, adduction, abduction and opposition.

It has five degree of freedom, three for coincidence of the point of contact, and two for full coincidence of the planes of the pulps.

Opposition is the essential movement of the thumb and brings the pulp of the thumb into contact with that of any finger, made of three components: anteposition, flexion and pronation of the oseoarticular column of the thumb.

Stability of the metacarpophalengeal joint of the thumb depends not only on the articular surface but also on its muscular cuff. Abnormal position of the hand and fingersThese can result from either underactivity or overactivity of the muscles described. The following condition cause abnormal position of the fingers:a) Tearing of the extensor expansionb) Rupture of the extensor tendon just proximal to its insertion into 3rd phalanx.

c) Rupture of the extensor tendon just proximal to metacarophalengeal joint.d) Rupture or paralysis of flexor digitorum sublimis .

e) Rupture or paralysis of flexor digitorum profundus tendon.

f) Paralysis of the interossei.Other abnormal deformeties Dupuyterns contracture [shortening of the pretendinous fibers of the central palmar aponeurosis, the finger are irreducibly flexed] Volkmanns contracture [ ischaemic contracture of the flexor muscles, finger assume hook-like position]

Massive ulnar drift [ fingers markedly deviated medially, mainly seen in rheumatoid arthritis]CLINICAL APPEARANCE OF PERIPHERAL NERVE INJURIES IN THE HAND

1. Median:

Often due to carpal tunnel sd.

Wasting of thenar eminence.

Decreased thumb function, especially opposition.

Thumb moves into plane of palm.

2. Ulnar:

Damage to ulnar nerve can occur with trauma to elbow region. Ulnar neuropathy is a frequent complication of diabetes mellitus

Wasting of web space and interosseous spaces.

Affects strength of intrinsic muscles of hand, so person can't hold a piece of paper between extended but adducted fingers

Affects adductor pollicis and ulnar head of FPB. A person who lacks strength in these muscles cannot grasp with the thumb unless he or she flexes the IP joint by substituting with the flexor pollicis longus. Paralysis of the ulnar nerve produces claw hand 3. Radial:

Associated with gunshot or stab wounds, fracture of humerus, "Saturday night palsy."

person demonstrates a "dropped wrist," and cannot reposition thumb.

lack of wrist extension may cause hand grip to be weak. ReferenceKapandji, I A. (1982). The Physiology of the Joints. New York: Churchill Livingstone.