Download Biomechanics of the Wrist and Hand

Biomechanics of the Wrist and Hand
The 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 wrist
The 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 joint
The 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 stretched
Extension  anterior ligament stretched
Muscles of the wrist
Motion
Wrist flexion
Wrist flexion
Assists Wrist flexion
Wrist extension
Wrist extension
Wrist extension
Muscle
Flexor Carpi Ulnaris
Flexor Carpi Radialis
Palmaris Longus
Extensor Carpi Radialis Longus
Extensor Carpi Radialis Brevis
Extensor Carpi Ulnaris
Wrist ulnar deviation
Wrist ulnar deviation
Wrist radial deviation
Wrist radial deviation
Flexor Carpi Ulnaris
Extensor Carpi Ulnaris
Flexor Carpi Radialis
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. Anteroposterior 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 position
When 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 adduction
When 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 abduction
Stabilization in the sagittal plane
In 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 flexion
During extension, the tendency for the carpal bones to be displaced
proximally and anteriorly is reinforced. (Figure 9)
figure 9. In extension
The 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 (boxer’s) 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 grasp
Two 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
Muscle
Action
abductor digiti
minimi (hand)
abducts the 5th digit
abductor pollicis
brevis
abducts thumb
adductor pollicis
adducts the thumb
extensor pollicis
brevis
extends the thumb at the metacarpophalangeal joint
flexor digiti
minimi brevis
(hand)
flexes the carpometacarpal and metacarpophalangeal joints of the 5th
digit
flexor pollicis
brevis
flexes the carpometacarpal and metacarpophalangeal joints of the thumb
flexor pollicis
longus
flexes 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,
palmar
flexes 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
minimi
opposes the 5th digit
opponens pollicis
opposes the thumb
palmaris brevis
draws 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 flexion
Mechanism for finger extension
We 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 structure
Although 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 extended
The 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 fingers
These 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 expansion
b) 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
 Dupuytern’s contracture [shortening of the pretendinous fibers
of the central palmar aponeurosis, the finger are irreducibly
flexed]
 Volkmann’s 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.
Reference
Kapandji, I A. (1982). The Physiology of the Joints. New York:
Churchill Livingstone.