biomechanics and hand trauma 2003

Biomechanics and hand trauma: what you needSteven L. Moran, MDa,*, Richard A. Berger, MD, PhDa,b

aDivision of Plastic Surgery, Division of Hand and Microsurgery, Mayo Clinic,

200 First Street SW, Rochester, MN 55905, USAbDepartment of Orthopaedic Surgery, Mayo Medical School, Rochester, MN 55905, USA

Mutilating hand injuries pose many challenges

to the hand surgeon. The variety and severity of

these injuries has led to the development of several

grading scales, ow charts, and algorithms to help

the surgeon organize his or her treatment plan [1

4,112]. These tools help the surgeon in preparation

for surgery, but fail to predict hand function fol-

lowing reconstruction accurately. It can be agoniz-

ing for the hand surgeon, especially the young

hand surgeon, intraoperatively to contemplate

accurately the functional loss imposed by imme-

diate joint fusion or digital amputation. Heroic

attempts are often made to salvage joints and dig-

its, whose loss results in little functional decit. In

addition, these severely injured ngers and joints

often become sti and insensate, requiring delayed

amputations. This not only prolongs patient re-

covery but also prolongs the surgeons anxiety.

Many articles dealing with the mutilated hand

contain experience-based protocols and reference

previous anecdotal reports [58]. Are there any

biomechanical principles of hand dynamics that

could help in deciding what must be preserved

and what can be discarded? Unfortunately, biome-

chanical studies involving mutilating hand injuries

are scarce. This article establishes a biomechanical

foundation for determining what anatomic com-

ponents are needed for hand function.

The essentials

In its most elemental form, the hand is com-

posed of a stable wrist and at least two digits that

can oppose with some power. One digit should be

capable of motion so it can grasp objects. The

other digit need only act as a stable post against

which the movable digit can pinch. To allow for

prehensile movements the digits require some form

of cleft to divide them, which allows for the accom-

modation of objects. The digits need to be sensate

and pain free or they provide little benet over

prosthesis [6,7,9]. Requirements for functional

sensation have been dened as two-point discrim-

ination of less than 10 to 12 mm [10].

The hand allows for prehension, which is the

ability to grasp and manipulate objects. As dened

by Tubiana et al [11], prehension may be dened

as all the functions that are put into play when an

object is grasped by the handsintent, permanent

sensory control, and a mechanism of grip. Pre-

hension requires that the hand be able to ap-

proach, grasp, and release an object [11,12]. If

only two sensate digits remain to oppose each

other, some prehension is possible.

In terms of biomechanical motion the hand

performs approximately seven basic maneuvers,

which make up most hand function:

1. Precision pinch (terminal pinch). This in-

volves exion at the distal interphalangeal

(DIP) joint of the index and at the interpha-

langeal joint (IP) joint of the thumb. The ends

of the ngernails are brought together as in

lifting a paper clip from a tabletop (Fig. 1).

2. Oppositional pinch (subterminal pinch). The

pulp of the index and thumb are brought

together with the DIP joints extended. This

allows for force tobe generated through thumb

opposition, rst dorsal interosseous contrac-

tion, and index profundus exion. This is often

measured with a dynamometer (Fig. 2).

* Corresponding author.

E-mail address: [email protected]

(S.L. Moran).

0749-0712/03/$ - see front matter 2003, Elsevier Science (USA). All rights reserved.doi:10.1016/S0749-0712(02)00130-0

Hand Clin 19 (2003) 1731

3. Key pinch. The thumb is adducted to the ra-

dial side of the middle phalanx of the index

nger. Key pinch requires a stable post (usu-

ally the index nger), which has adequate

length and a metacarpal phalangeal (MP)

joint, which can resist the thumb adduction

force (Fig. 3).

4. Directional grip (chuck grip). The thumb, in-

dex, and long nger come together to sur-

round a cylindrical object. When using this

grip, a rotational and axial force is usually

applied to the held object (ie, using a screw-

driver) (Fig. 4).

5. Hook grip. This requires nger exion at the

IP joints and extension at the MP joints. It is

the only type of functional grasp that does

not require thumb function. This grip is used

when one lifts a suitcase (Fig. 5).

6. Power grasp. The ngers are fully exed while

the thumb is exed and opposed over the

other digits, as in holding a baseball bat.

Force if applied through the ngers into the

palm (Fig. 6).

7. Span grasp. The DIP and proximal interpha-

langeal (PIP) joints ex to approximately 30

degrees and the thumb is abducted. Force is

generated between the thumb and ngers, dis-

tinct to power grasp where force is generated

between the ngers and the palm. Stability is

required at the thumb MP and IP. This grip

is used to lift cylindrical objects (Fig. 7)

[11,13,14].

Postoperatively, the hands ability to adopt

these positions and exert force through them

impacts how well the patient rehabilitates. These

maneuvers are predicated on good sensation in

the ngers and thumb. Through the preoperative

history, the hand surgeon can determine which

hand functions benet the patient most in

Fig. 1. Precision pinch (terminal pinch).

Fig. 2. Oppositional pinch (subterminal pinch).

Fig. 3. Key pinch.

Fig. 4. Directional grip (chuck grip).

18 S.L. Moran, R.A. Berger / Hand Clin 19 (2003) 1731

returning to their previous employment or activ-

ities, and direct the reconstruction appropriately.

Many classication schemes divide hand

trauma into dorsal, volar, radial, and ulnar inju-

ries [1,3]. When assessing the eects of mutilating

trauma on hand mechanics, however, it may be

easier to think of the hand as containing four func-

tional units: (1) the opposable thumb; (2) the index

and long nger, whose stable basal joints serve as

xed posts for pinch and power functions; (3) the

ring and small nger, which represent the mobile

unit of the hand; and (4) the wrist. It may also help

to think of only two major forms of hand motion,

as opposed to seven: thumb-nger pinch and digi-

topalmar grip. Pinch requires preservation of the

thumb unit and a stable post. If the patient is able

to add a third digit to pinch, they can achieve more

precision. Pinch function tends to be preserved

when the median nerve is intact and the thumb

and index-long units of the hand are salvageable.

Without median nerve function, thumb sensation

and thenar function are lost, making ne motor

movements negligible. In comparison, ulnar nerve

function and the ring-small nger unit are more

important for digitopalmar grip, where exion and

sensation in the ulnar digits are essential. Thumb

preservation is also important in power grasp to

provide stability and control of directional forces.

With these principles in mind this article now

examines how digital loss aects hand function.

The biomechanical impact of amputation

Partial or complete amputations are present in

most mutilating hand injuries. It has been recom-

mended that immediate amputation be performed

when four of the six basic digital parts (bone, joint,

skin, tendon, nerve, and vessel) are injured [8,15

20]. It is important to consider amputation in these

situations because long-term stiness and pain in a

salvaged digit can severely hamper the rehabilita-

tion of the remaining hand. When performing an

amputation, however, one should understand how

digital loss impacts overall hand function.

The thumb

The functional importance of each digit has

been debated. If one were to prioritize the digits

to be saved following mutilating injury, the thumb,

with its importance in prehension and in all forms

of grasp, takes top priority [109]. It provides 40%

of overall hand function in the uninjured setting

[2123]. Following mutilating trauma, when digits

are missing or sti, the thumb can account for

greater than 50% of hand function [24]. Its unique-

ness and versatility in humans is caused by the

Fig. 5. Hook grip.

Fig. 6. Power grasp.

Fig. 7. Span grasp.

19S.L. Moran, R.A. Berger / Hand Clin 19 (2003) 1731

position of the thumb axis. The thumb axis is

based at the trapeziometacarpal (TMC) joint and

is pronated and exed approximately 80 degrees

with respect to the other metacarpals in the hand

[25]. This positioning allows for circumduction,

which permits opposition [2629].

Opposition of the thumb is necessary for all

useful prehension and its preservation provides

the basis for successful salvage procedures. Oppo-

sition of the thumb is the result of angulatory

motion, which is produced through abduction at

the TMC joint, and exion and rotation of the

TMC and MP joints [30]. Multiple muscles are

required for functional opposition. These include

the abductor pollicis brevis, the opponens pollicis,

and the supercial head of the exor pollicis bre-

vis. These muscles act simultaneously on the

TMC joint and theMP joint. The abductor pollicis

brevis provides the major component of opposi-

tion, with the opponens pollicis and exor pollicis

brevis providing secondary motors for opposition.

All measures should be directed toward preserving

or reconstructing the abductor pollicis brevis if

possible [25,2832]. The extensor pollicis longus

(EPL) and adductor pollicis (ADD) are antago-

nists to thumb opposition providing a supinating

extension and adduction force.

The priorities of thumb reconstruction vary

with the level of amputation, but at all levels recon-

struction should attempt to restore opposition and

pinch (Fig. 8). Injuries distal to the IP joint (zone 1

injuries) may produce little functional decit,

because oppositional length tends to bemaintained

[33,34]. Residual insensibility and dysesthesia from

trauma produce more functional problems at this

level than the mechanical loss of length [35,36].

Subterminal pinch and precision pinch are com-

promised if an unstable or painful scar is present

at the thumb remnant. Loss of the distal phalanx

and IP joint (zone 2 injuries) may also not require

reconstruction. Functionmay be preserved if TMC

and MP motion is maintained [37].

Level three injuries, through the level of the

MP, are the most common and do represent a sig-

nicant loss of function. Unreconstructed injuries

result in a decrease in pinch dexterity and grip

strength [38]. The MP joint of the thumb has no

other mechanical equivalent in the hand. It has

three degrees of freedom; it represents a ball and

socket joint in extension, but when the joint is

exed, the tightening of the collateral ligaments

causes the MP joint to function more like a hinge.

The intrinsic muscles provide motion but also pro-

vide dynamic stability to the joint.

Fig. 8. Diagram depicting levels of thumb injury, as originally described by Hentz [31]. Zone 1 injuries result in tissue

loss distal to the IP joint. Zone 2 injuries result in thumb loss distal to MP joint. Zone 3 injuries result in loss of the MP

joint but preservation of thenar musculature. Zone 4 injuries occur distal to TMC joint with loss of thenar musculature.

Zone 5 injuries result in loss of the TMC joint. The zone of injury determines reconstructive priorities.


In injuries proximal to the MP joint one may

proceed with a free toe transfer, which is the gold

standard. The great toe metatarsal phalangeal

joint can reproduce the exion and extension arc

of the MP joint, but fails to reproduce the MP

joints 15 to 20 degrees of supination [35]. Func-

tional opposition is also possible with a toe wrap-

around ap. This reconstruction only allows for

TMCmotion. Excellent results have been obtained

when the fusion angles with bone graft were 30

degrees of exion and 45 degrees of internal

rotation. These fusion angles allowed for pinch

between all ngers and produced pinch and grip

strengths of 60% and 97%, respectively [39]. Non-

microsurgical methods for reconstruction of level

three defects can include deepening of the rst

web space, but any injury to the adductor or the-

nar musculature should be signicantly discour-

aged in an already traumatized thumb.

Level four injuries result in damage to the thenar

muscles,with resultant instability to theTMC joint.

This produces a major stumbling block in thumb

reconstruction, because TMC stability is required

for any successful thumb reconstruction. Injuries

at this level often require some form of soft tissue

reconstruction for restoration of opposition and

pinch [38,40]. In its most primitive form pinch can

be recreated, as in the tetraplegic patient, with

fusion of the IP and MP and reconstruction of the

adductormusculature. For reconstruction of oppo-

sitional pinch, however, tendon transfers may be

necessary. In a study by Cooney et al [27], muscle

cross-sectional area andmoment arm analysis were

used to determine the best donor muscle for oppo-

sitional transfer. The exor digitorum supercialis

(FDS) of the long nger and the extensor carpi

ulnaris (ECU) muscles closely approximated

thenar muscle strength and potential excursion.

Abduction from the palm was greatest after trans-

fer of the FDS from the long and ring ngers

and after ECU and extensor carpi radialis longus

(ECRL) transfers. Pulley location was found to

inuence the motion and strength of transfers in

both the exion and abduction planes. Both

Bunnell [41] and Cooney et al [27] stress the im-

portance of directing the force of the transfer

toward the pisiform. Transfers that are distal to

the pisiform, such as those using the extensor digiti

minimi (EDQ) or abductor digiti minimi (ADQ),

produce more exion than abduction. Transfers

proximal to the pisiform, such as the FDS using

the exor carpi ulnaris (FCU) loop as a pulley, pro-

duce more abduction and less metacarpal exion

(Fig. 9).

Level ve injuries represent a loss of the TMC

joint. In these cases restoration of TMC mobility

is probably best achieved by index ray polliciza-

tion, if available. The TMC joint is mechanically

equivalent to a universal joint [28,30,42]. The

TMC joint allows for thumb circumduction and

thumb extension with associated supination, and

pronation with thumb exion. The TMC joint is

very complex because of its inherent instability at

the radial aspect of the wrist with no bony stabil-

izers proximal (mobile scaphoid). This inherent

instability accounts for the large number of liga-

mentous supports that surround the joint (Fig.

10.). There are ve major internal ligamentous

stabilizers of the TMC joint: (1) dorsal radial

ligament, (2) posterior oblique ligament, (3) rst

intermetacarpal ligament, (4) ulnar collateral liga-

ment, and (5) the anterior oblique ligament. The

dorsal radial ligament prevents lateral subluxa-

tion. The posterior oblique ligament provides

stability in exion, opposition, and pronation.

The rst intermetacarpal ligament is taut in abduc-

tion, opposition, and supination; it holds the rst

metacarpal tightly against the second metacarpal.

The intermetacarpal ligament is joined by the

ulnar collateral ligament, which prevents lateral

subluxation of the rst metacarpal on the tra-

pezium and controls for rotational stress. The base

of the index metacarpal should be spared during

any type of ray resection to preserve the intermeta-

carpal ligament [43,44]. The fth and most impor-

tant ligament is the volar anterior oblique ligament

Fig. 9. Diagram depicting the use of the supercialis

tendon from the long nger for restoration of thumb

opposition. Tendon transfers directed proximal to the

pisiform tend to produce greater metacarpal abduction

and less metacarpal exion as compared with transfers

directed distal to the pisiform. The supercialis tendons

from the long and ring ngers closely approximate the

excursion and strength of the original thenar mus-

culature, and provide for an ideal tendon for transfer.

FDS exor digitorum supercialis; FCU exorcarpi ulnaris; P pisiform.


with its deep and supercial components. The lig-

ament arises from the volar tubercle of the tra-

pezium and inserts on the volar aspect of the

thumb. The anterior oblique ligament is taut in

extension, abduction, and pronation; it controls

pronation stress and prevents radial translation.

The deep anterior oblique ligament serves as a

pivot point for the TMC joint and guides the meta-

carpal into pronation while the thenar muscles

work in concert to produce abduction and exion.

These bers limit ulnar translocation of the meta-

carpal during palmer abduction while working

with the supercial anterior oblique ligament to

constrain volar subluxation of the metacarpal.

The anterior oblique, intermetacarpal, and dorsor-

adial ligaments are the most critical for preserva-

tion and reconstruction [4244].

The index nger

The index nger may be of next highest impor-

tance because of its exion and extension inde-

pendence, its ability to abduct, and its closeness

to the thumb. It has a major role in precision pinch

and directional grip [11,13,45,46]. A good range of

motion for the index nger is more important than

length. Amputation through the PIP leaves all

remaining stump exion to the control of the

intrinsics. This allows for exion to approximately

45 degrees. It may be shortened to the end of the

proximal phalanx and still participate in direc-

tional grip, span grasp, and lateral pinch [13].

The body, however, is quick to bypass the digit

for the long nger if it becomes insensate or sti.

The long nger replaces the index for terminal

and subterminal pinch if amputation exists below

the DIP level.

Elective loss of the index ray has been well

studied. Murray et al [47] studied patients who

underwent elective ray amputation. The study

found that power grip, key pinch, and supination

strength were diminished by approximately 20%

following surgery. Patients with persistent dyses-

thesia following ray amputation experienced larger

losses in grip strength. In addition, pronation

strength was diminished by 50% following ray

resection. Pronation strength is used for direc-

tional grip. This large decrease in pronation

strength is caused by a shortening of the palms

lever arm. In the intact hand, the width of the grip

extends from the hypothenar region to the index

nger. The ulnar aspect of the palm represents

the internal fulcrum and the radial aspect of the

palm represents the external fulcrum of move-

ment. With the loss of the index nger ray the ful-

crum is decreased by approximately 25% (Fig. 11).

This results in a loss of stability and a decrease

in mechanical advantage. Despite the loss of

strength, all patients in this study, without postop-

erative dysesthesia, believed that their overall hand

function had been improved, especially in regard

to prehension with the thumb [47]. This suggests

that the ability to perform precise activities is more

important for postoperative patient satisfaction

than the preservation of grip strength. In compar-

ison, a recent study of patients with traumatic

proximal phalanx amputations of the index nger

and patients with elective index ray resections

found that patients with amputation through the

proximal phalanx demonstrated a better func-

tional outcome when assessed with the DASH

questionnaire. A 30% decrease in pinch and grip

strength was seen in both groups. Cosmesis was

believed to be better with ray amputation [48].

Overall, it seems that a remaining proximal pha-

lanx stump does provide a benet in terms of grip

Fig. 10. Diagram of the trapezio-metacarpal joint

showing the outlay of the dorsal and volar ligaments.

Special attention must be given to preservation of this

joint for adequate thumb stability. The most important

ligaments for reconstruction and preservation are the

dorsal radial ligament (DRL), posterior oblique ligament

(POL), ulnar collateral ligament (not depicted), rst

intermetacarpal ligament (IML), and the anterior oblique

ligament, deep and supercial heads (DAOL and SAOL).

APL abductor pollicis longus; DIML dorsal inter-metacarpal ligament; DT-II MC dorsal trapezio-IImetacarpal; DTT dorsal trapeziotrapezoid.


strength and overall hand function. In light of the

high rate of postoperative dysesthesia associated

with ray resection, it seems that immediate index

ray resection should be reserved for very proximal

injuries where there is little chance of postopera-

tive MP motion.

The long and ring ngers

The long nger does provide the most nger

exion force when tested individually [49,50]. Its

central position allows it to participate in power

grip and precision grip. Patients are easily able to

substitute this digit for terminal and subterminal

pinch following the loss of the index nger. The

middle ray does lack the specialization of the rst

dorsal interosseous muscle when performing pinch

functions. Transfer of the rst dorsal interosseous

to the insertion of the second dorsal interosseous

has been suggested following rst ray resection;

however, studies have shown that this does not sig-

nicantly increase pinch strength [47,51]. In addi-

tion, this transfer can lead to the development of

an intrinsic plus deformity in the long nger

[47,52]. The ring nger has less strength than either

the index or long. It is also rarely used for precision

pinch or grip. As an individual digit, Tubiana et al

[11] believe the ring ngers loss leaves the least

functional decit in the hand. When this nger is

combined with the small as a functional unit, how-

ever, it can provide for adequate power grip and

replace the index and long for pinch maneuvers

should both digits be lost.

Central ray deletion, or loss of both ring and

long ngers, may produce scissoring of the remain-

ing digits because of instability of the transverse

metacarpal ligament and compromised inteross-

eous function. Three-point chuck pinch is com-

promised, as is hand competence, because small

objects may fall through the central defect [53

55]. Acute central ray resection with repair of the

transverse metacarpal ligament may still result in

scissoring of the neighboring digits, inadequate

closure of the gap, and loss of abduction of the

small ray [54,56,57]. In cases of central digital loss,

a ray transposition may alleviate hand incompe-

tence and reduce scissoring of the digits. Results

of strength testing following ray transposition for

central digital loss have found an average decrease

in grip and pinch strength of 20%, with larger

decreases in function being seen for index to long

transfer when compared with small to ring trans-

fers. Loss of motion was only 9% following

transfer [56]. Although ray amputation may be

indicated in cases of central digital loss, it seems

most prudent to perform this procedure in a

delayed fashion, after a discussion has been carried

out with the patient regarding his or her needs with

regard to hand strength and motion.

The small nger

The small nger has the least strength in ex-

ion; however, its loss can have broader implica-

tions on hand function. In digitopalmar grip the

fth ray presses objects and tools into the palm.

This is caused by the additional motion provided

by its carpal-metacarpal (CMC) joint, which can

move forward 25 degrees. Stabilization is also

added by the hypothenar muscles, which augment

the exion of the rst phalanx of the small nger.

In addition, the small ngers abduction capabil-

ities signicantly enhance span grasp. Tubiana

et al [11] believe the fth nger, with its metacarpal,

has the greatest functional value after the thumb.

Digital loss

For the most part single digit amputation, with

the exception of the thumb, does not result in the

loss of essential hand function. Brown [18] studied

183 surgeons who suered partial or total digital

amputations. Only four surgeons were unable to

continue operating following their injuries. Most

Fig. 11. Diagram showing the resultant eects of ray

excision on pronation and supination strength. Resec-

tion of the metacarpal narrows the palms. This shortens

the palms lever arm and decreases the hands mechan-

ical advantage during pronation and supination.


surprising was the nding that 15 surgeons who

had experienced thumb amputations through the

metacarpal or MP joint level were able to continue

operating with only minimal adaptation in their

surgical practice. Brown [18] concluded that the

motivation of the patient is more important than

the actual number of retained digits when attempt-

ing to predict functional outcome for digital

amputation. Of note, none of these surgeons had

to perform repetitive strenuous activity with the

hand and grip strength presumably was not a

major issue.

Unlike single-digit amputation, the amputation

of several digits still remains a challenging prob-

lem. Unfortunately, in the mutilated hand, multi-

ple digital losses are the norm, because severely

crushed and avulsed digits preclude replantation.

Preservation of the thumb and a single digit allows

for some prehensile grasp, but for optimal func-

tion the reconstruction of an additional digit is

recommended [24,5860]. The preservation or

reconstruction of the thumb and two digits allows

for the possibility of chuck pinch, which is stronger

than subterminal pinch. The use of a third digit

confers lateral stability in power pinch. A third

digit also allows the patient to perform hook grip

and power grasp. Span grasp is now possible

because functional palmar space is increased

allowing for grasp of larger objects [24,5860].

Wei and Colony [24] have found it preferable to

place toes next to remaining mobile ngers or in

the interval between them. They believe the adja-

cent digits contribute to cosmesis, help coordinate

movement, and smooth oppositional contact.

In injuries where there is loss of all ngers but

sparing of the thumb, reconstructive goals should

attempt to maintain useful thumb web space and

an opposable ulnar post of adequate length. Addi-

tional digits may be created with microvascular toe

transfer [24,5962]. Other options include the

transfer of remaining functional digits to more

useful positions. Transferring salvageable digits

to the ulnar side of the hand maintains the width

of the palm, and allows for power grasp and

the incorporation of pinch [21,22,24]. The radial

placement of reconstructed digits is more cosmeti-

cally pleasing but fails to take advantage of the

added power provided by intact hypothenar mus-

culature and the motion provided by the fth

CMC joint. In cases where there has been loss of

all digits including the thumb, microvascular

reconstruction of the thumb is required with the

additional creation of a stable ulnar post. The pre-

vious practice of constructing a cleft hand has been

shown to provide little benet for hand function. It

often has no eective prehension or grasp and does

not adequately compare with the results obtain-

able with microsurgical reconstruction [24,5962].

The biomechanical impact of fusion

There are several instances where the severity of

the trauma precludes any anatomic restoration of

the joint surface. These situations may require

fusion. Unfortunately, change in a single joint has

implications on the balance of the entire digit, and

the biomechanics of the hand. How do fusions

impact overall hand function?

Finger fusion

Of all fusions, DIP fusions are well tolerated

and probably impart the least detriment to hand

function. Fifteen percent of intrinsic digital exion

occurs at the DIP joint but the DIP joint contrib-

utes only 3% to the overall exion arc of the nger

[63]. Recent mechanical testing has shown that

after simulated DIP fusion of the index and middle

nger, there is a 20% to 25% reduction in grip

strength when compared with prefusion values.

The decrease in grip strength may be secondary

to the limited excursion of the profundus tendon

following fusion; this can create a quadriga eect.

It has been suggested that fusion in a more exed

position creates additional slack in the profundus

tendon, decreasing the loss of grip strength; how-

ever, this has not been shown clinically [64]. For

most individuals, with the exception of musicians,

arthrodesis is preferred over arthroplasty at the

DIP level.

The PIP joint produces 85% of intrinsic digital

exion and contributes 20% to the overall arc of

nger motion. Littler and Thompson [65] de-

scribed this joint as the functional locus of n-

ger function. PIP joint impairment can adversely

aect the entire hand; however, a full range of PIP

joint motion is not essential for hand function. An

arc extending from 45 to 90 degrees can provide

relatively normal function [66,108]. In addition,

mild exor contractures at the PIP level can be

compensated for through hyperextension of the

MP joint. This allows the nger to move out of

the plane of the palm when attempting to lay the

hand at or when placing objects into the palm.

A PIP fusion is often well tolerated in the index

nger because the indexs relatively independent

profundus function does not impose a signicant

quadriga eect on the other ngers during power


grasp. PIP fusion of the long nger, however, has

been shown to decrease the excursion of all pro-

fundus tendons, reducing grip strength. PIP fusion

restricts profundus excursion to a greater extent

than DIP or MP fusion [47,67,68]. In a study by

Lista et al [67], a signicant decrease in grip

strength occurred when the PIP joints of the index

and small nger were xed at less than 45 degrees

and when the long and the ring were fused in a

position of less than 60 degrees of exion. If both

MP and PIP joints are injured, salvage of the

MP joint through arthroplasty or other measures

is preferred over PIP joint arthroplasty. Grip

strength is decreased because of a quadriga eect,

but prehension can be maintained as long as the

thumb or border digit is capable of opposition. It

is important to remember that two consecutive

fusions increase stress at the next proximal joint,

because of an increase in the lever arm working

across the joint. This accelerates the degeneration

of adjacent joints if they are also injured.

Delayed arthroplasty of the PIP joints in cases

of trauma maintains motion and improves grip

strength [69]. Classic teaching has suggested that

index PIP joint arthrodesis be performed instead

of silicone arthroplasty, to provide stability for

key pinch. Surface replacement arthroplasty, how-

ever, may provide adequate stability for index

nger PIP arthroplasty. PIP stability has been pre-

served following surface replacement arthroplasty

with loads up to 22 N in experimental cases where

there was preservation of 50% of the index collat-

eral ligaments [70].

The MP joints probably represent the most

important joint for hand function. They contrib-

ute 77% of the total arc of nger exion [63,

65,66,71,72]. Unlike the giglymoid IP joint, which

functions like a sloppy hinge joint, the condyloid

MCP joint is diarthrodial, allowing for exion-

extension, abduction-adduction, and some rota-

tion [71,7375]. Most prehension grips require

that the digits extend and abduct at the MP joint

[74,76]. Precision pinch requires exion, rotation,

and ulnar deviation at theMP joint [73,74]. During

pinch the radial intrinsics and the collateral liga-

ment to the index must resist the stress applied

by the thumb. According to the American Medical

Associations Guide to the Evaluation of Perma-

nent Impairment, fusion of the MP joint results

in a 45% impairment of the involved nger [77].

Some have suggested that a single sti MP joint

can impair the entire hands function [78]. A full

range of motion, however, is not required for hand

function. Most activities of daily living require

only 50% of normal joint motion [73,79,80]. Stud-

ies have shown that obtaining 35 degrees of

motion at the MP is satisfactory if the arc of

motion is within the functional range and the

joint is stable [73]. Many rheumatoid patients

who have had PIP and DIP fusions maintain a

useful hand through the preservation of MP

motion. Previously, MCP arthrodesis was recom-

mended for border digits in heavy laborers; how-

ever, these indications may be reconsidered with

the availability of new surface replacement arthro-

plasty [70,80].

Wrist fusion

Although less common than nger fusion,

immediate limited wrist fusion or total wrist fusion

may be necessary following penetrating ballistic

trauma, punch presstype injuries, or in cases of

gross carpal instability. A stable wrist is necessary

for power grasp. In addition, a stable wrist pre-

vents the dissipation of nger exion and exten-

sion forces as tendons pass over the carpus.

What are the requirements for a functional wrist

and what eect does fusion have on wrist and hand

function?

The requirements for functional wrist motion

have been debated. Palmer et al [81] found that

the normal wrist had an average exion-extension

arc of 133 degrees, but only 5 degrees of exion

and 30 degrees of extension were needed for most

activity. Brumeld and Champoux [82] found that

10 degrees of exion and 35 degrees of extension

allowed one to complete the activities of daily liv-

ing. Ryu et al [83], however, found in 40 normal

patients that most activities of daily living could

be accomplished with 40 degrees of exion, 40

degrees of extension, 10 degrees of radial devia-

tion, and 30 degrees of ulnar deviation.

Limited carpal fusions consist of intercarpal

fusions and radiocarpal fusions (Fig. 12). Mechan-

ical studies by Meyerdierks et al [84] show that

fusions that cross the radiocarpal joint produce

the greatest loss of motion. On average radiolu-

nate, radioscapholunate, and radioscaphoid fu-

sions decrease the exion extension arc by 55%.

Recent studies have suggested that removal of

the distal pole of the scaphoid in radiocarpal

fusions unlocks the capitate, allowing unhindered

midcarpal motion. In the laboratory setting this

has produced exion extension arcs that are equiv-

alent to normal wrist motion [85]. Fusions that

cross the midcarpal joint result in the next largest

loss of wrist motion. Scaphocapitolunate and


capitolunate fusion can produce a 35% loss of the

exion and extension arc and up to a 31% loss

of radial and ulnar deviation. Scaphotrapezial-

trapezoid fusion produces a 23% decrease in the

exion extension arc and 31% decrease in radial

and ulnar deviation, whereas scaphocapitate

fusion results in a 19% loss in the exion extension

arc and a 19% loss in radial and ulnar deviation.

Inclusion of the lunate within partial wrist fusions

was found to nearly double the resultant loss of

wrist motion when compared with fusions that

did not include the lunate [84]. Fusion within the

same carpal row tends to have a minimal eect

on overall wrist motion, with average loss of only

12% of the exion and extension arc.

The choice for total wrist fusion must be care-

fully contemplated. Removal of all wrist motion

results in the loss of the benecial eect of tenode-

sis for any subsequent tendon transfer. In addi-

tion, wrist dorsiexion is important for pushing

o, rising from a chair, and power grasp. In those

cases where there is substantial carpal loss, how-

ever, fusion may be the only option.

Wrist fusion can have a negative impact on MP

motion and thumb motion presumably because of

extensor adhesion [86]. A 25% decrease in grip

strength may be seen [86,87]. Strength with key

pinch, subterminal pinch, and directional grip are

better maintained at approximately 85% of the

normal side. Maximum preservation of power grip

is found to occur in 15 degrees of extension and

15% of ulnar deviation [88]. Weiss et al [89] found

that patients believed they were able to accomplish

85% of the activities of daily living following total

wrist fusion. Patients were least able to use a

screwdriver and perform perineal care. Overall,

skills that presented the most diculty were those

that required signicant wrist exion in a small

space, where compensatory movements by the

shoulder and elbow are eliminated.

In severely mutilating trauma, the preservation

of wrist mobility imparts some function to a fore-

arm stump with the addition of prosthesis. Mod-

ern prosthetic techniques allow the incorporation

of the prosthesis to the wrist so that proximal

straps and attachment to the elbow are unneces-

sary. Preservation of wrist motion also eliminates

the need to incorporate a wrist articulation into

the prosthetic unit [6,17,90]. In addition, preserva-

tion of the distal radio-ulnar joint (DRUJ) further

improves function, because 50% of forearm rota-

tion can be transferred into the prosthesis [91].

Tendon requirements

Tendon injuries are present, in some aspect, in

all cases of mutilating hand trauma. Tendons

may be divided, avulsed, or have large segmental

gaps that prohibit immediate repair. It is impor-

tant to understand how tendon loss aects hand

function.

Extensor tendons

Multiple authors have pointed to the diculties

in obtaining excellent results with extensor tendon

Fig. 12. Diagram depicts the multiple sites for limited wrist fusions. (1) Four corner fusion or midcarpal fusion.

(2) Scaphotrapezialtrapezoid (STT) fusion. (3) Radioscapholunate fusion (radiocarpal fusion). (4) Scaphocapitate (SC)

fusion. (5) Lunotriquetral (LT) fusion. Fusions involving the radiocarpal joint result in the greatest loss of motion.

Fusions involving the same carpal row result in a 12% to 15% loss of motion.


injuries [9294]. The supercial position of the

extensor tendons, their complex architecture, and

paucity of surrounding subcutaneous tissue often

result in postoperative adhesions, which limit ex-

ion and produce extensor lags [94,111]. It has been

shown that injuries in the distal zones (1 through 5)

result in poorer outcomes and greater postopera-

tive extension decits. Extensor tendon injuries

also carry a signicantly worse prognosis when

associated with underlying fractures [19,94].

The extensor mechanism has less excursion

than the exor system [95]. In addition, it has less

ability to compensate for signicant shortening

because of the interconnections between the intrin-

sic and extrinsic mechanisms. Extensor tendon

excursion in the region of the PIP joint is only

between 2 and 5 mm. There is little margin for

adherence or shortening if a reasonable result is

expected [95,96]. If signicant shortening takes

place following repair and the lateral bands and

oblique retinacular ligament are intact, one can

opt to leave the central extensor mechanism unre-

paired. This may avoid exion loss, without pro-

ducing a PIP or DIP extension lag. Loss of long

extensor function can destabilize the MP joint,

however, resulting in a loss of active nger ab-

duction-adduction [97]. Further biomechanical

studies are required to determine the absolute

requirements for functional nger extension.

Maximizing intrinsic function helps in the pres-

ervation of full nger extension. Intrinsic function

can be compromised after metacarpal fractures.

Metacarpal shortening or fracture angulation

beyond 30 degrees can result in a shortening of

intrinsic muscle ber length [98]. Muscle ber

length determines the potential excursion of the

intrinsic tendon [31]. With metacarpal malre-

duction or shortening, potential excursion force

is wasted as slack in the muscle. Starting muscle

tension is also decreased. Both of these factors

decrease intrinsic tendon excursion and joint

motion [98,99]. This loss of intrinsic function

emphases the need for preservation of metacarpal

length and the anatomic reductions of fractures in

cases of signicant hand trauma.

Extensor tendon injuries proximal to the junc-

tura produce less postoperative decits. Quaba

et al [100] examined long-term function in patients

who had lost nger extensors in zones 6 and 7.

In the nine patients studied, no attempt was made

to reconstruct the extensor tendons. Soft tissue

coverage alone was provided to the dorsum of

the hand. In long-term follow-up, there was a

26% decrease in total active nger motion, most

evident at the MP joint. DIP and PIP extension

were preserved because of intact intrinsic function.

Active motion at the MP joint was only 60% of

normal. Surprisingly, patients reported a 90% sat-

isfaction rate with hand function. Diculty was

noted with tying knots and unscrewing lids. All

patients did maintain the extension of their thumb

and wrist extensors. This emphasizes the impor-

tance of thumb abduction and extension for pre-

hensile function when MP motion is limited. The

ability to move the thumb out of the palm allows

for the accommodation and prehension of objects

even with a moderate digital exion stance. The

loss of the central extensors decreases power grip

by approximately 30%, whereas severance of wrist

extensors results in a 50% reduction in grip

strength [97,100].

Flexor tendons

Loss of profundus function prevents subtermi-

nal and terminal pinch, unless the DIP joint is

fused. If the profundus tendon becomes adherent

to the remaining sublimis tendon or fracture callus

it may tether the profundus tendons of adjacent

uninjured ngers, preventing full digitopalmar

grip [14,101]. Classically this quadriga eect

applies only to the long through small ngers,

because of their common muscle belly. The quad-

riga eect can also extend to the index nger, how-

ever, because heavy synovium at the level of the

carpal tunnel, termed the bromembranous retinac-

ulum, can link the index profundus tendon to the

other three [102].

Power grip and forceful pinch are still possible

with supercialis loss. Loss of the supercialis with

preservation of the profundus tendon may result

in hyperextension of the PIP joint in supple indi-

viduals. This phenomenon is called recurvatum. In

exaggerated cases, this may produce delayed nger

exion. Patients may have to help the involved n-

ger initiate PIP exion with the adjacent digits

before active exion can ensue. Recurvatum can

be avoided by leaving the portion of the supercia-

lis distal to the chiasm [14]. With loss of both pro-

fundus and supercialis tendons, exion of theMP

joint to 45 degrees may be possible if intrinsic func-

tion is intact.

Retraction of the profundus tendon, following

more proximal amputations, may result in short-

ening and contracture of the corresponding lumbr-

ical. During exion, contraction of the profundus

muscle belly places stretch on the shortened lum-

brical, which results in paradoxical extension of


the PIP joint. This is termed the lumbrical plus

deformity. This deformity can be oset by divid-

ing the lumbrical or suturing the profundus ten-

don to the exor sheath in a relaxed position

[14,110].

With multiple digital amputations, retraction

of the exor mechanism can lead to lumbrical

migration into the carpal tunnel. Proximal lumbr-

ical migration may then lead to compression of the

median nerve and development of carpal tunnel

syndrome [6,103]. These patients may not present

with classic digital paresthesias if there has been

signicant digital soft tissue loss. Patients may

instead complain of generalized pain within the

wrist and palm, which may be exacerbated by the

standard provocative maneuvers. Carpal tunnel

release should be pursued in such instances.

During any exor tendon surgery it is impor-

tant to preserve the A2 and A4 pulleys [104107].

If either is divided the exor tendon moves away

from the phalanx, leading to bowstringing. The

A2 and A4 pulleys are located over the bony shafts

of the proximal and middle phalanx. This ana-

tomic conguration prevents the bowstringing

that occurs with joint exion and the bowstringing

that can occur over the phalanx shaft. Palmer plate

pulleys (A1, A3, and A5) have a variable relation-

ship to the joint axis depending on joint position,

and restrain only the joint-type of bow stringing.

They also shorten up to 50% with nger exion,

which reduces their eciency. Cruciate pulleys

vary the most in their anatomic position and have

little eect on restraining bowstringing [105,107].

Bowstringing increases the exion moment arm

at the PIP and MP joints. A longer moment arm

allows the exor mechanism to overcome the

extension forces, resulting in a exion deformity.

A longer moment arm also means the tendon must

move through a longer distance to obtain the

same motion at the joint, decreasing mechanical

eciency. As in the quadriga eect, grip strength

is decreased because full excursion is now

limited [107].

Summary

Mutilating hand trauma presents the surgeon

with many reconstructive challenges. This article

establishes some biomechanical guidelines to help

the surgeon evaluate the hand trauma patient.

Through a basic understanding of hand biome-

chanics, the surgeon may access more accurately

what motion and function can best be salvaged.

By understanding how amputation, fusion, and

tendon loss impact on postoperative hand motion,

the surgeon can better focus his or her reconstruc-

tive eorts to achieve the highest functional out-

come for the patient.

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Biomechanics and hand trauma: what you needThe essentialsThe biomechanical impact of amputationThe thumbThe index fingerThe long and ring fingersThe small fingerDigital loss

The biomechanical impact of fusionFinger fusionWrist fusion

Tendon requirementsExtensor tendonsFlexor tendons

SummaryReferences

biomechanics and hand trauma 2003

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