Hand Clin 21 (2005) 363–373

Biomechanics and Biology of External Fixationof Distal Radius FracturesRandy R. Bindra, MD, FRCS

Center for Hand and Upper Extremity Surgery, University of Arkansas for Medical Sciences,

4301 West Markham Street, Slot 531, Little Rock, AR 72205, USA

External fixation of distal radius fractures isonly one of several ways to approach this complex

injury. In fact, widespread analysis of clinicaloutcomes of distal radius fractures still fails toshow improved clinical outcomes from any spe-cific surgical treatment modality [1]. Loss of

reduction, the commonest complication of distalradius fractures, can be prevented with the appli-cation of an external fixator, a simple technique

that is surgically minimally invasive. Complica-tions such as pin loosening and pin track in-fection, wrist and finger stiffness, and complex

regional pain syndrome, however, have beenreported following the use of external fixationfor the management of distal radius fractures

[2,3]. In contradistinction to internal fixation ofa fracture with an intramedullary nail or plate byfollowing a defined operative technique, applica-tion of an external fixator entails considerable

planning with regard to insertion of the anchoringpins and construction of the external supportingframe. In addition, external fixator use also

requires closer supervision and follow-up in thepostoperative period for mechanical (clamp re-tightening, angular adjustment) and biologic (pin

site care) reasons.This article reviews themechanical properties of

external fixation and current concepts of the bi-ology of bone healing and the pin–bone interface.

In addition, the concept unique to the treatmentof distal radius fractures—ligamentotaxis—is dis-cussed, with attention to mechanics of reduction

The author has not received any funding or support

for this article.

E-mail address: BindraRandyR@uams.edu

0749-0712/05/$ - see front matter � 2005 Elsevier Inc. All rig

doi:10.1016/j.hcl.2005.02.007

and the adverse effects of prolonged traction on softtissues.

Types of application

Application of an external fixator to a trauma-

tized extremity may be intended as a temporarymeasure or as a definitive one until the fracturehas healed.

Temporary external fixation

Occasionally an external fixator is appliedtemporarily to an injured extremity with theintention of removal after a few days, at which

time it can be replaced by other methods offracture stabilization, such as internal fixation.Indications for such use are:

1. Initial management of severe grade openfractures with extensive soft tissue loss. The

external fixator provides stability while allow-ing access to wound care. Once soft tissueshave healed or cover has been achieved with

tissue transfer, the external frame can besubstituted for internal fixation.

2. Temporizing measure to resuscitate a poly-

traumatized patient. This use is almostexclusive to the pelvis, where an externalfixator applied emergently can significantlyreduce internal hemorrhage, constituting an

important adjunct to primary resuscitation.3. Pending transfer to a tertiary referral facility

for fracture management. The application of

an external fixator may be considered for theinitial management of severely comminutedfractures, such as those of the distal radius,

if expert management is not immediately

hts reserved.

hand.theclinics.com

364 BINDRA

available locally. The application of thefixator well away from the zone of injurydoes not interfere with subsequent open

reduction maneuvers and facilitates internalfixation by allowing better imaging studies,such as CT scanning, and maintains the softtissues at the correct length.

Definitive external fixation

The fixator also may be left in place for theduration of fracture healing rather than just asa temporizing measure pending soft tissue repair.

Soft tissue management is not usually the primaryconcern, and external fixation is applied for themanagement of closed fractures that are deemed

too comminuted to consider open reduction andinternal fixation. In distal radius fractures, thishas become by far the more common application,and the external fixator remains in place until the

fracture is judged to be healed sufficiently.

Basic mechanics of an external fixator

Complexity of external fixator designs hasvaried since initial descriptions of use of thefixator in 1943 [4]. Initial fixator designs consisted

of transfixing pins passed through the extremitywith a frame on either side, the so-called ‘‘bilateralframes.’’ By the late 1960s improved biomechan-ical understanding and metallurgy led to the

development of sturdier but less complicatedframes applied on one side of the limb usingthreaded Schanz screws: the unilateral frames [5].

With the introduction of newer concepts in limblengthening and three-dimensional deformity cor-rection, the design has become complex once

again with ring fixators that encircle the extremityand are anchored with thin transfixion K-wires.Although described by Ilizarov in 1951, wide-spread use of the ring fixator system only began in

the1990s. Most recently various hybrid fixatorsconsisting of combinations of distal transfixionpins and proximal Schanz screws are being pro-

posed for use in distal radius fractures [6].

Components

An external fixator is a modular system thatrequires assembly at the time of use to create

a stable construct. External fixators can be usedfor the management of the injured extremity orfor reconstructive procedures, such as correction

of deformity and limb lengthening followingcongenital or acquired problems. Fixator framesvary considerably in their appearance, but all have

the same basic components: an external frameconsisting of longitudinal bars that are connectedby clamps to pins that are anchored into the bone.Basically a fracture is immobilized by inserting

pins into each fragment and then in turn securingthe pins to a scaffold that is constructed outsidethe extremity. The longitudinal bars provide the

stable frame and the pins, the bone fixation; bothplay an important role in stability of the con-struct.

Anchoring pins

Anchoring pins vary from 2.5–6.0 mm for usewith different bones. Because they are subjected to

bending forces, the pins should be sufficientlylarge and strong but should not exceed a third ofthe bone diameter to prevent secondary pinhole

fractures. Ring frames use small diameter (1.5–2.0 mm) K wires placed under tension. Unilateralframes use one half threaded pins that areanchored into the bone from one side, whereas

bilateral frames and ring frames use transfixionwires that pierce the extremity from one side tothe other.

Connecting rods and joints

The longitudinal connecting rods are thestrongest elements of the frame and may be

constructed of metal or lighter radiolucent mate-rial, such as carbon fiber. The rods can be morecomplex in design with a built-in articulation to

allow angular correction or they may have a com-plex telescopic design that allows changes inlength for distraction or compression. The largest

design variation among fixators from differentmanufacturers is in the way the clamps joinanchoring pins to the connecting rods. A simple

articulation or joint connects a single pin toa longitudinal rod. A joint with multiple degreesof freedom is referred to as a universal joint. Someframes incorporate clamps for connecting multi-

ple anchoring pins to the longitudinal rods. Theseclamps typically accommodate two or more pinsthat must be inserted parallel and at a set distance

to each other to fit into the clamp. Rod–rod jointsallow longitudinal rods to be connected to eachother at varying angles to create more complex

frame configurations. A ring frame has ring–rodjoints to connect longitudinal rods across severalcircular rings placed around the extremity.

365EXTERNAL FIXATION OF DISTAL RADIUS FRACTURES

Frame configurations

The modularity of most external fixationsystems allows the creation of several differentconstructs with varying stability. It is obvious that

use of more pins and connecting rods providesimproved stability but potentially causes moresoft tissue tethering and may increase difficulty ofmanagement of the pin tracks and open wounds.

It is thus important to achieve a balance betweenthe mechanics and biology to provide a frame thatis suitably strong but with minimal interference

with soft tissues. Most of the research in mechan-ics of external fixation has examined its use in thetibia and the principles learned can be applied

easily to the radius.The basic unilateral and bilateral fixator place-

ment with one-plane and two-plane constructs isillustrated in Fig. 1. With the introduction of

threaded half-pins (Schanz pins) placed bicorti-cally, adequate stability can be achieved withunilateral frame configurations [5]. Most fixator

designs in use for distal radius fractures havea four-pin unilateral configuration. In situationswith extreme instability, such as in the presence of

significant bone loss, two unilateral frames may be

combined to create a unilateral two-plane tri-angular frame with significant increase in stability.

The commonest application of external fixa-tion for distal radius fractures is across the wrist

joint or bridging fixator. In fractures with minimalcomminution and sufficiently large distal fragmentit may be possible to achieve fixation in the

proximal and distal fragments without immobiliz-ing the wrist; this is referred to commonly as thenonbridging or radio–radial fixator [7]. The con-

cept of treating extra-articular distal radius frac-tures using a fixator that does not span the wristjoint was proposed first by Jenkins and later

supported by Melendez et al [8,9]. Because thereis insufficient length of distal fragment to accom-modate two parallel pins for a unilateral frame,nonbridging frames usually are designed as a tri-

angular construct with two unilateral two-pinframes connected to each other (Fig. 1).

Frame stability

Different fixator designs have differing degreesof strength and stability based on the design of theframe and connecting clamps. It is important,

however, to understand the basic factors that

1- plane 2- plane 1- plane 2- plane

Bridging fixator Non bridging fixator

Fig. 1. Basic external fixator constructs and their application to the distal radius.

366 BINDRA

govern the stability of a generic external fixator.Stability of a fixator construct is determined bythe following variables [5] (Fig. 2):

1. Frame configuration (unilateral, bilateral, ortriangular) and design

2. Pin size, number of pins, and pin spread alongbone

3. Pin–bone interface4. Decreased frame–bone distance5. Fixator placement along plane of major

displacement6. Injury characteristics: anatomic reduction,

comminution of fracture, and use of bone

graft7. Supplemental fixation with K-wires, augmen-

tation with graft

Several of these factors can be controlled bythe surgeon and hence it is of utmost importancethat surgical principles of pin insertion, clamp

application, and frame construction are followedmeticulously.

The strength of the fixator depends on the

rigidity of the connecting rods and the clamps.Rod diameter and strength must be weighedagainst their weight. The connecting rod must be

placed as close to the extremity as possible, andadditional rods may be added for increasedstability [10].

The pins that are part of the external fixator

construct are subjected to constant bending forcesand minimal pullout forces. Modern threadedpins hence are designed with a larger core di-

ameter and less core-thread diameter difference toallow the pin to withstand bending. Furthermore,when the pin engages both cortices, it is mainly

the far cortex that is subjected to pullout forces,whereas bending forces act on the pin fixation atthe near cortex. A pin with a short thread placedbicortically such that the threads engage the far

cortex and the thicker shaft engages the proximalcortex thus provides the best pin–bone fixation.The pins selected must be strong and as large as

possible proportionate to the bone in which theyare to be inserted [11,12]. In radius fixationcompared with 3-mm pins, 4-mm self-tappinghalf-pins are 145% stronger in bending and have

significantly higher pull-out strength of 76% andonly 8% decrease in torsional load strength of thebone [13]. Cylindric pins (also known as Schanz

screws) are preferable to tapered or triangularpins. The latter were first designed to allow furthertightening by advancing a wider part of the screw

into the cortex at first signs of loosening. Thesetapered screws, however, have the drawback thatonce inserted too deep cannot be backed out,because they will be loose.

Bone can tolerate compression better thantension or shear forces. One way to reduce pulloutand increase compression forces at the pin–bone

interface is to preload or prestress the pins beforefixing to the external fixation system [10]. Whena fixator is applied to neutralize forces in an

unstable fracture configuration, the pins in thetwo fracture fragments cannot be compressedagainst each other. Compression at the pin–bone

interface can be generated by pre-stressing thepins of each fragment against themselves as theyare attached to the frame. This is done by elasticdeformation of the pins as they are attached to the

frame by squeezing them together (compression)or separating them (distraction). Although it isa sound biomechanical principle, preloading a pin

can cause excessive unilateral cortical pressureand subsequent necrosis and loosening, and its usein the radius no longer is recommended [14].

Another method of applying compression atthe pin–bone interface is to create a tight-fit of thepin in the bone, referred to as radial preload. Pre-drilling the hole for pin insertion with a smaller

Pin spread

Clamp rigidity

Pin diameter

Pin-bone interface

Supplemental K-wire fixationFracture reduction

and stability

Rod-bone distance

Rod strength

Additionalconnecting rod

Fig. 2. Factors affecting fixator stability.

367EXTERNAL FIXATION OF DISTAL RADIUS FRACTURES

hole than the pin has been recommended for mostfixation systems. Drilling removes bone debrisfrom the hole and heat generation is minimal.Animal studies have shown that bone resorption

occurs at areas of the bone that are not subjectedto compression, and radial preload is moreeffective than bending preload in reducing this

resorption [15]. The optimum amount of radialpreload to be applied by pre-drilling is not clear.Significant microstructural damage can occur with

excessive preloading that exceeds the elastic limitof bone and has been shown experimentally tooccur after insertion of pins oversized by more

than 0.4 mm [16].Stress concentration is another adverse factor

in stability of a construct that is subjected toloading. It is advantageous to spread the force

evenly across the entire shaft of the bone bycreating a wide separation of the anchoring pins inthe shaft. To achieve stable fixation and reduce

the lever arm of displacing forces, however,fixation also should be gained close to the fracturesite. It follows that the optimal and minimal pin

placement would be with at least two pins in eachfragment, one pin as close to the fracture aspossible and the second as far as feasible along

the shaft of the bone [5]. Some fixator designshave multiple pin clamps in which the pins have tobe inserted at predetermined distances, limitingthe ability to create a good pin spread.

To increase fixation in the bone with a four-pinframe, best fixation can be achieved with two pinsplaced in the proximal radius. Distal metacarpal

fixation can be enhanced with a six cortical hold byinserting the proximal metacarpal pin through thebase of the index and long metacarpals without

violating the interosseous musculature [17].

Augmentation of fixation

Augmentation of external fixation with percu-taneously placed K-wires has been shown toincrease the stability of a distal radial fracture.Augmentation of fixation also reduces the need

for excessive traction [18–20]. In addition, theK-wire helps maintain palmar tilt that can bedifficult to restore with external fixation alone. A

single dorsal transfixion K-wire has been shown toproduce the greatest reduction in fragment mo-tion in the flexion–extension plane [20]. The use of

an external fixator with two supplemental styloidpins provides stability that approaches thatachieved with a 3.5-mm dorsal AO plate [21].

The additional stability of a single 0.065-in(1.6-mm) K-wire passed from just Lister tuberclein a proximal direction to exit from the volarcortex at an angle of 45( in the sagittal plane

seems to outweigh the importance of fixatordesign and inherent fixator rigidity [22].

Normal forces through radius and rehabilitation

It is important to take fracture stability intoaccount when planning rehabilitation. Force ap-plied when mobilizing digits is magnified as it istransmitted to the distal radius. A cadaveric study

has estimated that for each 10 N of grip force, 26–52 N of force is transmitted through the distalradius metaphysis, depending on the wrist posi-

tion [23]. The average male grip with a strength of463 N can result in more than 2000 N force at thedistal radius, a much higher force than can be

tolerated in a fresh fracture that is internally orexternally fixed [24]. Wrist fixators have beenfound to compress 3 mm with forces rangingfrom 55–729 N. Assuming maximal force trans-

mission through the radius, the maximal saferehabilitation grip force in early phase of fracturehealing should not exceed 10–140 N to avoid

fixation failure [23].Care also must be taken when initiating

mobilization of the forearm. By virtue of the

pull of the brachioradialis muscle and force trans-mission through the distal radioulnar joint, fore-arm rotation causes much larger magnitudes of

fragment motion than finger mobilization [21].

Static external fixation

The fundamental goal of external fixation is toobtain and maintain an acceptable reduction until

the fracture has gained sufficient stability. Thefixator can be applied before or after reduction isachieved. One method of application is closedreduction by the time-tested maneuver of traction,

flexion, and pronation. The fracture then can bestabilized by percutaneous pins, the wrist broughtto a neutral position without distraction, and

fixator applied as a neutralizing device. Anothermethod is to insert the anchoring Schanz pins anduse the fixator to achieve indirect reduction. The

ability to reduce the fracture after fixator appli-cation varies with the fixator clamp and framedesign, because some fixator clamps do not have

sufficient degrees of freedom in all axes. In theusual bridging construct, no mobilization of thewrist is possible until after removal of the fixator.

368 BINDRA

Dynamic external fixation

Early motion of an intra-articular distal radiusfracture not only minimizes stiffness of the wrist, italso may facilitate articular cartilage repair [25].

Jones in 1977 was the first to suggest that it waspossible to move the wrist during bridging externalfixation by placing a flexible tube between connect-ing rods [26]. A decade later Clyburn designed

a fixator frame with a ball joint for the samepurpose [27]. Other similar dynamic external fix-ators have been designed, all based on a frame that

allows movement at a ball joint that is aligned withthe capitate [28,29]. For an external fixator to betruly dynamic and to allow joint movement during

fracture healing, it should be kinematically com-patible with the wrist joint to allow unconstrainedmovement. Several of the commercially availabledynamic external fixation devices for treatment of

distal radial fractures, however, do not replicatenormal wrist kinematics that involves rotationaland sliding movements. Movement with these

fixators in place thus risks forcing the carpal bonesinto an abnormal pattern of movement or causingdisplacement of fracture fragments [30]. A clinical

comparative study has demonstrated poorer resultswith loss of reduction and increased complicationswith the use of ball joint-type external fixator

compared with static fixation [31].A frame with a single ball joint can be aligned

with the center of rotation of the wrist only aboutone axis. Only one type of movement (flexion–

extension or radioulnar deviation) thus can besynchronous with the center of rotation of thewrist. A new fixator design has been proposed

(Flexafix fixator, AO Research Institute, Davos,Switzerland) that uses two sliding discs connectedwith a screw. This creates a sliding mechanism

with a center of rotation that is projected 50 mmaway from the fixator over the capitate. Further-more the sliding mechanism simultaneously al-lows rotation about all three axes without

a change of the center of rotation [32,33]. Cadav-eric studies with this fixator compared withconventional ball joint designs have confirmed

the kinematic similarity with the sliding discmechanism and absence of increased loads at thepins in all planes of wrist motion [33].

Ligamentotaxis

Principles and biomechanics

In 1944 Anderson and O’Neil were the first todescribe the use of sustained traction by an

extraskeletal device anchored to the radius andfirst metacarpal for the closed treatment ofcomminuted distal radius fractures [34]. In a ca-

daveric study DePalma demonstrated that the softtissue envelope around the radiocarpal and distalradioulnar joints was preserved in artificiallycreated comminuted fractures of the distal radius

[35]. Straight traction of the hand with the wrist infull supination was capable of anatomic reposi-tioning of the fragments except for the volar tilt.

The popularization of articular fracture reductionby distraction is credited to Vidal et al, whodemonstrated that ligamentotaxis could be used

to reduce fractures around the wrist, ankle, hip,and knee [36].

Radial length and inclination usually are re-stored easily because of the pull on the radial

styloid by the attachments of the strong volarligaments. Several clinical series, however, haveshown that palmar tilt often is restored inade-

quately. Excessive application of a longitudinaldistraction force with the wrist in palmar flexioncauses tension in the extrinsic long extensor

muscles and produces a clinically evident clawingof the digits [4]. The inability to restore normalpalmar tilt with ligamentotaxis also has been

demonstrated experimentally in a cadaveric model[37]. Even 30 lb of traction with up to 30( of wristflexion does not restore palmar tilt of the distalradius fragment. This is because the palmar

ligaments are short, thick, and more longitudi-nally aligned than the dorsal carpal ligaments,which are arranged in a dorsal V with the apex at

the triquetrum. When distraction is applied to thewrist the palmar ligaments become taut and resistfurther distraction, leaving the dorsal ligaments

loose. Only if the palmar ligaments are releasedcan tension be applied to the dorsal ligaments,resulting in distraction of the dorsal lip and returnof the palmar tilt with minimal traction and

without the need for wrist flexion.Agee has refined further the concepts of liga-

mentotaxis as applied to the distal radius [4].He has

termed conventional ligamentotaxis that is appliedin one plane as uniplanar ligamentotaxis. Unipla-nar ligamentotaxis does not achieve restoration of

the palmar tilt. Longitudinal traction can becombined with radioulnar and dorsopalmar trans-lation, however, to provide multiplanar ligamen-

totaxis that is capable of restoration of normalanatomy of the distal radius. For this purpose,Agee has developed an external fixation system, theWristJack (Hand Biomechanics Lab Inc, Sacra-

mento, California), that has a gear mechanism

369EXTERNAL FIXATION OF DISTAL RADIUS FRACTURES

incorporated into the longitudinal supportingframe to allow supplemental translational forcesafter application of distraction. In this technique,after longitudinal traction is applied the hand is

translated in a palmar direction, producing a pal-mar vector at the midcarpal joint. The volardisplacement of the capitate creates a rotatory force

on the lunate; the distal radius fragment follows thelunate and tilts palmar-ward, restoring the normalpalmar inclination (Fig. 3). Traction then is re-

duced until the fingers can be fully passively flexedinto the palm. The final maneuver consists of ulnartranslation of the carpus to create a radial soft

tissue hinge that helps restore radial inclination.Clinical studies also have shown that distrac-

tion by ligamentotaxis alone is not capable ofreducing volar marginal intra-articular fractures

(AO type B or volar Barton pattern). Thesefractures require an additional volar buttress plate[38,39]. In addition, severely impacted fragments

may not reduce with traction and require percu-taneous manipulation using supplementary K-wires [17].

Biologic effects of distraction

Excessive prolonged distraction of the radio-

carpal ligaments may cause adverse effects on the

A

B

C

Fig. 3. Principle of ligamentotaxis. (A) Initial fracture

displacement. (B) Uniplanar traction restores length, but

as palmar ligaments are stretched fully, palmar tilt is not

restored. (C) Additional palmar displacement of the

capitate rotates the lunate that carries the distal

fragment into palmar tilt.

ligaments themselves and on the hand and wrist.Increased distraction and duration of distractionhave been associated with adverse outcomes, witha linear correlation with worse outcomes in

function, pain, motion, and grip strength [3].Distraction of the wrist can result in strains ashigh as 20% in the volar and dorsal ligaments

[40]. This may contribute to wrist stiffness byligament fibrosis from compromise of circulationor micro-failure of the already injured ligaments.

Overdistraction of the wrist also has been associ-ated with finger and wrist stiffness and withadverse outcome with poorer scores for function,

pain, motion, and grip strength [3]. Clinicallyoverdistraction can be avoided by checking thatall fingers can be fully flexed into the palm afterapplication [4]. Alternatively, radiographs can be

used to determine carpal height ratio index [3] orrelative distraction of the radiocarpal joint incomparison with the midcarpal space [17]. These

methods have been shown to be unreliable in anexperimental model, however [41]. The carpalheight ratio increases with distraction for the first

10–20 lb of traction, but then remains staticdespite increasing tension. Similarly the abilityto passively flex fingers into the palm is not lost at

higher loads of distraction.Fractures of the distal radius have a higher

incidence of carpal tunnel syndrome and complexregional pain syndrome. The development of

carpal tunnel syndrome may be related to in-creased pressure within the carpal tunnel [42].Distraction of the wrist has been shown to cause

a linear increase in carpal tunnel pressure, withpressures exceeding 40 mm Hg over baseline after2.72 kg of distraction force with the wrist in

neutral. Placing the wrist in extension furthermagnifies this effect [43].

Bone healing with external fixation

It has been established that bone heals bydirect Haversian remodeling and without callusformation after rigid internal fixation. In less rigid

environments, the healing process includes anintermediate fibrocartilaginous phase or callusformation. The relative motion between the bone

fragments determines the morphologic features ofthe repair tissue. The exact mechanism of this isnot known but may be related to the interfrag-

mentary strain [44].Lewallen et al compared bone healing with the

application of a unilateral frame with that

370 BINDRA

achieved by dynamic compression plating ina canine tibia model [45]. Both methods resultedin healing of the tibial osteotomy when the

animals were sacrificed at 120 days. Internallyfixed osteotomies were significantly stronger andhealed with endosteal bone formation. The heal-ing process in specimens treated with a fixator was

less mature, with a high bone turnover and lessdirect healing. Experimental studies of bone heal-ing after controlled osteotomies stabilized by

external fixation have suggested that healing isa combination of different processes [46]. Bonehealing mechanisms are different and depend on

the rigidity of the device used. With more rigidfixation achieved by six-pin bilateral frames, thereis early clinical union with similar appearance tothat of internal fixation. On the other hand, with

four-pin unilateral frames, periosteal callus for-mation and local bone resorption is significantlyincreased because of the less rigid fixation. The

distribution of callus is greater in the biomechan-ically weaker plane—the anteroposterior plane ifa fixator is applied along the mediolateral plane

[47]. A longer period is required for fracture repairand remodeling when external fixation with lessrigidity is used [48]. The abundance of callus

formation after external fixation allows the returnof sufficient stability to allow early removal of thefixator in comparison with internal fixation [49].

When the fixator is used in distraction as in the

wrist, overdistraction or actual bone void fromimpactionmay result in secondary loss of reductionin 10%–50%of cases after fixator removal [2,38]. A

significant void in subchondral bone must be filledwith bone graft or substitute to prevent thisproblem [17]. Bone graft or substitute placed in

the subchondral defect provides additional stabil-ity, allows the defect to fill in with bone, preventsingrowth of fibrous tissue, and has been shown toreduce secondary collapse in clinical studies [50,51].

Biology of the pin–bone interface

The pin–bone interface is the link between the

patient and the fixator [17]. Failure of this linkaffects not only the outcome of the fracture, but itmay result in serious additional complications,

such as osteomyelitis with additional morbidityfor the patient. Pin loosening leads to failure offixation and predisposes the pin track to infection

(Fig. 4). Pins holding an unstable fracture andthose subjected to static loading are more likelyto loosen. Histologically, loose pin tracks

demonstrate inflammatory exudates and extensivebone resorption [52]. Rehabilitation should take

into account the fracture stability to minimizeexcessive pin loading and subsequent loosening.Recent clinical and laboratory studies using an-

choring pins coated with hydroxyapatite suggestthat osseointegration of the pins can help preventthe loss of pin–bone fixation over time. It is also

possible that the coating creates a roughness thatincreases the initial interference fit of the pins [53–55]. Such pins are certainly advantageous whena fixator is to be placed for prolonged periods of

time, such as for bone lengthening applications,but are of doubtful value in short-term applica-tions, such as for wrist fractures.

Infection around anchoring pins is one of thecommonest complications of external fixation,and the reported incidence ranges from 0.5%–

30%. The incidence of more serious infectionleading to osteomyelitis is much lower and rangesfrom 0%–4%. External fixation wires and pins are

colonized with bacteria, usually Staphylococcusaureus and Staphylococcus epidermidis [56]. Asmuch as 75% of screw tips have positive culturesafter external fixator removal with a higher rate of

gram-positive bacteria [57]. The incidence of pintrack infection in ring fixators may be lower thanthat seen in hybrid or unilateral frames [58].

Recent measures to lower incidence of pin trackinfection being studied are silver coatings [56],hydroxyapatite and chlorhexidine coatings, and

antibiotic pin sleeves [59,60].Thermal damage to local tissues at the time of

anchor pin insertion is believed to be one of theimportant factors in pin loosening in cortical

bone. The damage is an effect of high temperatures

Thermaldamage

Skintension

Uneven corticalpressure

Excessiveloading

Local cortical resorption

Pin loosening

Sepsis

Bacterialinvasion

Fig. 4. Pathogenesis of pin track infection.

371EXTERNAL FIXATION OF DISTAL RADIUS FRACTURES

and the duration of exposure to high temper-atures. Most animal studies indicate that temper-atures of approximately 50(–55( C can causenecrosis in skin and bone. Heat adversely affects

bone by weakening collagen inter-linking, inacti-vation of alkaline phosphatase, and osteocytedeath [61,62]. Matthews et al examined the

thermal effects of pin insertion in cortical bone[63]. The single highest temperature recorded was185( C when a trocar-point pin was inserted at

700 revolutions per minute. Manual drillingresulted in increased duration of exposure andproduced higher temperatures than drilling at

3000 revolutions per minute. Manual insertionof pins into predrilled holes produced the leastincrease in temperature, exceeding 55( C only inthe immediate vicinity of the pin. Thermal effects

may be magnified if surgery is performed undertourniquet because of the lack of cooling effect ofthe circulation. Other investigators also have

demonstrated significant heat generation withpin insertion and stress the need for cooling withsaline irrigation at the time of pin insertion [64].

Pin care also may have a significant impact onoverall incidence of pin track sepsis. Opinions onthis vary widely and there is no universal consen-

sus on the best way to take care of the pin–skininterface. At one extreme is the recommendationfor daily showers without any specific care of thepins sites [65], and at the other is twice-daily pin

site cleansing with hydrogen peroxide and povi-done-iodine [66]. Necrotic skin and bone at thepin site interface also provide a medium for the

growth of bacteria, and it is important to ensurethat skin tension around the pin is relieved by anadequate incision.

Additional other factors that may contributeto pin tract infection include the thickness of thesoft tissue mantle between the skin and bone andmobility at the pin–skin interface. Pin track

infection is more common because of earlymobilization of the wrist that is possible withnonbridging fixators. Wrist immobilization in

a splint between periods of exercise has beenrecommended to reduce this complication [7].

Summary

External fixation is a versatile and useful toolfor management of complex fractures. There is

little to choose between the various types ofcommercially available fixators, and it is impor-tant to use one that allows the surgeon adequate

versatility and follows sound biomechanical prin-ciples. Ligamentotaxis can be used effectively toreduce the most difficult fractures; however, over-distraction and prolonged traction are harmful

and should be avoided. Certain types of fracturesdo not respond to treatment with ligamentotaxisalone and require adjunctive treatment, such as

limited internal fixation. A single K-wire signifi-cantly adds to the stability of fixation and shouldbe considered in all cases. Understanding the basic

mechanical principles and respect for pin–bonebiology allow for successful use of external fixationwith minimal complications.

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[3] Kaempffe FA, Wheeler DR, Peimer CA, Hvisdak

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