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Thebiophysicsofmandibularfractures:Anevolutiontowardunderstanding

ARTICLEinPLASTICANDRECONSTRUCTIVESURGERY·MARCH2008

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The Biophysics of Mandibular Fractures: AnEvolution toward UnderstandingRandal H. Rudderman, M.D.

Robert L. Mullen, Ph.D.John H. Phillips, M.D.

Alpharetta, Ga.; Cleveland, Ohio; andToronto, Ontario, Canada

Background: Predicting outcomes based on a variety of fixation techniques re-mains problematic in the treatment of mandible fractures. There is inherent dif-ficulty in comparing the hundreds of published articles on the subject because ofthe large number of variables, including injury patterns, assessment techniques,treatment approach, device selection and application, and definition of outcome.Methods: The authors review the behavior of the human mandible. Behavior of theintact mandible, multiple fracture scenarios, and small and large (single and mul-tiple) plating applications are reviewed.Results: Several misconceptions in the literature are clarified. Factors that willresolve the dichotomy between clinical results and current biomechanical theoriesare presented such that a more logical biomechanical model may be used toapproach fixation of the mandibular fracture being treated.Conclusions: Current mandibular biomechanics theory must be expanded to re-flect the complex nature of the system and to more accurately describe conditionsthat exist in the physical world. Otherwise, further analysis in advancements inoutcome and treatment will be relegated to chance. (Plast. Reconstr. Surg. 121: 596,2008.)

Treatment of mandible fractures before themid twentieth century was consistent in theconcept involving application of splinting tech-

niques to achieve maxillomandibular fixation. In-terest in fixation devices, providing the option forearly return to function, was stimulated in part by theorthopedic success with internal fixation1 that pro-vided for adequate healing and consistent resultswhile reducing the associated consequences of im-mobilization of the active joint.2–4 The variety ofinternal fixation techniques resulted in significantdifferences in success rates. Numerous in vitro testswere conducted to describe the biomechanical be-havior of facial structures to confirm or support thevarious fixation techniques.5–9 Although bench test-ing uniformly indicated increased stiffness andstrength in multiple plate systems repair versus sin-gle-plate applications, the technique using singlesmall plates for treatment of fractures produced con-sistent favorable clinical results.10 In the 1970s and1980s, the knowledge of biomechanics of the facialskeleton suggested a model of mandible behavior

consisting of a tension zone at the upper margin anda compression zone at the lower margin. This modelrepresented an oversimplification of the system andtoday remains inadequate for describing fracturebehavior, device behavior, and variations in clinicalresults.

A more accurate description of mandibularbiomechanics will have relevance in resolving theparadox of similar clinical success obtained withuse of reconstruction plates and the small platetechniques on the same fracture scenarios. Un-derstanding the science of biomechanics is nec-essary to optimize current treatment systems anddirect decisions regarding future steps needed tosignificantly improve outcomes.

This article expands on previous descriptionsof mandibular behavior and reviews two commonfracture patterns: posterior body/angle fracturesand symphyseal fractures. The expected displace-ment behavior of the fractures exposed to two biteforces—incisor loading (midline) and molar load-ing (posterior)—are described.

From private practice; the Department of Civil Engineering,Case Western Reserve University; and the Craniofacial Cen-ter for Care and Research, The Hospital for Sick Children.Received for publication April 1, 2005; accepted September1, 2005.Copyright ©2008 by the American Society of Plastic Surgeons

DOI: 10.1097/01.prs.0000297646.86919.b7

Disclosure: None of the authors has a financialinterest in any of the products, devices, or drugsmentioned in this article.

www.PRSJournal.com596

FRACTURE STABILIZATIONThe behavior of an intact system (normal man-

dible) differs from the behavior seen when a fractureis present. An intact mandible develops tension andcompression zones during normal function, andthese zones are dynamic and contingent on the bitetarget location and muscle recruitment pattern. De-vices are applied during treatment of a mandiblefracture to stabilize the segments in the proper an-atomical orientation so that, ultimately, healing willoccur and normal function will follow. For livingbone to heal, either a callus is formed providing aninternal split limiting motion at the fracture zone, ordevices are applied (internal or external) limitingmotion at the fracture site during healing. Eachcondition helps provide the environment necessaryfor further healing to occur at the fracture site andto restore the ability to carry normal functionalloads. The goal of device application is to constructan environment that functions normally duringhealing.

Techniques that include application of a de-vice for the treatment of a mandible fracture whileallowing function substantially affect the behaviorof the overall system. The fixation device acts totransfer forces across the fracture zone. Both ten-sile and compressive stresses can be generated ata fracture site when devices are applied. Each de-vice application scenario will therefore modify thestress conditions that occur with function and willaffect stresses generated at the fracture site. Eventhe application of seemingly simple devices mayhave profound effects on the entire system. Iden-tifying which techniques potentially interfere withand which ones promote soft-tissue contributionsto stability is not an obvious and simple venture.

Early devices used to stabilize fractures func-tioned by approximating segments (i.e., wire loop)or restricting movement when loaded (i.e., arch bar,splints, or plating system). An arch bar functions bytransfer of forces from one segment to the otherthrough the bar during loading. If the segments aresubject to displacement, the bar will serve to preventdistraction and will be loaded in tension. Most thinconstructs of metals will deform by stretching undertensile loads and will deform (fail) by buckling un-der compressive loads. Arch bars, because of inher-ent dimensions and material properties, are signif-icantly more efficient in tensile conditions thancompression.

The bone segments approximated and stabi-lized by an arch bar contribute to load sharing undercompressive forces if the segments are in contact.When maxillomandibular fixation is applied, the

restricted motion largely eliminates biomechanicalbehavior of function.

Plates and screws applied to a fracture site mod-ify the stresses during loading. Screws are insertedinto the bone and contact the plate. As the screwtightens, compressive forces increase between theplate and the bone surface. Movement of the platerelative to the bone will not occur unless sufficientforce is applied to overcome friction between theplate and bone. When the friction force is exceeded,force is transferred by the bearing of the plateagainst the side of the screw. More screws increasethe frictional force on the bearing area between theplate and the bone and increase the force needed todisrupt the construct. Systems that are locking (thescrew locks into the plate using additional threads inthe plate) rely on the screw/bone interface and thescrew/plate interface for stability. These systems be-have in a manner similar to an external fixationdevice (where there is no reliance of the plate/boneinterface for stability) but necessitate soft-tissue dis-turbance during application. In each system, stabilitydepends on the screw/bone contact, and local fail-ure here will result in system failure.

Screw/Bone Stress Factors for System StabilityThe screw/bone interface is critical in main-

taining device stability. This construct must sustainloading conditions without inadvertent concen-tration of stress that will result in bone damage.Any damage to the bone at the screw insertion sitethat results in micromotion will contribute to fu-ture instability. Because forces flow along areas ofgreatest stiffness, the optimal system for device ap-plication would consist of materials and geometrythat simulate the behavior of the bone. If the systemis too stiff (rigid), concentration of stress in excess ofthat tolerated by the bone in contact with the innerscrews may result in bone damage leading to mo-bility at the screw/bone interface and possible sys-tem failure. During and following healing withoutscrew loosening, the screw/plate/screw load pathwill continue to carry most of the force across thefracture site. This pattern of stress distribution issignificantly altered from the prefracture conditionbut still allows for normal function.

It is a general misconception that it is alwaysbest to repair a broken structure with the stiffestmaterials. Materials and applications that simulatethe original structure, and do not interfere withfunction and healing while providing adequate stiff-ness to resist excess motion, should generate themost reproducible results in treatment and create anappropriate environment for healing to occur.

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All forces must remain in equilibrium duringfunction regardless of the number or type of platesapplied. A complete force circuit will be estab-lished, with stress distributions seen along regionsthat provide greatest resistance to deflection andare therefore stiffest. Because no single positionalong the bone surface in the human mandible issubject to only one type of stress, any plate appliedwill have to maintain stability in a variety of stressconditions (compression and tension) while re-sisting rotation and shear forces during loading atvarious bite locations.

Clinical evidence suggests that there is a needfor stress in a region of fracture healing for ade-quate maturation of the bone to occur. The ortho-pedic literature refers to loss of loading because ofexcess plate application as stress shielding of a frac-ture zone (a condition consistent with structural me-chanics). Any device used to stabilize a fracture thatis stiffer than the native bone will transfer stress awayfrom the fracture and therefore results in stressshielding. The stiffer the device, the larger the me-chanical effect of stress shielding. The debate is notwhether stress shielding occurs in treatment of themandible, but whether or not there is clinical sig-nificance during or following healing.11

If a fracture site must experience some loadfor maturation, one of two conditions must existduring healing: (1) the external dynamic forcespresent after injury and plate application are ini-tially significantly lower than normal, then in-crease toward normal during healing to contrib-ute to greater stress at the fracture site; and (2) theplate screw systems gradually lose some stability atthe screw/bone interface, reducing the force flowthrough the plate as the bone/bone interaction atthe fracture bears more load. This second scenariodoes not mean that the screws become clinicallyloose, but that some micromotion occurs allowingfor change in stress flow patterns. In select con-ditions, seemingly weak plates may contribute toconditions promoting adequate fracture healing.These less stiff plates may allow for earlier loadingat the fracture site and earlier transfer of stressacross the injury area (less loading of the screw/plate/screw system).

There remains today difficulty in comparingtechniques because of the variability in reportedresults and an incomplete theory of biomechanics.Some of the confusion persists because of a mis-understanding of basic mechanical principles. Thisbecomes compounded by treatments based on con-clusions from simple in vitro scenarios. One of themost difficult concepts for researchers to deal withis the relationship between the stability of a system

in its natural functional state following repair andthe requirements of the repair device. The often-quoted logical approach is to apply the largest, stiff-est system so that the injury has the longest time toheal before the repair system fails. When inanimateobjects are damaged (fractured), repair consists ofreplacing material that is damaged or applying ma-terials to reconstitute the segments to allow for re-turn of the originally intended function. The pa-rameters considered in evaluation of a repairstrategy include the identification of consistent pa-rameters of strength and stiffness. Techniques forrepair can be too weak or too stiff, altering the stressdistribution and resulting in system failure. The stiff-ness required for stabilization may not be equivalentto the conditions required for healing. In evaluatingthe biomechanics of fracture treatment, one mustask how our understanding of plate stiffness relatesto strength and healing of the fracture zone.12

Most techniques of internal fixation for man-dible fracture describe single or multiple plateapplications (with the exception of lag screw tech-nique and mesh plate techniques) and relate treat-ment results to the device. Traditional mandibularbiomechanics describes plate placement for frac-ture repair by defining a tension band (plate)along the upper margin and a compression platealong the lower margin. This is an oversimplified,incorrect model that is not proven by mechanicaltesting, as is discussed.

FRACTURE SCENARIOSThe most basic fracture conditions are reviewed:

(1) posterior body/angle fracture with bite load an-terior, posterior, and contralateral to the fracture;and (2) symphyseal fracture with midline and pos-terior bite load.

Posterior Body/Angle FracturesIncisor Loading (Midline Load)This scenario involves a fracture position at

the posterior body/angle region with a central(incisor) bite target (Fig. 1). The bite target is thepoint of force transition between the upper (max-illa) and lower (mandible) dental segments. Thebite target completes a force circuit between themandible and midface, where the load is trans-ferred through this substance secondary to forcegenerated by muscular actions.

As muscular contraction occurs, the masse-teric sling (masseter and medial pterygoid mus-culature) generates an upward movement of theposterior mandible. Most obvious movement oc-curs at the fracture site with the mouth open. The

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midline load position (target) acts as a constraintaround which the mandible rotates. When thefracture is anterior to the attachment of the mas-seter, regardless of the orientation of the fracture(oblique, oriented anterosuperior or anteropos-terior), the segment posterior to the fracture willrotate, resulting in separation along the upper mar-gin and less separation, or relative compression, ofthe lower margin. The result is tension at the upperborder not on the bone but on immediately adjacentsoft tissue. Bone at the fracture site cannot experi-ence tensile surface stress if surfaces created by thefracture are not in contact during distraction. Softtissue (i.e., fascia, periosteum, or muscle) that re-mains adherent to each fracture segment may ex-perience tensile loads that can be communicatedbetween each bone segment by soft-tissue attach-ments. The inferior mandible margin will experi-ence some degree of compression, only if the seg-ments are in contact, during movement (Fig. 2).Traditional diagrams of the anterior mandible seg-ment moving downward because of a midline forceare misleading, as the bite target itself does not gen-erate force. This type of force component can onlyoccur if the anterior segment is actively pulled down-ward by submental musculature or by an additionalexternal force.

Molar Loading (Posterior Body)Conditions change dynamically as the bite tar-

get moves posteriorly approaching the fracturelocation (Fig. 3). Displacement will be noted at thelower border as muscle activation occurs, placinglocal soft tissue under tension. Compression at theupper surface is experienced if bone segments arein contact. When the fracture is anterior to the bite

target, a shear component may be seen in combi-nation with the rotatory movement, effecting furtherdisplacement of the fracture segments (Fig. 4).

When the bite target is contralateral to the frac-ture of the body/angle region and the fracture iswithin the attachment region of the muscle, theipsilateral soft-tissue/muscle components may assistin stabilizing the fracture from additional movementcaused by muscle contraction, depending on frac-ture conditions.

Fig. 1. Posterior body/angle fracture with incisor loading. Fig. 2. Posterior body body/angle fracture with incisor loadingwill result in intact soft tissues surrounding the fracture on theupper margin experiencing tensile forces and lower margin tis-sues and bone experiencing compressive forces.

Fig. 3. Posterior body/angle fracture with more posterior (mo-lar) loading.

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Symphysis FractureIncisor Loading (Midline Load)This scenario involves a central incisor bite

target with the fracture position at the symphysisregion (Fig. 5). Analysis of symphysis fractures re-veals a behavior pattern significantly different fromthat predicted by accepted tension/compressioncantilever theory. Cantilever theory has generallybeen depicted as a hemimandible loaded at the mid-line with the implied region of tension along theupper margin.13 A curved structure, suspended bysoft tissue, with the active component of the forcegeneration laterally positioned (human mandible),presents with behavior more consistent with a sus-pended beam14 (Fig. 6). Finite element analysis stud-ies (and in vivo studies in primates) indicate tensilestress at the midline in an intact system, with greatertensile stress along the lingual surface than along thebuccal surface.15,16

When a midline fracture is present, the incisorload position (target) acts as a constraint aroundwhich the mandible rotates. Activation of the mas-seteric sling will produce a rotation around an an-teroposterior axis of a hemimandible (fracture at themidline) because of the point of attachment of themuscle and the curved structure of the mandible.The effect of this rotation and movement will beseen at the midline as separation of the lower borderof the mandible greater than separation of the up-per border (Fig. 7).

A compressive force along the upper mandibleborder will occur if the segments are in contact.

The inferior margin of bone will not experiencetension unless substantial soft tissue remains incontact with the fracture segments.

When a bite load is generated at the fracturelocation, three effects contribute to displacementof the segments at the fracture site: (1) rotatory,(2) axial (translation), and (3) shear.17 If the bitetarget is adjacent to the fracture, one segment maymove vertically relative to the other, resulting insliding (shear) at the fracture location. There is noscenario clinically or experimentally derived thatsupports a tensile stress at the upper symphysismargin when an incisor bite load condition exists.

Molar Load (Posterior Load)A midline fracture will experience similar rel-

ative displacement patterns with an incisor bite

Fig. 5. Symphyseal fracture with incisor loading.

Fig. 6. Drawing of a suspended beam loaded in the midlinewith force generated laterally (human mandible), which re-sults in tension on lower border tissues and compression atthe upper border.

Fig. 4. Posterior body/angle fracture with posterior (malar) load-ing displacement will occur at the lower border, with the softtissues experiencing a tensile force. Compressive forces will oc-cur at the upper border.

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target or molar bite target. With no bony contactat the central segment fracture site (symphysis),18

the lateral mandibular segments will rotate withmidline distraction opposed by soft-tissue attach-ments spanning the fracture. The end effect willbe displacement at the inferior border (tensionof the soft tissue) and compression of the upperborder.

Variations in effects experienced at the frac-ture site are significant, contingent on bite targetlocations. In three of the four common scenariosdescribed—(1) symphysis fracture with incisorbite position, (2) symphysis fracture with molarbite position, and (3) body/angle fracture withmolar bite position—the significant tensile com-ponent is at the lower margin and occurs withinthe soft tissue (and minimally at the bone sur-faces if there is significant tissue spanning thefracture gap).

Tensile stresses are predictably generated atthe upper mandible margin when a body/anglefracture is exposed to incisor bite load conditions.Bone (or any solid structure) that is in multiplesegments (two or more) cannot experience tensileforces across segments other than those devel-oped by attached adjacent soft tissue. Fracturedsegments may develop compressive stress onlywhen the bone surface is in contact.

SOFT-TISSUE CONTRIBUTION TOSTRESS DISTRIBUTION (CIRCUIT

THEORY)Forces generated by muscle contraction of

the masseter sling affect mandible movement.What is neither obvious nor often referenced isthe change in stress distribution within the softtissues that generates the forces within the bone.

Consider the masseter attachments that origi-nate at the zygoma and insert at the mandible. Ac-tivation of the masseteric initiates mandible move-ment as muscle contraction generates force. Atactivation, tensile stresses develop at the sites of mus-cle attachment to the bone (origin and insertion).Stress that is tensile at the muscle attachment is con-verted to compressive stress at the bite target. Asforce increases to modify the bite target, force isdistributed (flows) through the stiffest componentsof the bone to the target point. (Force flows alongregions that are most resistant to deflection.) At thetarget, the teeth and adjacent bone experience themaximal compressive stress. This compressive stressincreases at the bite target until the geometry of thetarget changes (i.e., the bite target is modified bycleaving or crushing).

Stress generated in the bone must remain inequilibrium at any moment to comply with laws ofphysical behavior. Significant load sharing is dis-tributed within the soft tissue as well during acti-vation of the system.

As contraction occurs, the muscle itself becomesstiffer (fiber alignment during activation occurs alonga predefined pattern based on geometry and phys-iology). A fully contracted muscle would be able totransmit forces to another target because of its morerigid nature during activation and therefore share inload distribution during contraction.19

Consider a molar bite load. The masseter mus-cle itself, when activated, becomes stiffer, and actsto carry some of the load in addition to generatingforce. The significant proportion of the force gen-erated is distributed locally, through the masseterto the maxilla above and mandible below, andthen to the bite target. The contralateral masse-teric sling generates force that acts to stabilize themandible from rotation. Forces are distributed fromthe contralateral masseter to the contralateral max-illa above, the contralateral mandible below, acrossthe midface (and palate), and through the mandibleto the target to complete the circuit (Fig. 8).

Facial force circuits must by definition remainin equilibrium and are present every momentmuscle contraction occurs. These circuits includemuscle forces generated and resulting stress pat-

Fig. 7. Incisor loading acts as a constraint around which themandible rotates. This results in tensile force on tissues and sep-aration at the lower border.

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terns established within all solid structures, includ-ing soft tissues. The stiffest components, includingbone, activated muscle fibers, and fascia, will allshare in some load distribution.

Local muscle contraction can effect some de-gree of increased stability at the fracture site dur-ing contraction when a fracture occurs withoutsignificant disruption of the periosteum and soft-tissue/muscle attachments. The effect is most sig-nificant when the fracture is within the attachmentregion of the muscle. The contracted muscle actsto carry some of the load generated, reducing theload on the adjacent bone. Muscle contractionand the resulting stiffness of the muscle can alsoprovide for additional stability at a fracture site byreducing the displacement during loading. A bitetarget anterior to the main vector of the masseterwill be associated with a greater degree of bendingstresses, and the effects of muscle support dimin-ish with incisor loads. The greater the lever arm(the longer the distance) from a posterior fractureto a bite target, the higher probability of motionat the fracture site and the less significant thecontribution of soft tissue for stability.

STABILIZATION TECHNIQUESCENARIOS

Force distribution and loading patterns on theplate/screw construct and local bone are substan-tially different when one versus two plates are usedfor treatment of mandibular fractures. In a two-plate system, one plate is typically placed at theupper border and one is placed at the lower bor-der. Consider a midbody fracture treated with ei-ther a single- or two-plate technique as follows.

Single-Plate SystemsUpper Border PlateWhen an incisor (midline) bite target is pres-

ent, the upper border plate at the fracture site willexperience primarily tensile loads, with a minimalbending component. In a system with four or morescrews, the distribution of force among the screwsdepends on very small (micron) changes in screwspacing. With appropriate screw insertion, thescrews adjacent to the fracture will experience themajority of the load (up to 90 percent of the stresson the central two screws). As a load is applied,tensile stresses begin to develop in the plate betweenthe screws across the fracture site. The stress is fur-ther directed into the bone by means of the bone/screw interface. The bone along the fracture expe-riences no load transfer from the opposite bonysurface. The local stress distribution is secondary tothe transfer of loads through the plate. As healingoccurs (bone growth and maturation at the fracturesite), the healing tissues gradually begin to contrib-ute to the transfer of forces generated during load-ing. The system at any point in time must remain inequilibrium. If the total load on the system is F, andthe load carried by the plate/screw system is P andthat of the healing bone of the fracture site is B, thenat the time of device application, F � P � B, whereB � 0 except in compression loading. Fractures areknown to be associated with alteration in the musclerecruitment following injury.20 These studies indi-cate the probability of gradually increasing biteforces with time after injury. Even when the fractureis completely healed, some of the load continues tobe carried by the plate. Therefore, the system doesnot return to the preinjury stress state while platesare present and remain firmly attached.

Lower Border PlateA single plate placed along the lower border of

a mandible body fracture, with an incisor biteload, will need to resist distraction at the upperborder. The load condition becomes more bend-ing, not pure tension or compression. In a four-screw/plate scenario, when the plate is subject to

Fig. 8. All stresses and vectors of force in the circuit must be inequilibrium. When a bite target is on the right posterior teeth,for example, the masseter on that side, when activated, be-comes stiffer and acts to carry some of the load in addition togenerating force. The contralateral masseter also generatesforce that acts to stabilize the mandible from rotation. A circuitof force is created.

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bending, all of the screws now are subjected toequal loading. If a single screw becomes loose andfails to carry a load before adequate healing in thiscondition, the entire construct is subject to failure.

If the plate is applied by means of an openexternal approach, soft tissue must be dissectedfor exposure and placement. Periosteal dissectionresults in muscle elevation as well when in theregion of the masseter. The soft-tissue elevationand disruption interferes with the potential for thesoft tissue to contribute to stability during func-tion. Plates placed along the inferior border bymeans of an intraoral approach also require soft-tissue elevation for plate application and reducethe potential contribution of the soft tissue forfunctional stability.

Two-Plate SystemWhen a midbody fracture is treated with a

two-plate application and an incisor bite target ispresent, the plate along the lower border of themandible is in a compression area during loading.As a load is applied, compressive stress begins todevelop within the bone/plate system. The centraltwo screws in a four-screw system carry the majorityof the compressive load. The plate will thereforeexperience varying amounts of compressive loadsdepending on the amount of load transferred atthe fracture site. If no bone-to-bone contact ispresent, all compressive loads will be transferredthrough the plate.

If any one of the inner two screws becomesmobile, the load then shifts to the outer screw.This effect will occur in either plate (upper orlower) under tensile or compressive load condi-tions. The possibility of loss of three-dimensionalstability significantly increases with screw mobility.When two plates are used in concert, neither platewill experience significant bending. The greaterthe distance between the upper and lower plates,the smaller the already small bending effect be-comes, even if one plate is smaller than the other.The significant contribution to greater resistanceto bending (increased stiffness) with two plates isdefined by general mechanics. This effect can becalculated and confirmed by bench testing. Im-proved healing, however, does not necessarily fol-low the same relationship as increased stiffness orstrength of the system.

ANGLE FRACTURES AND SOFT-TISSUESTABILIZATION

The treatment of angle fractures is among themost problematic in mandibular trauma because

of the frequency of injury, variability in severity,difficulty in approach and application, and vari-ability in plating techniques and soft-tissue disrup-tion during application. This all leads to variablecomplication rates.21,22 The angle is considered tobe a weaker region of the mandible and thereforesuccumbs to fracture at a high rate during injury.However, the posterior body/angle region duringnormal function is the region where the highestloads are measurable at the occlusal surface. It isimportant not to equate the ease of fracture oc-currence in a region to the functional attributesunder normal conditions.

The angle geometrically is a thinner constructthan the anterior mandible. The molar regionfunctions with an efficiency greater than otherregions. The masseter–medial pterygoid sling inthe posterior body–angle region is oriented toprovide for mostly vertical force during function.As muscle contraction occurs with a bite target inthis location, the majority of stress in the mandibleis attributable to compressive effects. The muscle,once activated, becomes stiffer, and the musclefascia component acts to stabilize the area. Therecruitment patterns of the muscles of masticationhave been studied in humans and primates, andthe general effect of timing of activation relates tothe ability to balance the structure and resist lat-eral rotation.

A single-plate application has been used suc-cessfully for the treatment of angle fractures. Theplates used for this application are typically con-sidered small plates. The location of plate appli-cation is often along the oblique line (upper bor-der) of the mandible. This plate when loadedexperiences stress conditions dissimilar to the singleplate placed along the inferior border. The upperplate becomes loaded primarily in tension with anincisor bite condition. The central two screws aremaximally loaded during function. A molar bite loadwould tend to distract the lower border. However, ifminimal displacement of the fracture occurs duringinjury, the soft tissue spanning the fracture zonemay help stabilize the fracture region. Activa-tion of the masseter will effect shortening andstiffening of the muscle, which may reduce mo-bility at the fracture site during function follow-ing application of the plate. Because applicationof this plate requires minimal dissection, a smallplate device placed along the superior margin,with minimal dissection along the fracture, mayprovide conditions where the nondisplaced softtissue can contribute to stability during function

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DISCUSSION

Divergent Techniques and Consistent ResultsMultiple plate application techniques have been

promoted in the past 30 years for the treatment ofmandible fractures. As treatment options shiftedfrom wire to plate applications, proponents of AOtechniques and those of Luhr promoted large plat-ing applications that were believed to be essential foruneventful and predictable healing. The techniquesof the AO/ASIF and of Luhr when followed havedemonstrated reproducible results.23–25 This ap-proach is consistent with repair strategies used ininanimate objects (where the largest plate is consid-ered to provide the greatest margin of safety).

In contrast, Michelet and Champy26–29 advo-cated the use of small plate applications from theearly 1970s, a technique that has continued to beused at present, with acceptable success rates.30–34

The review by Ellis in 1999 of 10 years’ experiencewith multiple plating techniques indicated diver-gent success and complication results as well.10

The study and review in the literature by Ellis issignificant in several respects.35–39 The procedureswere performed or proctored by the same indi-vidual in 95 percent of the cases. The outcome andresults were reviewed by the same individual, dem-onstrating consistency over the time period. Thetreatment methods were applied to a somewhatconsistent patient population over a 10-year pe-riod. It is apparent that many other studies reportdiffering success and complication rates comparedwith this study. The significance is not only the spe-cific rate of complications and reliability of proce-dure, but the range of rates quoted for any particulartreatment regimen. The study by Ellis reveals themost optimal success rates with the smallest plateplaced intraorally and the largest plate placed bymeans of the open approach.40,41 These findingspresent a challenge to accepted biomechanics the-ory of the mandible, because of the apparent para-dox in clinical results.

When conditions in the physical world are notexplained by theory and results defy logic, thedetails on which the theories are based must bescrutinized to advance understanding of behavior.To elucidate the behavior of biological systems,additional parameters must be considered beyondmechanical models. The primary parameters forconsideration are those that facilitate an environ-ment that allows the organism to repair itself. Theessential and critical factors that enhance predict-ability in determining fracture repair in biologicalsystems include enhanced knowledge of (1) theuninjured system behavior, (2) the motion that

can be tolerated at the injury site, (3) the amountof load that can be tolerated or that is requiredfor adequate healing, and (4) the amount ofsoft-tissue alteration (effecting soft-tissue stabi-lization) and interruption of direct blood supplyor nutrient pathways that can be tolerated andresult in predictable healing failures.

Quantitative knowledge regarding uninjuredsystem behavior exists. Modeling of mandible be-havior is complicated by the difficulty of exploringmultiple load conditions and accurately establishingboundary conditions. As modeling technology hasadvanced, finite element analysis techniques havebecome more accessible for behavioral analysis.42

Finite element analysis models require that bound-ary conditions be specified to arrive at a solution andwill not describe complete behavior without exten-sive variable load iterations.43 Finite element analysismodels helped confirmed both human models andprimate models of tension and compression varia-tions in the mandible dependent on load position.These finite element analysis techniques used toevaluate the structural dynamics of the mandiblesuggest an extremely complex behavior.44 The sim-plified concept of static tension and compressionzones used to describe mandible behavior is incon-sistent with the geometry and boundary conditionsof the mandible. Kroon had performed an eleganttest confirming that tension and compression zonesreverse depending on load position. In vivo studiesin primates serve to confirm stress zones consistentwith the complex suspended-curved-beam behaviordescribed by finite element analysis and general me-chanics theory.45 Primate studies and evaluation ofhominoid facial structures indicate other aspects ofcomplex behavior of the structure consistent withthe theory of variations in qualitative stress compo-nents based on bite locations.46–48 Currently, onlyqualitative knowledge regarding the three other crit-ical factors exists: (1) excess motion at the fracturesite may result in nonunion, fibrous union, or in-fection; (2) inadequate load applied at the fracturesite may result in bone atrophy, bone absorption,and suboptimal ossification; and (3) compromisedblood supply may result in tissue death and failureto heal.

If increased system stiffness or strength is theobjective in fracture treatment, the results of Ellisdemonstrating divergent techniques resulting insimilar success rates cannot be confirmed by anycurrent mechanical model. Evaluation of thesetwo divergent techniques described by Ellis revealthe following: (1) differences in the amount ofsoft-tissue disruption, (2) variations in surgical ap-proach, and (3) differences in qualitative load and

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stress distribution of plates and screws applied, allaffecting the stress distribution during functionalloading of the bone, soft tissue, and device duringhealing. An expected reduction in bite forces onthe side of the fracture is not sufficient alone toaccount for the apparent paradox in behavior.The divergent results demonstrate that the modelsof behavior are incomplete, not including the ef-fects of soft-tissue stabilization during functionand the effects of soft-tissue disturbance duringtreatment. These soft-tissue effects are both pas-sive (by anatomical attachment) and active (loadcarrying and potential stabilization by contrac-tion) at the fracture site. In vitro testing would beexpected to confirm that two plates will resist mo-bility when loaded as a beam construct. The in-consistent findings with two miniplates, includinga report of increased complications in a groupwith two plates and maxillary mandibular fixationversus two miniplates alone, suggest that theremay be other significant factors at play beyondthose of operator technique. Findings that max-illary mandibular fixation with two miniplates didnot reduce the complication rate further pointtoward the requirement to include soft-tissue ef-fects in biomechanical models. Furthermore, frac-ture treatments should be evaluated in terms ofproviding not the greatest stiffness across the frac-ture but the optimal force transmission across thefracture. It may not yet be feasible to quantitativelydefine all of these components, but to omit theircontributions is to relegate further advancementsin technology to chance.

CONCLUSIONSBiomechanics is the study of the function of

living materials. The inability to explain divergentresults of human mandible fracture treatment isattributable to incomplete understanding of the fac-tors affecting biomechanics. Existing clinical expla-nations of divergent findings are incomplete, over-simplified, and confusing. This confusion is notsimply a result of the difficulties encountered incomparing inconsistent patient populations or com-plication definitions, or difficulty in comparing re-ports, but is a result of the unavailability of an ac-curate model for understanding bone healing. Anytheory on mandible behavior will be incomplete if itignores the effects of soft tissue, including the effectsof the fascial and periosteal attachments, and theeffects of muscle contraction in distracting and sta-bilizing fractures. The forces are transmitted notonly through bone but through soft tissues, creatingcircuits of force.

The results associated with the smallest, mostflexible devices do not invalidate biomechanicsbut serve to demonstrate a complexity of behaviorappreciated but not fully delineated.48 Completebiomechanics theory includes not just bone/de-vice interaction but also nutrition and metabo-lism, bone healing, and application techniques,including operator skill. Techniques may preserveor disrupt soft tissue, altering the contribution ofmuscle contraction to stability, stress, and loaddistributions and the overall outcome. In fracturerepair, the reduction process should not contrib-ute to additional system damage. The stabilizationprocess should provide for a functional constructthat can adequately heal while the patient partic-ipates in near normal activities. The fixation sys-tem should provide adequate stiffness and strengthto allow for early return to function. In addition, thesystem should not continue to significantly modifythe stress distributions after healing has occurred.What is regarded as adequate fixation of specificfractures? Conventional wisdom indicates that morerigid fixation provides for a greater chance of un-eventful fracture healing. More careful consider-ation suggests that the minimum amount of stiffnessto achieve immediate return to function and long-term return to preinjury conditions may representthe optimal treatment option. Science advances bydiscarding constructs that defy logic and provideconstructs that survive examination, confirmingmore accurate description of the physical world.When mandible biomechanics are described accu-rately, the seeming dichotomy of clinical observa-tions is explained.

John H. Phillips, M.D.Division of Plastic Surgery

The Hospital for Sick Children555 University Avenue, Room 5429

Toronto, Ontario M5G 1X8, Canadajphillips002@sympatico.ca

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study of mandibular form. Am. J. Phys. Anthropol. 112: 541,2000.

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Online CME CollectionsThis partial list of titles in the developing archive of CME article collections is available online at www.PRSJournal.com. These articles are suitable to use as study guides for board certification, to help readers refamiliarizethemselves on a particular topic, or to serve as useful reference articles. Articles less than 3 years old can be taken for CMEcredit.

Pediatric/Craniofacial

The Use of Perioperative Corticosteroids in Craniomaxillofacial Surgery: A Survey—Themistocles L. Assimesand Lucie M. LassardEndoscopically Assisted Reconstruction of Orbital Medial Wall Fractures—Chien-Tzung Chen et al.Subunit Principles in Midface Fractures: The Importance of Sagittal Buttresses, Soft-Tissue Reductions, andSequencing Treatment of Segmental Fractures—Paul Manson et al.Maxillary Reconstruction: Functional and Aesthetic Considerations—Arshad Muzaffar et al.Cleft Lip: Unilateral Primary Deformities—James D. Burt and H. Steve ByrdOptimal Timing of Cleft Palate Closure—Rod J. Rohrich et al.Efficacy of Preoperative Decontamination of the Oral Cavity—Adam N. Summers et al.Primary Repair of Bilateral Cleft Lip and Nasal Deformity—John B. MullikenCorrection of Secondary Deformities of the Cleft Lip Nose—Samuel Stal and Larry HollierCorrection of Secondary Cleft Lip Deformities—Samuel Stal and Larry HollierCommon Craniofacial Anomalies: The Facial Dystoses—Jeremy A. Hunt and Craig HobarCommon Craniofacial Anomalies: Conditions of Craniofacial Atrophy/Hypoplasia and Neoplasia—JeremyA. Hunt and Craig HobarSubciliary versus Subtarsal Approaches to Orbitozygomatic Fractures—Rod J. Rohrich et al.Management of Craniosynostosis—Jayesh Panchal and Venus UttchinThe Management of Orbitozygomatic Fractures—Larry H. Hollier et al.Common Craniofacial Anomalies: Facial Clefts and Encephaloceles—Jeremy A. Hunt and Craig HobarVelopharyngeal Incompetence: A Guide for Clinical Evaluation—Donnell F. Johns et al.Distraction Osteogenesis of the Craniofacial Skeleton—Jack C. Yu et al.Cleft Rhinoplasty—Allen L. Van Beek et al.The Management of Frontal Sinus Fractures—Reha Yavuzer et al.The Spectrum of Orofacial Clefting—Barry L. Eppley et al.The Pediatric Mandible I: A Primer on Growth and Development—James M. Smartt et al.The Pediatric Mandible II: Management of Traumatic Injury or Fracture—James M. Smartt et al.Two Hundred Ninety-Four Consecutive Facial Fractures in an Urban Trauma Center: Lessons Learned—Patrick Kelley et al.Aesthetic Management of the Nasal Component of Naso-Orbital Ethmoid Fractures—Jason K. Potter et al.Management of Mandible Fractures—David Heath Stacey et al.

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