Biomechanical rationale and evaluation of an implant system for rib fracture fixation

Transcription

1 Eur J Trum Emerg Surg (2010) 36: DOI /s REVIEW ARTICLE Biomechnicl rtionle nd evlution of n implnt system for ri frcture fixtion M. Bottlng S. Wlleser M. Noll S. Honold S. M. Mdey D. Fitzptrick W. B. Long Received: 22 June 2010 / Accepted: 23 August 2010 / Pulished online: 24 Septemer 2010 Ó The Author(s) This rticle is pulished with open ccess t Springerlink.com Astrct Bckground Biomechnicl reserch directed t developing customized implnt solutions for ri frcture fixtion is essentil to reduce the complexity nd to increse the reliility of ri osteosynthesis. Without simple nd relile implnt solution, surgicl stiliztion of ri frctures will remin underutilized despite proven enefits for select indictions. This rticle summrizes the reserch, development, nd testing of specilized nd comprehensive implnt solution for ri frcture fixtion. Methods An implnt system for ri frcture fixtion ws developed in three phses: first, reserch on ri iomechnics ws conducted to etter define the form nd function of ris. Second, reserch results were implemented to derive n implnt system comprising ntomicl pltes nd intrmedullry ri splints. Third, the functionlity of ntomic pltes nd ri splints ws evluted in series of iomechnicl tests. Results Geometric nlysis of the ri surfce yielded set of ntomicl ri pltes tht trced the ri surfce over distnce of cm without the need for plte contouring. Structurlly, the flexile design of ntomic pltes did not increse the ntive stiffness of ris while restoring 77% of the ntive ri strength. Intrmedullry ri splints with M. Bottlng (&) S. Wlleser M. Noll S. Honold S. M. Mdey W. B. Long Biomechnics Lortory, Legcy Clinicl Reserch nd Technology Center, 1225 NE 2nd Avenue, Portlnd, OR 97215, USA e-mil: mottln@lhs.org D. Fitzptrick Slocum Center for Orthopedics nd Sports Medicine, Eugene, OR, USA rectngulr cross-section provided 48% stronger frcture fixtion thn trditionl intrmedullry fixtion with Kirschner wires. Conclusion The ntomic plte set cn simplify ri frcture fixtion y minimizing the need for plte contouring. Intrmedullry fixtion with ri splints provides less-invsive fixtion lterntive for posterior ri frcture, where ccess for plting is limited. The comintion of ntomic pltes nd intrmedullry splints provides comprehensive system to mnge the wide rnge of frctures encountered in flil chest injuries. Keywords Antomic pltes Intrmedullry splints Ri frcture Flil chest Introduction Surgicl stiliztion of ri frctures hs een successfully performed for pin mngement of multiple ri frctures [1], fixtion of chroniclly pinful nonunions [2], reduction of overriding ris [3], nd for stiliztion of flil chest injuries [4, 5]. Prticulrly in the cse of fil chest stiliztion, surgicl fixtion is of gret vlue, s it cn reduce ventiltor time [4, 5], pneumoni [4, 5], mortlity [4], nd medicl costs [5] while gretly improving functionl outcomes nd qulity of life compred to nonopertive mngement [5]. Despite these clinicl enefits nd over 40 yers of clinicl experience, ri frcture fixtion remins n underutilized procedure [3]. In 2009 survey, 77% of surgeons supported the need for ri frcture fixtion for select indictions, ut only 26% of surgeons hd conducted or ssisted in ri frcture fixtion [6]. The survey ttriuted this strk discrepncy in prt to the lck of reserch on optiml fixtion techniques.

2 418 M. Bottlng et l. While vriety of implnts for ri frcture fixtion hve een introduced, iomechnicl studies tht evlute nd optimize their function re rre t est. In the sence of vlidted implnt solution, ri fixtion remins unnecessrily complex nd exhiits persistent complictions nd limittions. Ri plting with stndrd pltes requires timeconsuming nd difficult plte contouring [7]. The high stiffness of stndrd pltes hs een linked to screw pull-out nd persistent discomfort [8 10], requiring hrdwre removl in 5 15% of ptients [10 12]. Furthermore, plting is not well suited for the fixtion of posterior frctures, where ccess is limited [7]. Intrmedullry fixtion with Kirschner wires hs een used for over 40 yers nd llows the stiliztion of posterior frctures in less invsive mnner. However, due to their smll circulr cross-section, Kirschner wires remin prone to wire migrtion nd cut-out [13 17]. In order to ddress these complictions nd limittions, we conducted series of iomechnicl studies to systemticlly develop n dvnced solution for ri frcture fixtion, guided y comprehensive review of the literture tht descries the clinicl experience on ri frcture fixtion over the pst 40 yers. This review identified essentil spects of ri frcture fixtion with pltes nd intrmedullry devices: n dvnced plting solution should provide low-profile fixtion construct [7], reduce the need for intropertive plte contouring [3], llow for spnning nd suspension of flil segment with long pltes [18, 19], mtch the low stiffness of ris to restore physiologic ri function [9, 20], nd it should deliver durle nd strong fixtion. A comprehensive strtegy for ri frcture fixtion should furthermore include n IM fixtion option for stiliztion of posterior frctures tht prevents the implnt migrtion nd cut-out oserved with Kirschner wires. These design spects were susequently integrted into novel implnt system tht comines n dvnced plting option with n intrmedullry (IM) fixtion option to deliver comprehensive solution suitle for the stiliztion of simple ri frctures s well s complex flil chest injuries. This mnuscript summrizes reserch pertining to the three principl phses in the evolution of this novel implnt system: (1) sic reserch on ri iomechnics pertinent to implnt design [21]; (2) extrpoltion of sic reserch results into dvnced plting nd IM solutions; nd (3) iomechnicl evlution of dvnced plting nd IM solutions [22 24]. The results of this reserch provide surgeons with scientific evidence on the function, fetures, nd performnce of this novel implnt system for ri frcture fixtion. Reserch on ri iomechnics Design gols for n dvnced implnt solution were extrcted y reviewing over 80 cse reports nd clinicl studies on ri frcture fixtion. For plte fixtion, the design gol ws ntomiclly shped pltes tht mtch the stiffness of the ntive ri. Such ntomicl pltes would support low profile fixtion, minimize the need for intropertive plte contouring, nd fcilitte the spnning of flil segments with long pltes. Pltes tht mtch the stiffness of ntive ris would furthermore restore physiologic function to decrese the incidence of chest tightness nd fixtion filure ssocited with overly stiff pltes [7, 9]. For IM fixtion, the design gol ws n ntomiclly curved IM device with design fetures tht provide rottionl stility nd tht prevent implnt migrtion or cutout. In order to meet these design gols, three sic reserch studies were conducted, chrcterizing: (1) the ri surfce geometry required for n ntomicl plte design; (2) the ri cross-sectionl geometry required for n IM implnt design; nd (3) the structurl properties of ntive ris, which re required to design durle implnts tht support norml ri function. Ri surfce geometry The ri surfce is twisted nd conicl, which cuses stndrd pltes to diverge from the ri upon ending to the overll ri curvture [21]. To derive n ntomic plte design, the surfce geometry of ris ws chrcterized y three principl prmeters: the generl ri curvture C G, the unrolled curvture C U, nd the longitudinl twist LT long ris. To ssess these three prmeters, the outer surfce of ris 3 9 of eight humn cdvers ws digitized in 2 mm increments. The generl curvture C G of the ris ws clculted from digitized point-triplets for loctions rnging from 15 to 85% of the ri length (0% = tuercle, 100% = costochondrl junction). The unrolled curvture C U descries the in-plne curvture tht plte must hve in order to trce the conicl surfce of ri upon ending to its pprent curvture. C U ws determined y outlining the ri contour on templte conformed to the ri s outer surfce. Susequently, this templte ws unrolled on flt surfce nd the curvture C U of the centerline from 15 to 85% of ri length ws mesured. The longitudinl twist LT of the ri surfce is geometric feture common to ll ris, wherey the right nd left ris re twisted in opposite directions. This twist LT ws quntified from 15 to 85% of the ri length. Results of the surfce geometry nlysis yielded three prmeters essentil for the ntomic contouring of pltes to the ri surfce (Fig. 1). The generl curvture results descried differences in curvture etween ris s well s chnges in curvture long ris, with C G rnging from 3.8 ± 1.5 m -1 t the lterl spect of ri 7 to 17.3 ± 1.7 m -1 t the nterior portion of ri 3. The unrolled curvture C U decresed grdully from ris 3 5,

3 Implnt system for ri frcture fixtion 419 Fig. 1 Three principl geometric prmeters of ris required for plte contouring: the generl curvture C G required for out-of-plne plte ending; the unrolled curvture C U required for inplne plte ending enles pltes to trce the conicl ri surfce; nd c the longitudinl twist LT is required to ensure tht the plte remins prllel to the ri surfce c nd incresed grdully with reverse orienttion from ris 6 9, rnging from 1.1 ± 0.8 m -1 in ri 6 to 6.9 ± 0.7 m -1 in ri 3. The longitudinl twist LT ws notly consistent etween ris, rnging from 41 to 45 in ris 3 7, nd from 58 to 60 in ris 8 nd 9. No sttisticlly significnt difference in twist etween ris ws found (P [ 0.05). In summry, these results delineted the complex ri surfce geometry into three sic prmeters for the contouring of ri pltes. A systemtic pproch for contouring stright plte to the ri requires sequentil ppliction of in-plne ending C U, longitudinl twist LT, nd out-ofplne ending C G. Alterntively, these prmeters cn redily e implemented into n ntomic ri plte design to reduce the time nd complexity of intropertive plte contouring. Ri cross-section geometry In second iomechnicl study, the IM cnl or ris ws chrcterized to support the development of n IM implnt solution tht resists migrtion nd cut-out. The cross-sectionl geometry of ris 3 9 ws exmined in five fresh frozen humn cdvers. Cross-sections of 2 mm thickness were excised t 5, 25, 50, nd 75% of ri length, with the 5% cross-section eing locted posteriorly etween the tuercle nd ngle. Contct rdiogrphs of cross-sectionl specimens were nlyzed to extrct the ri height (h) nd width (w), the cortex thickness (t c ) t the superior, inferior, inner nd outer spects, nd the cross-sectionl re of the medullry cnl (A M ). The results of this study descried for the first time differences in IM cnl size nd shpe etween ris nd long ris (Fig. 2). The cross-sectionl re of the ris ws nerly constnt long the ris. However, the size of the intrmedullry cnl incresed y 38% from posterior to nterior. The results furthermore descried the cortex thickness, which ws 37.5% greter t the inner cortex (1.1 ± 0.5 mm) thn t the outer cortex (0.8 ± 0.4 mm, P \ 0.01). In comintion, these results provide guidnce for the design nd scling of intrmedullry implnts for ri frcture fixtion. Structurl properties of ris In third iomechnicl study, the stiffness nd strength of ntive ris ws determined. Chrcterizing the structurl function of the intct ri is crucil to designing implnts of the pproprite stiffness, since stiffer implnts my not provide etter fixtion. Prticulrly for ri frcture fixtion, the use of overly stiff implnts hs een linked to screw pull-out, fixtion filure, persistent discomfort nd chest tightness [7, 9, 25, 26]. The stiffnesses nd strengths of 20 humn ris 4 9 were ssessed. To replicte qusi-physiologicl loding in

4 420 M. Bottlng et l. Fig. 2 Chnges in cross-sectionl re nd shpe of the intrmedullry cnl long the ri diphysis; ris hve unique ility to tolerte lrge mounts of flexion. The sme ri is shown unloded nd xilly loded mteril test system, polymer spheres of dimeter 25 mm were pplied to oth ends of ech ri specimen, simulting ntomic constrints t the costroverterl nd sternocostl rticultions. The ctutor of the test system pplied xil lods tht induced two-point ending of the ri, representtive of the principl loding mode in vivo [27 29]. The verge stiffness of ris ws 10.2 ± 6.2 N/mm, nd vried y over one order in mgnitude, rnging from 1.5 to 20.1 N/mm. The ris tolerted lrge mount of flexion efore frcturing (Fig. 2), nd frctured t ending moment of 3.0 ± 1.8 N m (rnge N m). The low stiffness of ris, comined with their unique ility to undergo lrge mounts of flexion, emphsizes the need for n implnt design tht supports the physiologicl flexion of ris nd prevents the fixtion filure seen with implnts tht re considerly stiffer thn ris. Implnt design The results of the iomechnicl nlysis of ris were used to derive n implnt system for ri frcture fixtion comprising ntomicl pltes nd intrmedullry implnts (Fig. 3). The flexiility provided y comining plting nd IM solution ws deemed essentil to ccommodte the rnge of frcture ptterns nd frcture loctions encountered in flil chest injuries. Antomicl plting solution Bsed on the results of the ri surfce nlysis, n ntomic plte set ws derived tht ccounts for the three principl prmeters defining the ri surfce: the generl curvture, the in-plne curvture, nd the twist. An nlysis of these prmeters reveled similrities etween ris tht were exploited to reduce their complex surfce geometry into set of six ntomic pltes tht would ccommodte the plting of right nd left ris 3 9. The six pltes vried in in-plne curvture nd twist while hving the sme generl curvture. The plte set ws mnufctured with generl curvture of 5.1 m -1. This generl curvture cn e redily incresed y out-of-plne ending the flexile pltes to ccommodte the incresed curvture of posterior ri segments. To ccount for the conicl ri surfce, the ntomic plte set comprised pltes with vrying in-plne curvtures of up to 5 m -1. To ccommodte for the longitudinl twist of the ri surfce, the three left pltes of the plte set were designed with clockwise twist of 1.5 /cm, nd the right pltes were designed with counterclockwise twist of the sme mgnitude. In order to reproduce the flexiility of ntive ris, lowprofile pltes were designed from elstic titnium. This plte design imed to mtch the stiffness of osteoporotic ris rther thn strong ris. Plte fixtion in osteoporotic one poses the gretest chllenge, wherey overly stiff pltes cuse incresed stress t the screw one interfce nd susequent fixtion filure y screw pull-out. Bending tests of prototype ri pltes demonstrted tht they were over three times less stiff thn titnium sternl locking pltes (Synthes CMF), nd over five times less stiff thn stinless steel 3.5 mm reconstruction pltes (Fig. 3). For durle fixtion in osteoporotic one, the pltes were designed with threded screw holes tht ccommodte locking screws with threded screw heds (Fig. 3c). Upon

5 Implnt system for ri frcture fixtion 421 Fig. 3 Implnt system for ri frcture fixtion comprising set of three left nd three right ntomicl pltes nd intrmedullry ri splints in three sizes; ntomic ri pltes re over three times less stiff thn titnium sternl locking pltes (Synthes CMF), nd over five times less stiff thn stinless steel 3.5 mm reconstruction pltes; c locking screws hve threded hed tht engges in threded plte holes to improve fixtion strength c insertion into the ri, these locking screws securely engge into the threded plte holes nd resist pull-out. Intrmedullry splint solution Bsed on the results of the ri cross-sectionl nlysis, ri splints for intrmedullry fixtion of ri frctures were developed (Fig. 4). The finl ri splint design hs thickness of 1 mm nd rectngulr cross-section to provide rottionl stility nd incresed cut-out resistnce. The ri splint is designed in widths of 3, 4, nd 5 mm to ccommodte the size rnge of the IM cnl otined in the cross-sectionl ri nlysis. The ri splint hs 75 mm long intrmedullry segment to stilize single frcture. The intrmedullry segment hs n out-of-plne curvture of 200 mm to minimize residul stress fter implnt insertion. The splint front section is tpered to reduce the insertion force (Fig. 4). The splint tip is sloped to guide the splint long the medullry cnl without penetrting the lterl cortex (Fig. 4c). Ri splints hve smll extrmedullry segment tht ids insertion nd llows fixtion with single locking screw to prevent implnt migrtion (Fig. 4d). Splints re designed for insertion through lterl entry portl, pplied t distnce of 30 mm from the frcture. A custom insertion tool ws developed tht rigidly connects to the extrmedullry splint segment, llowing for the controlled insertion of the ri splint y tpping onto the insertion tool with mllet (Fig. 4e). Implnt evlution Prototypes of the implnt system were mnufctured nd iomechniclly tested in humn cdveric ris. Antomic pltes were evluted in regrd to their fit to the ri, nd the stiffness, durility nd strength of the plte fixtion constructs were ssessed [23]. IM splints were evluted in direct comprison to the trditionl pproch of IM fixtion with Kirschner wires. Evlution of ntomic pltes First, the longitudinl fit of the ntomic plte set ws ssessed in 109 humn ris y mesuring the plte length l P

6 422 M. Bottlng et l. Fig. 4 The ri splints hve n intrmedullry shft with rectngulr cross-section to provide rottionl stility nd cut-out resistnce, while mintining flexile fixtion;, c the tpered nd sloped splint tip fcilittes insertion nd guides the splint long the intrmedullry cnl; d the ri splints re inserted through n entry portl t distnce of 30 mm from the frcture, nd re secured with locking screw to prevent splint migrtion; e the splints re inserted with custom tool tht cn e rigidly connected to the splint c d e over which pltes trced the ri surfce in the sence of mnul plte contouring (Fig. 5). To ssess the enefits of ntomic pltes over stndrd pltes, the sme mesurement ws otined for stndrd pltes tht were ent to the generl curvture of the ris ut tht hd no in-plne curvture or twist. The results demonstrted tht the ntomic pltes could trce the surfce of ris 3 9 over plting length l P rnging from 12.5 to 14.7 cm without the need for contouring. Compred to stndrd pltes, the ntomic pltes significntly incresed the plting length l P y 79% for ri 3, y 67% for ri 4, nd y 65% for ri 9. In ddition to the longitudinl fit, the surfce fit of the ntomic pltes ws ssessed. The congruency etween the ri surfce nd the plte surfce ws mesured y compring the ntomic twists of 8 nd 16 cm long ri segments with the twists of 8 nd 16 cm long sections of the ntomic pltes. An 8 cm long plte is suitle for spnning single frcture, while the 16 cm plte is suitle for spnning multiple frctures of flil segment. This nlysis demonstrted tht the surfce of ris 3 9 twisted on verge y 8 ± 13 over n 8 cm segment, nd y 33 ± 11 over 16 cm segment. The ntomic pltes pproximted the twist of the ri surfce within 3.7 on verge for n 8 cm long plte, nd within 8.7 on verge for 16 cm long plte (Fig. 5). These findings demonstrte tht smll set of ntomic ri pltes cn minimize the need for intropertive plte contouring y providing n incresed plting length l P over which pltes trce the ri surfce, nd y pproximting the twist of the ri surfce. Antomic ri pltes cn therefore reduce the time nd complexity of ri frcture fixtion, nd fcilitte the spnning of flil segments with long pltes. Furthermore, the inherent congruency etween ntomic pltes nd ris is essentil to chieving lowprofile fixtion constructs nd will contriute to durle fixtion with screws tht cn relily e inserted long the ri midline. The mechnicl function of ntomic plte constructs ws chrcterized y ssessing construct stiffness, durility, strength, nd filure modes in 20 humn cdveric ris (donor ge: 69 ± 19 yers). Ri segments were prepred for loding in the form of the two-point ending

7 Implnt system for ri frcture fixtion 423 Fig. 5 Antomic pltes with in-plne curvture cn trce the conicl ri surfce over longer plte distnce l P thn pltes without in-plne curvture, which thus tend to diverge from the ri surfce; the improved congruency provided y the ntomic plte twist compred with stndrd plte without twist is depicted to scle for 8 nd 16 cm long pltes representtive of the principl physiologicl loding mode, s previously descried for stiffness ssessment in ntive ris (Fig. 6). Specimens were sujected to sequence of four tests to determine the strength of intct ris, the stiffness of plte constructs, the durility of plte constructs under exggerted dynmic loding, nd the residul strength nd filure mode of constructs fter dynmic loding. First, the ris were loded to filure to determine the strength of the intct ris nd to induce cliniclly relistic frcture pttern. Second, frctures were stilized with 7-hole ntomic pltes, using three icorticl locking screws on ech side of the frcture while retining one empty screw hole over the frcture. The stiffness of plte constructs ws ssessed equivlent to the stiffness ssessment of ntive ris. Third, the plte constructs were dynmiclly loded for 360,000 cycles to simulte respirtory loding history until frcture stiliztion y cllus formtion cn e expected [26, 30]. Dynmic loding ws pplied with n exggerted respirtory loding mgnitude of 200 N mm, representing five times the ending moment mesured in vivo on humn ris during physiologic respirtion [26, 28]. Fourth, ri fixtion constructs were qusi-stticlly loded to filure to determine their residul strength nd to nlyze the filure mode. The results demonstrted tht the stiffness of the plte constructs (7 ± 4 N/mm) remined on verge elow the stiffness of ntive ris (10 ± 6 N/mm). The finding tht the pltes did not increse the stiffness of the ris held true for the wekest ri tested (1.5 N/mm ri stiffness, 1.3 N/mm construct stiffness) s well s for the strongest ri tested (20 N/mm ri stiffness, 11 N/mm construct stiffness). All plting constructs survived exggerted dynmic loding without encountering screw loosening or fixtion filure. Susequent loding to filure yielded residul strength for plte constructs of 2.30 ± 1.17 N m, demonstrting tht plting restored 77% of the strength of ntive ris (2.97 ± 1.80 N m). This residul construct strength Fig. 6 Test setup used to evlute the stiffness, durility nd strength of fixtion constructs under xil loding controlled y mteril test system; polymer spheres simulte physiologic constrints t the costroverterl nd sternocostl rticultions ws 58 times greter thn physiologic lods during norml respirtion. Eighteen of the 20 constructs filed y ri frcture djcent to the plte end, nd two constructs filed y plte ending over the frcture. Mechnicl testing results demonstrted tht flexile ri pltes did not increse the ntive stiffness of ris, regrdless of whether the frctures were stilized in osteoporotic or strong ris. By comining flexile plting with locking screw fixtion, the ntomic ri pltes effectively prevented screw loosening nd pull-out while restoring 77% of the ntive ri strength. Evlution of IM splints The mechnicl function of the ri splint constructs ws chrcterized nlogously to tht of the ntomic plte constructs y ssessing construct stiffness, durility, strength, nd filure modes in 22 pired ris [24]. Pired

8 424 M. Bottlng et l. Fig. 7 Ri frctures stilized with 1.5 mm Kirschner wire nd ri splint; c ctstrophic filure of Kirschner-wire construct y cutting through the medil cortex, cusing loss of reduction, instility, nd protrusion of the wire from the ri; d filure of the ri splint construct y frcture long the superior nd inferior cortices, shown in the stressed position; e fter lod removl, splint constructs recovered elsticlly nd retined functionl reduction nd fixtion, suggesting tht this filure mode would remin cliniclly symptomtic c d e testing ws performed to llow for direct comprison in mechnicl function etween ri splint constructs nd conventionl Kirschner-wire constructs for IM stiliztion of ri frctures. Frctures in right ris were stilized with 80 mm long Kirschner wires of 1.5 mm dimeter tht were inserted through n entry portl t distnce of 30 mm from the frcture (Fig. 7). Frctures in left ris were stilized with 4 mm wide ri splints (Fig. 7). There ws no significnt difference in stiffness etween the ri splint constructs (2.0 ± 1.0 N/mm) nd the Kirschner-wire constructs (2.5 N/mm, P [ 0.05). All constructs sustined dynmic loding without filure. After dynmic loding, the residul strength of the ri splint constructs remined 48% greter thn tht of the Kirschner-wire constructs, nd ws 26 times greter thn the ending lods under physiologic respirtion [28]. Five of the 11 Kirschner-wire constructs filed ctstrophiclly y cutting through the medil cortex, leding to complete loss of stility nd wire migrtion through the lterl cortex (Fig. 7c). In contrst, no splint construct filed ctstrophiclly nd ll of the splint constructs retined functionl reduction nd fixtion (Fig. 7d, e). In summry, ri splints provided superior strength nd prevented the complictions of implnt migrtion nd cutout seen with Kirschner wires. Discussion This line of reserch descried the ntomic foundtion, design fetures, nd iomechnicl evlution of novel implnt system for the stiliztion of ri frctures. By comining ntomic pltes nd intrmedullry splints, this system provides comprehensive solution tht ccommodtes the lrge vriety of frcture ptterns nd frcture loctions encountered in complex flil chest injuries. Most recently, this system hs een further refined nd hs een mde ville for clinicl use y Synthes CMF (Mtrix- RIB, Synthes, West Chester, PA, USA). The system represents conservtive solution sed on estlished techniques which were systemticlly enhnced to support the unique requirements for ri frcture fixtion while preventing the complictions reported for the trditionl techniques. Plte osteosynthesis nd intrmedullry fixtion historiclly represent the two most common techniques for ri frcture stiliztion. Compred to trditionl plting, the ntomic plte set reduces the chllenge of intropertive plte contouring, provides flexile stiliztion, nd employs locking screws to enhnce fixtion in osteoporotic ris. It therefore not only reduces the time nd complexity of the opertive procedure ut lso provides durle, low-profile fixtion with decresed need for implnt removl. Antomic pltes lso support the use of long pltes to llow the ridging of comminuted frctures, the spnning of multiple frctures, nd the suspension of fil segments [18, 19]. The results of the plte fit evlution demonstrted tht the ntomic plte set lrgely elimintes the need for intropertive plte contouring y providing the pproprite in-plne curvture nd longitudinl twist. Locking pltes provide improved fixtion strength y rigidly connecting locking screws to the plte nd ri, while conventionl plting constructs rely on plte compression onto the ri surfce to chieve stle fixtion [31]. By eliminting the need for plte compression to the one surfce, locking pltes support iologicl fixtion while preserving

9 Implnt system for ri frcture fixtion 425 periostel perfusion [32, 33]. Locking pltes provide stle fixtion even if the plte is not perfectly contoured to the ri surfce. However, to void the unintended elevtion of locking pltes over the ri surfce, the pltes should e pproximted to the ri surfce efore the hed of the locking screw engges into the plte. Ri splints represent n enhnced version of the trditionl pproch of intrmedullry ri fixtion with Kirschner wires. The iomechnicl evlution of the ri splint constructs demonstrted tht the design fetures of the ri splint relily prevented the implnt migrtion nd cut-out seen with Kirschner wires, while delivering improved construct strength. Therefore, ri splints re n ttrctive intrmedullry lterntive for the less-invsive stiliztion of ri frctures, especilly in the cse of posterior ri frctures, where ccess for plting is limited. Unlike ntomic ri pltes, ri splints re not designed to spn multiple or severely comminuted frctures. While ri splints enle less-invsive pproch compred to plte fixtion, sufficient ccess is required to ensure tht ri splints cn e inserted tngentil to the ri surfce. Both ntomic plte nd ri splint implnts re designed for flexile fixtion in order to provide durle stiliztion, to restore ri function, nd to promote frcture heling y cllus formtion. Elstic implnts with low stiffness cn minimize pek stresses t the one implnt interfce, mking them prticulrly suitle for frcture fixtion in osteoporotic one [20]. Specific for ri fixtion, Litzke et l. [9] emphsized tht stiff implnts cn restrict respirtory motion nd re prone to fixtion filure due to stress concentrtions. Unlike implnts tht re primrily designed for lod ering, they stted tht the principl function of ri implnts is to restore chest wll integrity y mintining frcture pposition without restricting respirtory kinemtics, which requires elstic fixtion constructs. In regrd to frcture heling, flexile fixtion enles interfrgmentry motion, which in turn promotes cllus formtion nd ony union [32]. Conversely, rigid fixtion cn suppress heling nd cn led to one resorption [34]. The mechnicl evlution of ntomic plte nd ri splint constructs hs severl limittions. Implnts were only tested in one loding mode, representing the principl loding of ris in vivo [28]. Testing ws limited to the fixtion of single trnsverse or olique frctures, nd did not ccount for severely comminuted frctures. Furthermore, constructs were tested in isoltion without ccounting for secondry stiliztion provided y djcent ris nd the surrounding soft-tissue envelope. To overcome the limittions inherent to ny iomechnicl study, prospective clinicl study will e essentil to further evlute the performnce of MtrixRIB implnts in vivo. In conclusion, reserch on ri iomechnics fforded detiled understnding of the form nd function of ris, which served s the foundtion for the design of specilized system for ri frcture fixtion. The resulting system comines set of ntomic pltes nd intrmedullry splints to ccommodte the rnge of frctures seen in complex flil chest injuries. By resolving the principl limittions nd complictions encountered with trditionl plting nd IM fixtion techniques, this comprehensive system for ri frcture fixtion will simplify the surgicl procedure, provide more relile stiliztion, nd will likely e etter tolerted y ptients. Conflict of interest One or more of the uthors receive consulting/ roylty pyments from Synthes CMF relted to technology discussed. Open Access This rticle is distriuted under the terms of the Cretive Commons Attriution Noncommercil License which permits ny noncommercil use, distriution, nd reproduction in ny medium, provided the originl uthor(s) nd source re credited. References 1. Ccchione RN, Richrdson JD, Seligson D. Pinful nonunion of multiple ri frctures mnged y opertive stiliztion. J Trum. 2000;48(2): Ng AB, Ginnoudis PV, Bismil Q, Hinsche AF, Smith RM. Opertive stilistion of pinful non-united multiple ri frctures. Injury. 2001;32(8): Richrdson JD, Frnklin GA, Heffley S, Seligson D. Opertive fixtion of chest wll frctures: n underused procedure? Am Surg. 2007;73(6): Grnetzny A, Ad El-Al M, Emm E, Shly A, Boseil A. Surgicl versus conservtive tretment of flil chest. Evlution of the pulmonry sttus. Interct Crdiovsc Thorc Surg. 2005;4(6): Tnk H, Yukiok T, Ymguti Y, Shimizu S, Goto H, Mtsud H, Shimzki S. Surgicl stiliztion of internl pneumtic stiliztion? A prospective rndomized study of mngement of severe flil chest ptients. J Trum. 2002;52(4): Myerry JC, Hm LB, Schipper PH, Ellis TJ, Mullins RJ. Surveyed opinion of Americn trum, orthopedic, nd thorcic surgeons on ri nd sternl frcture repir. J Trum. 2009;66(3): Engel C, Krieg JC, Mdey SM, Long WB, Bottlng M. Opertive chest wll fixtion with osteosynthesis pltes. J Trum. 2005;58(1): Friedrich B, Redeker H, Kljucr S. The unstle thorcic wll: possiilities for tretment. Helv Chir Act. 1991;58(1 2): Litzke R. Erly thorcotomy nd chest wll stiliztion with elstic ri clmps (uthor s trnsl). Zentrll Chir. 1981;106(20): Lrdinois D, Krueger T, Dusmet M, Ghislett N, Gugger M, Ris HB. Pulmonry function testing fter opertive stilistion of the chest wll for flil chest. Eur J Crdiothorc Surg. 2001;20(3): Pris F, Trzon V, Blsco E, Cnto A, Csills M, Pstor J, Pris M, Montero R. Surgicl stiliztion of trumtic flil chest. Thorx. 1975;30(5): Voggenreiter G, Neudeck F, Aufmkolk M, Oertcke U, Schmit- Neuerurg KP. Opertive chest wll stiliztion in flil chest outcomes of ptients with or without pulmonry contusion. J Am Coll Surg. 1998;187(2):130 8.

10 426 M. Bottlng et l. 13. Ahmed Z, Mohyuddin Z. Mngement of flil chest injury: internl fixtion versus endotrchel intution nd ventiltion. J Thorc Crdiovsc Surg. 1995;110(6): Alrecht F, Brug E. Stiliztion of the flil chest with tension nd wires of ris nd sternum (uthor s trnsl). Zentrll Chir. 1979;104(12): Menrd A, Testrt J, Philippe JM, Grise P. Tretment of flil chest with Judet s struts. J Thorc Crdiovsc Surg. 1983;86(2): Moore BP. Opertive stiliztion of nonpenetrting chest injuries. J Thorc Crdiovsc Surg. 1975;70(4): Shh TJ. On internl fixtion for flil chest. J Thorc Crdiovsc Surg. 1996;12(3): Hsler GB. Open fixtion of flil chest fter lunt trum. Ann Thorc Surg. 1990;49(6): Snchez-Lloret J, Letng E, Mteu M, Cllejs MA, Ctln M, Cnlis E, Mestres CA. Indictions nd surgicl tretment of the trumtic flil chest syndrome. An originl technique. Thorc Crdiovsc Surg. 1982;30(5): Lill H, Hepp P, Korner J, Kssi JP, Verheyden AP, Josten C, Dud GN. Proximl humerl frctures: how stiff should n implnt e? A comprtive mechnicl study with new implnts in humn specimens. Arch Orthop Trum Surg. 2003;123(2 3): Mohr M, Arms E, Engel C, Long WB, Bottlng M. Geometry of humn ris pertinent to orthopedic chest-wll reconstruction. J Biomech. 2007;40(6): Bottlng M, Helzel I, Long W, Fitzptrick D, Mdey S. Lessinvsive stiliztion of ri frctures y intrmedullry fixtion: iomechnicl evlution. J Trum. 2010;68(5): Bottlng M, Helzel I, Long WB, Mdey S. Antomiclly contoured pltes for fixtion of ri frctures. J Trum. 2010;68(3): Helzel I, Long W, Fitzptrick D, Mdey S, Bottlng M. Evlution of intrmedullry ri splints for less-invsive stilistion of ri frctures. Injury. 2009;40(10): Mouton W, Lrdinois D, Furrer M, Regli B, Ris HB. Long-term follow-up of ptients with opertive stilistion of flil chest. Thorc Crdiovsc Surg. 1997;45(5): Reer PU, Kniemeyer HW, Ris HB. Reconstruction pltes for internl fixtion of flil chest. Ann Thorc Surg. 1998;66(6): Grnik G, Stein I. Humn ris: sttic testing s promising medicl ppliction. J Biomech. 1973;6(3): Rehm KE. Die Osteosynthese der Thorxwndinstiliteten. Hefte zur Unfllheilkunde 1986; Sles JR, Ellis TJ, Gillrd J, Liu Q, Chen JC, Hm B, Myerry JC. Biomechnicl testing of novel, minimlly invsive ri frcture plting system. J Trum. 2008;64(5): Bulger EM, Arneson MA, Mock CN, Jurkovich GJ. Ri frctures in the elderly. J Trum. 2000;48(6): discussion Fitzptrick DC, Doornink J, Mdey SM, Bottlng M. Reltive stility of conventionl nd locked plting fixtion in model of the osteoporotic femorl diphysis. Clin Biomech (Bristol, Avon). 2009;24(2): Perren SM. Evolution of the internl fixtion of long one frctures. The scientific sis of iologicl internl fixtion: choosing new lnce etween stility nd iology. J Bone Joint Surg. 2002;84(8): Tn SL, Blogh ZJ. Indictions nd limittions of locked plting. Injury. 2009;40(7): Uhthoff HK, Poitrs P, Bckmn DS. Internl plte fixtion of frctures: short history nd recent developments. J Orthop Sci. 2006;11(2):