Ots en fx supraintercondileas en osteoporsis

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2010;92:1442-1452. doi:10.2106/JBJS.H.01722 J Bone Joint Surg Am.Thomas Mückley Dirk Wähnert, Konrad L. Hoffmeier, Geert von Oldenburg, Rosemarie Fröber, Gunther O. Hofmann and

BoneInternal Fixation of Type-C Distal Femoral Fractures in Osteoporotic

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Internal Fixation of Type-C Distal FemoralFractures in Osteoporotic Bone

By Dirk Wahnert, MD, Konrad L. Hoffmeier, Dipl-Ing, Geert von Oldenburg, Dipl-Ing, Rosemarie Frober, MD,Gunther O. Hofmann, MD, Dr rer nat, and Thomas Muckley, MD

Investigation performed at the Department of Traumatology, Hand and Reconstructive Surgery, Friedrich Schiller University Jena, Jena, Germany

Background: Fixation of distal femoral fractures remains a challenge, especially in osteoporotic bone. This study wasperformed to investigate the biomechanical stability of four different fixation devices for the treatment of comminuteddistal femoral fractures in osteoporotic bone.

Methods: Four fixation devices were investigated biomechanically under torsional and axial loading. Three intramed-ullary nails, differing in the mechanism of distal locking (with two lateral-to-medial screws in one construct, one screw andone spiral blade in another construct, and four screws [two oblique and two lateral-to-medial with medial nuts] in thethird), and one angular stable plate were used. All constructs were tested in an osteoporotic synthetic bone model of anAO/ASIF type 33-C2 fracture. Two nail constructs (the one-screw and spiral blade construct and the four-screw construct)were also compared under axial loading in eight pairs of fresh-frozen human cadaveric femora.

Results: The angular stable plate constructs had significantly higher torsional stiffness than the other constructs; theintramedullary nail with four-screw distal locking achieved nearly comparable results. Furthermore, the four-screw distallocking construct had the greatest torsional strength. Axial stiffness was also the highest for the four-screw distal lockingdevice; the lowest values were achieved with the angular stable plate. The ranking of the constructs for axial cycles tofailure was the four-screw locking construct, with the highest number of cycles, followed by the angular stable plate, thespiral blade construct, and two-screw fixation. The findings in the human cadaveric bone were comparable with those inthe synthetic bone model. Failure modes under cyclic axial load were comparable for the synthetic and human bonemodels.

Conclusions: The findings of this study support the concept that, for intramedullary nails, the kind of distal interlockingpattern affects the stabilization of distal femoral fractures. Four-screw distal locking provides the highest axial stabilityand nearly comparable torsional stability to that of the angular stable plate; the four-screw distal interlocking constructwas found to have the best combined (torsional and axial) biomechanical stability.

Clinical Relevance: The enhanced distal interlocking stability of four-screw distal locking construct may be beneficial tothe stabilization of complex distal femoral fractures, especially in osteoporotic bone.

Since the late 1970s, distal femoral fractures, especiallyunstable or intra-articular patterns, have been managedwith surgery. Open reduction and internal fixation with

a blade plate, as recommended by the AO/ASIF (Associationfor the Study of Internal Fixation) in the mid-1970s, soonbecame the gold standard. However, over the past decades,intramedullary devices have been increasingly used in themanagement of distal femoral fractures1,2. The distal third ofthe femur is involved in 6% of femoral fractures3. In these

cases, there are apparently two different patient groups. Thefirst group consists of younger people—predominantly menwho are less than forty years old—with high-velocity trauma.The second group consists of older people—predominantlywomen—with osteoporotic fractures as a result of low-velocitytrauma4-6.

The AO/ASIF-defined goals7 of restoration of the anat-omy, stable fracture fixation, preservation of blood supply, andearly mobilization are demanding and difficult to attain in

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants inexcess of $10,000 from Stryker Trauma GmbH. In addition, one or more of the authors or a member of his or her immediate family received, in any oneyear, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Stryker TraumaGmbH).

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COPYRIGHT � 2010 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED

J Bone Joint Surg Am. 2010;92:1442-52 d doi:10.2106/JBJS.H.01722

osteoporotic bone stock, which often does not allow stabledevice fixation8-11. In response to this challenge, novel implants(intramedullary nails with special distal interlocking features,and angular stable devices) and modified (less invasive) sur-gical techniques have been developed.

This study was performed to investigate the biome-chanical stability of different fixation devices for the treatmentof comminuted distal femoral fractures. We evaluated the ef-fect of three intramedullary distal locking methods in com-parison with a locking plate on fracture fixation stability inosteoporotic bone, using both a cadaveric model and a syn-thetic osteoporotic bone surrogate model.

Materials and MethodsImplants

Three titanium-alloy intramedullary nails with different in-terlocking configurations in the distal fragment were eval-

uated. They included (1) a 320-mm-long and 12-mm-diameterfemoral nail (T2 Femoral Nail [T2]; Stryker, Schonkirchen,Germany), with two lateral-to-medial 5-mm-diameter partiallythreaded locking screws (Fig. 1, a); (2) a 320-mm-long and12-mm-diameter supracondylar nail (T2 Supracondylar Nail[SCN]; Stryker), with one anteromedial-to-posterolateral andone anterolateral-to-posteromedial 5-mm interlocking screw inaddition to two lateral-to-medial condylar bolts (Fig. 1, b); and(3) a 320-mm-long and 12-mm-diameter distal femoral nail(DFN; Synthes, Umkirch, Germany), with a 6-mm fully threadedscrew and a lateral-to-medial spiral blade (Fig. 1, c). Proximally,the three intramedullary devices were locked in the femoralshaft with two anterior-to-posterior screws. The fourth devicewas a steel-alloy 310-mm-long fourteen-hole angular stableplate (AxSOS; Stryker, Duisburg, Germany) anchored in thedistal fragment with two lag screws and five angular stablescrews. Proximal plate fixation to the femoral shaft was by

Fig. 1

Distal interlocking patterns of the devices used in this study. a: T2 Femoral Nail

(T2). b: Supracondylar Nail (SCN). c: Distal Femoral Nail (DFN). d: Angular stable

plate (AxSOS).

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TH E J O U R N A L O F B O N E & JO I N T SU R G E RY d J B J S . O R G

VO LU M E 92-A d NU M B E R 6 d J U N E 2010IN T E R N A L F I X AT I O N O F TY P E -C DI S TA L FE M O R A L F R AC T U R E S

I N OS T E O P O R O T I C B O N E

means of two angular stable screws and two cortical screws(AxSOS) (Fig. 1, d). All constructs were performed as recom-mended by the implant manufacturer.

Fracture ModelThe biomechanical properties of the constructs were in-vestigated with a simulated AO/ASIF type 33-C2 fracture12

(Fig. 2). The pattern was produced with use of a transverseosteotomy, which created a 1.5-cm fracture gap at a point6.5 cm proximal to the joint line, and a sagittal intercondylarosteotomy.

Synthetic Bone ModelThird-generation composite femora (Sawbones Europe, Malmo,Sweden) were modified by the replacement of their distal,condylar portion with anatomic polyurethane foam condylesfabricated at our laboratory with use of H200-AT two-component structural foam (Vosschemie, Uetersen, Germany).A mean foam density (and standard deviation) of 150 ± 5 kg/m3

was chosen to simulate osteoporotic bone stock13 and to morerealistically model construct failure conditions in the con-dylar region. Each implant was tested under torsional andaxial loading with use of five synthetic bone specimens, for atotal of forty synthetic bone specimens. Separate specimenswere used for torsional and axial loading tests.

Human Cadaveric Bone ModelEight pairs of fresh-frozen human femora, harvested postmortem with consent, were used for a matched-pair com-parison of the axial cycles to failure of the SCN and DFNconstructs. The mean age of the donors (four male and fourfemale) at the time of death was 74 ± 9 years. The femorawere completely stripped of soft tissues. The bones under-

went biplanar radiography to rule out bone disease, andbone mineral density was determined with dual x-ray ab-sorptiometry (Lunar Prodigy Advance; GE Healthcare,Madison, Wisconsin). The specimens were stored in vacuumpacks at –27�C and were thawed at room temperature fortwenty-four hours before testing. The specimen lengths of thefemora were standardized to 455 mm by cutting the shaft ifrequired.

Test SetupThe test setup, in particular the principle of eccentric axialloading, was adapted from Milne and Latta14. The specimens

Fig. 2

Schematic of the fracture model (AO/ASIF type 33-

C2) that was used in this study.

Fig. 3

Test setup showing eccentric loading of the specimen, with

1 indicating the upper universal joint; 2, proximal fixation;

3, anatomic axis; 4, mechanical axis; 5, a specimen; 6,

distal fixation with polyethylene insert and cage; and 7, the

lower universal joint (fixed to constrain the specimens to

coronal plane movement—only medial-lateral translation

was possible).

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were mounted, by means of two universal joints, on a biaxialelectrohydraulic material testing machine (model 8874; Ins-tron, High Wycombe, Bucks, United Kingdom) in such a wayso as to align the mechanical axis of the femur with the ma-chine axis (eccentric loading) (Fig. 3). The upper joint allowedtilting in the coronal and sagittal planes, and the lower jointwas fixed to constrain the specimens to coronal plane move-ment such that only medial-lateral translation was possible.The distal end of the specimen was mounted on an ultra-highmolecular weight polyethylene tibial insert of a total knee re-placement. During the torsion tests, the distal, condylar por-tion was additionally restricted from rotation by a metal cagewith four set screws. The screws secured the distal portion byclamping the condyles from the front and back. The set screwpositions were arranged such that they would not contact or

otherwise interact with the implant. The proximal portions ofthe synthetic femora were embedded, over a distance of 10 cm,in a split aluminum-polyurethane resin shell. The human ca-daveric specimens were proximally potted in a two-componentcast resin (RenCast FC 52; Huntsman Advanced Materials,Monthey, Switzerland) inside a split aluminum shell.

Time, displacement, load, angle, moment, and numberof cycles were acquired and plotted with use of MAX software(version 9.2; Instron, Canton, Massachusetts).

Test ProtocolSynthetic Bone and Human Cadaveric SpecimensFor torsional loading, a static compressive axial load of 20 Nwas applied and the specimens were cyclically torqued for tencycles, in external and internal rotation around the mechanical

TABLE I Torsional Test Results for the Synthetic Bone Model Constructs*

T2 SCN DFN AxSOS

Property MeanStandardDeviation

StandardError ofMean Mean

StandardDeviation


StandardDeviation


StandardDeviation

StandardError ofMean

Stiffness(Nm/deg)

3.11 0.44 0.21 3.90 0.37 0.18 1.50 0.16 0.03 4.42 0.34 0.16

Range ofmotion(deg)

6.33 0.90 0.43 5.03 0.47 0.22 13.2 1.41 0.31 4.17 0.34 0.16

Neutralzone(deg)

1.34 0.45 0.20 0.89 0.28 0.13 4.98 1.80 0.45 0.75 0.22 0.10

*T2 = T2 Femoral Nail, SCN = Supracondylar Nail, DFN = Distal Femoral Nail, and AxSOS = angular stable plate.

Fig. 4

Torsional stiffness of the constructs used in the synthetic bone model, with the values

given as the mean and the standard deviation. All differences were significant (p < 0.001).

The sample size included five specimens per construct. T2 = T2 Femoral Nail, SCN =

Supracondylar Nail, DFN = Distal Femoral Nail, and AxSOS = angular stable plate.

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axis of the femoral shaft, to 10 Nm, at a loading rate of 1 Nm/s.Torsional stiffness, range of motion, and neutral zone, as de-fined by Wilke et al.15, were obtained. The torsional stiffnesswas calculated with regression analysis of the linear portion ofthe torque-angular rotation curve. The range of motion re-flects the deformation, in degrees, under a given maximumtorque (in this study, at ±10 Nm), and the neutral zone rep-resents the laxity of the construct, which we calculated in acorridor of ±0.5 Nm around 0 Nm. For these parameters, themean of the ten cycles was calculated.

Following torsional testing, static external rotation wasapplied to failure, at a rate of 1 Nm/s, to determine the ultimatetorsional strength of the constructs. With the test setup used inthis study, ultimate static torsional strength could be deter-mined for the nailed constructs only. During torsional loadingof the constructs with the AxSOS plate, contact occurred be-tween the plate and the metal cage of the antirotation fixture,inhibiting the calculation of the ultimate torsional strength.

Cyclic axial loading was performed incrementally,starting with an axial load oscillating between 20 N and 200 N,

Fig. 5

Range of motion (ROM) and neutral zone (NZ) of the constructs used in the synthetic bone

model, with the values given as the mean and the standard deviation. All differences were

significant (p < 0.001). T2 = T2 Femoral Nail, SCN = Supracondylar Nail, DFN = Distal

Femoral Nail, and AxSOS = angular stable plate.

Fig. 6

The initial axial stiffness of the constructs in the synthetic bone and in the human ca-

daveric bone model. All differences were significant (p < 0.001). T2 = T2 Femoral Nail,

SCN = Supracondylar Nail, DFN = Distal Femoral Nail, and AxSOS = angular stable plate.

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at a rate of 2 Hz, and raising the lower limit by 10 N and theupper limit by 100 N after each 500-cycle step. After com-pleting a loading level, the machine was manually set to themean of the subsequent load level. The necessary readjustmentrequired a pause of less than thirty seconds.

The initial axial stiffness of the constructs was obtainedfrom cycles 50 through 59 of the first cyclic loading step. For thatpurpose, the slope of the linear area of the load-displacementcurve was calculated for each of these cycles by linear regressionand then we determined the mean and standard deviation foreach construct.

Construct failure as a result of cyclic loading was definedas deformation of ‡5 mm at an axial load of 190 N. Thedeformation was determined from machine displacement. Thenumber of cycles when 5 mm of deformation was achieved wasrecorded.

Human Cadaveric SpecimensThe SCN and the DFN constructs were also evaluated in eightpairs of human cadaveric femora to study the cyclic axialstiffness and strength of the constructs in a matched paircomparison. The cyclic testing protocol was the same as thatfor the synthetic femora.

For the human cadaveric bone constructs, failure wasdefined as deformation of ‡2.5 mm because the SCN con-structs only reached a mean of 3.15 mm of deformation after12,500 cycles. To compare the results of the human and thesynthetic bone, we used the 2.5-mm criterion for both.

Statistical AnalysisData analysis was conducted with use of Excel (Microsoft 2003;Microsoft, Unterschleissheim, Germany). Regression analyseswere performed for calculation of the mean and standard

deviation of torsional and axial stiffness. For statistical analysis,SPSS for Windows (version 13.00; SPSS, Chicago, Illinois)was used. Torsional stiffness and strength and axial stiffnessand strength were analyzed for normal distribution with theKolmogorov-Smirnov and the Shapiro-Wilk test. For our data,no property demonstrated a normal distribution in any group.Thus, the groups were tested for differences with use of theMann-Whitney test. Significance was set at p < 0.05.

Source of FundingThis study received outside funding from Stryker Trauma(Schonkirchen-Kiel, Germany). They supported us by paying forall implants and synthetic bone models. Also, a donation for thehuman cadaveric bones was given to the Institute of Anatomy.

ResultsSynthetic Bone ModelTorsional Tests

The AxSOS plate construct achieved the greatest torsionalstiffness (p < 0.001). The SCN construct had 88% of the

torsional stiffness of the AxSOS plate construct; the T2 con-struct, 70%; and the DFN construct, 34% (Fig. 4). Of theintramedullary nails, the SCN and the T2 femoral nail pro-duced significantly stiffer constructs than did the DFN nail(p < 0.001; Table I).

The AxSOS plate construct had the smallest range ofmotion (4.17�; p < 0.001) of all of the constructs tested. Of theintramedullary nails, the SCN construct had the smallest rangeof motion (p < 0.001), which was, however, 21% greater thanthat of the AxSOS construct. The mean range of motion for theT2 construct was 52%, and that for the DFN construct was217%, of that of the AxSOS plate construct. All differenceswere significant, with p < 0.001 (Fig. 5, Table I).

Fig. 7

The mean number of cycles to failure (‡5 mm of deformation) in the synthetic bone

model. The sample size included five specimens per construct. T2 = T2 Femoral Nail,

SCN = Supracondylar Nail, DFN = Distal Femoral Nail, and AxSOS = angular stable plate.

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The AxSOS plate construct had the smallest neutral zone(p < 0.001), whereas the neutral zone of the DFN constructwas 6.7 times larger than that of the plate constructs. The SCNnail and T2 constructs had a significantly smaller neutral zonethan the DFN construct (p < 0.001; Fig. 5; Table I).

With regard to static ultimate torsional strength, fail-ure first occurred in the T2 construct (38 ± 14 Nm), fol-lowed by the DFN construct (40 ± 5 Nm). The SCN

construct did not fail at the torque cell limit of 100 Nm.With use of the torque cell limit, the SCN construct had thehighest static ultimate torsional strength, which was signif-icantly higher than that for the other two constructs (p =0.008). The T2 and DFN constructs failed by disruption ofthe distal bone model in the frontal plane. All specimensfailed by splitting first from medial to lateral and then fromdistal to proximal.

Fig. 8

Failure modes in the incremental cyclic axial test (synthetic bone model). The T2 construct (a) and

DFN construct (b), showing widening of the distal fracture gap and proximal displacement of the

medial condyle (top) and disruption of the medial condyle in the coronal plane (bottom). The SCN

construct (c) and AxSOS angular stable plate (d) showing loosening of the distal interlocking device.

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Incremental Cyclic Axial TestsThe SCN construct achieved the greatest initial axial stiffness(280 ± 10 N/mm; p < 0.001, Fig. 6) followed by the DFNconstruct (257 ± 8 N/mm) and then the T2 construct (235 ±15 N/mm). The AxSOS plate construct achieved 111 ± 11 N/mm, which was only 40% of the stiffness of the SCN con-structs, and thus had the least axial stiffness (Fig. 6). All dif-ferences were significant (p < 0.001).

With regard to axial cycles to failure, the SCN constructhad the highest axial cyclic deformation limit, with a mean of6212 ± 909 cycles to failure and a mean failure load of 1400 ±122 N (p = 0.008, Fig. 7). The AxSOS plate construct with-stood 3504 ± 750 cycles, thus achieving 56% of the axial cyclesto failure of the supracondylar nail construct (p = 0.008). Thismade the AxSOS plate construct significantly stronger in thistesting protocol compared with the other two nails (p = 0.032for the DFN construct, and p = 0.008 for the T2 construct).The DFN and the T2 constructs had the lowest number of axialcycles to failure, failing after 2504 ± 366 and 2090 ± 196 cycles,respectively; the difference between these two constructs wasnot significant (p = 0.056). The mean failure loads were 900 ±160 N for the AxSOS plate, 700 ± 80 N for the DFN constructs,and 600 ± 49 N for the T2 constructs.

The failure modes in the incremental cyclic axial fatiguetest differed markedly among the implants used. The T2 (Fig.8, a) and the DFN (Fig. 8, b) constructs showed distal-to-proximal widening of the intercondylar fracture gap, withsubsequent proximal displacement of the medial condyle.With both devices, this condyle was also disrupted in thecoronal plane. The SCN and the AxSOS plate constructs failedby medial screw loosening in the bone model (Fig. 8, c and d).

Human Cadaveric Bone ModelThe mean bone density (and standard deviation) of the spec-imens was 0.822 ± 0.15 g/cm2 in the SCN group and 0.831 ±0.16 g/cm2 in the DFN group; the difference was not significant(p = 0.725). According to the literature, our specimens can becharacterized as mildly osteoporotic16. The mildly osteoporoticbone quality was also confirmed by the obtained median Tscoreof –217.

The initial axial stiffness was 382 ± 58 N/mm for theSCN construct and 358 ± 37 N/mm for the DFN construct.The 6% difference was significant (p < 0.001) (Fig. 6).

With regard to axial cycles to failure, the end point of‡2.5 mm of deformation was reached after a mean of 10,715 ±1499 cycles by the SCN construct and after a mean of 6210 ±1690 cycles for the DFN construct (Fig. 9). The 42% differencewas significant (p = 0.001).

With regard to failure modes, the SCN constructs hadminimal observed loss of reduction in the intercondylar fracturegap; screw loosening was seen in only one specimen. The DFNconstructs behaved as in the synthetic bone model, with rapidloss of reduction in the intercondylar fracture gap and distal gapwidening. In one specimen, this was followed by medial screwloosening and coronal plane disruption of the medial condyle.

Synthetic Bone Compared with Human Cadaveric BoneConstructsBoth the initial axial stiffness (Fig. 6) and the axial cycles tofailure (Fig. 9) were markedly higher, in absolute terms, in thehuman cadaveric bone constructs. In the human bone model,the SCN constructs had 36% greater stiffness and the DFNconstructs had 39% greater stiffness than did the constructs in

Fig. 9

The mean number of cycles to failure (‡2.5 mm of deformation) of the SCN and the DFN

constructs in the synthetic bone and in the human cadaveric bone model; the difference

between the SCN and DFN constructs in the synthetic bone and between the SCN and

DFN constructs in human bone was significant (p < 0.001 for both). The sample size

included five specimens per construct in the synthetic bone model and eight specimens

per construct in the human bone model.

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the synthetic bone model. These differences were significant(p < 0.001). The human bone constructs withstood 2.2 times(SCN constructs) to 2.7 times (DFN constructs) the number ofcycles survived by the synthetic bone constructs (p = 0.002).However, the differences between the DFN and the SCNconstructs were of a similar magnitude in the two models. Inthe synthetic bone model, the SCN construct had 8.9% greaterstiffness than the DFN construct; in the human bone model,the difference was 6.7%. The number of cycles to failure ofthe SCN construct was 1.7 times (human bone model) to 2.1times (synthetic bone model) greater than that of the DFNconstruct.

Discussion

This study investigated the biomechanical characteristics offour fixation devices, which mainly differed in their distal

interlocking patterns, for the treatment of comminuted distalfemoral fractures in osteoporotic bone. This study supports theconcept that distal locking has an influence on biomechanicalbehavior for this fracture model. The magnitude of the dif-ferences found in this study may be clinically important.

Distal femoral fractures may be, and commonly are,managed with intramedullary nailing or with plating9,18-22.Implant selection is guided by factors such as fracture pattern,joint involvement, bone stock quality, and soft-tissue condi-tion21. Biomechanical studies have provided guidance as to theadvantages and drawbacks of different fracture fixation23,24.Especially in osteoporotic bone, sufficient implant fixation ofdistal femoral fractures is a challenge. Fixation failures arecommon in osteoporotic distal femoral fractures, and the morestable the construct, the more likely a good clinical result25.

A newly developed intramedullary implant for distalfemoral fractures featuring distal condylar interlocking bolts, theStryker SCN, has not, to date, been the subject of many bio-mechanical studies. Hohle et al.26 did not find any significantdifference with regard to axial and torsional stiffness among theSCN supracondylar nail, the LISS-DF (Less Invasive Stabiliza-tion System-Distal Femur; Synthes) nail, and the DFN (Syn-thes). In contrast to our study, Hohle et al. used an A3 fracturemodel, a fracture without joint involvement, although the distalinterlocking was the same as that in our investigation. The re-sults of Hohle et al. showed no significant difference betweenthe SCN and DFN construct for torsional and axial stiffness.

The enhanced torsional stability and strength of the SCNcompared with the DFN construct may be attributed to thecompressive forces exerted on the fracture fragments by thecondylar bolts, as well as to the three-plane configuration ofthe distal interlocking devices, which minimizes the neutralzone and thus provides enhanced stability. The DFN constructhad the least stability, with the lowest torsional stiffness and thelargest neutral zone. Since this device does not exert any in-terfragmentary compression, the fragments were able to moveand displace along the intercondylar fracture site. Grass et al.9

compared the DFN construct, which has a spiral blade, withthe Green-Seligson-Henry nail (Smith and Nephew, Memphis,Tennessee), which has two distal locking screws, and with an

angled blade plate. They found the DFN construct to havesignificantly poorer torsional stability.

There is agreement, throughout a range of studies of distalfemoral fracture management9-11,27-29, that plating providesgreater torsional stiffness, whereas intramedullary devices havegreater axial strength. In our study, all intramedullary devicesshowed similar axial stiffness compared with each other. Sincethe nails tested were of the same material, length, and diam-eter, the differences in their biomechanical behavior must havebeen due to the different distal interlocking patterns. Theadvantage of the spiral-blade design of the DFN device over aconventional two-screw interlocking design was demonstratedby Ito et al.30, who found the spiral blade to confer a significantincrease of 41% in axial stiffness. In our study, the stiffnessachieved with the DFN construct was 10% greater than thatobtained with the T2 construct. In their comparison of theDFN device and the Green-Seligson-Henry nail, Grass et al.9

also found spiral-blade interlocking to provide better axialstiffness. The plated constructs in our study achieved 40% to50% of the axial stiffness provided by the intramedullary nails.Zlowodzki et al.11 found the angled blade plate constructs toconfer only 25% of the axial stiffness achieved with an intra-medullary nail. Using a Synthes condylar plate, Ito et al.31

found the device to provide only 10% of the axial stiffnessachieved with a Synthes intramedullary nail. These results maybe accounted for by the design principle of the plate (a laterallyeccentric applied device) and by the design features (material,thickness, and size) of the plate. The axial stiffness of platedconstructs is a clinical concern: delayed union and even non-union of plated distal femoral fractures have been reported,especially in cases where there was no medial support19,32,33.These adverse events have been attributed to poor axial stiff-ness permitting movement in the fracture gap34.

The axial strength of the implants in our study was in-fluenced by the distal interlocking pattern, with a larger implant-bone contact area of the distal interlocking devices beingassociated with greater strength. Thus, failure occurred first inthe T2 construct, followed by the DFN construct, the AxSOSplate, and finally the SCN construct. The implant-bone contactarea of the spiral blade is 75% greater than that of a conventionalinterlocking screw. The combination of a spiral blade with aninterlocking screw provides a 38% larger contact area than twoconventional interlocking screws30. In our study, this constructappeared to result in enhanced stiffness and strength.

Ito et al.30 investigated the biomechanical properties ofthe spiral blade in comparison with conventional locking boltsin static compression testing. They found a 13% to 21% in-crease in axial strength of the spiral blade compared withconventional locking bolts. These results agree with the find-ings in our study, in which the DFN (spiral blade) constructswithstood 21% more cycles to failure than did the T2 (con-ventional locking bolt) constructs. The highest cyclic axialstrength was achieved with the SCN construct, which has athree-plane configuration of four distal interlocking devices,two of which are condylar bolts with washers and nuts. Thediagonal bolt arrangement of the SCN construct provides

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more secure fixation in the denser bone of the posteriorcondyles, a feature described as beneficial by Ingman35.

The failure modes in the incremental cyclic axial testobserved by us have been reported by other authors, both insynthetic bone models and in human femora. Trough forma-tion in the bone was found by several authors28-31; plate failurewas found by Ito et al.31 and by Zlowodzki et al.11.

For the validation of our synthetic bone model, the cyclicaxial test was repeated in human cadaveric bone with use of theDFN and SCN intramedullary nails. The synthetic bone modelresults were found to be qualitatively reproducible in the ca-daveric bones. While the cadaveric bone constructs had higheraxial stiffness and numbers of cycles to failure, the pattern ofthe results observed in the human cadaveric bone model wassimilar to that seen in the synthetic bone model. For the SCNand DFN implants, the failure modes under cyclic axial loadingwere comparable between the synthetic and human bones. Thedifferences observed between the models are likely attributableto the comparatively high bone density of the human speci-mens. While the density of the structural foam condyles hadbeen specially chosen to simulate osteoporotic bone stock, themean bone density of the cadaveric bones available for ourstudy was 0.826 g/cm2, which is well above the severe osteo-porotic bone densities in the literature. Koval et al.28,36 usedspecimens with a bone mass density of 0.3 to 0.5 g/cm2 and of0.35 to 0.675 g/cm2 for biomechanical testing on three dif-ferent plate constructs for osteosynthesis. Our findings suggestthat our synthetic bone model lends itself to a first-line com-parative biomechanical evaluation of a range of implants.However, the results obtained would need to be validated, ineach case, with human bone.

The synthetic bone model (Sawbones third-generationcomposite femoral shafts and polyurethane foam condyles)used in our study was derived from earlier studies with use offoam cortical shell (Sawbones Europe) and Sawbones third-generation composite femora. In those studies, the two com-mercially available synthetic bone models had shown atypicalfailure modes1,4,37. Therefore, we decided to customize thedistal part of our bone model using a self-made anatomicalpolyurethane foam model without a cortical shell.

Our test setup was adapted from the one used by Milneand Latta14; similar setups have been used in biomechanicalevaluations of constructs for the management of distal femoralfractures11,26,38 and of proximal tibial fractures39. The loadingregimes chosen for our study were based on the results ofin vivo telemetry studies40,41, as well as on one biomechanical

study42, and thus, were reasonably representative of physiologicpostoperative loading of the distal end of the femur. Incre-mental axial loading was established as a testing methodologyby Marti et al.24 and has been used by several authors11,21,43.

Our biomechanical study, however, had some limita-tions. (1) The simplified test setup did not simulate soft-tissueforces (capsule, ligaments, and muscles); (2) 500 cycles perloading step was not truly representative of a postoperativeloading pattern; and (3) we were unable to repeat the full rangeof tests in the human cadaveric bone model.

In this study, we demonstrated that, with intramedullarynails, the distal interlocking pattern is important to the stabili-zation of distal femoral fractures, in terms of both the primarybiomechanical stiffness and the strength of the constructs.

The results obtained in the synthetic bone model for theSCN and DFN constructs were also found in a human ca-daveric bone model, allowing for differences in bone density.

Because the SCN construct provided significantly higheraxial stability than the other constructs and torsional stabilitythat was nearly comparable with that of the AxSOS plate, theSCN construct had the best combined biomechanical stability.Clinically, we suggest that, if predominantly torsional stabilityis needed, e.g., for patients who are confined to bed, the AxSOSplate will be sufficient, since treatment of distal femoral frac-tures by plating is fast and straightforward. In contrast, thefindings of our study support the concept that the SCN con-struct should be considered for mobile patients for whom earlypostoperative mobilization for rehabilitation is desired. n

Dirk Wahnert, MDKonrad L. Hoffmeier, Dipl-IngGunther O. Hofmann, MD, Dr rer natThomas Muckley, MDDepartment of Traumatology, Hand and Reconstructive Surgery,Friedrich Schiller University Jena,Erlanger Allee 101, D-07747 Jena, Germany.E-mail address for D. Wahnert: [email protected]

Geert von Oldenburg, Dipl-IngStryker Trauma GmbH, Prof.-Kuntscher-Strasse 1–5,D-24232 Schonkirchen/Kiel, Germany

Rosemarie Frober, MDDepartment of Anatomy I, Friedrich Schiller University Jena,Teichgraben 7, D-07743 Jena, Germany

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Ots en fx supraintercondileas en osteoporsis

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