Injury, Int. J. Care Injured 47 (2016) 2688–2693

Impact of cement-augmented condylar screws in locking plateosteosynthesis for distal femoral fractures — A biomechanical analysis

Christopher Bliemel, MD*, Ludwig Oberkircher, MD, Benjamin Bockmann, MD,Eric Petzold, Rene Aigner, MD, Thomas Jan Heyse, MD, Steffen Ruchholtz, MD,Benjamin Buecking, MDCenter for Orthopaedics and Trauma Surgery, University Hospital Giessen and Marburg, Location Marburg, Germany

A R T I C L E I N F O

Keywords:Distal femoral fracturePolyaxial angular stable plateosteosynthesisCement augmentationOsteoporosisBiomechanical analysis

A B S T R A C T

Introduction: Compromised bone quality and the need for early mobilization continue to lead to implantfailure in elderly patients with distal femoral fractures. The cement augmentation of screws mightfacilitate improving implant anchorage. The aim of this study was to analyse the impact of cementaugmentation of the condylar screws on implant fixation in a human cadaveric bone model.Material and methods: Ten pairs of osteoporotic femora (mean age: 90 years, range: 84–99 years) wereused. A 2-cm gap osteotomy was created in the metaphyseal region to simulate an unstable AO/OTA 33-A3 fracture. All specimens were treated with a polyaxial locking plate. Specimens randomly assigned tothe augmented group received an additional cement augmentation of the condylar screws using bonecement. A servohydraulic testing machine was used to perform incremental cyclic axial loading using aload-to-failure mode.Results: All specimens survived at least 800 N of axial compressive force. The mean compressive forcesleading to failure were 1620 N (95% CI: 1382–1858 N) in the non-augmented group and 2420 N (95% CI:2054–2786 N) in the group with cement-augmented condylar screws (p = 0.005).Deformation with cutting out of the condylar screws and condylar fracture were the most commonreasons for failure in both groups. Whereas axial stiffness was comparable between both osteosyntheses(p = 0.508), significant differences were observed for the plastic deformation of the constructs (p = 0.014).Conclusion: The results of the present study showed that the cement augmentation of the condylar screwsmight be a promising technique for the fixation of distal femoral fractures in elderly patients withosteoporotic bones.

ã 2016 Elsevier Ltd. All rights reserved.

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Introduction

Osteoporosis-associated fractures represent one of the largestsocio-economic burdens in western industrial countries [1].Because of ongoing demographic changes, the incidence ofosteoporotic fractures will most likely continue to increase. Inthis context, distal femoral fractures occurring among elderlypatients are typically considered to be related to osteoporosis.

* Corresponding author at: Center for Orthopaedics and Trauma Surgery,University Hospital Giessen�Marburg, Marburg, Baldingerstrasse, 35043 Marburg,Germany.

E-mail addresses: bliemel@med.uni-marburg.de (C. Bliemel),oberkirc@med.uni-marburg.de (L. Oberkircher), bockmann@med.uni-marburg.de(B. Bockmann), ericpetzold@gmx.de (E. Petzold), aignerr@med.uni-marburg.de(R. Aigner), heyse@med.uni-marburg.de (T.J. Heyse),ruchholt@med.uni-marburg.de (S. Ruchholtz), buecking@med.uni-marburg.de(B. Buecking).

http://dx.doi.org/10.1016/j.injury.2016.10.0130020-1383/ã 2016 Elsevier Ltd. All rights reserved.

Because these fractures are less frequent than fractures of theproximal humerus, distal forearm, spine, or hip, they might beunderestimated [2]. Nevertheless, fractures of the distal femur areparticularly difficult to treat, especially in geriatric patients withreduced bone quality.

Such challenges in the treatment of geriatric distal femoralfractures, which are mostly related to conventional plate osteosyn-thesis, have led to innovations in implant technology [3–5].Currently, locking plates are a standard tool to achieve fracturefixation in osteoporotic bone. Clinical and biomechanical trialshave demonstrated that angular stable plates are superior in termsof axial stability and load to failure compared to conventionalplates [5–9]. Whereas first-generation locking plates provided onlymonoaxial screw fixation, further developments led to theintroduction of polyaxial angular stable plates. These plates aredistinguished by their increased flexibility in the positioning of thescrews, allowing the regions with the best bone stock to be

C. Bliemel et al. / Injury, Int. J. Care Injured 47 (2016) 2688–2693 2689

accessed, thus enabling firmer implant anchorage in both weakcancellous bone and complex anatomical conditions, such as thecondyle region of the distal femur [10].

Despite these innovations in implant technology, problemscontinue to affect the fixation of far-distal femoral factures,especially in severely osteoporotic bone. A relatively new approachto improving implant anchorage is to increase the interfacebetween the implant and the rarefied bone by cement augmenta-tion of screws, as performed in osteoporotic fractures of theproximal humerus, spine and hip in the clinic [11–13]. The resultsare promising, suggesting the potential development of thisstrategy for the fixation of distal femoral fractures [14]. In thiscontext, Fig. 1a presents the clinical case of an 82-year-old womanwith highly osteoporotic bone structure and a peri-implant distalfemoral fracture. Fig. 1b shows the fracture fixation via plateosteosynthesis and cement-augmented condylar screws.

The purpose of the present biomechanical study was tocompare a polyaxial locking plate with augmented condylarscrews and one with non-augmented screws for the fixation ofdistal femoral fractures in a human cadaveric bone model.

The cement augmentation of condylar screws was hypothe-sized to improve implant fixation in osteoporotic bone and,therefore, increase the load-to-failure values of the osteosynthe-ses.

Materials and methods

Specimens

This study was conducted on 10 pairs of adult femora. Thesefemora were obtained from human cadavers that had beenembalmed with a solution of 96% ethanol and <2% formaldehyde,as previously described [15]. The specimens were stored for at leastone year before use. The femora were provided by the Institutes of

Fig. 1. Clinical case of an 82-year-old women with highly osteoporotic bonestructure and the presence of a peri-implant distal femoral fracture (a). Fracturefixation was conducted with a plate osteosynthesis and cement augmentation ofthe condylar screws (b).

Anatomy and Cell Biology of Philipps University, Marburg,Germany. All donors gave written consent by their own free willfor the use of their body for research purposes. All experimentswere conducted in accordance with the local ethics committee (AZ157/14).

The samples originated from 2 male and 8 female adults with anaverage age of 90 years (range: 84–99 years). After the surroundingsoft tissue was stripped off, the specimens were wrapped in towelsmoistened with the aforementioned embalming solution andstored in a cooling chamber at 4 �C to avoid artefacts from drying.

Assessment of bone quality

To exclude damage related to pre-existing fractures orosteolyses, all femora were subjected to a clinical examinationand an X-ray examination using a c-arm unit. Subsequently, thebone mineral density of each femur was measured by Dual-energyX-ray Absorptiometry (DXA).

Implants and surgical treatment

The implant used in this study was the Non-Contact Bridgingplate for Distal Femur (NCB-DF1, Zimmer Inc., Winterthur,Switzerland) (CE marketing 0086). The NCB-DF1 is a polyaxiallocking device made of titanium alloy. This plate is anatomicallypreformed to ensure an optimum fit to the lateral cortex of thedistal femur. In the present setup, only NCB-DF1 nine-hole plateswere used. Shaft fixation was bicortically performed using fivescrews (diameter: 5 mm) located in the proximal five holes of theplate. Five screws (diameter: 5 mm) were also positioned in thecancellous bone of the condylar region. All osteosyntheses wereperformed by the same study surgeon (BC). Subsequently, astandardized osteotomy was created using a surgical oscillatingsaw. An unstable supracondylar femoral fracture was simulated(33-A3 according to the Orthopaedic Trauma Association OTA/AOclassification) as the fracture model. A 2-cm gap osteotomy wascreated perpendicular to the anatomic axis of the femur. The distalcut was made at the level of three-quarters of the width of thedistal femur above the intercondylar notch. One osteosynthesis ofeach pair of femora was randomly assigned to the augmentedgroup, and the other was assigned to the non-augmented group.The specimens randomized into the augmented group receivedadditional cement augmentation of the five cancellous screws inthe distal part of the construct. Cement augmentation wasperformed by removing one cancellous screw at a time andinjecting 2 ml of polymethylmethacrylate (PMMA) bone cement(Kyphon1 HV-R1 bone cement; Medtronic Inc., Sunnyvale, CA,USA) (CE marketing 0473) into each screw hole. All of the surgicalsteps were performed under fluoroscopic imaging in two planes to

Fig. 2. Typical postoperative x-ray pictures showing a pair of samples with cementaugmented condylar screws in the right femur (a) and non-augmented screws inthe left femur (b)-.

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assure accurate hardware positioning and proper fit of the injectedcement without any leakage (Fig. 2a and b).

Test setup and loading

After osteosynthesis, the proximal part of each femur wasresected with an oscillating saw. The osteotomy was made 6 cmproximal to the plate. Subsequently, the proximal part of eachspecimen was embedded in special containers using Technovit3040 (Heraeus, Wehrheim, Germany), which is a thermosettingresin. Using a custom-made holding device, the femora werepositioned upside down in an Instron 5566 universal servohy-draulic testing machine (Instron Cor., Darmstadt, Germany). Ametal plate that was movable in two directions (anterior-posteriorand medial-lateral) was used to exert the pressure. After thefemora were aligned in an anatomical, slightly valgus position of 5–7�, loading was applied along the mechanical axis of the femur(Fig. 3). Before testing, the samples were thawed at roomtemperature.

Subsequently, each femur was subjected to cyclic loadingfollowing a standardized protocol. Each femur was subject to apreload of 100 N to compress the specimen and avoid deformationartefacts in the bone construct. Cyclic loading tests were thenconducted at a constant speed of 125 mm/min. Loading wasapplied at this speed until the desired maximum force wasachieved; a maximum cycling frequency of 1 Hz was reached. Asthe compression force increased, the frequency declined. At theend of the test, the loading machine stopped automatically. Thetest sequence started with 500 cycles at 600 N, and the load wasincreased in steps of 200 N every 500 cycles until the osteosyn-thesis failed. Construct failure was defined as a sudden loss ofmeasured force (>30%) and major deformation of the boneconstruct (>20 mm). Testing was conducted in displacementcontrol mode.

Fig. 3. Test setup demonstrating force application over the condylar region with ametal plate. The femoral shaft was statically fixed in containers embedded withTechnovit.

Data collection and statistical analysis

The data were collected at 100 ms intervals using theinstrument-specific Bluehill Software. In all tests, plastic deforma-tion, as a measure of irreversible deformation under the influenceof force, was recorded automatically as the maximum value in theprevious cycle.

Loading (N), compression set (mm), and the number of cycleswere also recorded. In addition, 95% confidence intervals (CIs) weredetermined. The stiffness of each osteosynthesis construct wascalculated from the compression set under the applied loading.

An a priori power analysis was performed: For the load tofailure (i.e., the primary outcome measure), a sample size of 10pairs of femora was calculated. A clinically meaningful differencewas defined as 100% failure of the osteosynthesis in non-augmented vs. 40% failure in augmented specimens at a load of2000 N. The power was set at 0.80 with an alpha error of 0.05. Thedata were statistically analysed using IBM SPSS statistics 22(Statistical Package for the Social Sciences, IBM Cooperation,Armonk, NY, USA). The data were examined for normality using theKolmogorow–Smirnow test. Depending on the result, a pairedStudent’s t-test or the Wilcoxon rank sum test for paired sampleswas applied for further data analysis. Statistical significance wasset at p < 0.05.

Results

Bone quality

DXA revealed compromised bone quality in all tested speci-mens with a mean T-score of �3.31 (CI: �3.65 to �2.96). Bonemineral density was comparable in all of the matched pairs ofcadaver femora examined (p = 0.543).

Cyclic testing and failure mode

All specimens survived at least 800 N of axial compressive force.Whereas 90% of the specimens with augmented condylar screwssurvived a load of 2000 N or more, none of the specimens in thenon-augmented group survived a load of 2000 N. The meancompressive forces leading to failure were 1620 N (95% CI: 1382–1858 N) in the non-augmented group and 2420 N (95% CI: 2054–2786 N) in the group with cement augmented condylar screws(Fig. 4). This difference was statistically significant (p = 0.005).

Fig. 4. Failure loads based on the cyclic loading tests, comparing plateosteosynthesis with and without augmentation of the condylar screws.

Fig. 5. Photographs showing the different failure modes. In the augmented group deformation with cutting out of the condylar screws (a), condylar fractures (b) and shaftfracture (c) occurred. In the non-augmented group deformation with cutting out of the condylar screws (a) and condylar fractures (b) were seen.

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Failure in the cement-augmented group occurred by condylarfractures in six cases, cutting out of the condylar screws in threecases, and shaft fracture in one case. In the non-augmented group,deformation with cutting out of the condylar screws and condylarfractures each occurred five times (Fig. 5a–c).

Axial stiffness and plastic deformation at a load of 800 N

Based on a load of 800 N (the highest value at which allspecimens remained intact), a separate analysis of the stiffness andplastic deformation of the constructs was performed. At this load,stiffness was comparable for osteosyntheses with augmentation ofthe condylar screws (mean: 2.71 kN/mm; 95% CI: 1.69–3.72 kN/mm) and without augmentation (mean: 2.07 kN/mm; 95% CI:1.55–2.59 kN/mm) (p = 0.508) (Fig. 6). As shown in the box-plot(Fig. 7), analysing the data revealed a higher variability in theplastic deformation of the construct in non-augmented osteosyn-theses (mean: 1.69 mm; 95% CI: 0.43–2.96 mm). In contrast,augmentation of the condylar screws led to smaller plasticdeformation (mean: 0.67 mm; 95% CI: 0.45–0.89 mm). Under aload of 800 N, this difference in the plastic deformation betweenthe two types of osteosynthesis was not significant (p = 0.098).

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Fig. 6. Stiffness calculation of plate osteosynthesis with augmented screws (blackdots) and plate osteosynthesis with non-augmented screws (grey dots) at a load of800 N.

The plastic deformation of the constructs was also testedbetween 600 N and 1800 N. Loads greater than 1800 N were nottested because all non-augmented specimens failed beforereaching this load. Analysing the plastic deformation in this rangerevealed significant differences between the two types ofosteosynthesis (p = 0.014) (Fig. 8). Osteosynthesis with augmentedcondylar screws exhibited a mean plastic deformation of 0.98 mm(95% CI: 0.68–1.28 mm), whereas osteosynthesis with non-augmented condylar screws had a mean plastic deformation of1.45 mm (95% CI: 1.17–1.73 mm).

Discussion

This biomechanical study analysed the impact of implantaugmentation using a polyaxial locking plate for the osteosyn-thesis of osteoporotic distal femoral fractures. The principlefindings of this study revealed that the cement augmentation ofcondylar screws significantly decreased the construct’s plasticdeformation and significantly increased its load to failure relativeto those of non-augmented specimens.

Fig. 7. Plastic deformation of plate osteosynthesis with augmented screws andplate osteosynthesis with non-augmented screws at a load of 800 N.

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Mean plasti c deformation (augmented screws)

Fig. 8. Mean plastic deformation of plate osteosynthesis with augmented and non-augmented condylar screws plotted against the applied load.For each mean value, half bars illustrating the standard deviation are shown.For the mean values of plate osteosynthesis with non-augmented condylar screws, the half bars go upwards. For the mean values of plate osteosynthesis with augmentedcondylar screws, the half bars go downwards.

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Previous studies on implant augmentation in distal femoralfractures from a biomechanical perspective were conducted byWähnert et al. using monoaxial locking plates in artificial andhuman cadaveric bones [14,16,17]. However, the existing studies onimplant augmentation in distal femoral fractures are limited bysome methodical drawbacks. In this context, it should be notedthat synthetic bone models do not capture interspecimenvariability and material inhomogeneity, both of which may affectimplant performance [18]. Additionally, particularly in severeosteoporotic bone, where the cement augmentation of implantsshould be considered for fracture fixation, polyaxial implants arepreferred. In this context, the main disadvantage of unidirectionalangular stable plates is related to the perpendicular configurationof the threads in the plate, which result in predetermined screwpositioning. Thus, screw positioning often occurs in areas withinferior bone quality, possibly leading to secondary loss ofrealignment and cutting out of the screws [10]. In a prospectivemulticentre study, monoaxial implants were shown to be inferiorto multidirectional angular stable plates for the fixation of distalfemoral fractures [19].

Having overcome the method-related drawbacks affectingprevious examinations, our study reports the first biomechanicalapproach with data that are as close as possible to the real situationin geriatric patients with osteoporotic bone structures. Augmen-tation of the condylar screws significantly increased the load tofailure in distal femoral fractures. These results can be explained bythe enlargement of the bone-implant interface by the cementaugmentation, leading to increased stability in osteoporotic bone[13]. Nevertheless, our results are contrary to those of Wähnertet al., likely because of the use of hybrid specimens consisting of acombination of fresh-frozen distal femora and a PMMA femoralshaft [17]. Unfortunately, no DXA scan was performed in theirstudy, and thus, it cannot be ruled out that the differences in theload-to-failure results are related to non-osteoporotic bonestructures in some of their specimens. The use of the hybridmodel itself might also explain these divergent findings.

The abovementioned enlargement of the bone-implant inter-face by the cement augmentation of the condylar screws may alsohave contributed to the failure mode of the osteosynthesis and the

significant differences in the plastic deformation of the two typesof osteosynthesis studied here. Whereas cutting out of thecondylar screws occurred in 50% of the non-augmented osteosyn-theses, it was less common in the cement-augmented group.Additionally, because of the increased pull-out strength, lessplastic deformation of the construct occurred in osteosyntheseswith cement-augmented screws. Reduced rates of cutting out ofaugmented condylar screws and minor plastic deformation havebeen reported in previous biomechanical investigations addressingdistal femoral fractures [14,16].

The present biomechanical analysis revealed that the cementaugmentation of condylar screws did not increase the stiffness ofthe construct. Therefore, most of the motion of the osteosynthesislikely occurs through the bending of the plate itself under theinfluence of axial force rather than through micromotion of thescrews in the bone. The fact that augmentation of the condylarscrews did not significantly change the stiffnesses of the osteosyn-thesis constructs must be considered to be a positive resultbecause secondary bone healing strongly dispends on theexistence of axial micro-movements [20].

Despite these promising biomechanical results, the discussionabout cement-related disturbances of bone metabolism remainsongoing. Because PMMA cement is hardened through an exother-mic reaction, the heat released during the cement augmentation ofthe condylar screws might increase the risk of bone and cartilagenecrosis. For example, temperatures up to 45 �C were observed inan in vitro investigation of cement-augmented hip screws [21].Further in vitro experiments addressing the cement augmentationof proximal humerus plate screws conducted by Blazejak et al.revealed that PMMA leads to locally limited heat development inthe cement cloud and the surrounding tissue [22]. Most of the heatreleased during polymerization of PMMA was dissipated by theimplant itself, and the rest accumulated mainly in the cementcloud. Therefore, the critical threshold values for cartilage andsubchondral bone necrosis were not reached, and cementaugmentation was determined to be safe. Although such inves-tigations have not yet been performed in the distal femur, theresults obtained in other parts of the body can be assumed to begeneralizable to this type of fracture.

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Limitations

Despite its careful design, the present study has somelimitations in terms of theoretical or computational analyses.Although a large scope exists for the validation of our study results,finite element analyses were not performed in this biomechanicalinvestigation and should be addressed in the future. Furthermore,the in vitro setup in which only axial loading of the specimen wastested also represents a potential limitation. These restrictions ofthe loading conditions with the absence of torsional and bendingloads might have influenced the results concerning the failuremode and load. Additionally, the number of tested samples is low.Nevertheless, among the existing studies on this topic, the presentone is the largest to date. Moreover, according to a power analysisperformed prior to testing, this study achieved significant results.

Conclusion

The results of this study showed that the cement augmentationof the condylar screws might be a promising strategy for theimplant fixation of distal femoral fractures in elderly patients withosteoporotic bones. Thus, the augmentation of implants, whichincreases their loading capacities, might contribute to winning therace between fracture healing and osteosynthesis failure.

Conflict of interest

The corresponding author declares on behalf of all authors thatthere are no conflicts of interest.

Acknowledgments

Zimmer Inc. Winterthur, Switzerland is acknowledged forproviding the implants. No further funding was received for thiswork. No additional financial support for the execution of the studywas received.

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