MUSCULOSKELETAL

Dynamic contrast-enhanced magnetic resonance imagingcan assess vascularity within fracture non-unions and predictsgood outcome

Oliver Schoierer & Konstantin Bloess & Daniel Bender &

Iris Burkholder & Hans-Ulrich Kauczor &

Gerhard Schmidmaier & Marc-André Weber

Received: 23 July 2013 /Revised: 23 September 2013 /Accepted: 1 October 2013 /Published online: 22 October 2013# European Society of Radiology 2013

AbstractObjectives To prospectively evaluate whether dynamiccontrast-enhanced (DCE) MRI can assess vascularity withinnon-unions and predicts clinical outcome in combination withthe clinical Non-Union Scoring System (NUSS).Methods Fifty-eight patients with non-unions of extrem-ities on CT underwent 3-T DCE MRI. Signal intensitycurves obtained from a region-of-interest analysis weresubdivided into those with more intense contrast agentuptake within the non-union than in adjacent muscle(vascularised non-union) and those with similar or lesscontrast uptake. The pharmacokinetic parameters of theTofts model K trans, K ep, iAUC and Ve were correlatedwith union at CT 1 year later (n =49).Results Despite inserted osteosynthetic material, DCE param-eters could be evaluated in 57 fractures. The sensitivity/specificity of vascularised non-unions as an indicator of goodoutcome was 83.9 %/50.0 % compared to 96.8 %/33.3 %using NUSS (n =49). Logistic regression revealed a signifi-cant impact of NUSS on outcome (P=0.04, odds ratio=0.93).At first examination, median iAUC (initial area under theenhancement curve) for the ratio non-union/muscle was10.28 in patients with good outcome compared with 3.77 in

non-responders (P=0.023). K trans, K ep and Ve within the non-unionwere not significantly different initially (n =57) or 1 yearlater (n =19).Conclusions DCEMRI can assess vascularity in fracture non-unions. A vascularised non-union correlates with goodoutcome.Key points• Dynamic contrast-enhanced magnetic resonance imagingcan assess vascularity within bony non -unions .

• Vascularised ununited fractures appear better at 1 -year CTthan poorly vascularised fractures .

• Non -union healing after osteosynthesis or osteoinductivedrugs fundamentally requires vascularity.

•DCEMRI predicts treatment outcome better than the clinicalNon -Union Scoring System .

• DCE MRI is clinically feasible to predict treatment outcomein bony non -unions .

Keywords Dynamic contrast-enhancedmagnetic resonanceimaging . Non-union . Vascularity . Fracture healing .

Non-Union Scoring System

AbbreviationsCEUS Contrast-enhanced ultrasoundCI Confidence intervalCT Computed tomographyDCEMRI

Dynamic contrast-enhanced magnetic resonanceimaging

FOV Field of viewiAUC Initial area under the enhancement curveKep Constant reflux between extravascular extracellu-

lar space and blood plasmaK trans Volume transfer constant between blood plasma

and extravascular extracellular spaceNUSS Non-Union Scoring System

O. Schoierer :D. Bender :G. SchmidmaierHeidelberg Trauma Research Group, Department of Orthopaedic andTrauma Surgery, University Hospital Heidelberg, SchlierbacherLandstr. 200a , D-69118 Heidelberg, Germany

K. Bloess :H.<U. Kauczor :M.<A. Weber (*)Department of Diagnostic and Interventional Radiology, UniversityHospital Heidelberg, Im Neuenheimer Feld 110,D-69120 Heidelberg, Germanye-mail: MarcAndre.Weber@med.uni-heidelberg.de

I. BurkholderDepartment of Nursing and Health, University of Applied Sciencesof the Saarland, Goebenstr. 40, D-66117 Saarbrücken, Germany

Eur Radiol (2014) 24:449–459DOI 10.1007/s00330-013-3043-3

ROC Receiver operating characteristicROI Region of interestSTIR Short tau inversion recoveryTE Echo timeTR Repetition timeTSE Turbo-spin-echoVe Extravascular extracellular volume fractionVIBE Volume interpolated breath-hold examination

Introduction

Fracture healing is influenced by the age and morbidity of theinjured patients, the fracture itself, the accompanying softtissue trauma and the chosen treatment [1–4]. Bone healingcan be visualized by conventional radiographs, but in the caseof an anatomical reduction of a fracture, normally no callusformation can be detected radiologically. Secondary bonehealing forms callus which is not missed on normal radio-graphs. But endosteal or intramedullary bone healing is diffi-cult to detect either with plain radiographs or even withcomputed tomography (CT) [4, 5]. CT helps in this contextby better analysing bone healing disorders, postoperative ro-tational malalignment or malposition of fractures [6]. Forinstance, CTshows very good diagnostic accuracy with a highintraobserver agreement and a sensitivity of 100 % fordetecting non-union of distal tibia fractures compared to nor-mal x-rays, which yield a lower sensitivity and specificity [7].

If no radiologically visible bone progress occurs within 12–16 weeks after surgical treatment, this is denominated delayedbone healing. If the bone has not consolidatedwithin 26weeksafter the first stabilization procedure, this is an accepted indi-cation for additional dedicated treatment. The treatment ofnon-union of long bones with an estimated incidence of 5–10 % is demanding [8]. The use of osteoinductive andosteoconductive materials is well established for the treatmentof both non-union and acute fractures [9–16]. The results ofnon-union surgery following the diamond bone concept [17],a tissue engineering approach for fracture non-union healingincluding the utilization of mesenchymal stem cells, scaffoldsand growth factors together with ensuringmechanical stabilityat the fracture site, are safe and reliable [13, 18]. But, thetreatment of non-unions nevertheless remains unpredictableeven with the use of new osteosynthesis techniques or appli-cation of osteoinductive drugs. The Non-Union ScoringSystem (NUSS) groups delayed bony union or non-union intothree categories with good, moderate and poor prognosisresponding to standard treatment methods [19]. But to the bestof our knowledge the NUSS score was never validated justlike the classification of Weber and Cech [20], which is rathera description of the radiological appearance of non-unions oflong bones.

In the diagnosis of bone healing disorders, the assessmentof blood flow of the affected bone has a decisive influence onthe choice of therapy. This is also reflected in the diamondconcept assigning vascularity a crucial role for bone healing[17] and its presence is considered as an important prerequisiteof the diamond concept [21]. Modern magnetic resonanceimaging (MRI) techniques, such as dynamic T1-weightedcontrast-enhanced (DCE) MRI, allow for the evaluation oftissue microcirculation [22] and can non-invasively visualizethe endosteal or periosteal blood supply [22–25]. Thus, DCEMRI appears to be a promisingmethod to assess vascularity offracture healing. Using DCE MRI significant differences incontrast medium enhancement patterns, indicative of alteredvascularisation, between acute and chronic scaphoid fractures[26] and a disturbed blood flow of the femoral head in patientswith dislocated fractures of the femoral neck have been re-ported [27]. Furthermore, a decreased perfusion in the proxi-mal femora in patients with osteoporosis compared toosteopenia or healthy controls has been described, where onlythe bone perfusion but not the perfusion of periarticular softtissue has been diminished, although the vascular supply is thesame [28, 29]. Also, capillary permeability in the bone and thebone healing process can be visualized, both in terms of therevascularisation of the fracture gap and visualization of callusformation [30, 31].

Since there are no DCEMRI studies on the vascularisationof fracture non-unions, we aim to analyse in this prospectiveclinical pilot study the vascular characteristics of non-unionsof long bones and evaluate whether DCE MRI can assessvascularity within non-union clefts and predicts clinical out-come better than the clinical NUSS. Our hypothesis is that avascularised non-union in DCE MRI has better clinicaloutcome.

Material and methods

Patient population and clinical assessment

We prospectively enrolled 58 patients with clinically and CT-proven non-union of an extremity fracture. All patients pre-operatively received a standardized T1-weighted DCEMRI, aradiological x-ray examination and CTof the affected extrem-ity. Owing to severe susceptibility artefacts because of theindwelling osteosynthetic material, DCE MRI parameterscould not be assessed in one female patient. Thus, a total of57 patients (23 female, 34 male) with non-union of the hu-merus (7 %), ulna (2 %), tibia (51 %) or femur (40 %) wereused for statistical analysis (Table 1). The mean age of ourpatient population was 48.7 years with a range from 22 to88 years (Table 1). The non-union was located mostly in themiddle third of the diaphysis of the humerus, ulna, femur ortibia in 29 of 57 cases (Table 2). Hypertrophic non-unions

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according to the classification of Weber and Cech [20] couldbe detected in 2 of the 4 humerus cases, in 5 of the 23 femurcases and in 3 of the 29 tibia cases. The Weber and Cechclassification [20] distinguishes between atrophic and hyper-trophic non-unions on the basis of x-ray findings. Whereasatrophic non-unions appear non-vital and non-reactive at thefracture gap, hypertrophic non-unions have a prominent andvital callus formation. The radiological appearance of the non-unions was nevertheless predominantly atrophic, especially inthe distal third of the tibia. Two non-unions of the femur andulna shaft were included after a post-radiation fracture. In allpatients the NUSS score [19] was calculated by a seniortrauma surgeon. DCE MRI was performed the day beforesurgery to determine the endosteal and periostealvascularisation of all patients using 3-Tesla MRI(MAGNETOM Verio, Siemens Healthcare, Erlangen,Germany). All 57 patients received standardized operativetreatment for the non-union of humerus, ulna, tibia or femurfollowing the diamond bone concept (n =37 as a one-stepprocedure, i.e. humerus (n =4), ulna (n =1), tibia (n =17),femur (n =15) [13, 17, 18]; and n =20 following theMasquelet or induced membrane technique requiring a two-step procedure, i.e. tibia (n =12), femur (n =8) [32–35]). Themedian time since the fracture and the administered therapywas 16 months (range 2–178 months, interquartile range 8–28 months). The postoperative clinical and radiological exam-ination was carried out at 6 weeks, 12 weeks, 6 months and12 months. The response rate to operative treatment was

evaluated by radiological signs of bone healing of thenon-union in consensus by a senior trauma surgeon andsenior musculoskeletal radiologist at the 12-monthfollow-up in CT with multi-planar reconstructions tooffer adjunct evaluation using additional imaging planestailored to favour fracture plane projection and individ-ual screw evaluation [36]. The CT criteria of fractureunionization were used according to previous recom-mendations [6, 36, 37]: At least 25 % of the fracturegap area filled with newly formed bridging trabeculaand/or at least 25 % of the non-union area filled withnewly formed cortical bridges across the fracture gap. Ifthese aforementioned signs of trabecular and/or corticalbridging were met, the patient was denominated as aresponder.

Seven of the 57 patients used for statistical analysis werelost at follow-up (2 femoral, 4 tibial non-unions and 1 ulnarnon-union; lost at follow-up rate, 12 %). Because in an 89-year-old woman with fracture non-union of the humerus onlyplain radiographs at follow-up were performed that showedfull consolidation of the non-union 1 year after surgery, at 1-year follow-up, correlation to CT outcome could be obtainedin n =49 (Table 3). A second T1-weighted DCE MRI wasscheduled 1 year postoperatively (or earlier in 7 cases ofbreakdown or collapse of the osteosynthetic material becauseof persistent non-union) and was available at the 1-yearfollow-up in 19 patients. The study was approved by the localethics committee (reference number S-380/2010) and wasconducted according to the declaration of Helsinki in thepresent form. All patients gave written informed consent priorto study inclusion after the nature of the procedure had beenfully explained.

DCE MRI examination and evaluation

After a localizer sequence, a coronal T1-weighted turbo-spin-echo (TSE) and an axial T2-weighted short tau inversionrecovery (STIR) sequencewere performed. Three unenhancedcoronal T1-weighted fat-saturated VIBE (volume interpolatedbreath-hold examination) sequences with different flip angles(9°, 5° and 13°) were measured before application of thecontrast medium. The contrast agent was injected with a flowof 2 ml/s from the second measurement via a peripheralvenous access (0.2 mmol/kg body weight of gadoteratemeglumine, Dotarem®, Guerbet, France). A coronal T1-weighted fat-saturated VIBE sequence was used for the T1-weighted dynamic images (repetition time (TR)/echo time(TE) = 3.75/1.35 ms, 17 measurements, 48 slices each,2 mm slice thickness). The field of view (FOV) varied de-pending on the anatomical region between 350 mm to amaximum of 450 mm. The measurement time of the MRIprotocol was 17 min and 6 s.

Table 1 Patient population with sex and age distribution

Non-union Gender Age

Localization Number (Female/male) (Mean, range)

Femur 23 11/12 46.5 (22–74)

Tibia 29 11/18 50.2 (27–71)

Humerus 4 2/2 52 (30–88)

Ulna 1 0/1 30

Total 57 24/33 48.5 (22–88)

Table 2 Localization and number of bony non-unions

Proximal Diaphysis Distal Total

Humerus 1 2 1 4

Ulna 0 1 0 1

Femur 3 15 5 23

Tibia 5 11 13 29

Mostly non-unions of the diaphysis or at the lower end of the diaphysiswere treated

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The DCE data sets were then displayed as colour-codedparameter maps on a separate workstation of the MRI unitusing the commercially available software Tissue 4D®(Siemens Healthcare, Erlangen, Germany). Motion correctionof the DCE MRI was performed if movements arose duringthe recording resulting in a more accurate evaluation. Fourregions of interest (ROIs) were determined on the latecontrast-enhanced VIBE MR images: ROI 1 = non-uniongap (bone defect area or zone of impaired bone healing),ROI 2 = proximal bony end adjacent to the gap, ROI 3 =distal bony end adjacent to the gap, ROI 4 = adjacent muscletissue appearing normal on STIR images. In the next step, thefollowing physiologically relevant pharmacokinetic parame-ters for the ROIs were obtained using the Tissue 4D® soft-ware, which uses the established Tofts model for DCE param-eter evaluation [38–40]: K trans = vessel permeability or bettervolume transfer constant between blood plasma and extravas-cular extracellular space, Ve = extravascular extracellular vol-ume fraction and K ep = the constant reflux between theextravascular extracellular space and blood plasma. An aver-age value was calculated for each ROI (or every pixel withinan ROI). The ROI placement was done by a resident undersupervision of a senior musculoskeletal radiologist. Besidesthe semi-quantitative analysis of DCE pharmacokinetic pa-rameters to assess vascularity within the non-union cleft, weevaluated signal intensity curves obtained in the 4 ROIs ofDCEMRI. The DCEMRI measurement in the non-union gapwas performed and referenced to the vascularisation in theadjacent muscle tissue. Signal intensity curves in the non-union cleft were subdivided in those with more intense con-trast agent uptake than muscle tissue (more than 20 % higherrelative enhancement) indicating a vascularised non-union(Fig. 1) and those signal intensity curves with similar or lesscontrast uptake than muscle tissue, indicating a non- or poorlyvascularised non-union (Fig. 2). Also, the parameter iAUC,the initial area under the enhancement curve, was calculatedseparately for all ROIs and also for the ratio ROI 1/ROI 4.

Statistics

The statistical analysis was performed using the statisticalsoftware SAS (Statistical Analysis System) for Windows ver-sion 9.3 (SAS Institute Inc., North Carolina) and R version2.14.1 (http://www.r-project.org). Demographic information

and characteristics of patients with non-unions were reporteddescriptively. All quantitative parameters were summarizedusing descriptive statistics (arithmetic mean, standard devia-tion, median, minimum and maximum), whereas frequenciesand percentages were reported for qualitative variables.Response rates were separated for the group of patients withvascularised non-unions and calculated for the group of patientswith non- or poorly vascularised non-union. Exact confidenceintervals were calculated for the response rates as well as fordifferences of the estimated response rates. The calculation ofthe exact 95 % confidence interval (CI) was based on theFarrington–Manning score statistics. A two-sidedWald asymp-totic test with continuity correction was used to compare theresponse rates. Median and mean of the Tofts pharmacokineticparameters (K trans, Kep, Ve) and the integral of the dynamicsignal intensity curve (iAUC) for the four ROIs were reportedusing descriptive statistics for the total population and separate-ly for CT responders and non-responders at the 12-monthfollow-up CT using the preoperative DCE MRI and the post-operative DCEMRI. Differences between responders and non-responders were tested using the Wilcoxon rank sum tests. Theimpact of the NUSS score on the clinical outcome was evalu-ated using a logistic regression model and results were reportedby means of the odds ratio and 95% confidence intervals of theestimates. In addition, a ROC (receiver operating characteristic)analysis was performed on the median iAUC for ROI 2 andROI 1/ROI 4, which was measured in the preoperative DCEMRI. The sensitivity and specificity were calculated first foreach median iAUC value observed and plotted in a standardROC curve (sensitivity as the y-coordinate and 1−specificity asthe x-coordinate). Also, the Youden index (i.e. sensitivity+specificity−1) was calculated for each median iAUC value.The optimal cut point was determined as the median iAUCshowing the maximum Youden index. All statistical tests wereconsidered explorative.P values less than 0.05were consideredstatistically significant.

Results

DCE MRI evaluation

Despite indwelling osteosynthetic material, DCE parameterswere assessable in 57 of 58 patients (98%, Fig. 1). Correlation

Table 3 Patient population withsex and age distribution and lo-calization and number of bonynon-unions at 1-year follow-upCT

Non-union (localization) Number Gender (female/male) Age (mean, range)

Humerus 3 1/2 39.7 (27–53)

Ulna 0

Femur 21 11/10 48.2 (23–63)

Tibia 25 10/15 57.5 (25–73)

Total 49 22/27 48.5 (23–73)

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a b dc

Fig. 1 A 72-year-old man with tibial non-union. Projection radiograph(a), DCEMRI parameter map for iAUC at first visit (b), VIBE sequenceof DCE MRI together with placement of the ROIs 1–4 at first visit (c),signal intensity curves over time obtained from the ROIs (d). The inset inFig. 1c shows an enlargement of the ROI positioning. Despite the external

fixator, DCE parameters and parameter maps could be calculated withgood quality in 98 % of all patients. This patient had clinical andradiological response and had a substantially higher contrast enhance-ment in the non-union cleft than in adjacent healthy muscle tissuerepresenting a vascularised non-union

a b dc

Fig. 2 A 53-year-old woman with tibial non-union. Projection radio-graph (a), CTat first visit (b), VIBE sequence of DCEMRI together withplacement of the ROIs 1 and 4 at first visit (c), signal intensity curves overtime obtained from the ROIs (d). This patient had radiological non-

response at 12-month follow-up and a comparable contrast enhancementin the non-union cleft to that in adjacent healthy muscle tissuerepresenting a poorly vascularised non-union

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at 1-year follow-up with response on CT could be obtained inn =49 patients and an additional second DCE MRI at the1-year follow-up was available in 19 patients.

A hypervascularised non-union was observed in 41 of the57 patients (71.9 %) and a poorly vascularised non-union in16 patients (28.1 %). Regarding the enhancement curve anal-ysis, a higher contrast uptake in the non-union cleft comparedto muscle tissue indicating a vascularised non-union was anindicator of good CT outcome (P=0.02, Table 4), whereaslower or comparable vascularisation in the non-union cleft tothat in the adjacent muscle in curve analysis indicating ahypovascularised non-union was predictive of a low responserate (Figs. 2, 3 and 4). At first examination, the mean of themedian iAUC values of the ratio ROI 1/ROI 4 was 10.28 inpatients with good outcome compared with 3.77 in non-responders (P =0.02, Fig. 3). ROC analysis yielded asensitivity/specificity of median iAUC for ROI 2 using theoptimal cut-off value (i.e. optimal cut point determined by themaximum Youden index) of 19.99 to predict outcome of73.3 %/43.8 % [95 % CI 54.1–87.7 % and 19.8–70.1 %,respectively] (Fig. 5) and a sensitivity/specificity of medianiAUC for ROI 1/ROI 4 using the optimal cut-off value of 5.89to predict outcome of 51.6 %/1.1 % [95 % CI 33.1–69.8 %and 0.1–34.7 %, respectively]. No significant differences orchanges of all pharmacokinetic parameters (K trans, Kep andVe) measured within the non-union cleft or the adjacent bonecould be observed both at first visit and at the 1-year control(Table 5).

NUSS

More than 75 % of the study population had a NUSS scorebetween 26 and 50 (n =45; 79.0 %), 8 patients had a NUSSscore between 51 and 75 (14.0 %) and 4 patients had a NUSSscore of 25 or lower (7.0 %). Logistic regression analysisrevealed a significant impact of NUSS on the outcome at the12-month follow-up (n =49, P=0.04, odds ratio = 0.93 [95 %CI 0.87–1.0]). From a clinical point of view, the DCE MRIcurve analysis yielded a better combination of sensitivity andspecificity compared with NUSS, which depending on thethreshold was either insensitive or of limited specificity(Table 6).

Radiological outcome

All humeral non-unions met the radiological criteria of union-ization within 6 months postoperatively. Within the 12-monthfollow-up period, 9 femoral and 10 tibial non-unions (33.3 %of the total study population) did not meet the radiologicalcriteria of unionization. In these patients, a median of 6operative procedures had been performed before inclusioninto our study (range 0–25 operations). Bacterial infection ofthe non-union was proven histologically and microbiological-ly in 5 of the 19 CT non-responding patients. The medianNUSS was 44.5 (range 16–72) in the CT non-respondinggroup and 38.8 (range 16–54) in the responding group at the12-month follow-up CT.

Discussion

In the present study, we answered the following two ques-tions: Can DCE MRI assess vascularity within fracture non-unions? Can DCE predict outcome at the 12-month follow-upCT better than the clinical NUSS? We could demonstrate thatDCE MRI is clinically feasible and can assess vascularity innon-union clefts. The non-union site, i.e. the bone defect area,symbolises the heart of the diamond conceptual framework.This zone of impaired bone healing is the area where all thecascade of events of bone repair processes must progress sothat bone continuity can be restored [21]. Thus, the non-uniongap is the centre of the highest biological activity, recentlyreferred to as the “biological chamber” of the diamond con-cept [21]. The presence of vascularity is a “must” prerequisitefor this biological chamber to promote bone healing, becausea good vascular bed guarantees transportation and delivery ofoxygen, nutrients, signalling molecules and osteoprogenitorcell migration [21]. According to our study results, avascularised non-union in DCE MRI is indicative of goodoutcome and thus substantiates the diamond concept. Sincevascularity is crucial for the healing process afterosteosynthesis or application of osteoinductive drugs (accord-ing to the diamond concept [17]), the correlation of goodvascularity of the non-union site demonstrated by DCE MRIand good outcome appears plausible. Also, recent concepts ofpolytherapy for the treatment of non-unions, namely the

Table 4 Results of the DCEMRIcurve analysis

Higher contrast uptake in the non-union cleft compared to muscletissue in DCE MRI indicates avascularised non-union, which inturn is an indicator of good out-come at 12-month follow-up CT

Total Hypervascular non-union Iso- or hypovascular non-union

Evaluable (n) 49 35 14

Responder (n) 31 26 5

Response rate (%) 63.3 74.3 35.7

95 % CI (%) [48.3–76.6] [56.7–87.5] [12.8–64.9]

P value 0.023

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simultaneous application of the three fundamental elements ofthe diamond concept, osteoprogenitor cells, growth factorsand osteoconductive scaffolds, fundamentally are impairedby poor vascularity in the non-union [8]. Thus, attempts topromote the regeneration of the microvascular system at the

fracture non-union site by administration of vascular endothe-lial growth factor have been undertaken [8, 41].

In our study, we also demonstrated that the clinical NUSSscore can predict the outcome and substantiate a previousrecent report [42], but DCE MRI predicts treatment outcome

ba d

e gf h

c

Fig. 3 A 42-year-old woman with tibial non-union. Projection radio-graph at first visit (a), postoperatively (d), at 6-month follow-up (e),1-year follow-up (f) and 2-year follow-up (h). CT at first visit (b) clearlydemonstrates the non-union cleft (arrow). DCE MRI parameter map foriAUC at first visit (c), and at 1-year follow-up (g), the reduction of

vascularity within the non-union cleft (arrows) is clearly depicted, to-gether with progressive fusion evidenced in projection radiographs (seearrow in f). This patient had clinical response and the 2-year control (h)showed complete bony fusion and removal of all osteosynthesis material

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of bony non-unions better and maybe more reproducibly thanthe clinical NUSS score given its better combination of sen-sitivity and specificity and the fact that it sometimes can bevery time consuming or impossible to obtain all items (e.g.

bone quality, original fracture characteristics being closed oropen, number of previous interventions, invasiveness of pre-vious interventions, adequacy of previous surgery, bone align-ment, presence of bone defect, the state of the soft tissues and

a b dc

e

Fig. 4 A 42-year-old man with tibial non-union. DCE MRI parametermap for iAUC at first visit (a), VIBE sequence of DCE MRI togetherwith placement of the ROIs within the non-union (ROI 1), within a vessel(ROI 2) and muscle tissue (ROI 4) at first visit (b), signal intensity curvesover time obtained from the ROIs (e). Projection radiograph at first visitjust before operation with osteosynthesis (c) and at 7-month follow-up

after surgery (d). This patient was a radiological non-responder (see thestill visible non-union cleft in the 7-month control, arrow in d) and had acomparable contrast enhancement in the non-union cleft to that in adja-cent healthy muscle tissue representing a badly vascularised non-union;note the high contrast enhancement curve obtained from the vessel (b)

Fig. 5 Receiver operating characteristic analysis (a). The sensitivity/specificity of iAUC for ROI 2 to predict outcome was 73.3 %/43.8 %.(b) Box plots illustrating the distribution of the median iAUC values in

clinical responders and non-responders at first visit. Response rate tooperative treatment was assessed at the 12-month follow-up using theCT criteria of bone healing as defined in the “Material and methods”

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the American Society of Anesthesiologists grade of the patient[19]) required to validly calculate the NUSS score. Dyke andAaron [30] postulated that a sufficient vascularisation of thebone defect is a prerequisite for proper healing, and thatgrowth factors, cytokines and bone morphogenetic proteinsas bone stimulating agents to enhance bone healing in

persistent fracture non-union [43] can then be used moreefficiently. When bringing these findings and arguments to-gether, DCE MRI is in our opinion a new diagnostic tool thatenables the trauma surgeon to estimate CT outcome andprognosis in bony non-unions. Of clinical relevance, the trau-ma surgeon may decide on the basis of a well-vascularisednon-union in DCEMRI to perform a stable osteosynthesis as aone-step procedure [13, 17, 18]. In the case of a poorlyvascularised non-union in DCE MRI, intensified treatment,such as the Masquelet or induced membrane technique requir-ing a two-steps procedure [32–35], should be taken intoconsideration. Also, the recent concepts of polytherapy forthe treatment of non-unions [8] and attempts to promote theregeneration of the microvascular system at the fracture non-union site by administration of vascular endothelial growthfactors [8, 41] may especially be helpful in poorlyvascularised non-unions. However, since our DCE MRI re-sults have to be substantiated by larger cohorts, all patientswhatever the DCE MRI results might have been should re-ceive standardized operative treatment for the non-union fol-lowing the diamond bone concept.

We could not demonstrate significant changes of the phar-macokinetic parameters K trans, Kep and Ve within the non-union cleft both at first visit and at the 1-year control, butsignificant changes in iAUC and curve analysis. As men-tioned previously [23], the latter parameters are more robust,and in our opinion also more clinically feasible to obtainduring daily routine work. The pharmacokinetic parametersK trans, K ep and Ve might be more influenced by changes intissue composition, especially the extravascular, extracellularspace in the granulation tissue or the reforming bone, thanparameters principally relying on tissue perfusion such asiAUC and the curve analysis. Of note, iAUC (i.e. the integralof the area under the enhancement curve) and the enhance-ment curve itself describe tissue vascularisation and thereforeideally both yield similar sensitivities and specificities.

In principle, contrast-enhanced ultrasound (CEUS) is also aclinically feasible method to assess tissue perfusion [44] and itmay be interesting to compare CEUS with the DCE MRI in afuture study, because CEUS has the advantage of being usedat the bedside.

Limitations

One limitation of our study was the relatively small number ofpatients examined by DCE MRI at the 1-year follow-up,mainly because patients did not show up for the MRI examor consented into the additional follow-up MRI, or they werefully lost at follow-up. Also, we chose not to analyse the entirebone defect using a histogram-type analysis as reported pre-viously for perfusion MRI data [45], but used an ROI analysiswith exactly predefined criteria for the ROI placement.Although user-induced variance in measured DCE data sets

Table 5 Medians and range (in parentheses) of parameters obtained fromROI analysis at first visit

Parameter CT responder CT non-responder P value

K trans

ROI 1 0.14 (0.01–1.21) 0.10 (0.03–0.28) 0.85

ROI 2 0.09 (0.03–1.32) 0.15 (0.06–0.63) 0.08

ROI 3 0.10 (0.01–0.8) 0.08 (0.03–0.65) 0.75

ROI 4 0.02 (0.01–0.05) 0.03 (0.01–0.05) 0.14

Kep

ROI 1 0.25 (0.09–2.22) 0.30 (0.04–3.23) 0.70

ROI 2 0.31 (0.06–22.44) 0.25 (0.21–2.91) 0.81

ROI 3 0.33 (0.07–20.63) 0.42 (0.16–3.25) 0.67

ROI 4 0 .15 (0 .03–0 .46) 0 .09 (0 .03–0 .24) 0 .03

Ve

ROI 1 0.43 (0.02–0.93) 0.59 (0.02–0.94) 0.51

ROI 2 0.40 (0.01–0.84) 0.53 (0.09–1) 0.25

ROI 3 0.34 (0.01–0.85) 0.32 (0.02–0.69) 0.58

ROI 4 0 .14 (0 .03–0 .65) 0 .28 (0 .09–0 .74) 0 .01

iAUC

ROI 1 14.03 (0.34–53) 10.96 (0.63–26.61) 0.83

ROI 2 14.12 (0 .18–41.96) 20.53 (3 .57–44.64) 0 .04

ROI 3 11.7 (0.1–42.17) 10.17 (0.68–31.82) 0.88

ROI 4 1 .55 (0 .38–5 .53) 2 .99 (1 .07–7 .96) 0 .01

ROI 1 /4 5 .89 (0 .34–57.19) 3 .41 (0 .08–10 .63) 0 .02

ROI 1 = fracture non-union gap, ROI 2 = proximal bony end adjacent tothe gap, ROI 3 = distal bony end adjacent to the gap, ROI 4 = adjacentmuscle tissue appearing normal on STIR images. Significant results are inbold

Table 6 Results of the Non-Union Scoring System (NUSS) compared toDCE MRI

Prognosis clinical outcome Sensitivity Specificity

NUSS (cut-off ≤25) 6.5 %(0.8–21.4 %)

88.9 %(65.3–98.6 %)

NUSS (cut-off ≤50) 96.8 %(83.3–99.9 %)

33.3 %(13.3–59.0 %)

DCE curve analysis 83.9 %(66.3–94.5 %)

50.0 %(26.0–74.0 %)

A NUSS between 0 and 25 yields the recommendation of standardtreatment. For a NUSS between 26 and 50, special treatment is recom-mended. For a NUSS between 51 and 75, special treatment and intensi-fied care are recommended. Patients with a NUSS of 76 or more may becandidates for consideration for primary amputation. 95 % confidenceintervals (CI) are given in parentheses

Eur Radiol (2014) 24:449–459 457

using conventional ROI analysis may cause systematic errorsand may impair the value of these techniques for treatmentmonitoring, its reproducibility has been demonstrated to beclinically acceptable [46]. Also, an ROI analysis is faster andmore easy to analyse and thus more feasible in a rou-tine clinical context. We have not performed a DCEMRI subgroup analysis to assess potential DCE MRIdifferences between non-unions of the femur and tibiagiven the pilot character of this study and the limitedpatient number in the subgroups.

We used normal appearing muscle neighbouring the non-union as reference, because this appeared clinically mostfeasible. This has led to the limitation, which may also explainthe differences in muscle iAUC between responders and non-responders, that some muscles that we had to use as referenceshowed higher signal intensities on STIR images than normalmuscle. This was the case in 2 of 31 CT responders (6.5 %)and in 6 of 18 CT non-responders (33 %). Muscular signalchanges on STIR MRI may be caused by denervation orinflammatory changes [44], which both may accompany al-tered tissue perfusion. Thus, since the STIR signal of themuscles used as reference was not balanced between the CTresponders and non-responders of our patient population, thishas to be acknowledged as a limitation. Of course, one maythink of other tissues or structures for normalization, e.g. otherauthors prefer the use of vessels for normalization whenperforming DCE MRI in soft tissue sarcomas and other mus-culoskeletal lesions [47–49]. We refrained from using vessels,because the non-union cleft was usually poorly perfused incontrast to the very high vessel perfusion (Fig. 4) and thus noclear distinction between different perfusion patterns of non-unions may be visualized. Thus, we chose a poorly perfusedtissue, such as the skeletal muscle at rest [50, 51]. In our study,we had subdivided signal intensity curves in non-union cleftsinto those withmore intense contrast agent uptake thanmuscletissue (more than 20 % higher relative enhancement) indicat-ing a vascularised non-union and those signal intensity curveswith similar or less contrast uptake than muscle tissue, indi-cating a non- or poorly vascularised non-union. Of note, ourthreshold of 20 % to discriminate vascularised from poorlyvascularised non-unions by using the signal intensity curveswas arbitrarily chosen on the basis of pilot results and arecommendation in musculoskeletal neoplasms to use such a20 % threshold to differentiate oedematous muscle from tu-mour tissue [52]. However, other authors [47, 48, 53] recom-mended higher thresholds in soft tissue and bone tumoursincluding multiple myeloma, but these (mostly highlyvascularised) tissues cannot be compared to the poorly per-fused tissue within non-union cleft. Finally, we had to assessthe relative enhancement using the Tissue 4D® DCE MRIanalysis software, because the software did not allow for thecalculation of absolute enhancement values that had been usedby recent musculoskeletal DCE MRI studies [49].

In summary, DCE MRI is a clinically feasible tool, espe-cially when performing an enhancement curve analysis, toassess vascularity in non-union clefts. A vascularised non-union in DCE MRI indicates good outcome in CT at the 12-month follow-up. Thus, the prediction of treatment outcomein bony non-unions now seems possible using DCE MRI andthis is of clinical relevance.

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