three-dimensional intercondylar notch volumes in a skeletally immature pediatric population: a...

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Three-Dimensional Intercondylar Notch Volumes ina Skeletally Immature Pediatric Population: A Magnetic

Resonance ImagingeBased Anatomic Comparison of KneesWith Torn and Intact Anterior Cruciate Ligaments

Vimarsha Gopal Swami, B.Sc., Myles Mabee, B.Sc., Catherine Hui, M.D., F.R.C.S.C.,and Jacob Lester Jaremko, M.D., Ph.D., F.R.C.P.C.

Purpose: To determine whether 3-dimensional notch volume, measured with magnetic resonance imaging (MRI), differssignificantly between knees with torn and intact anterior cruciate ligaments (ACLs) after sports injury in a skeletallyimmature pediatric population. Methods: MRI studies of 50 pediatric patients (age range, 10 to 17 years) with ACL tearswere compared with 50 age- and sex-matched intact-ACL control patients. All patients had open physes and underwentMRI after a sports injury. Notch volume was calculated through manual segmentation of notch boundaries seen on axial1.5-T proton densityeweighted images. Two-dimensional (2D) measurements (notch width and notch width index) weremade on coronal proton densityeweighted MRI studies. Notch volume was compared between groups by use of theMann-Whitney U test. Pearson correlation coefficients were also calculated between indices. Results: Notch volume wassignificantly lower in knees with ACL tears than in control knees (5.5 � 1.1 cm3 v 6.4 � 1.5 cm3, P ¼ .002), whereas 2Dnotch width and notch width index did not differ significantly between these groups. Girls had significantly smaller notchvolumes than boys (5.4 � 1.2 cm3 v 6.5 � 1.3 cm3, P < .001). Notch volume was not correlated with age but wasmoderately correlated with 2D notch width (r ¼ 0.485, P < .001). Conclusions: In adolescent patients with sportsinjuries, the 3-dimensional notch volume was significantly smaller in knees with ACL tears than in intact-ACL controlknees. Notch volume was also significantly smaller in girls than in boys and did not vary significantly with age. Level ofEvidence: Level III, case-control study.

arrow femoral intercondylar notch sizes have
Nbeen previously associated with an increased riskof anterior cruciate ligament (ACL) rupture.1 The ACLcontacts the top of the intercondylar notch in full kneeextension, leading to increased forces on the mid-substance of the ligament.2,3 In flexion the ACL isstretched over the inner margin of the lateral femoral
From the Department of Radiology and Diagnostic Imaging (V.G.S., J.L.J.),epartment of Biomedical Engineering (M.M.), and Division of Orthopedicurgery, Department of Surgery, (C.H.), University of Alberta, Edmonton,lberta, Canada.The authors report that they have no conflicts of interest in the authorship

nd publication of this article.Received April 4, 2013; accepted August 24, 2013.Address correspondence to Jacob Lester Jaremko, M.D., Ph.D., F.R.C.P.C.,epartment of Radiology and Diagnostic Imaging, 2A2.41 WC Mackenzieealth Sciences Centre, University of Alberta, 8440 112 St, Edmonton,lberta T6G 2B7, Canada. E-mail: [email protected]� 2013 by the Arthroscopy Association of North America0749-8063/13223/$36.00http://dx.doi.org/10.1016/j.arthro.2013.08.031

954 Arthroscopy: The Journal of Arthroscopic and Related Surg

condyle.4 Bony intercondylar notch morphology mayespecially contribute to ACL injury risk in the skeletallyimmature pediatric age group, whose growth plateshave not yet fused.5

Although there have been many studies looking at theassociation between notch size and the risk of ACLrupture (Table 1), most used 2-dimensional (2D) indicesof notch size. Most studies relating 2D notch dimensionstoACL tear status have been in adults,with few studies ina pediatric population.5-7 These studies present variedresults,with somefinding associations between2Dnotchsize and ACL injury risk5,8-11 and others finding no suchtrends.12-14 These inconsistencies may be due in part tothe use of 2D indices, which may not adequately repre-sent the complex 3-dimensional (3D) structure of thenotch.4,15,16 We know of only 2 studies, both in adults,comparing 3D notch geometry in populations with andwithout ACL tears. Simon et al.17 found smaller notchvolumes in ACL-injured knees, whereas van Eck et al.15

observed the opposite trend; however, neither trendreached statistical significance.

ery, Vol 29, No 12 (December), 2013: pp 1954-1962

mailto:[email protected]

http://dx.doi.org/10.1016/j.arthro.2013.08.031

Table 1. Overview of Studies on Notch Size and Their Relation to ACL Injury

AuthorsMeasurement

Type Methodology Population Population Size Findings Additional Comments

Andersonet al.4

(1987)

2D Retrospective: ratio ofnotch width to condylar

width (computedtomography)

Adults 48 (31 injuredand 17

uninjured)

Narrower notch indices in injuredpatients

No age/sex matching ofcohorts

Domzalskiet al.5

(2010)

2D Retrospective: NWI(MRI)

Skeletallyimmaturepatients

90 (46 injuredand 44

uninjured)

NWI significantly smaller in injuredpatients

Age matching of controls

Vyas et al.6

(2011)2D Retrospective: NWI

(radiography)Skeletallyimmaturepatients


uninjured)

NWI not significantly different betweengroups

Small sample size; no age/sexmatching

LaPrade andBurnett8

(1994)

2D Prospective 2-yr follow-up: NWI (radiography)

Adultathletes


uninjured)

NWI significantly smaller in patientsinjured during pivoting or cutting sports

Shelbourneet al.9

(1998)

2D Prospective: notchwidth (radiography)

Adults 714 (all withunilateral injury,

27 withcontralateral

injury)

Higher rate of contralateral ACL injuryin patients with narrow notch width

Shelbourneet al.10

(1997)

2D Retrospective: notchwidth (radiography)

Adult andpediatricpatients


uninjured)

Notch width significantly smaller ininjured patients


Uhorchaket al.11

(2003)

2D Prospective 4-yr follow-up: notch width andNWI (radiography)

Adultcadets


uninjured)

Notch width and NWI significantlysmaller in injured patients

Results held true whenanalyzing male and female

patients separatelyHerzog

et al.12

(1994)

2D Retrospective: notchwidth and NWI (MRI)

Adultathletes


uninjured)

Notch width and NWI not significantlydifferent between groups

Age and sex matching ofcontrols

Lombardoet al.13

(2005)

2D Prospective 11-yrfollow-up: NWI(radiography)

Adultathletes


uninjured)

NWI not significantly differentbetween groups

Male patients only

van Ecket al.15

(2011)

3D, 2D Retrospective: 3D notchvolume and

arthroscopic notchwidth and height (MRI)


uninjured)

No significant difference in notchvolume between groups; smaller notchvolume in men; no correlation between

notch volume and width


Simonet al.17

(2010)

3D, 2D Prospective: 3D notchvolume, notch width atinlet, and notch width

at outlet (MRI)


uninjured)

Smaller notch volume in injured group,but not statistically significant; smaller

notch width in injured group

Small sample size; nocalculation of statistical

power; method of measuringnotch volume not described

Charltonet al.20

(2002)

3D, 2D Prospective: 3D notchvolume and notch

width (MRI)

Adults 48 (alluninjured)

3D notch volume smaller in women,related to smaller height in women

No injured patients included

Sonnery-Cottetet al.22

(2011)

2D Retrospective: NWI(MRI)


uninjured)

NWI significantly smaller in injuredpatients

Age and sex matching ofcontrols

3D NOTCH VOLUMES IN KNEES WITH ACL TEARS 1955

An understanding of the morphologic risk factors forACL injury is particularly important in skeletallyimmature patients because of the long duration ofmorbidity from an ACL tear and the potential for age-related changes in notch development. ACL injuriesare also becoming increasingly common in teensbecause of increased participation in high-levelsports.18,19 Identification of risk factors could lead tomore effective strategies for the prevention of ACL tearsin these patients, and the use of 3D indices of notchdimension may prove more useful than prior 2Dindices. No previous studies have evaluated the asso-ciation between 3D notch volume and ACL injury ina skeletally immature pediatric population.

The purpose of this study was to determine whether3D notch volume, measured using magnetic resonanceimaging (MRI), differs significantly between knees withtorn and intact ACLs after sports injury in a skeletallyimmature pediatric population. We hypothesized thatknees with torn ACLs would have a smaller notchvolume than knees without ACL injury.

Methods

Patient SelectionEthics approval was granted by our institutional re-

view board for this retrospective case-control study. Wesearched for patients imaged at 3 tertiary hospitals who

Fig 1. Flow diagram outlining patientsearch strategy and selection of patientcohorts.

1956 V. G. SWAMI ET AL.

(1) had routine knee MRI performed between July2005 and July 2010 to assess a sports-related injury, (2)were aged 10 to 17 years at the time of MRI, and (3)had open distal femoral and proximal tibial physes onMRI with a line of high T2 signal centrally in the physesindicating a lack of confluent ossification across thegrowth plates. This search yielded 763 patients and isoutlined in Fig 1. We excluded 45 patients with priorknee surgery, 32 with known previous knee ligamentinjury, 14 with morphologic knee abnormalities, 13with displaced fractures, and 8 with imaging orarthroscopic evidence of degenerative joint disease. Ofthe remaining patients, 113 had MRI evidence of ACLinjury but 51 were excluded because of lack ofarthroscopic confirmation of a full-thickness tear. Ofthe 62 eligible patients with an arthroscopically con-firmed full-thickness acute ACL tear visible on preop-erative MRI, 50 patients (22 girls) were randomlyselected for inclusion in this study. The control groupconsisted of patients who met the previously men-tioned inclusion and exclusion criteria, except that theyhad (1) a normal-appearing intact ACL and no dis-placed meniscal tear or other ligament rupture on MRI,(2) no evidence of a positive anterior drawer test and/orLachman test in the clinical chart, and (3) eithersurgical confirmation of an intact ACL (e.g., from latersurgery for other pathology such as meniscal tear) or norecord of subsequent surgical knee procedures ina medical record database. Of the 538 patients withoutACL injury, we excluded 57 with displaced meniscaltears and 36 with other ligamentous pathology. Fromthe remaining patients, we sequentially selected the

first 50 patients for the control group whose age and sexcould be matched to a corresponding patient with anACL tear.

Sample Size CalculationOn the basis of our preliminary analysis and prior

studies by van Eck et al.15 and Charlton et al.,20 weexpected notch volumes to average 5 cm3 with an SD of1.4 cm3. Assuming a 1-cm3 (20%) difference in notchvolumes between knees with intact and torn ACLs to beclinically significant, at a ¼ 5% and b ¼ 20% (80%power), we required 62 knees. Our sample size (N ¼100) exceeded this requirement.

Magnetic Resonance ImagingRoutine clinical knee MRI was performed on 1.5-T

scanners (Siemens AG, Munich, Germany), consistentwith standard practice at our center. Scans were co-localized (i.e., all voxels in each sequence were in thesame 3D coordinate system). For notch calculations, weused coronal proton densityeweighted images (thick-ness, 3.0 mm; matrix size, 512 � 256; 16 � 16ecm fieldof view; repetition time/echo time, 1,800 milliseconds/13 milliseconds) and axial proton densityeweightedimages with fat saturation (thickness, 4.0 mm; 384 �230 matrix; 16 � 16ecm field of view; repetition time/echo time, 3,000 milliseconds/42 milliseconds).

Anatomic MeasurementsAll image analysis was performed offline with

OsiriX open-source image processing software (Geneva,Switzerland). The 3D notch volume was measured

Fig 2. Notch volume measurementtechnique shown on axial MRI slices.The borders of the notch are outlined inred, and the posterior notch border isformed by a line connecting the pointson the internal surface of the femoralcondyles where the cartilage ends. Thefirst image (A) is the most distal slicewith bony continuity between thefemoral condyles. The last image (D) isthe most proximal slice with bothfemoral condyles clearly visible. Notchvolume was calculated by summing theareas bounded by the red line on eachslice and multiplying by slice thickness.


from axial images as described by Charlton et al.20 andvan Eck et al.15 (Fig 2). First, the femoral intercondylarnotch border was manually segmented on all 2D axialMRI slices showing the notch with boundaries definedfrom previously described anatomic landmarks.15 Tocalculate notch volume, the notch area on each axialslice was summed and multiplied by slice thickness.We also assessed the correlation between notch

volume and MRI-based 2D notch measurements (notchwidth and notch width index [NWI]) in this youngpopulation. The 2D notch measurements were per-formed using the technique described by Souryalet al.21 and applied to coronal MRI slices, per Sonnery-Cottet et al.22 The coronal slice best showing thepopliteal groove was selected. We recorded the width ofthe intercondylar notch, in centimeters, as well as theNWI, representing the notch width divided by thebicondylar width, on the same image (Fig 3).

Reliability AnalysisWe assessed the reliability of 3D notch volume

measurement as described by van Eck et al.15 To assessinterobserver reliability, 2 observers with extensivetraining in knee anatomy on MRI under the guidance ofa dual pediatric and musculoskeletal fellowshipetrained

radiologist each separately segmented notch volumes forthe entire dataset (N¼ 100). For intraobserver reliability,observer 1 performed analysis on 2 separate occasions 8weeks apart. The observers were blinded to clinical anddemographic information, as well as to numeric notchvolumes during the current or previous reading session,while drawing contours. They could not be blinded to thestatus of the ACL because this was obvious on inspectionof the images. After the reliability analysis, all other dataanalyses in this study were performed using themeasurements made by observer 1.

Statistical AnalysisData were tabulated in Microsoft Excel (2010 version;

Microsoft, Redmond, WA) and statistics calculated withSPSS software (version 19; SPSS, Chicago, IL). Contin-uous variables were described as mean � standarddeviation. Using a level of significance of P ¼ .05, wetested differences between means of continuous vari-ables using unpaired, 2-tailed Student t tests whenapproximately normal distributions could be confirmedfrom review of frequency tables and using the Mann-Whitney U test otherwise. Bivariate Pearson correla-tion coefficients and their level of significance werecalculated between clinical indices, ACL tear status, and

Fig 4. Distribution of notch volumes for torn-ACL group (n ¼50) (clear bars) and control group (n ¼ 50) (solid gray bars).

Fig 3. Two-dimensional measurements of notch geometry.The NWI is the ratio of the intercondylar notch width (a) tothe bicondylar width of the distal femur (b) at the level of thepopliteal groove.

Table 2. Demographic Features and 2D and 3D NotchMeasurements in Patients With Torn and Intact ACLs

Torn ACL Intact ACL P Value

Sex 28 boys and22 girls

28 boys and22 girls

Not applicable

Age (yr) 14.8 � 0.9 14.2 � 0.7 .001Notch width (cm) 2.03 2.04 .836NWI 0.283 0.287 .3973D notchvolume (cm3)

5.5 � 1.1 6.4 � 1.5 .002*

*P value calculated with Mann-Whitney U test.


notch indices. We determined the correlation betweennotch indices and age using Pearson correlation coeffi-cients, and we assessed differences in notch volumebetween boys and girls using Student t tests. Interob-server and intraobserver reliability of 3D notch volumemeasurement was assessed by calculating intraclasscorrelation coefficients (ICCs) for single measures usinga 2-way mixed-effects model.

ResultsPatient demographic characteristics and 2D and 3D

notch measurements are shown in Table 2. The 2Dnotch width measurements were not significantlydifferent between the torn-ACL and intact-ACLcohorts, but the 3D notch volume was significantlylower in knees with ACL tears compared with controlknees (5.5 � 1.1 cm3 v 6.4 � 1.5 cm3, P ¼ .002). Thedistribution of 3D notch volumes was somewhatbimodal for both cohorts, with a peak near 5 cm3 (tornACL) and 6 cm3 (control) and a second smaller peaknear 10 cm3 (Fig 4). Because this was not a classicalnormal distribution, we tested the significance of thedifference between means using a nonparametricMann-Whitney U test, allowing use of t tests. Post hocanalysis of statistical power showed a 91.1% power,given our sample size, to detect a 0.9-cm3 difference in3D notch volume between torn-ACL and intact-ACLknees. The 2D notch indices were closer to normal indistribution.

Notch volume was not correlated with age (r ¼ �0.070,P ¼ .507). There were no significant differences innotch width or NWI between the torn-ACL and intact-ACL groups. Subgroup analysis was performed by sex(Table 3). Girls had significantly smaller notch volumesthan boys overall, with a mean of 5.4 cm3 versus 6.5 cm3

(P < .001). The trend toward smaller notch volumes inpatients with torn ACL ligaments was significant in boys(P ¼ .002) but did not reach significance in girls (P ¼ .13)for this sample size.Table 4 shows correlations between notch volumes

and other 2D morphologic parameters. Notch volumewas not correlated with NWI (r ¼ 0.126, P ¼ .213).Notch volume was moderately correlated with notchwidth, and this correlation was statistically significant(r ¼ 0.485, P < .001).

ReliabilityInterobserver reliability of 3D notch volume mea-

surement was high. The ICC was 0.890 (95% confidenceinterval [CI], 0.841 to 0.925) for the entire dataset, 0.931(95% CI, 0.882 to 0.960) for the torn-ACL group, and0.875 (95% CI, 0.790 to 0.927) for the control group.Intraobserver reliability of 3D notch volume measure-ment was also high. The ICC was 0.923 (95% CI, 0.887to 0.947) for the entire dataset, 0.864 (95% CI, 0.772 to0.921) for the torn-ACL group, and 0.946 (95% CI, 0.907to 0.969) for the control group. For consistency, all dataanalyses were performed using measurements made byobserver 1. As an internal check, the main finding of this

Table 3. Sex-to-Notch Volume Subgroup Analysis

No. ofPatients

Notch Volume (cm3)(Mean � SD)

P Value(Mann-Whitney

U test)

Boys 56 6.5 � 1.3 <.001Girls 44 5.4 � 1.2Boys with

torn ACLs28 5.9 � 1.0 .002

Boys withintact ACLs

28 7.0 � 1.4

Girls with tornACLs

22 5.0 � 1.0 .134

Girls withintact ACLs

22 5.7 � 1.3


article also held true for the measurements of observer 2,which were made independently of observer 1, furthervalidating the results of this study: 3D notch volume wassignificantly lower in knees with ACL tears than incontrol knees (P < .05). There were no significant dif-ferences in notch volume measurements made byobserver 1 versus observer 2 (P > .05).

DiscussionACL injuries are becoming increasingly common in

young patients,23 and it is important to understand theanatomic risk factors for ACL injury in this age groupbecause of the long duration of morbidity from an ACLtear and the potential for age-related changes in notchdevelopment. This study shows that 3D intercondylarnotch volume was significantly lower in knees withACL tears than in healthy control knees with intactACLs in a skeletally immature pediatric population.This is the first study of 3D notch volumes in pediatricknees with open physes.There have only been 2 previous studies examining

the predisposing anatomic factors leading to ACL injuryusing 3D parameters. Both studies were performed in anadult population. Similar to our findings, Simon et al.17

found a smaller 3D MRI notch volume in their ACL-injured patients compared with the control group.However, their results did not reach statistical signifi-cance (P ¼ .24), likely because of a smaller sample size(N ¼ 54). The opposite trend was found by van Ecket al.15: the 3D notch volumes tended to be larger inknees with torn ACLs. Again, their results did not reach

Table 4. Pearson Correlation Coefficients (r) Between 3D Notch

ACL Tear Status Notch Volume

ACL tear status 1 �0.315 (P ¼ .001)* 0Notch volume 1 0Popliteal widthNotch widthNWI

*Significant correlation (P < .05).

statistical significance (P ¼ .054). Furthermore, theACL-injured and control groups were not sex matched,and the ACL-injured group contained more men thanthe control group. Because men have been shown tohave larger notch volumes than women, this mayaccount for the trend toward larger notch volumes inthe ACL-injured group in their study. In addition, thecontrol group included patients with meniscal injurywho did not have healthy knees.We accounted for thesefactors in the design of our study because our ACL-injured and control groups were matched for sex andage and all of our control patients had normal or nearlynormal MRI findings without evidence of ligamentousinjury, displaced meniscal tears, or bony pathology,therefore adding to the strength of our study results.We confirmed that 3D notch volumes could be

determined on routine clinical MRI studies with highinterobserver and intraobserver reliability. In addition,the notch volume magnitudes measured in our studywere similar to those previously reported in adults,15,20

and notch volume did not vary significantly by patientage in our study.Our notch volume measurement technique, which

involved manual segmentation of MRI slices andmultiplication of the segmented area by slice thickness,was based on the available evidence and has previouslybeen validated in a porcine model for measurement ofACL volume.24 Because previous studies have shownthat notch volume measurement on high-resolutionresearch-grade MRI scans is not superior to usingroutine clinical MRI scans,15 we used routine scans inthis study. Notch contours were drawn per the methodsof previous reports.15,20 Studies have shown that theposterior notch border, at the edge of the condylarcartilage, can be reliably identified,20,25,26 and this wasconfirmed by our observers.Despite extensive study (Table 1), 2D indices of notch

geometry have shown inconsistent associations withACL injury in the literature,5,6,22 possibly because oflimitations of 2D measurements in characterizing 3Dnotch geometry. The 3D notch volume may be morerelevant than a single 2D measurement because thenotch-ACL interaction changes throughout the range ofmotion. Our findings suggest that a small intercondylarnotch volume may predispose skeletally immaturepatients to ACL injury and serve as motivation for

Volume and 2D Notch Measurements on MRI

Popliteal Width Notch Width NWI

.068 (P ¼ .504) �0.021 (P ¼ .836) �0.086 (P ¼ .397)

.535 (P < .001)* 0.485 (P < .001)* 0.126 (P ¼ .213)1 0.676 (P < .001)* �0.069 (P ¼ .494)

1 0.685 (P < .001)*1


future prospective work. Precautionary measures alongwith proper education and counseling may be consid-ered when one is managing adolescent athletes,particularly those with small MRI notch volumes whoare participating in high-risk sporting activities. Forinstance, if an active athlete is found to have a smallnotch volume below age- and sex-adjusted populationvalues on a knee MRI scan obtained to assess this or forother reasons, precautionary measures such as activitymodification, additional training regimens, or quadri-ceps and hamstring muscle strengthening may be rec-ommended to reduce the risk of ACL injury,27 perhapswith more urgency than in the absence of anatomic riskfactors. Understanding intercondylar notch morphol-ogy and its association with ACL injury risk is alsoimportant for surgeons performing arthroscopic ACLreconstruction. If a narrow notch is noted intraop-eratively, a notchplasty may be performed by the sur-geon with the goal of widening the notch to decreasethe risk of graft impingement,21 although the efficacy ofthis has not been firmly established.Future studies are warranted to explore the reasons

why 3D notch volume was smaller in skeletallyimmature knees with torn ACLs, as well as how it maypredispose patients to ACL injury. A smaller notch mayresult in an increased concentration of stresses on themidsubstance of the ACL during knee extension whenthe ACL contacts the top of the notch, as well asincreased force on the ligament when it is stretchedacross the lateral femoral condyle in flexion. Ourfinding of a significant association between a low 3Dnotch volume and ACL tears can serve as the basis forbiomechanical studies to assess the strain on the ACLwith simulated weight bearing in knees with varyingnotch volumes and shapes. In addition, Simon et al.17

reported that notch volume was correlated with ACLvolume and that smaller notches and ACL volumesmay predispose patients toward ACL injury. Differencesbetween our findings and those of van Eck et al.15 in anadult population may be because of more rigorouspatient selection (age and sex matching) in our studyand/or morphologic differences between skeletallyimmature and adult knees. Our study design did notpermit us to assess this, and future work could includedirect comparison of 3D notch geometry betweenskeletally immature adolescents and adults.The secondary aims of this study were to correlate 3D

notch volume to demographic factors (age, sex) andMRI-based 2D notch measurements (notch width, NWI)in our pediatric population. We found that girls hadsignificantly smaller notch volumes than boys, confirm-ing trends previously observed among adults.15,17,25,26

Consistent with our findings, it is known that femalepatients have smaller notch sizes,20,28-30 and this mayexplain why female patients have a greater propensityfor ACL injury than male patients.31,32 However, it is

important to note that the smaller notch volumesobserved in female patientsmay have been due in part todifferences in height and weight, which could not bemeasured or controlled for in this retrospective study.There was a statistically significant difference in age

between the ACL-injured and control patients in thisstudy (Table 2). Exact month-by-month age matchingwas not possible because of limitations in patientavailability. However, the actual difference in agebetween groups was small: on average, patients withACL tears were 7 months older than control patients(mean age, 14.8 years v 14.2 years). This difference isunlikely to be meaningful, especially because notchvolume was not significantly correlated with age,consistent with findings in adults.15

Notch volume was also not significantly correlatedwith NWI, the most commonly used method of evalu-ating 2D notch size on radiographs21 and MRI scans.5,22

This is consistent with prior findings in cadavers.25 Apotential explanation may be that NWI is a ratio thattakes into account the size of the knee, dividing notchwidth by the bicondylar width of the femur, whereasnotch volume is an absolute measurement. Althoughthere are no validated techniques described in theliterature to date to express notch volume in relation toknee size, it is possible that such an index may showa correlation to NWI.Notch volumewasmoderately correlated toMRI-based

2D notch width (r ¼ 0.485, P < .001). These findingsdiffer from the results of van Eck et al.,15 who found nocorrelation between MRI 3D notch volume and arthros-copic measurements of notch width. However, anotherstudy on cadavers found a moderate correlation(r ¼ 0.551), similar to the level observed in our study,between notch volume measured on computed tomog-raphy and notch width at the middle of the notchmeasured arthroscopically.26 It would seem logical fornotch width and volume to be correlated because notchwidth is a determinant of volume. The reason for thesediscrepant results is not clear and may relate to differingor non-standardized locations for notch width measure-ment during arthroscopy. To our knowledge, there is nodescribed technique in the literature to assess 3D notchdimensions arthroscopically. Given that we found nosignificant differences in 2D measures (notch width andNWI) between the ACL-injured and control groups, 3Dindices of notch morphology on MRI may prove moreuseful in the prediction of ACL injury risk than these 2Dindices have to date.

LimitationsThis study had limitations. First, although we showed

a significant difference in notch volume between theACL-injured and control groups, our retrospective-cohortstudy design shows only associations, not causation. Aprospective study would be necessary to more fully assess


the influence of 3D notch morphology on the risk ofdeveloping ACL injury. Second, as with all studiescomparing morphologic features in knees with intact andtorn ACLs, there is no way to ascertain whether patientsin the intact-ACL group have not gone on to injure theirACL beyond the study period. Third, the status of the ACLcould not generally be confirmed arthroscopically for allcontrol patients. However, MRI has been shown previ-ously to be highly sensitive and specific in the diagnosis ofACL tears.31,32 Furthermore, we reviewed the compre-hensive medical record database, and none of the patientsincluded in our control group had documented physicalexamination findings of an ACL tear or any record ofsubsequent knee injury on MRI or knee surgery. Patientheight and weight were not taken into account because ofthe retrospective nature of our study. It has previouslybeen shown that body mass index is not correlated withnotch volume,15 but there is some evidence for a corre-lation between patient height and notch volume.20

Although the age and sex matching in this study wasmore rigorous than prior work on 3D notch volume15 andlikely minimized differences in patient height betweencohorts, future prospective work should also directlymatch patients for height and weight. In addition, thevalidity of the MRI notch volume measurements taken inthis study was not compared with arthroscopic parame-ters. As we noted earlier, an accurate technique tomeasure notch volume arthroscopically has not beenestablished. Moreover, the methods used in this studyhave recently been used and validated in determiningACL volume from MRI scans.20,24 Lastly, there may bemerit in developing and validating a measurement tool infuture studies to express notch volume in relation to kneesize, similar to the NWI.

ConclusionsIn adolescent patients with sports injuries, the 3D

notch volume was significantly smaller in knees withACL tears than in intact-ACL control knees. Notchvolume was also significantly smaller in girls than inboys and did not vary significantly with age.

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three-dimensional intercondylar notch volumes in a skeletally immature pediatric population: a...

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