Elbow Position Affects Distal Radioulnar Joint Kinematics

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SCIENTIFIC ARTICLE

Elbow Position Affects Distal Radioulnar Joint

KinematicsEric Fu, BS, Guoan Li, PhD, J. Sebastiaan Souer, MD, Santiago Lozano-Calderon, MD,

James H. Herndon, MD, Jesse B. Jupiter, MD, Neal C. Chen, MD

Purpose Previous in vivo and in vitro studies of forearm supination–pronation suggest thatdistal radioulnar joint kinematics may be affected by elbow flexion. The primary hypothesestested by this study were that, in vivo, ulnar variance changes with elbow flexion and forearmrotation, and the arc of forearm rotation changes in relationship to elbow flexion.

Methods Changes in radioulnar kinematics during forearm supination–pronation and elbow flexion(0° to 90°) were studied in 5 uninjured subjects using computed tomography, dual-orthogonalfluoroscopy, and 3-dimensional modeling. Analysis of variance and post-hoc testing was performed.

Results Proximal translation of the radius was greatest with the elbow flexed to 90° with the armin midpronation. With the arm in midpronation, the translation of the radius was significantlygreater at 0° versus 45° of elbow flexion (0.82 � 0.59 mm vs 0.65 � 0.80 mm, F: 4.49, post hoc:0.055; p � .05) and significantly smaller at 45° versus 90° of elbow flexion (0.65 � 0.80 mm vs0.97 � 0.35 mm, F: 4.49, post hoc: 0.048; p � .05). Proximal translation of the radius inmidpronation was significantly greater than when the forearm was in a supinated position whenthe elbow was at 0° or 90° flexion (F: 14.90, post hoc: �0.01; p � .01, F: 19.11, post hoc: �0.01,p � .01). The arc of forearm rotation was significantly decreased at 0° compared with 90° ofelbow flexion (129.3° � 22.2° vs 152.8° � 14.4°, F: 3.29, post hoc: 0.79; p � .09). The centerof rotation shifted volarly and ulnarly with increasing elbow extension.

Conclusions Elbow position affects the kinematics of the distal radioulnar joint. The kinemat-ics of the distal radioulnar joint are primarily affected by forearm rotation and secondarilyby elbow flexion. These findings have clinical relevance to our understanding of ulnarimpaction, and how elbow position affects the proximal–distal translation of the radius. Thesefindings have implications for the treatment of ulna impaction, radiographic evaluation of thedistal ulna, and future biomechanical studies. (J Hand Surg 2009;34A:1261–1268. © 2009Published by Elsevier Inc. on behalf of the American Society for Surgery of the Hand.)

Key words Elbow, distal radioulnar, kinematics, biomechanics, forearm.

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ADIOGRAPHIC AND CADAVERIC studies have beenused to investigate in vivo and in vitro ulnarvariance changes with forearm supination and

ronation, in healthy and pathologic subjects.1–4 Ulnarariance is standardized in these studies, where thehoulder is placed in 90° of abduction and the elbow

From the Hand and Upper Extremity Service and the Bioengineering Laboratory, Department of Ortho-paedics, Massachusetts General Hospital, Boston, MA.

The authors thank Jeffrey Bingham, Shaobai Wang, and Michal Kozanek for their technical assistance.

Received for publication July 8, 2008; accepted in revised form April 20, 2009.

Support was received from the American Foundation for Surgery of the Hand/the American Societyfor Surgery of the Hand (N.C.C., J.H.H., J.B.J., and G.L.). The authors gratefully acknowledge the finan-cial support of the Department of Orthopaedic Surgery at Massachusetts General Hospital. J.B.J. re-
ceived support from the AO Foundation, Smith and Nephew, Wright Medical, Small Bone Innova- d
exed to 90° with the forearm in neutral rotation. How-ver, it is unclear whether the position of elbow flexionntrinsically affects ulna variance.

Tay et al. used finite helical axis analysis to describehe in vivo kinematics of the forearm and found prox-mal–distal translation of the radius relative to the ulna

ions, Joint Active Systems, and the Orthopaedic Trauma Association. G.L. received support from theational Institutes of Health. J.S.S. received support from Stichting Annafonds, Leiden, The Nether-

ands;andStichtingMichaelvanVlotenFonds,Rotterdam,TheNetherlands.Thisstudywasfundedbybasic science grant from the American Foundation for Surgery of the Hand and the American Society

or Surgery of the Hand.

orresponding author: Neal C. Chen, MD, Medsport, University of Michigan, 24 Frank Lloyd Wrightrive, P.O. Box 0391, Ann Arbor, MI 48106; e-mail: [email protected].

363-5023/09/34A07-0014$36.00/0

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© Published by Elsevier, Inc. on behalf of the ASSH. � 1261

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1262 ELBOW POSITION AFFECTS RADIOULNAR KINEMATICS

with rotation of the forearm,5 suggesting that elbowposition may affect distal forearm kinematics. How-ever, because imaging techniques such as computedtomography (CT) and magnetic resonance imaginghave geometric limitations, and plain radiography is notas precise as these other techniques, the effect of elbowflexion on the distal radioulnar (DRU) joint has notbeen well studied.

Recently, a combined dual fluoroscopic and mag-netic resonance–CT imaging technique was developedthat allows measurement of joint motion without con-straints of the target joint.6,7 This technique has beensuccessfully applied to the lower extremity.8–10 In thisstudy, we used this imaging technique to explore therelationship of DRU joint kinematics with elbow andforearm positions. The primary hypotheses tested arethat proximal-distal translation of the radius relative tothe ulna changes in relationship to elbow flexion andforearm rotation, and that the arc of forearm rotationchanges in relationship to elbow flexion.

MATERIALS AND METHODSThis study was approved by the institutional reviewboard and informed consent was obtained for eachsubject. Five right-dominant upper extremities withouta history of previous injury from right hand–dominantmale volunteers (ages 24–31 years) were imaged by CTwhile in neutral forearm rotation and elbow extension(GE Light Speed Pro 16-Slice scanner; General Elec-tric, Milwaukee, WI). Parallel axial CT images sepa-rated at 0.625 mm with a resolution of 512 � 512 pixelswere obtained. Each CT image was processed using aCanny filter programmed in a commercially availablemathematics software package (Matlab; Mathworks,Canton, MA). The Canny filter calculates gradients inpixel intensity to detect edges between objects. Thecalculated edges were used to help trace the outlines ofthe radius and ulna within the axial plane image usingsolid-modeling software (Rhinoceros; Robert McNeel& Associates, Seattle, WA). The contours were thenplaced in the appropriate plane in a 3-dimensionalspace, and the surface was meshed using the solid-modeling software.

After we constructed CT image-based computer3-dimensional models, each subject was imaged using 2orthogonally placed fluoroscopes (BV Pulsera; Philips,Bothell, WA) at elbow flexion angles of 0°, 45°, and90° as the subject rotated his forearm from maximumsupination, 50% supination, neutral, 50% pronation,and maximum pronation (Fig. 1). The wrist was placedin 0° of extension and 0° of radioulnar deviation, and
the metacarpophalangeal, proximal interphalangeal,
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and distal interphalangeal joints were held in extension.These positions were confirmed with a goniometer.Each subject practiced the testing positions 5 timesbefore actual testing. The degree of pronation–supinationwas estimated from the preconditioning trials with theunderstanding that the true amount of rotation would bedefined with the modeling techniques. During imaging,the plane of the C-arm was coplanar to the floor and thesubject was seated with the elbow stabilized on a radiolu-cent table (Fig. 1). The subject held each position, whichwas confirmed with a goniometer, and then the image wasacquired. The entire testing procedure took about 10 min-utes.

The orthogonal images were then imported into solidmodeling software (Rhinoceros) and used to determinethe in vivo forearm positions at each of the targetedflexion angles. The orthogonal images were processedin the software to reproduce the positions of the 2intensifiers of the fluoroscope during image acquisition.The forearm model was imported into the virtual spaceand was viewed simultaneously from 2 orthogonal di-rections corresponding to the positions of the x-raysource of the fluoroscope during image acquisition. The3-dimensional forearm model was then manipulated in6 degrees of freedom (6DOF) inside the 3-dimensionalC-arm until its projections viewed from 2 orthogonaldirections matched the outlines of the fluoroscopic im-ages obtained, thus reproducing the in vivo forearmposition using the 3-dimensional anatomical models.The method has been previously validated on a kneemodel to an accuracy of 0.1 mm position and 0.1°rotation.6,7

To describe the 6DOF kinematics of the DRU joint,anatomically based ulnar and radial Cartesian coordi-nate systems similar to those reported previously in theliterature were constructed for each subject (Fig. 2). Theposition of the forearm in neutral position during CTscanning was used as a reference. For the ulnar coor-dinate system, the longitudinal axis (z-axis) was definedas the line passing through the longitudinal axis of theulna. The y-axis was defined as the line passing fromthe center of the ulnar head through the center of theulnar styloid base. The x-axis was an axis perpendicularto those 2 axes pointing dorsally. The radial longitudi-nal axis (z-axis) was the long axis of the radius. They-axis was defined as the line passing through theanatomic center of the distal radius to the tip of theradial styloid. The x-axis was an axis perpendicular tothose 2 axes pointing dorsally. In this manner, a coor-dinate system for each wrist could be established in aconsistent way. The axes used to describe the radius are
similar to those used clinically to describe the forearm.
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ELBOW POSITION AFFECTS RADIOULNAR KINEMATICS 1263

The 6DOF kinematics of the wrist was de-scribed by the relative position and orientation ofthe distal radius with respect to the ulnar head

FIGURE 1: Dual fluoroscopic imaging system. Subject with elbowat elbow flexion angles of 0°, 45°, and 90° with the forearmmaximum pronation.

FIGURE 2: The coordinate systems used to describe radioulnarthe center of the ulna styloid, was used to delineate supination

using a script written by our laboratory for Rhi-

JHS �Vol A, Sep

noceros solid modeling software. The 3-dimen-sional positions were determined by the position ofthe origin of the distal radius coordinate system in

45° flexion and forearm in neutral rotation. Images were capturedaximum supination, midsupination, neutral, midpronation, and

matics. The ulnar y-axis, a line the center of the ulnar head topronation.

atat m

kine

the ulnar head coordinate system. The orientation

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was represented by the relative orientation of thedistal radius coordinate system with respect to theulnar styloid coordinate system using 3 Euler an-gles (in x-y-z sequence) (Fig. 3). In this study,6DOF was expressed using the radial displace-ments along the x, y, and z-axes of the ulna foranterior–posterior, ulnar–radial, and proximal–distal displacements, and rotation about the z-axisfor pronation–supination. Proximal translation ofthe radius relative to the ulna was defined as neg-ative, whereas distal translation was positive.

To characterize the overall effect of flexion androtation at a specified elbow flexion angle, weexamined the difference observed in proximal-distal translation between full supination and fullpronation (terminal difference) versus the differ-ence observed between the peak proximal and peakdistal translation (peak difference).

To compare the actual path of rotation of theradius with the expected path of a perfect circle,we defined a circular path about the center of theulnar coordinate system using the center of theradial coordinate systems and the “best fit circle”function within Rhinoceros (Fig. 4). Lines extend-ing from the center of the ulnar coordinate system

FIGURE 3: Change in ulnar variance was measured from theneutral position for each angle of elbow flexion. Ulnarvariance was measured as the distance along the ulnar z-axis between the center of the radial coordinate system andthe center of the ulnar coordinate system. Proximaltranslation of the radial coordinate system relative to theulnar coordinate was defined as negative, whereas distaltranslation was positive.

were extended to the centers of the radial coordi-

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nate systems such that they intersected with thepath of the best fit circle. The difference in dis-tance between the actual and expected path ofrotation were then measured along this line.

Because of the small sample size of the cohort,significance was predefined using an alpha of 0.10. Weused analysis of variance to compare the motion of theforearm among the different elbow flexion and rotationangles with Statistica 6.0 (StatSoft, Tulsa OK). Post-hoc testing using Neuman Keul’s test was performed toevaluate differences between groups.

RESULTS

Proximal– distal translation

Traditional ulna variance is measured at 90° of elbowflexion and neutral rotation. In our subjects, ulna vari-ance based on the model would be negative 0.24 � 0.57mm.

The proximal and distal translation of the radiusrelative to the ulna varied with elbow flexion. Withthe arm in midpronation, the translation of theradius was significantly greater at 0° versus 45° ofelbow flexion (0.82 � 0.59 mm vs 0.65 � 0.80mm, F: 4.49, post hoc: 0.055; p � .05), and sig-nificantly smaller at 45° versus 90° of elbow flex-ion (0.65 � 0.80 mm vs 0.97 � 0.61 mm, F: 4.49,post hoc: 0.048; p � .05) (Fig. 5).

When the elbow was at 0° or 90° flexion, the prox-imal and distal translation of the radius relative to theulna was not maximal at extremes of pronation andsupination. Rather, peak displacement was observed atmidsupination and midpronation. With the elbow in fullextension, translation of the radius was significantlymore in midsupination than full supination (–1.10 �0.35 mm vs –0.24 � 0.34 mm, F: 14.90, post hoc:0.005; p � .01). With the elbow in full extension,translation of the radius was also significantly greaterwhen in midpronation than in full pronation (0.82 �0.59 mm vs 0.3 � 0.24 mm, F: 14.90, post hoc: 0.06;p � .01). Translation of the radius in midpronation wassignificantly greater when the forearm was in eithermidsupination or terminal supination with the elbow at0° or 90° flexion (F: 14.90, post hoc: �0.01; p � .01,F: 19.11, post hoc: �0.01, p � .01). Similarly, trans-lation of the radius in midsupination was significantlyless than any other position of forearm rotation with theelbow at 0° or 90° flexion (F: 14.90, post hoc: �0.01;p � .01, F: 19.11, post hoc: �0.01, p � .01).

In general, at each flexion angle, the peak differencewas greater than the terminal difference. At 0° of elbow
flexion, the terminal difference was significantly
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smaller than peak differences at all elbow flexionangles (F: 5.49, post hoc: �0.05 for all flexion an-

FIGURE 4: Center of rotation was determined by finding theDeviations from this circular path were random and minor.

FIGURE 5: Proximal–distal translation of the radius relative todefined as negative, whereas distal translation was positive.

gles; p � .002). At 90° flexion, the peak difference

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was significantly greater than the terminal differ-ences at all elbow flexion angles (F: 5.49, post hoc:

fit circle of the center points of the radial coordinate system.

lna. Proximal translation of the radius relative to the ulna was

best

the u

�0.05 for all flexion angles; p � .002).

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Arc of pronation–supination

From maximal supination to maximal pronation, theDRU joint demonstrated an average of 141.8° � 18.4°of total rotation for all angles of elbow flexion. Total arcof rotation increased with progressive elbow flexion. Aselbow flexion increases, end-range pronation increases(F: 3.67, post hoc 0° vs 45°: 0.10, post hoc 0° vs 90°:0.049; p � .07). The arc of rotation was significantlysmaller at 0° compared with 90° of elbow flexion(129.3 � 22.2° vs 152.8 � 14.4°, F: 3.29, post hoc:0.79; p � .09) (Table 1).

The radius rotated about the ulna in a circular pathwith a center of rotation located in the ulnar head, withonly minor deviations in the radius of rotation. At 90°of elbow flexion, the center of rotation was nearlycoincident with the anatomic center of the ulnar head.The center of rotation of the radius shifted volarly andulnarly with increasing elbow extension (Table 2, Fig.6). The location of the center of rotation was signifi-cantly different when the elbow was flexed to 90°compared with the elbow in full extension (F: 3.31, posthoc: 0.09; p � .08). The diameter of the circular pathtraversed by the radius relative to the ulna also changed

TABLE 1. Total Arc of Rotation, MaximumPronation, and Maximum Supination ComparedAcross Elbow Flexion Angle

ElbowFlexion 0° 45° 90°

Total arc ofrotation

129.3 � 22.2 143.4 � 11.7 152.8 � 14.4

Maximumpronation

41.1 � 15.5 57.7 � 8.3 57.3 � 5.9

Maximumsupination

88.2 � 12.3 85.7 � 8.0 95.5 � 18.2

There was a significant difference (p � .09) between the range ofrotation at 0° elbow flexion and 90°.

TABLE 2. Ulnar and Volar Changes (mm) in theCenter of Rotation of the Radius About the Ulna

ElbowFlexion

UlnarDisplacement

VolarDisplacement

Arc of RotationDiameter

0° 3.1 � 2.3 1.7 � 1.2 57.9 � 8.3

45° 1.5 � 1.4 0.5 � 0.8 54.3 � 4.4

90° 0.5 � 1.5 0.1 � 0.6 55.0 � 5.5

There was a significant difference (p � .10) in the ulnar displacementof the center of rotation between 45° and 90° of elbow flexion.

with elbow flexion. The diameter was significantly dif-

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ferent between 0° versus 45° (57.9 � 8.3 mm vs 54.3 �4.4 mm, F: 3.13, post hoc 0° vs 45°: 0.07; p � .10)(Table 2).

DISCUSSIONThis study examined the kinematics of the forearmusing a combined dual fluoroscopy imaging system andCT imaging technique. To our knowledge, this is thefirst application of this type of technology to under-standing the kinematics of the forearm. Strengths of thisnoninvasive imaging system are that this techniqueallows examination of a distal joint while a proximaljoint is unconstrained, and examination of forearm mo-tion in the entire functional range of motion.6–10

The first hypothesis, that ulnar variance changes withelbow flexion, was supported by our findings. Our datasuggest that when the elbow is at 45° flexion, the radiustranslates proximally less than when the elbow is at fullextension or 90° flexion. The maximum amount ofdistal or proximal translation of the radius relative to theulna did not occur at terminal pronation or supination,but at midarc. One possible reason why maximal trans-lation occurs during midarc is that as the forearm ap-proaches terminal rotation, the corresponding distal ra-dioulnar ligament becomes taut and restrictslongitudinal translation. Proximal structures such as thebiceps and brachialis may also limit translation as theforearm approaches terminal supination with elbow ex-tension.

Previous studies have suggested that elbow flexionand forearm rotation influence ulna variance. For ex-ample, Schuurman et al. examined ulna variance withthe forearm in neutral rotation with the elbow at 90° offlexion, compared with the forearm in full supinationwith the elbow at full extension, and found differencesof –0.18 � 1.56 mm versus –0.86 � 1.47 mm.1 Yeh etal. studied ulnar variance at maximum pronation, neu-tral rotation, and maximum supination using anteropos-terior radiographs and found a total proximal–distaltranslation of 0.6 mm with confidence intervals of 0.4 to0.8 mm, but they also did not control for elbow flexionother than in neutral rotation.2 Using static CT scans,Tay et al. found proximal–distal translation of 1.67 mmwhen moving from full pronation to full supination.5 Wefound a similar magnitude of mean maximum changebetween maximum pronation and maximum supination.

When comparing previous data, it is not clear towhat degree elbow flexion or forearm rotation contrib-utes to translation of the radius relative to the ulna. Ingeneral, we found that forearm rotation has a greaterrelative contribution to proximal–distal translation than
elbow flexion. Rotation leads to changes of up to 2 mm,
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whereas elbow flexion leads to changes of up to 0.5mm. Ultimately, our data suggest that ulna impactionon the carpus is most likely to occur when the forearmis in midpronation and the elbow is at 90° of flexion orfull extension.

The second hypothesis, that arc of rotation changeswith elbow flexion, was supported statistically. Thetotal arc of rotation increases with increasing elbowflexion. We observed that as elbow flexion increased,the center of rotation of the radius approached theanatomic center of the ulna head; in addition, the diam-eter of that arc of rotation decreased significantly withelbow flexion. This suggests that the precession of theradius about the ulna changes with elbow flexion. Usinga custom jig, Shaaban et al. found that total arc ofrotation increased significantly from 0° to 45°, re-mained constant from 45° to 90°, and then decreased atfull flexion.11 This was generally consistent with ourresults. The decrease of range of motion in extensionmay be a result of the biceps and other soft-tissuerestraints around the proximal radioulnar joint becom-ing tight at this position.

These findings are relevant to radiographic evalua-tions of the DRU joint and ulna variance and under-standing injuries of the forearm. Studies have examinedvarious methods of quantifying DRU joint dorso-volartranslation and ulna variance.12,13 In evaluating ulnavariance, there is emphasis on elbow flexion and neutralforearm rotation, and the effect of proximal and distaljoint positioning on radiographic evaluation has been

FIGURE 6: Center of rotation of the radius about the ulna shcenter of rotation from 45° to 90° of flexion.

recognized by previous authors1,14,15; however, this has

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not been systematically studied previously. Our resultssuggest that maximal distal radioulnar translation oc-curs when the elbow is flexed or fully extended and inmidpronation.

Similar to Baeyens et al., we observed no volar–dorsal translation at the DRU joint at either maximumsupination or pronation.16 Tay et al. demonstrated volar–dorsal translation of the radius relative to the ulna invivo during resisted supination and pronation whilegripping an external handle.5 Schuind et al. alsodemonstrated a mean change in ulnar variance of 0.5mm during maximal grasp of a dynamometer.15 Wedid not alter finger or wrist flexion– extensionthroughout the experiment, suggesting that duringelbow flexion, muscle forces across the DRU jointare such that the ulna remains centered in the sigmoidnotch in vivo if the wrist and hand are not closed orexperiencing an external load.

Limitations

A limitation of this study was the small number ofsubjects available for study. Ideally there would be alarger number of subjects and female subjects included;however, practical considerations limited the number ofparticipants and female subject enrollment. The smallcohort also had no outliers with an ulna variance �2mm. The inclusion of these subjects may give insightinto how deviations affect our measurements of trans-lation or forearm kinematics. A second limitation wasthat it was not possible to perform fluoroscopy that

with elbow flexion. There was a significant ulnar shift in the
ifted
encompassed the entire elbow and wrist joint at the

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same time because of the receiver size on the fluoro-scopes. This would allow simultaneous analysis of theproximal and DRU joints. In addition, the position ofthe elbow could be more accurately described ratherthan using a goniometer for positioning. Finally, anideal study would also include more positions of elbowflexion and points along the rotational arc and underdynamic forearm motions.

In conclusion, application of 3-dimensional solidmodeling techniques in conjunction with dual orthogo-nal fluoroscopy demonstrates that the translation of theradius relative to the ulna, the arc of forearm rotation,and the size of the arc of forearm rotation changes withelbow flexion. These findings have implications in thetreatment of ulna impaction, radiographic evaluation,and understanding of forearm kinematics. Future stud-ies examining integrating soft-tissue structures and dy-namic forearm loading may be helpful in understandingthe contribution of the DRU ligaments, triangular fibro-cartilage, and interosseous membrane. These observa-tions could help further elucidate our understanding ofthe in vivo kinematics of the DRU joint.

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5. Tay SC, Berger RA, Tomita K, Tan ET, Amrami KK, An KN. Invivo three-dimensional displacement of the distal radioulnar jointduring resisted forearm rotation. J Hand Surg 2007;32A:450–458.

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9. Li G, Papannagari R, Li M, Bingham J, Nha KW, Allred D, et al.Effect of posterior cruciate ligament deficiency on in vivo translationand rotation of the knee during weightbearing flexion. Am J SportsMed 2008;36:474–479.

10. Jordan SS, DeFrate LE, Nha KW, Papannagari R, Gill TJ, Li G. Thein vivo kinematics of the anteromedial and posterolateral bundles ofthe anterior cruciate ligament during weightbearing knee flexion.Am J Sports Med 2007;35:547–554.

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Elbow Position Affects Distal Radioulnar Joint Kinematics

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