Glenohumeral translation during active and

shoulder joint. 2000 Elsevier Science Ltd. All rights reserved. ... In a second step, the in#u- ence of ... environment (Haubner et al., 1997) in which a sphere was. "tted to the ... 100 positions, the computational matching was only ap- .... Page 5 ...
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Journal of Biomechanics 33 (2000) 609}613

Glenohumeral translation during active and passive elevation of the shoulder * a 3D open-MRI study Heiko Graichen!,*, Tobias Stammberger", Harald Bonel#, Karl-Hans Englmeier", Maximilian Reiser#, Felix Eckstein! !Muskuloskeletal Research Group, Anatomische Anstalt, Ludwig Maximilians Universita( t Mu( nchen, Pettenkoferstr. 11, D 80336 Mu( nchen, Germany "Institut fu( r Medizinische Informatik und Systemforschung, GSF Forschungszentrum Neuherberg, Ingolsta( dter Landstr. 1, 85764 Oberschlei}heim, Germany #Institut fu( r Radiologische Diagnostik, Klinikum Gro}hadern, Marchioninistr. 15, D 81377 Mu( nchen, Germany Accepted 28 August 1999

Abstract Despite its importance for the understanding of joint mechanics in healthy subjects and patients, there has been no threedimensional (3D) in vivo data on the translation of the humeral head relative to the glenoid during abduction under controlled mechanical loading. The objective was therefore to analyze humeral head translation during passive and active elevation by applying an open MR technique and 3D digital postprocessing methods. Fifteen healthy volunteers were examined with an open MR system at di!erent abduction positions under muscular relaxation (30}1503 of abduction) and during activity of shoulder muscles (60}1203). After segmentation and 3D reconstruction, the center of mass of the glenoid and the midpoint of the humeral head were determined and their relative position calculated. During passive elevation, the humeral head translated inferiorly from #1.58 mm at 303 to #0.36 mm at 1503 of abduction, and posteriorly from #1.55 mm at 303 to !0.07 mm at 1503 of abduction. Muscular activity brought about signi"cant changes in glenohumeral translation, the humeral head being in a more inferior position and more centered, particularly at 90 and 1203 of abduction (p(0.01). In anterior/posterior direction the humeral head was more centered at 60 and 903 of abduction during muscle activity. The data demonstrate the importance of neuromuscular control in providing joint stability. The technique developed can also be used for investigating the e!ect of muscle dysfunction and their relevance on the mechanics of the shoulder joint. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Shoulder; Humeral head translation; MR imaging; Virtual reality

1. Introduction Shoulder joint stability joint requires adequate coordination of all passive and active stabilizers (Bowen and Warren, 1991; Warner et al., 1992; Pagnani et al., 1995; Walch, 1996) and pathologic changes of them can lead to unphysiologic translation of the humeral head relative to the glenoid cavity. Quantitative assessment of glenohumeral translation has so far been performed in vitro in cadaveric shoulder specimens (Harryman et al., 1990; McMahon et al., 1995; WuK lker et al., 1995) or with "nite element models (van der Helm and Pronk, 1995), but the elimination of the natural shoulder girdle motion, and the unknown force relationship between the di!erent * Correspondence address. Orthopaedic Department, University of Frankfurt Marienburgstr. 2, 60528 Frankfurt, Germany. Tel.: #49-696705-0; fax: #49-69-6705-375.

shoulder muscles make it problematic to transfer these in vitro data to the situation in the living. In vivo analyses with conventional radiography (Howell et al., 1988; Paletta et al., 1997; Poppen and Walker, 1976) are of limited value due to projectional artifacts and to the restriction to two planes (van der Helm and Pronk, 1995). In CT and high-"eld MR systems (Kiss et al., 1997) the arm cannot be investigated in the clinically relevant positions (Kessel and Watson, 1977; Hawkins and Hobeika, 1983). These problems can be potentially overcome by using an open MR system, which allows to investigate the shoulder joint in functional positions during abduction (Graichen et al., 1998) and under the in#uence of muscle activity (Graichen et al., 1999). The objective of this study was to analyze 3D humeral head translation relative to the glenoid cavity during passive and active elevation in healthy volunteers by applying speci"c 3D imaging and postprocessing techniques. The speci"c

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H. Graichen et al. / Journal of Biomechanics 33 (2000) 609}613

questions to be answered were: (1) Is there a reproducible pattern of superior}inferior or anterior}posterior glenohumeral translation during passive elevation (30}1503) in healthy volunteers. (2) Is the position of the humeral head relative to the glenoid altered by the action of abducting muscle forces.

2. Material and methods An open MR scanner (Magnetom Open, Siemens, Erlangen, Germany) was used and an optimized T1weighted, 3D gradient recalled echo-sequence (TR" 16.1 ms, TE"7.0 ms, FA"303) at a spatial resolution of 1.88]0.86]1.56 mm3 (FOV"220 mm2, aquisition time of 4 min and 26 s) was applied. Fifteen healthy volunteers were "rst examined without muscle activity (passive elevation), positioning the arm at "ve di!erent abduction angles (30}1503). In a second step, the in#uence of muscle activity on glenohumeral translation was investigated by examining the volunteers at 60, 90 and 1203 of arm abduction, a 1 kg mass with an adducting load direction being applied perpendicular to the distal humerus during imaging, and the abductor muscles of the shoulder being contracted isometrically. All parts of the study were approved by the local ethic commitee. The 3D image data sets were then transferred to a multiprocessing computer (ONYX, Silicon Graphics Inc., Moutain View, CA), semiautomatic segmentation of the humerus, and the scapula being performed with a region-growing algorithm. After 3D reconstruction, the articular surface of the glenoid was seperated interactively from the body of the scapula, and its center of mass (CM) calculated. Finally, a plane was "tted to the glenoid which served as a local reference system, the upper edge of the articular surface de"ning the superior direction. The reproducibility of the CM determination was tested by performing this step six times on one scapula. For determining the center point (CP) of the humeral head, the data were transferred into a virtual reality (VR) environment (Haubner et al., 1997) in which a sphere was "tted to the central part of the articular surface of the humeral head. The size of the transparent sphere was adapted interactively to the size of the humeral head (VR-matching) and used as a starting point for an automatic matching algorithm (computational matching), which computes the least-squares distance between the sphere and the articular surface of the humerus head as a measure of similarity (s (m , m , m , r)): x y z S (m , m , m , r)" x y z + g(h )(J(hi !m )2#(hi !m )2#(hi !m )2!r, zi x x y y z z i (1)

where h "(hi , hi , hi ) are the sampled points on the x y z i surface of the humeral head and g g(Z, Z )"e~(z~z#. )2@2s2 (2) #. is a Gaussian weighted function, to reduce the in#uence of partial volume e!ects in the anterior and posterior parts of the humeral head. The function S(m , m , m , r) x y z was then minimized, the parameter space being restricted to a close neighborhood of the starting parameters m and r : S S S ((m, r)"min with (m, r)3M(m, r)DDDm!msE)d ,Dr!r D(d N. (3) m 4 3 The accuracy of the VR- and the computational matching was compared by calculating the least-squares distance of both methods in 20 positions. In the remaining 100 positions, the computational matching was only applied when the degree of similarity was lower than 0.017 mm. The reproducibility of the VR- and the computational matching was tested by performing the measurement on one shoulder six times. Finally, the CP of the humeral head was projected prependicularly onto the plane that had been "tted to the glenoid, its location relative to the CM of the glenoid being computed in all dimensions (Fig. 1).

3. Results During passive elevation (30}1503), the humeral head showed a slightly superior position relative to the glenoid, which decreased signi"cantly from #1.58$1.2 mm at 303 of abduction to #0.36$1.6 mm at 1503 (Fig. 2). With muscle activity, the humeral head was in a superior position at 603 of abduction (#1.0$1.3 mm) (Fig. 2), but at 903 (#0.04$1.3 mm) and at 1203 (!0.02 $1.4 mm) of abduction it was centered (Fig. 2). Direct comparison of values obtained at muscular activity with those at muscular relaxation showed an inferior translation of 0.8$1.5 mm (p(0.05) at 603, of 0.8$1.5 mm (p(0.01) at 903 and of 1.0$1.4 mm (p(0.01) at 1203 of abduction. During passive elevation, the midpoint of the humeral head showed a signi"cant anterior translation from 303 (1.55$2.2 mm) to 903 (2.38$1.8 mm) of abduction (Fig. 3), but from 903 to 1503 (!0.07$1.4 mm) it translated posteriorly (Fig. 3). With muscle activity, the humeral head was localized between 1 and 1.8 mm anterior relative to the CM of the glenoid (Fig. 3). Direct comparison of the values obtained at muscular relaxation with those obtained at muscular relaxation showed a posterior translation of 0.88$ 2.6 mm (p(0.05) at 603, of 1.22$2.0 mm (p(0.01) at 903, and an anterior translation of 0.37$2.0 mm at 1203 (p"n.s.).

H. Graichen et al. / Journal of Biomechanics 33 (2000) 609}613

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Fig. 2. Position of the humeral head relative to the glenoid during passive and active elevation in superior/inferior direction. During passive elevation, a continous inferior translation can be observed. Under muscle activity, the humeral head is translated inferiorly relative to the images obtained under muscular relaxation, taking a more central position.

Fig. 1. After 3D determination of the midpoint of the humeral head this point is projected onto the local coordinate system of the glenoid and the position to the CM is calculated in all three dimensions.

The determination of the CM of the glenoid and that of the CP of the humeral head by the VR method were shown to be highly reprodubible (Table 1), the computational optimization improving the mean least-squares distance only slightly (0.015}0.012 mm).

4. Discussion The objective of this study was to three-dimensionally analyze the translation of the humeral head relative to

Fig. 3. Position of the humeral head relative to the glenoid during passive and active elevation in anterior/posterior direction. During passive elevation, the humeral head is always localized anteriorly, except at 1503 of abduction. Under the in#uence of muscle activity the humeral head is more posterior than under muscular relaxation at 603 and at 903 of abduction taking a more central position, but this is not observed at 1203 of abduction.

the glenoid during elevation of the shoulder in vivo during passive elevation and under isometric muscle activity. In previous studies, the importance of passive stabilizers (O'Brien et al., 1990; Pagnani et al., 1995; Warner et al., 1992; Habermayer and Schuller, 1990; Lazarus et al., 1996; Matsen et al., 1994) have been examined with in vitro shoulder models. However, the assumptions regarding the force relationship of the shoulder muscles in those

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H. Graichen et al. / Journal of Biomechanics 33 (2000) 609}613

Table 1 Reproducibility of determining the glenoid center of mass (CM), the humeral head midpoint (MP) with the VR- and with the computational method CM glenoid

x 1 2 3 4 5 6 CV (%)

97.2 96.6 96.7 97.2 97.5 97.4 0.3

VR MP humeral head

Computational MP humeral head

y

z

x

y

z

x

y

z

173.6 172.8 172.8 172.9 172.8 172.7 0.2

10.3 11.2 11.1 10.3 10.2 10.2 4.0

118.6 118.3 119.0 118.5 118.6 118.5 0.6

184.4 184.1 183.9 183.9 183.5 183.6 0.3

91.3 91.7 91.2 91.3 91.1 91.6 0.2

119.1 118.8 119.5 119.0 119.1 119.0 0.6

183.5 183.1 182.9 182.9 182.5 182.6 0.3

91.8 92.2 91.7 91.8 91.6 92.1 0.2

models are di$cult to justify (van der Helm and Pronk, 1995; WuK lker et al., 1995), and most biomechanical models have been restricted to the analysis of glenohumeral motion (Poppen and Walker, 1976; van der Helm and Pronk, 1995). These problems could be overcome in the present study by investigating healthy volunteers in vivo, in whom the physiologic neuromuscular activation patterns were preserved and the scapula free to move. Some limitations arise from the fact that the scapula touched the table during imaging, but the positioning of the arm was not carried out in the imaging device, but in a sitting position outside the scanner. In a previous study Graichen et al. (2000) could con"rm that the abduction angle taken outside are highly reproducible, and that a normal scapulo-humeral rhythm is preserved with the MR method employed. The problems involved in the analysis of glenohumeral translation with conventional radiography (Deutsch et al., 1996; Howell and Galinat, 1989; Poppen and Walker, 1976) were overcome by developing a three-dimensional postprocessing method. Poppen and Walker (1976) observed from X-rays that the humeral head moved upward relative to the glenoid between 0 and 303 of abduction and translated inferiorly by 2}3 mm throughout the rest of abduction. In our study, the inferior translation observed from 30 to 1503 was only 1.2 mm. Deutsch et al. (1996) described a more centered position of the humeral head in healthy volunteers, showing no signi"cant change of the position during abduction. In their radiographic study, the volunteers held a weight equal to 2.5% of body weight. In our analysis we also "nd a more centered position of the humeral head under muscle activity, but systematic differences are observed between the di!erent arm positions, however, the di!erence of positioning the volunteers in this study and other conventional radiographic studies should be mentioned. While at 603 of abduction the humerus is still 1 mm superior to the glenoid, it is more centered at 90 and 1203. The superior position at 603 may be caused by the dominance of the deltoid with its cranial force direction, while at 903 and 1203 the rotator cu! muscles with their centralizing e!ect

are more active (Graichen et al., 1999; Kronberg et al., 1990; Sporrong et al., 1996). In another radiographic study, Howell et al. (1988) described a nearly centered position of the huneral head in anterior/posterior direction at full extension, this being in agreement with our data. In all other positions during abduction, we found an anterior position of the humeral head during passive elevation, but again a centralizing e!ect of muscle activity. The technique described here can be readily extended to other positions of the joint and with other external force directions, its advantage being that the neuromuscular control mechanisms of each individual are preserved. It should be noted that the study was performed under isometric and not under isotonic muscle activity, which may yield di!erent results, but it is currently not possible to obtain three-dimensional information in real time. In conclusion, we have developed a MR-based technique that makes it possible to investigate humeral head translation three dimensionally in functionally important arm positions with and without muscle activity. The study demonstrates the relevance of neuromuscular control in providing joint stability and can be used as a basis for investigating the e!ect of muscle dysfunctions and their relevance for joint mechanics in shoulder patients. Acknowledgements We would like to express our thanks to the Deutsche Forschungsgemeinschaft (DFG) for their support (GR 1638/1-2). References Bowen, M.K., Warren, R.F., 1991. Ligamentous control of shoulder stability based on selective cutting and static translation experiments. Clinics in Sports Medicine 10, 757}782. Deutsch, A., Altchek, D.W., Schwartz, E., Otis, J.C., Warren, R.F., 1996. Radiologic measurement of superior displacement of the humeral head in the impingement syndrome. Journal of Shoulder Elbow Surgery 5, 186}193.

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