Effect of abducting and adducting muscle acitivity on glenohumeral translation, scapular kinematics and subacromial space width in vivo
Introduction
A reduction of subacromial space width represents a relevant factor in the pathogenesis of the impingement syndrome (Michener et al., 2003; Neer, 1972). Therefore, the general aim of conservative or surgical therapy has been to enlarge this space (Morrison et al., 1997; Soyer et al., 2003). Several factors have been identified that reduce the width of the space below the acromion, such as acromion morphology (i.e. hooked acromion) (Bigliani et al., 1991), scapular morphology (Anetzberger and Putz, 1996), protraction of the shoulder girdle (Solem-Bertoft et al., 1993), elevation of the arm (Flatow et al., 1994; Graichen et al., 1999a), alteration of the scapulo-humeral rhythm (Graichen et al., 2001; Paletta et al., 1997), and others. These factors have also partly been shown to conincide with the presence of clinical symptoms of impingement syndrome (Bigliani et al., 1991; Flatow et al., 1994; Graichen et al., 2001).
Retraction of the shoulder girdle, in contrast, has been shown to cause a widening of the subacromial space (Solem-Bertoft et al., 1993). Some of the physiotherapeutic approaches for treating the impingement syndrome are therefore based on the assumption that strengthening of the adductor muscles may be beneficial to enlarge the subacromial joint space (Morrison et al., 1997; Werner et al., 2002). We have previously shown that abducting muscle activity causes a reduction of the subacromial joint space (Graichen et al (1998), Graichen et al (1999a)) and centering of the humeral head relative to the glenoid (Graichen et al., 2000b). It is, however, currently unclear whether such potential alterations of the subacromial space are caused by downward translation of the humeral head, or by alterations of scapular position. Theoretically, an increased tilt of the scapula or a decreased scapulo-humeral rhythm may affect the acromiohumeral distance, albeit the position of the humeral head relative to the glenoid remains constant.
In this study we employ state-of-the-art open MR imaging and 3D postprocessing technology (Graichen et al (1998), Graichen et al (2000a), Graichen et al (2000b)) to address these questions directly in vivo. We test the specific hypotheses that (1) adducting muscle activity causes an increase of scapular tilting and a decrease of the scapulo-humeral rhythm, which indirectly causes an enlargement of the subacromial space, and that (2) adducting muscle activity causes a downward translation of the humeral head relative to the humerus and thus also widens the subacromial space width directly.
Section snippets
Open MR imaging
The shoulders of 12 healthy volunteers (21–33 years) were analyzed. The volunteers had no history of musculo-skeletal disorders, shoulder pain or injury. MR imaging was performed with an open MR system (0.2 T; Magnetom Open; Siemens; Germany) and a T1-weighted 3D gradient recalled echo sequence (TR 16.1, TE 7.0 ms, FA 30°). Image acquisition was performed in an oblique coronal orientation (slice thickness 1.875 mm) perpendicular to the glenoid cavity. The in-plane resolution was 0.86 mm and the
Subacromial space width
In all arm positions, adducting muscle activity led to a significant increase of the minimal acromio–humeral distance compared with abducting muscle activity.
Scapular kinematics
Under adducting muscle activity, a constant scapulo-humeral rhythm of 2.3–2.8 was observed during the entire range of arm elevation (Fig. 2A). During abducting muscle activity, values ranged from 2.2 to 2.7 between 60° and 150°. Only at 30°, the scapulo-humeral rhythm was somewhat higher (3.1) (Fig. 2A). Statistical comparison showed no
Discussion
In this study we have analyzed the effect of adducting and abducting muscle activity on the subacromial space width during elevation of the arm. Moreover, the study was designed to clarify whether potential changes in subacromial space width are caused by alterations of scapular kinematics and/or humeral head position. We hypothesized that both scapular kinematics and glenohumeral head position were associated with subacromial joint space width. Whereas the position of the humeral head relative
Acknowledgements
We would like to express our thanks to the Deutsche Forschungsgemeinschaft (DFG; GR 1638/5-1, 5-2) and the Klein foundation for support.
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