PRACTICAL SIGNIFICANCE

This study demonstrated that unconstrained active joint position sense is similar in subjects with and without unstable shoulders.  This similarity may be due to an increased reliance on musculotendinous receptors in instability patients to maintain joint stability.

STUDY BACKGROUND

The glenohumeral joint is the most commonly dislocated joint in the body. Anterior dislocations are usually caused by a wrenching movement in abduction and external rotation. Structural damage can result from these injuries, as well as damage to associated mechanoreceptors responsible for providing proprioceptive feedback.  Resulting abnormalities can lead to recurrent dislocations in 70% of untreated cases and 25-96% of conservatively treated cases.  Joint position sense has been shown to be diminished in unstable shoulders using position replication testing in internal and external rotation.  However, unconstrained testing may provide valuable information regarding the effect of anterior instability on functional ability.

OBJECTIVE

To examine the effect of shoulder elevation angle on repositioning error in healthy and unstable shoulders.

DESIGN AND SETTING

A 2 X 5 factorial design was used to compare repositioning errors across groups (instability and healthy) and elevation angles (30°, 45°, 60°, 75°, and 90°).  This study took place in the Orthopaedic Biomechanics Laboratory at University of Oregon.

SUBJECTS

Twelve patients (6 male, 6 female) with unilateral traumatic anterior shoulder instability, aged 29.42 (± 10.64) years, and twelve age- (28.33 ± 11.24 years) and gender-matched subjects without shoulder pathology participated in this study.

MEASUREMENTS

Participants attempted to actively replicate various target shoulder positions without visual feedback. Positions were presented via custom-made LabVIEW software through a head-mounted display (figure 1).  Target positions consisted of various elevation angles (30°, 45°, 60°, 75°, and 90°), all in the scapular plane.  Repositioning error was defined as the angle between the humerus (with respect to the thorax) in the presented and reproduced positions.

RESULTS

No group X elevation angle interaction effect was observed for repositioning error (P = .58).  No main effect was observed for elevation angle (P = .18).  Lastly, there was no significant difference between repositioning errors in the unstable and healthy groups (P = .32; figure 2).

CONCLUSIONS

These data suggest that patients with anterior shoulder instability may rely more on musculotendinous receptor feedback, compared to healthy subjects, to maintain joint stability during functional movements, which may help to compensate for diminished capsuloligamentous receptor function.

	Figure 1: Experimental Set-up.
	Figure 2: Effect of Elevation Angle on Repositioning Error across Groups.

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