Atraumatic Shoulder Instability - Pathophysiology – what are we really treating?

Authors: E Wiedemann

References: SECEC 2005, Rome


The etiologic factors responsible for atraumatic instability of the shoulder joint are not known.  Mechanisms in discussion include an intrinsic connective tissue disorder responsible for capsular and ligamentous redundancy, a skeletal anomaly like a small or flat glenoid, or some neuromuscular imbalance.


In most if not all cases of atraumatic instability there is more than one direction of instability.  Within the cases of multidirectional instability Gerber differentiates patients with general hyperlaxity from those without.  Hyperlaxity itself is not a disease, however.  This is why with and without hyperlaxity treatment of shoulder instability aims at the ability of the patient to centre the humeral head within the glenoid under normal loads.


In patients suffering from atraumatic instability Bailey differentiates those with structural lesions (polar ii) from those with non-structural imbalances that he calls “muscle patterning” (polar iii).  We compared 11 patients suffering from such a non-structural atraumatic instability to ten healthy subjects who had no history of trauma or pain at their shoulder.


During isokinetic movements in three planes (Cybex, 60°/s), the EMG of eight muscles was recorded in parallel by intramuscular fine-wire electrodes made from platinum.  The muscles studied were the supraspinatus (SSP), Infraspinatus (ISP), Subscapularis (SUB), major pectoralis (PECT), anterior and posterior deltoid, upper part of trapezius, and anterior serratus (SA).  The emg and mechanographic data (angle of movement and torque) were stored on FM-tape and evaluated off-line.  Distribution-free statistics were applied in order to calculate the confidence intervals of the emg data.


In the planes of movement studied, the emg activity of patients suffering from atraumatic instability showed significant differences in relation to normal subjects only in specific muscles:



Increased activity

Decreased activity




-Forward flexion:



-Internal rotation:

PECT, Post. delt.


-External rotation:

SSP, Post. delt.



There was no difference in the EMG activities of the anterior deltoid and the upper part of the trapezius muscle in any plane of movement.


The different patterns of EMG activity in patients suffering from atraumatic instability are ambidextrous: they may be the primary reason for that instability or compensate for another reason.  Taking this into account, the emg patterns may be explained by a number of hypotheses:


1.                  The increased EMG activities of the supraspinatus and Subscapularis muscles during abduction and forward flexion might help to stabilise the joint anteroinferiorly.

2.                  The increased EMG activity of the posterior deltoid muscle during rotational movements might stabilize the joint dorsally.

3.                  The EMG activity of the Subscapularis muscle was diminished during internal rotation, and the EMG activity of the Infraspinatus muscle was diminished during external rotation, although these muscles are thought to be the prime movers in the respective direction.

4.                  Positioning of the scapula by increasing EMG activity of the anterior serratus muscle is of key importance for the stability of the glenohumeral joint.


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