Which shoulder motions cause subacromial impingement? Evaluating the vertical displacement and peak strain of the coracoacromial ligament by ultrasound speckle tracking imaging
These authors studied 16 normal male shoulders (average age 28.6 years). They measured the vertical displacement and peak strain of the coracoacromial ligament by a motion tracing program during these active motions: (1) forward flexion in the scapular plane, (2) horizontal abduction in the axial plane, (3) external rotation with the arm at 0° abduction (ER0), (4) internal rotation with the arm at 0° abduction (IR0), (5) internal rotation with the arm at 90° abduction (IR90), and (6) internal rotation at the back (IRB).
An experienced sonographer examined the shoulders using 2-dimensional speckle tracking echocardiography (2D STE). Dynamic ultrasonography with the motion tracking program was used to evaluate the vertical displacement and peak strain of the coracoacromial ligament during the active motions. The coracoacromial ligament was identified with the ultrasonographic transducer placed perpendicular to the skin between the coracoid process and the acromial tip. The coracoacromial ligament was traced throughout each shoulder motion. The vertical displacement of the coracoacromial ligament with various degrees of superior bulging away from the surface of the rotator cuff could be seen, and degree of vertical displacement was measured from the vertex of the coracoacromial ligament convexity to a line connecting the acromion and coracoid process. The longitudinal peak strain of the coracoacromial ligament was calculated by the horizontal fractional change.
The mean vertical displacement of the coracoacromial ligament during forward flexion (2.2 mm), horizontal abduction (2.2 mm), and IR90 (2.4 mm) was significantly greater than that during the other motions (ER0, −0.7 mm; IR0, 0.5 mm; IRB, 1.0 mm; P < .003). The mean peak strain was significantly higher in forward flexion (6.88%), horizontal abduction (6.58%), and IR90 (4.88%) than with the other motions (ER0, 1.42%; IR0, 1.78%; IRB, 2.61%; P < .003).
The authors concluded that normal shoulders without any pathologic change could result in physiologic contact beneath the coracoacromial ligament. The coracoacromial ligament was vertically displaced during some shoulder motions in all subjects. These findings indicate that there is physiologic contact between the coracoacromial ligament and the rotator cuff in normal shoulders.
Comment: This is a very interesting and important study. It demonstrates that loading of the coracoacromial ligament by the subjacent rotator cuff occurs in normal movements of the normal shoulder. Nature's design is marvelous in that rather than having a rigid unyielding bony bridge between the coracoid and the acromion, the coracoacromial ligament provides a conforming 'spring' ligament between these two structures that can accommodate loading by the cuff and minor variations in the shape of the proximal humeral convexity.
In previous studies, we have demonstrated the loading of the coracoacromial arch by the rotator cuff in vivo and in vitro. Superiorly directed forces are applied to the coracoacromial arch by the subjacent rotator cuff (for example when pushing down on the arms of a chair to stand).
Loss of the coracoacromial ligament by surgery to 'decompress' the rotator cuff can result in anteriorsuperior escape.
Contact between the cuff and the ligament, "impingement", is normal!
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