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PURPOSE
It was the purpose of this study to identify and document normal and abnormal supraspinatus tendon function in vivo using real-time ultrasound.

METHODS
We defined 4 groups of 20 individuals each: partial tear (group 1), full-thickness tear (group 2), successfully repaired tear (group 3), and healthy asymptomatic controls (group 4). Except for group 4, all patients underwent magnetic resonance arthrography to confirm the diagnosis. All underwent ultrasound imaging of the supraspinatus tendon with the adducted arm at rest and under maximal isometric abduction. Tendon deformation was dynamically assessed and measured with tendon thickness changes at 0.5, 1, 1.5, and 2 cm from the tendon insertion. The clinical assessment consisted of absolute and relative Constant score, subjective shoulder value, and strength measurements.

RESULTS
Without muscle contraction, the tendons of the 4 groups were not of significantly different thickness, with the least variation at 1.5 cm from the insertion site. On contraction, the normal tendon thickness significantly increased at a distance of 2 cm, whereas it did not for the full-thickness and partial supraspinatus tears. Thus contraction of the muscle resulted in measurable deformation of the tendon.

CONCLUSIONS
Partially torn supraspinatus tendons can be functionally incompetent, leading to a biomechanical deformation of the musculotendinous unit that is not different from that of a unit with a full-thickness tendon tear. The dynamic sonographic finding of a successful repair of a supraspinatus tendon is similar to that of a normal tendon, even though the previously injured muscle appears unable to generate the same strength as a normal muscle.

LEVEL OF EVIDENCE
Level III, case-control study.