Rotator cuff ruptures are frequently associated with loss of strength
of the shoulder. However, the characteristics of the rotator cuff
tear that are responsible for the loss of force generation and transmission
have not yet been identified. The purpose of this study was to compare
the effects of supraspinatus tendon detachments, tendon defects,
and muscle retractions on in vitro force transmission by the rotator cuff
to the humerus.
The rotator cuff tendons from ten cadaver shoulders were loaded
proportionally to the respective cross-sectional areas of their
muscles. A fiberglass rod was cemented into the medullary canal
of the humerus and connected to a three-component load cell for
the measurement of the forces transmitted by the rotator cuff to
the humerus. This study was performed with the humerus in a hanging
arm position and with various sizes of supraspinatus tendon detachments,
tendon defects, and muscle retractions.
Detachment or creation of a defect involving one-third or two-thirds
of the supraspinatus tendon resulted in a minor reduction in the
force transmitted by the rotator cuff (≤5%), while detachment
or creation of a defect involving the whole tendon resulted in a
moderate reduction (11% and 17%, respectively). Simulated muscle
retraction involving one-third, two-thirds, and the whole tendon
resulted in losses of torque measuring 19%, 36%, and 58%, respectively.
Side-to-side repair of the one-third and two-thirds defects nearly restored
the force transmission capability, whereas a deficit remained after
side-to-side repair following complete resection.
Our results support the rotator cable concept and correspond to
the clinical observation that patients with a small rupture of the
rotator cuff may present without a loss of shoulder strength. Muscle
retraction is potentially an important factor responsible for loss
of shoulder strength following large rotator cuff ruptures.
Supraspinatus muscle retraction diminishes glenohumeral abduction
torque significantly more than either a defect in the tendon or
a simple detachment of the tendon from the tuberosity. In cases
of irreparable defects, side-to-side repair may be worthwhile to
restore muscle tension and the integrity of the rotator cable.