UC Irvine

Automobile drivers with a diagnosed supraspinatus (SSP) tendon tear following a low-speed rear-end collision often allege the collision is responsible for causing their shoulder injury. The goal was to determine the propensity for acute tear of a healthy SSP tendon during a low-speed rear-end automobile collision. To accomplish this, first the scapula and humerus positions were measured in twenty-one volunteers while in a driver seat. Then these positions were averaged and simulated in a cadaver shoulder to explore three possible injury mechanisms for acute isolated SSP tendon tear: humeral head displacement, tendon impingement, and tendon tension.

With a balanced rotator cuff muscle load, the SSP muscle had the largest change in length for 5 of 8 directions of humeral head displacement; however, the muscle length in 7 of 8 humeral head displacements was less than the muscle length at maximum ROM and changes in SSP tendon length were minimal (0±1 mm) for all humeral head displacements. No acromial impingement of the distal SSP tendon occurred with any of the humeral head displaced positions. Tensile testing of the humerus-SSP tendon complex resulted in a fracture of the humerus at 904 N.

These findings and review of literature suggest that a low-speed rear-end collision may not be an event that predisposes a healthy SSP tendon to tear. Tendon degeneration, non-uniform tendon loading, and pre-existing asymptomatic partial or full-thickness tendon tears may alter the risk of a SSP tendon tear or onset of pain in low-speed rear-end collisions.