Morphology of the torn rotator cuff.

The morphological characteristics of shoulders with torn rotator cuffs were determined using 41 embalmed specimens. The following parameters were measured in the supraspinatus (SSP), infraspinatus (ISP) and subscapularis (SSC) muscles: the length, thickness and width of the extramuscular tendon; the length of the intramuscular tendon; the length and width of a tear, if present, muscle fibre length; and muscle volume. The cross-sectional area (CSA) of the tendon was measured on the photographic image of slices of the tendon using an image analysis system, and the CSA of the muscle was calculated by dividing the muscle volume by muscle fibre length. The rotator cuff was intact in 11 shoulders. A partial-thickness tear of the cuff was present in 12 shoulders, a full-thickness tear of the SSP in 11 shoulders, and a full-thickness tear of more than 2 tendons in 7. Overall incidence of full-thickness tears of the rotator cuff was 44%, and that of partial-thickness tears 29%. With increase of tear size, the functional tendon length (extramuscular tendon length plus tear length) increased by a statistically significant amount in the SSP, ISP and SSC, whereas muscle fibre length decreased in SSP and ISP. It is concluded that the increased functional tendon length and decreased muscle fibre length are the main morphological changes that make the rotator cuff a physiologically abnormal unit. Surgical repair of the torn cuff would be expected to improve these anatomical changes and restore the kinetics of the glenohumeral joint.(ABSTRACT TRUNCATED AT 250 WORDS)