A histoanatomical study of the fiber bundle forming the ‘Comma Sign,’ a critical marker of the torn edge of the subscapularis tendon

Abstract

The "Comma sign" is a well-known indicator of the subscapularis torn edge of the shoulder. We undertook a histoanatomical study of the fiber bundle forming Comma sign (FBCS) to determine why FBCS is maintained even in cuff tear cases. Part 1: five tissue blocks including the supraspinatus tendon (SSP), rotator interval (RI), and subscapularis tendon (SSC) out of 5 cuff-intact cadavers were histologically observed in serial sections. Part 2: another tissue blocks of 6 cuff-intact cadavers were serially sectioned along the estimated FBCS direction based on the Part 1 findings. Additionally, 5 tissue blocks of cuff-torn cadavers including the three components, SSP, FBCS, and SSC, were serially sectioned along the apparent FBCS. In one slice clearly demonstrating FBCS fibers out of each section series, the components were measured of the sound speed and visualized through a scanning acoustic microscope (SAM). At the lateral portion, RI tissue with the joint capsule became thick and tightly surrounded SSP. Similarly, thicker RI tissue adhered to SSC from the superior and bursal side. More laterally, the borders of SSP/FBCS and FBCS/SSC were unclear with intermingled fibers. At the lateral most portion, RI tissue formed a fiber bundle, FBCS, extending from SSP to the bursal side of SSC. The sound speeds of SSP and SSC were significantly faster than FBCS in both cuff-intact and cuff-torn slices. In SAM images of cuff-torn specimens the FBCS borders were all unclear. As FBCS extends from the capsule beneath SSP and to the bursal surface of SSC, the FBCS connection to SSP and SSC is hardly lost, even though SSP or SSC detaches from the greater or lesser tubercle, respectively. Additionally, as degeneration make the elasticity difference gradual, the stress concentration at the borders may be diminished, leading to less breakage of FBCS.