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

Abstract

Introduction

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.

Materials and methods

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).

Results

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.

Conclusions

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.

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Acknowledgements

We, the authors, would like to thank to Drs. M Hagiwara and A Takeuchi and the technicians in the Center for Anatomical, Pathological and Forensic Medical Research, Graduate School of Medicine, Kyoto University for preserving cadavers and creating histological sections. We are also grateful to Ms. Mary Shibuya for proofreading the manuscript and language help.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Ryuzo Arai. Special measurements were performed by Yoshihiro Hagiwara and Yoshifumi Saijo. Supervision of all the procedure and monitoring of the steady progress were done by Shuichi Matsuda. The first draft of the manuscript was written by Ryuzo Arai and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Ryuzo Arai.

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Conflict of interest

None of the authors, their immediate family, and any research foundation with which they are affiliated did receive any financial payments or other benefits from any commercial entity related to the subject of this article.

Ethical approval

This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by Kyoto University Graduate School and Faculty of Medicine, Ethics Committee, the Ethics Committee of University (R0379-2).

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Arai, R., Hagiwara, Y., Saijo, Y. et al. A histoanatomical study of the fiber bundle forming the ‘Comma Sign,’ a critical marker of the torn edge of the subscapularis tendon. Arch Orthop Trauma Surg (2021). https://doi.org/10.1007/s00402-021-03775-6

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Keywords

  • Anatomy
  • Histology
  • Shoulder
  • Rotator cuff
  • Subscapularis
  • Comma sign