Pectoralis major tendon and enthesis: anatomic, magnetic resonance imaging, ultrasonographic, and histologic investigation.

Background: This study evaluates the pectoralis major (PM) tendon humeral insertion, using imaging and histologic assessment in cadaveric specimens. Current descriptions of the pectoralis major tendon depict a bilaminar enthesis, and clarification of the anatomy is important for diagnostic and surgical considerations.
Materials and Methods: Fourteen fresh-frozen whole upper extremity specimens were used in this study. Magnetic resonance (MRI) and ultrasonographic (US) imaging of the PM muscles, tendons, and entheses were performed, followed by anatomic dissection and inspection. Morphology of the lateral tendon and entheses were evaluated, focused on the presence of layers. In 11 specimens, the lateral 3 cm of the PM tendon was carefully dissected from the footprint, whereas in 3 specimens, the tendon and humeral insertion were preserved and removed en bloc. Histology was performed in axial slabs along the medial-lateral length of the tendon and also evaluated for the presence of layers.
Results: The superior-inferior and medial-lateral lengths of the PM footprint were 75 ± 9 mm and 7 ± 1 mm respectively. In all specimens, the clavicular and sternal head muscles and tendons were identified, with the clavicular head tendon generally being shorter. The medial-lateral length of the clavicular head tendon measured 19 ± 8 mm superiorly and 9 ± 3 mm inferiorly. The medial-lateral length of the sternal head tendon measured 38 ± 8 superiorly and 41 ± 18 mm inferiorly. All specimens demonstrated a unilaminar, not bilaminar, enthesis with abundant fibrocartilage on histology. Three specimens demonstrated interspersed entheseal fat and loose connective tissue at the enthesis on MRI and histology.
Conclusion: The PM tendon humeral insertion consists of a unilaminar fibrocartilaginous enthesis. US, MRI, and histology failed to identify true tendon layers at the enthesis. Delaminating injuries reported in the literature may originate from a location other than the enthesis.
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