Structural Variations of Tendons: A Systematic Search and Narrative Review of Histological Differences Between Tendons, Tendon Regions, Sex, and Age.

Tendons are force-transmitting structures which facilitate musculoskeletal functioning. Characterizing variations between different anatomical tendons, regions within tendons, as well as between the sexes and with age can improve understanding of tendon physiology and pathology. A systematic search of the literature was conducted to identify and summarize microscopic structural (histological) variations in normal/healthy tendons in relation to these variables (Tendon, Region, Age, Sex, and Other). Regional differences within individual tendons have been investigated in numerous studies, however investigations comparing histological variations between a range of different tendons are sparse, with most focusing on a few select tendons. When injured, ageing tendons typically have a greater degree of pathological changes than younger tendons, but few studies have documented variations in tendon histology throughout typical (uninjured) ageing or across large age spans. Similarly, sex-related observations of tendon structure are underreported. This narrative review summarizes studies on these topics and explores interactions between these variables, as well as the implications of these in the context of selecting control samples for studies of tendon pathology. Future studies should endeavour to improve knowledge of tendon structural variations-specifically focusing on normal tendons-to facilitate understanding of tendon structure-function relationships, physiological mechanisms involved in tendon damage/healing, and to aid clinical research and practice.