Functional anatomy of entheses and enthesis organs: A celebration of Professor Mike Benjamin's contribution to enthesis biology.

Abstract

This review celebrates the work of Professor Mike Benjamin, whose anatomical research transformed our understanding of entheses - the sites where tendons, ligaments and other connective tissues attach to bone. This review aims to provide an overview of Professor Benjamin's foundational concepts, including the enthesis organ, functional entheses and the synovio-entheseal complex and their relevance to musculoskeletal health and disease. Entheses are biomechanically complex regions that accommodate the transition between compliant soft connective tissues and rigid bone by natural macroscopic and microscopic adaptations that reduce stress concentration. Macroscopically, tendons and ligaments often flare near their attachment sites, increasing surface area. Microscopically, entheses are classified as fibrous or fibrocartilaginous, with the latter displaying a zonal organisation that includes uncalcified and calcified fibrocartilage. These zones provide a graded transition in stiffness, reducing the risk of tissue failure and enables gradual bending of collagen fibres. Mechanical loading is essential for the normal development of the enthesis and is required to maintain its biomechanical properties in the adult. The enthesis organ concept, one of Professor Benjamin's most significant contributions, recognises that entheses are rarely isolated structures. Instead, they are part of a functional unit comprising adjacent tissues including sesamoid and periosteal fibrocartilages, bursae, fat pads and retinaculae which collectively dissipate mechanical stress. Adipose tissue and synovium at these sites may also play immunological and proprioceptive roles, and its involvement in neurovascular invasion has implications for pain and pathology. However, beyond direct tendon-bone attachments, functional entheses describe regions where tendons and ligaments interact with bone at a distance from the insertion but share structural and functional characteristics with classical entheses. The development of these concepts highlights Professor Benjamin's integrative approach to research and will continue to underpin research in musculoskeletal biology, pathology and tissue engineering, as well as inspire generations of anatomists.