The enthesis: a review of the tendon-to-bone insertion

Abstract

The integration of tendon into bone occurs at a specialized interface known as the enthesis. The fibrous tendon to bone enthesis is established through a structurally continuous gradient from uncalcified tendon to calcified bone. The enthesis exhibits gradients in tissue organization classified into four distinct zones with varying cellular compositions, mechanical properties, and functions in order to facilitate joint movement. Damage to tendinous insertions is common in the field of orthopaedic medicine and often involves surgical intervention that requires the attempted recreation of the natural organization of tendon into bone. The difficulty associated with recreating the distinct organization may account for the surgical challenges associated with reconstruction of damaged insertion sites. These procedures are often associated with high failure rates and consequently require revision procedures. Management of tendinous injuries and recon-struction of the insertion site is becoming a popular topic in the field of orthopaedic medicine.

Kartogenin Enhances Collagen Organization and Mechanical Strength of the Repaired Enthesis in a Murine Model of Rotator Cuff Repair

“…In our study, we continued to observe the generation of a fibrovascular scar at the tendon—bone junction, which is biomechanically inferior and more prone to failure compared with the native 4-zone enthesis. 30,31 Differential expression of numerous cytokines, transcription factors, and other signaling molecules at the healing tendon—bone enthesis that recapitulate the environment present during embryonic development is likely required to direct cell phenotype and promote the formation of the structure and composition of the enthesis. 2 Mechanical loading is critical to the processes of mineral accumulation, fibrocartilage formation, and collagen fiber formation that are intimately involved in a healing enthesis.…”

Tissue‐specific endothelial cells: a promising approach for augmentation of soft tissue repair in orthopedics

“…In addition, it is known that mechanical factors are vital to the proper development of the tendon–bone interface—known as the enthesis—a parameter that is especially difficult to control. The need for multiple biological factors and mechanical influences in combination with the specific composition, function, and mechanical properties of fibrocartilaginous entheses makes it extremely difficult to recreate this native construct …”

Bone Surface Micro‐Topography at Craniofacial Entheses: Insights on Osteogenic Adaptation at Muscle Insertions

“…The relative tissue composition of the enthesis as either fibrous or fibrocartilaginous (Benjamin et al, ; Doschak and Zernicke, ; Benjamin et al, ; Hieronymus, ; Sanchez et al, ) is the most commonly used criteria to demarcate type. The majority are considered to be of the fibrocartilaginous type where zones of uncalcified and calcified fibrocartilage are interposed between the dense fibrous tissue of the tendon and a bone matrix (Apostolakos et al, ). These “fibrocartilaginous” forms have been considered by some to be restricted to long bone epiphyses where they are often associated with bone eminences that act as pulleys providing mechanical advantage (Benjamin et al, ; Lu and Thomopoulos, ; Apostolakos et al, ).…”

“…The majority are considered to be of the fibrocartilaginous type where zones of uncalcified and calcified fibrocartilage are interposed between the dense fibrous tissue of the tendon and a bone matrix (Apostolakos et al, ). These “fibrocartilaginous” forms have been considered by some to be restricted to long bone epiphyses where they are often associated with bone eminences that act as pulleys providing mechanical advantage (Benjamin et al, ; Lu and Thomopoulos, ; Apostolakos et al, ). The “fibrous” type are thought not to contain fibrocartilage and are considered to be associated with insertions into diaphyseal and metaphyseal regions of long bones.…”

“…These attachments are either “direct” with a zone of mineralized fibrous insertion into the bone matrix or as “indirect” via periosteal fibrous attachments. The bone‐penetrating extrinsic fibers known as Sharpey's fibers are considered as the defining feature when these fibrous tendons are directly anchored to the bone (Benjamin et al, ; Aaron, ; Apostolakos et al, ).…”