Microdamage and altered vascularity at the enthesis–bone interface provides an anatomic explanation for bone involvement in the HLA–B27–associated spondylarthritides and allied disorders

Objective To describe the basis for entheseal‐associated bone disease in the spondylarthritides, by analyzing microanatomic and histopathologic relationships between soft tissue, bone cortex, and adjacent trabeculae. Methods Serial sections from 52 entheses were examined; these entheses encompassed small and large insertions in the upper limb (n = 21), lower limb (n = 27), and spine (n = 4) from 60 cadavers. Enthesis microdamage (fissuring) as well as vascular and reparative changes were evaluated. Contact radiographs were used to ascertain the relationship between entheses and the trabecular network. Results At virtually all fibrocartilaginous entheses, the deep cortical boundary was extremely thin (typically 50–600 μm) or indistinguishable, and 96% of entheses had small holes in the cortical shell (typically 100–400 μm wide). Such regions were frequent sites of bone formation and renewal (96%) and microdamage (31%); these changes were more common in the lower limb. The presence of blood vessels near holes in the cortical shell was common; in 85% of attachments, blood vessels were present on the soft tissue side of the enthesis. Highly orientated trabeculae were more obvious in the lower limb than the upper limb (59% versus 29%). Conclusion The trabecular network supporting the cortical shell is an integral part of the enthesis, transferring load to an extensive skeletal region. In many cases, tendons/ligaments are anchored directly to such networks. This functional integration is associated with microdamage and repair at the hard tissue–soft tissue interface. These findings have implications for understanding bone involvement in SpA and for the SpA concept in general, especially the hypothesis that enthesis–bone architecture may be important in disease initiation.