Heterotopic mineral deposits in intact rat Achilles tendons are characterized by a unique fiber-like structure

Heterotopic mineralization entails pathological mineral formation inside soft tissues. In human tendons miner-alization is often associated with tendinopathies, tendon weakness and pain. In Achilles tendons, mineralization is considered to occur through heterotopic ossification (HO) primarily in response to tendon pathologies. However, refined details regarding HO deposition and microstructure are unknown. In this study, we charac-terize HO in intact rat Achilles tendons through high-resolution phase contrast enhanced synchrotron X-ray tomography. Furthermore, we test the potential of studying local tissue injury by needling intact Achilles tendons and the relation between tissue microdamage and HO. The results show that HO occurs in all intact Achilles tendons at 16 weeks of age. HO deposits are characterized by an elongated ellipsoidal shape and by a fiber-like internal structure which suggests that some collagen fibers have mineralized. The data indicates that deposition along fibers initiates in the pericellular area, and propagates into the intercellular area. Within HO deposits cells are larger and more rounded compared to tenocytes between unmineralized fibers, which are fewer and elon-gated. The results also indicate that multiple HO deposits may merge into bigger structures with time by accession along unmineralized fibers. Furthermore, the presence of unmineralized regions within the deposits may indicate that HOs are not only growing, but mineral resorption may also occur. Additionally, phase contrast synchrotron X-ray tomography allowed to distinguish microdamage at the fiber level in response to needling. The needle injury protocol could in the future enable to elucidate the relation between local inflammation, micro-damage, and HO deposition.

Automated summary

Heterotopic mineralization is common in human tendons and is associated with tendinopathies, tendon weakness, and pain. In this study, heterotopic ossification (HO) was observed in intact rat Achilles tendons, with HO deposits characterized by an elongated shape and a fiber-like internal structure. Additionally, needle injury caused tissue microdamage and may be related to the deposition of HO. This research provides insights into the relationship between local inflammation, microdamage, and HO deposition.