Histological Evaluation of the Relationships Between Distal Interphalangeal Psoriatic Arthritis and Nail Psoriasis: Correlations With Anatomical Studies of the Normal Nail Unit and the Main Radiological Data Concerning Distal Interphalangeal Psoriatic Arthritis.

To determine whether distal interphalangeal joint psoriatic arthritis (DIP PsA) and nail psoriasis are anatomically linked, we studied 2 fingers taken from a cadaver presenting a typical cutaneous and nail psoriasis in the setting of a dactylitis limited to the fourth toe. This comprehensive study of the inflammatory pattern of DIP PsA is discussed in the context of the controversial theory of the nail as a musculoskeletal appendage. Both the extensor and flexor entheses were focally and quite markedly infiltrated by lymphocytes and showed variable fibrosis and neovascularization. In addition, some clusters of giant cells were seen. Synovial perivascular inflammation was focally relatively dense. Discrete periostitis and bone inflammation of the intertrabecular spaces were seen, maximally at the insertion of the extensor and flexor tendons. The retained superficial fibrocartilaginous and tendinous cuff separated the inflamed extensor enthesis from the surrounding connective tissues. The thick proximal periosteum constituted a barrier between the inflamed bone and the matrical hypoderm. The lateral sections showed inflammation at 3 levels as follows: the enthesis of the interosseous ligament and collateral ligament, periosteum, and nail epithelium. In the 3 specimens, the inflammatory foci involving entheses and nails were prominent and never contiguous. This suggests that DIP PsA is not merely an extensor enthesitis and that the nail unit remains a microanatomical structure independent from the extensor enthesis, even with severe DIP PsA.