The nail as a musculoskeletal appendage--implications for an improved understanding of the link between psoriasis and arthritis.

The nail is considered to be a specialised modification of the skin--an 'epidermal appendage'. However, the phrase 'hanging on by your fingernails' belies a vital aspect of nail microanatomy that has been overlooked, namely that the nail is functionally integrated with the musculoskeletal system. The current article reviews how the nail is functionally linked to the distal phalanx and several distal interphalangeal joint structures, including extensor tendon fibres and the collateral ligaments. New histological images exploring this link are provided. The extensor tendon in particular continues from its bony insertion to envelop the nail root, and the collateral ligaments form an integrated network on the sides of the joint, helping to anchor the nail margins. This virtual continuum of connective tissue structures merges with a thick periosteum on the distal phalanx and with the numerous cutaneous ligaments that anchor the fatty pads of the finger pulp to the skin.