Background  Lichen planus (LP) is a common skin disorder of unknown aetiology that affects the skin, mucous membranes and nails. Although metal allergies have been implicated in the development of oral LP (OLP), the contribution of these allergies to nail LP (NLP) has yet to be studied in detail.

Objective  To elucidate the link between metal allergy and NLP.

Methods  We retrospectively analysed 115 LP patients with respect to the contribution of metals to either NLP or OLP. We also attempted to detect the specific metals involved in these nail lesions.

Results  Of the 79 patients that received a metal patch test (PT), 24 (30%) were positive for at least one of the metal compounds tested. Notably, the prevalence of positive reactions to metals in the NLP patients was significantly higher as compared with the OLP patients (59% vs. 27%, P < 0.05). Among the 10 PT-positive patients with NLP, improvement of the skin lesions was seen in six of the patients after removal of dental materials containing causative metals or systemic disodium cromoglycate therapy. On the other hand, only 3 of 16 PT-positive patients with OLP exhibited improvement after the removal of dental materials. Causative metals in the dental fillings/braces were detected in the involved nail tissues.

Conclusion  This study suggests that metal allergies are more closely associated with NLP vs. OLP, and that deposited metals in the nail apparatus contribute to the development of lichenoid tissue reactions in the nail bed and matrix.