A large and significant effect on perceived pain in patients with first carpometacarpal joint osteoarthritis was observed after 4 weeks of splint use. Differences in treatment effects were found with regard to muscle cross-sectional areas, but these were not significant. Clinical relevance Custom-made splints may be recommended for the treatment of first carpometacarpal joint osteoarthritis. Moderate to large but non-significant treatment effects were found with regard to muscle cross-sectional areas.