Background: There is poor understanding regarding functional limitation of thumb strength and dexterity following thumb metacarpophalangeal (MCP) and interphalangeal (IP) arthrodesis. The purpose of this investigation was to quantitatively evaluate the functional limitations of MCP and IP joint fusion using an orthosis to simulate fusion. Methods: Thirty-two healthy volunteers underwent simulated MCP and IP thumb arthrodesis using custom-molded orthoses. Each volunteer was independently assessed for hand and thumb function using the Jebsen-Taylor Hand Function test, lateral pinch strength, tip pinch strength, and Grooved Pegboard test. Comparisons between the 3 groups in all functional tests were carried out. Results: The mean lateral pinch strength was significantly greater in the unsplinted group (8.3 kg) compared with the MCP- (6.3 kg) and IP-splinted (5.7 kg) groups. Mean tip pinch strength was also significantly higher in the unsplinted group than in MCP- and IP-splinted thumbs (4.6 kg vs 4.1 and 3.9 kg). There was no difference in the Jebsen-Taylor or Grooved Pegboard test between the 3 groups. Conclusion: Our study suggests that with a fused MCP joint the lateral and tip pinch strength will decrease by 24% and 10%, respectively, compared with a healthy nonsplinted thumb. A fused IP joint will decrease lateral and tip pinch by 31% and 16%, respectively. This information does not take into account an arthritic thumb. We found that the Jebsen-Taylor test and Grooved Pegboard test were not affected by simulated thumb MCP and IP fusion.