Trapeziectomy is performed for trapeziometacarpal (TMC) arthritis but decreased lateral pinch strength is a major source of discomfort after the surgery. The magnitude of the decrease is unclear, however, and how the pressure changes in the TMC joint is unknown. To investigate this relationship, we designed a cadaveric study to measure TMC joint pressure using a lateral pinch model, and quantitatively evaluated the effect of trapeziectomy on the pressure measurements. For 10 cadaveric forearms, physiologic forces were applied across the thumb TMC joint by loading five tendons, thereby simulating lateral pinch. Using pressure sensors, we measured the lateral pinch pressure and TMC joint pressure, which averaged 10.1 (range, 4.2–16.2) kg/cm² and 2.0 (range, 0.8–4.4) kg/cm², respectively. A significant correlation between the measurements was found, with an average ratio of 19% (range, 10%–27%). After trapeziectomy and interposition of the tendon ball using flexor carpi radialis, the pressure measurements were repeated under the same conditions. Significant changes were found, which averaged 5.1 (range, 1.7–10.7) kg/cm² for lateral pinch pressure and 15.0 (range, 5.6–25.6) kg/cm² for TMC joint pressure. In conclusion, TMC joint pressure could be measured as the ratio relative to lateral pinch pressure using a cadaveric model. After trapeziectomy, the lateral pinch strength decreased, whereas the TMC joint pressure increased.