The NeuFlex joint implant is, like the Swanson prosthesis, a silicone-spacer-implant for metacarpophalangeal arthroplasty in rheumatoid arthritis or osteoarthritis. The implant design differs essentially through a preformed, 30 degrees anatomically neutral angle and palmar lying movement center, which reduces strain across the hinge by 35%. The Swanson prosthesis is straight with a dorsal lying movement center. The advantage of this new design is an improved overall function arc and range of movement in the MCP-joint with a better grip-function. Results of simulator and laboratory tests are presented and interpreted. In 50 patients (168 MCP-joints), early results show an average functional arc of movement of 61 degrees and an average extensor lag of 12 degrees. The 30 degrees pre-bend angle, corresponding to the normal resting angle of the hand, and the improved flexor tendon excursion and moment arm appear to be the main reasons for the substantially improved flexion following use of the NeuFlex device.