Soft tissue reconstructive surgery for rheumatoid-related proximal interphalangeal joint deformities frequently fails to produce the long-term predicted results. Detailed information on the biomechanics of this joint, under both normal and pathological conditions, is required to assess the efficacy of such surgical intervention. A biomechanical model of the proximal interphalangeal joint has been developed to investigate tendon and joint loading during real life three-dimensional activities. Based on a rigid body mechanics approach, the model uses high resolution MRI scans to obtain anatomical tendon and bone geometries in conjunction with three-dimensional kinematic and loading data. The model incorporates an optimisation routine which minimises overall maximum tendon stress in the eight individual elements considered. Radial and ulnar joint force components are included at the proximal interphalangeal joint level. Two simulated pathological versions of the mathematical model are developed to accommodate the altered anatomic relationships after tendon reconstructive surgery. Joint forces of up to 450N and common usage of the extensor mechanism during normal pinching and grasping activities are predicted. The ulnar lateral bands of the extensor tendon are generally loaded to a greater extent than the radial bands. Extensor tendon and joint forces in the simulated pathological models are significantly higher than those in the normal model. Combined with the poor tendon quality of rheumatoid arthritis patients generally, these amplified internal forces may lead to further joint deformation.