This study presents a finite element model of the C4-C7 segment in healthy conditions and after implantation of a disc prosthesis at a single level, in order to investigate of the influence of disc arthroplasty on the biomechanics of the cervical spine. A nonlinear finite element model of the C4-C7 segment in intact conditions was developed and run in flexion and extension. A detailed model of the Bryan disc prosthesis, including contacts between the different components of the device, was built and positioned at C5-C6. The calculated segmental motion resulted preserved after disc arthroplasty, with respect to the model of the intact spine, in both flexion and extension. A general preservation of the forces transmitted through the facet joints was obtained; a minor force increase at the implanted level was detected. The analysis of the instantaneous centers of rotation (ICR) in flexion-extension showed the preservation of a physiological kinematics. The mechanical behaviour showed an asymmetry between flexion and extension, probably due to the removal of the anterior longitudinal ligament and the anterior part of the annulus fibrosus, and the preservation of the posterior structures. In general, the disc prosthesis showed to be able to reproduce a nearly physiological motion. However, other important mechanical aspects, such as the possible micromotion at the bone-implant interface and the possible degenerative conditions of the spine, need to be evaluated before drawing a conclusion about total disc arthroplasty from an engineering point of view.