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Study design: Biomechanical study in cadaveric specimens. Background:The commercially available lumbar disc prostheses do not reproduce the intact disc's Instantaneous centre of Rotation (ICR), thus inducing an overload on adjacent anatomical structures, promoting secondary degeneration.Aim: To examine biomechanical testing of cadaveric lumbar spine specimens in order to evaluate and define the ICR of intact lumbar discs.Material and Methods: Twelve cold preserved fresh human cadaveric lumbosacral spine specimens were subjected to computerized tomography (CT), magnetic resonance imaging (MRI) and biomechanical testing. Kinematic studies were performed to analyse range of movements in order to determine ICR.Results: Flexoextension and lateral bending tests showed a positive linear correlation between the angle rotated and the displacement of the ICR in different axes.Discussion: ICR has not been taken into account in any of the available literature regarding lumbar disc prosthesis. Considering our results, neither the actual ball-and-socket nor the withdrawn elastomeric nucleus models fit the biomechanics of the lumbar spine, which could at least in part explain the failure rates of the implants in terms of postoperative failed back syndrome (low back pain). It is reasonable to consider then that an implant should also adapt the equations of the movement of the intact ICR of the joint to the post-surgical ICR.Conclusions: This is the first cadaveric study on the ICR of the human lumbar spine. We have shown that it is feasible to calculate and consider this parameter in order to design future prosthesis with improved clinical and biomechanical characteristics. * Corresponding author.spine [78] . When the intervertebral disc degenerates, it loses its capacity to transmit this load and thus may become a source of low back pain [66] .Lumbar disc arthroplasty was introduced in 1960 [27] , although results were not promising until the 1980 ′ s [7 , 36 , 60 , 115] . The aim of this surgical procedure is to preserve motion as well as to avoid spinal fusion related complications and side-effects. The first successfully implanted device was the SB Charitè [7 , 15] . The initial suboptimal results, which occurred due to mechanical failure [7 , 19 , 52 , 101 , 102] , were mitigated by repeatedly improving on the design [60] . Ever since, many other disc