Segmental spine instability — especially that of torsional nature — is difficult to diagnose in a degenerative lumbar spine in vivo. The motion of the lumbar segment is of a coupled nature and is described by a total of six degrees of freedom. Relative motion between two bodies in space is described by translations and rotations, and this includes the concept of the instantaneous axis of rotation (IAR). References 1–3 are representative reports of the many available in the literature about calculations of the IAR in cadaveric tissue, usually single motion segments [1–3]. In contrast, results of the IAR for lumbar motion measured in vivo (and by extension, for the whole lumbar spine) are actually scarce. Based on the preliminary report presented by group [4], in the present study we describe the characteristics of the IAR for lumbar spine torsion measured in vivo in a much larger cohort of normal subjects through the application of the method originally described by Kinzel et al. [5].

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