Reproducible reference frame for in vitro testing of the human vertebrae

Definition of an anatomical reference frame is necessary for in vitro biomechanical testing. Nevertheless, there is neither a clear recommendation, nor consensus in the literature concerning an anatomical reference frame for in vitro testing of the human vertebrae. The scope of this work is to define a reference frame for the human vertebrae for in vitro applications. The proposed anatomical reference frame relies on alignment of well-defined points on the endplates, and on two landmarks on the posterior wall. The repeatability of the proposed alignment procedure has been tested in vitro by 5 operators, on 7 specimens. Furthermore, the feasibility and repeatability of the proposed procedure was assessed in silico, using CT-scans of the same specimens. Variations between operators were slightly larger than between repetitions by the same operator. The intra-operator in vitro repeatability was better than 3° for all angles. The inter-operator in vitro repeatability was better than 9° for all angles. The lateral tilt was the most repeatable angle, while anterior-posterior tilt was least repeatable. The repeatability when alignment was performed in silico on CT-scans was comparable to that obtained in vitro, on the physical specimens. This is the first time than an anatomical reference frame is formally defined and validated for the human vertebrae. The adoption of this reference frame will provide more reproducible alignment of the specimens and of the test load. This will enable better in vitro biomechanical tests, and comparisons with numerical models.