Introduction Central placement of a total disc arthroplasty (TDA) in the coronal plane will result in equivalent facet joint loading, less tendency for lateral core migration, optimum kinematics, and better outcomes. This study was performed to determine which of the radiographic markers – the vertebral body, the pedicles, or the spinous process – provides the most accurate guide to the coronal midline, so to optimise coronal TDA. The coronal midline was defined as the perpendicular bisector of a line drawn between the midpoints of the two facet joints.
Methods Axial CT images were reconstructed from 35 abdominal CT’s to show the relevant anatomy at L4, L5, and S1. Measurements were taken comparing the consistency of the midpoints of the vertebral body, the pedicles, and the spinous processes, in relation to the coronal midline.
Results The mean distance from the coronal midline to the vertebral body midpoint was 0.55mm (SD 0.45), to the interpedicular midpoint was 0.19mm (SD 0.40), and to the spinous process midpoint was and 1.30mm (SD 1.30). 16% of the distances from the coronal midline to the spinous process midpoint were greater than or equal to 3mm, compared with 0% of the distances to the interpedicular midpoint or the vertebral body midpoint. The interpedicular midpoint was significantly closer to the coronal midline than the spinous process midpoint or the vertebral body midpoint at all levels (p<
0.001).
Discussion The interpedicular midpoint is the most accurate guide to the coronal midline. We recommend this landmark be used in preference to the spinous processes or the vertebral body midpoint when placing the implant in TDA. The close location of the interpedicular midpoint to the implant, compared with the more posteriorly located spinous process, means the likelihood of parallax error, by rotation of the patient or the C arm, is reduced using the interpedicular midpoint.
