Biomechanics Lab

PROPOSAL FOR DEFINITION OF A JOINT COORDINATE SYSTEM

FOR THE SPINE

Ian Stokes, Wafa Skalli, and André Plamondon

This proposal is based on recommendations made in a report presented to the Scoliosis Research Society by THE SCOLIOSIS RESEARCH SOCIETY WORKING GROUP ON 3-D TERMINOLOGY OF SPINAL DEFORMITY (Ian A. F. Stokes, Chair) (published in Spine 19:236-248, 1994).

These recommendations deal with some special cases of axis definition for use in biomechanics relevant to the axial skeleton. Biomechanics of the spine sometimes deals with the individual vertebrae (for which a local axis system is defined) and sometimes with the entire trunk (for which a spinal axis system is defined). For considerations of spinal deformity, a regional axis system is also defined in Spine 19:236-248, 1994.

The axis system uses the principle of ISO 2631 in which X is forward, Y left and Z up. A right-hand convention is used. (Note that in the axial skeleton there is no possibility of using different systems for the two sides of the body). This is different from the ISB recommendation (Journal of Biomechanics 28:1257-1261, 1995) which has X forward, Y up and Z right. However, the system presented here could readily be adapted to the ISB system, using the same landmarks, but altered conventions for axis names and directions

The choice of points to define axis systems is based on a compromise between using bony landmarks (which are often inaccessible), and using surface markers (which are often imprecise).

Local axis systems (Vertebra axis system xv, yv, zv) (Figure 1a).

This axis system aligns with the assumed plane of symmetry of a vertebra. In deformed or asymmetric vertebrae it is aligned with identifiable landmarks on the vertebral endplates and pedicles.

Definition: The origin is at the centroid of the vertebral body (half way between the centers of the two endplates), the local ‘z’ axis passes through the centers of the upper and lower endplates, the ‘y’ axis is parallel to a line joining similar landmarks on the bases of the right and left pedicles.

Spinal axis system (spinal xs, ys, zs) (Figure 1b)

The spinal axis system has the origin and sagittal plane defined by the pelvis, with a line parallel to the plane containing the anterior superior iliac spines (ASIS) defining the transverse global (Y) direction. (For static x-ray studies alignment would be achieved by positioning the ASIS parallel to the film plane.) The apparently more logical approach of aligning the axes with the sacrum or hip joint was not accepted for practical reasons.

Definition: The origin is at the center of the upper endplate of S1, the ‘z’ axis passes through the center of a specified vertebral body (usually C7 or T1, and the ‘y’ axis is parallel to the vertical plane containing the ASIS.
Figure 1: The two coordinate systems defining spinal geometry; (a) Local coordinates based on a vertebra. (b) Spinal coordinates, defined by the zs axis passing through the centers of the most caudal and cephalad vertebrae of the entire spine with its origin at the base of the spine (SI), and the zs axis directed towards a chosen vertebra at the top of the spine.

Submitted by: Ian Stokes [email protected]

Wafa Skalli [email protected]

André Plamondon [email protected]

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