Spinalogic Line Drawing – The Science and Maths
Spinalogic has advanced x-ray and posture analysis built in. This post describes the maths behind this analysis. (It’s for the more technically minded reader.)
There is a lot of research done on x-ray analysis published in the peer-reviewed literature. The analysis done by Spinalogic is based on this research. This description is in no way meant to be an exhaustive review of the literature.
General
The standard x, y and z anatomical axes are used for calculations with the standard mathematical interpretation of rotation using the right-hand rule. All rotations are measured in degrees. All translations in mm.
DICOM images are assumed to be calibrated and level.
Images acquired by camera are calibrated by the inclusion of a horizonal or vertical calibration bar. Spherical geometric distortion of phographic images require extra care with interpretation.
Normals are taken from the literature where available and estimated where they are not. Normals are editable by the user so you can use the normals that you believe to be of the most clinical value. It is always encumbent on the practitioner to interpret any data in the clinical context to determine its relevance to any individual case.
Lateral Cervical
The primary measurements of the lateral cervical xray are the intersegmental rotations and translations of C2 through C7, and the inclination of the atlas and C7 with respect to the horizon.
The rotational alignment of C2 through C7 is based on the posterior tangent line of each vertebral body as this has been shown to be the most represtative of actual vertebral alignment in the literature. Spinalogic reports the relative rotations by comparing these posterior tangent lines.
The rotational alignment of C1 is based on a line drawn through the anterior and posterior tubercles and expressed with respect to the horizon.
Translational alignment of C2 through C6 with respect to the vertebrae below is calculated as the translation of the inferior most point of the superior vertebrae wrt the projection of the posterior tangent line of the inferior vertebrae in the horizontal plane.
Global translation of the cervical spine is calcluated as the mm translation of the inferior posterior body of C2 wrt the superior posterior body of C7.
Global rotation of the cervical spine is the relative rotation of the posterior tangent lines of C2 and C7.
Lateral Thoracic
The global rotation of the thoracic spine is the relative rotation of the posterior tangent lines of T3 and T10.
The thoracic spine z translation is mm translation of the superior posterior body of T3 and the inferior posterior body of T10.
Lateral Lumbar
Their are a number of important measurements in the lateral lumbar.
First there are the intersegmental translations and rotations of T12 through S1. These are calculated in the same manner as described for the lateral cervial.
The lumbar global rotation is calculated from the relative rotation of the posterior tangent lines of L1 and L5.
The lumbar global translation is mm translation of the inferior posterior body of S1 and the inferior posterior body of T12.
The Sacral Base Angle is the rotation of the sacral base wrt the horizon.
The Angle of Pelvic Incidence (API) is a measure of the morphology of the pelvis and sacrum and allows better estimation of the ‘normal’ sacral base angle and lumbar global rotation for an individual case. Spinalogic calcluates both the raw API as well as interpreting this to give the patient’s preferred sacral base angle and lumbar global rotation.
The API is calculated according to the method described by Legaye et al in Eur Spine J. 1998;7(2):99-103. It is the angle formed between the line joining the Hip Axis and the center of the sacral base and the perpendicular to the center of the sacral base. The Hip Axis is the centroid of the superior-most point of the acetabluar cup.
The sacral base angle (SBA) patient normal is equal to 7.3 + 0.63 x API, and the global lumbar rotation -0.9 x SBA, based on the work of Vialle et all J Bone Joint Surg Am. 2005 Feb;87(2):260-7.
Anterior Posterior Cervico-Thoracic
The primary points measured for the APCT image are the tips of the spinous processes from C2 through T12 and the center of the C2 odontoid process when they can be visualized.
The calculated values include the x translation of each spinous wrt the spinous below expressed in mm; and the ‘Cervico-Dorsal Angle’ which is the angle between the line of best fit of the spinouses of C2-7 and that of T1-T4. The line of best fit is calculated using least-squares regression.
Anterior Posterior Lumbo-Pelvic
Similar to the APCT, intersegmental spinous process x displacement is calculated for any visible spinous from T1 down to L5.
The lumbo pelvic angle is the angular displacement of the line of best fit of L3-L5 from the sacral base line. Normal of couse is 90 degrees. The line of best fit is again calculated using least-squares regression.
Ferguson
The Ferguson view is an APLP with the tube dropped low to better visualize the sacral base. It is of particular value to calculate anatomical leg length inequalities to prescribe shoe or ischial lifts.
The femoral head width is the mm separation of the superior most points of the femoral heads.
The ‘Total Lift’ is the number of mm of lift required under the foot to result in a level sacral base. It assumes a magnification factor of 25% due to the tube tilt.
The ‘Ischial Lift’ is the number of mm of lift required under the ischium when seated to result in a level sacral base.
It highly recommended that before prescribing any lifts that a re-xray is taken with the proposed lift to confirm correct calculations.
Digital Postural Analysis – Anterior Posterior
By taking a digital photograph of the patient including a calibration line, you can get accurate postural analysis.
The AP view calculated values include the translation of the pelvis wrt the feet, the thorax wrt the pelvis and the head wrt the thorax; and the rotation of the pelvis, thorax and head in the coronal plane.
Digital Postural Analysis – Lateral
Calculated values are the mm of z displacement of the greater trochanter wrt the ankle, the acromion wrt the greater trochanter, and the external acoustic meatus wrt the acromion.
