There are several technical considerations in obtaining and interpreting radiographic studies to evaluate AOMSI, including magnification, film quality and normal thresholds for translation and angular motion. AOMSI as defined in the Guides is assessed by plain film flexion-extension radiographs only. Source to image distance (SID) for flexion-extension X rays of the thoracic and lumbar spine is not defined in the literature or medical practice; however, 40" SID imaging parameters are most commonly used in the lumbar spine. The relative translation measurements are described in this text for each spinal region, corrected for magnification.


Cervical spine AOMSI is defined using flexion/extension X rays (Figures 17-5 and 17-6 below). A diagnosis of AOMSI in the cervical spine by translation measurements requires greater than 20% anterior or greater than 20% posterior relative translation of one vertebra on another, on flexion or extension radiographs, respectively; or angular motion of more than 11° greater than each adjacent level on the flexion radiograph. Alternatively, there may be complete or near-complete loss of motion of a motion segment due to developmental fusion; successful or unsuccessful attempts at surgical arthrodesis, including dynamic stabilization; or preserved motion with disk arthroplasty.


Thoracic spine AOMSI is defined using flexion/extension X rays (Figures 17-5 and 17-6) below. A diagnosis of AOMSI in the thoracic spine by translation measurements requires at least 2.5 mm anterior or 2.5 mm posterior translation of one vertebra on another, on flexion or extension radiographs respectively; or successful or unsuccessful attempts at surgical arthrodesis, including dynamic stabilization.


Figure 17-5, Loss of Motion Segment Integrity, Translation
A dot is placed at the posterior superior corner of the lower vertebra, and a separate dot is placed at the posterior-inferior corner of the upper vertebra. The distance (A) is measured as illustrated by the figure, using two parallel lines. Measurements are obtained in flexion and extension. Measure the A-P sagittal plane diameter at the midlevel of the superior vertebra (B). Distance A is then compared to distance B; determine % or distance in mm as specified for each region. AOMSI is established if the regional criteria are met.


Lumbar spine AOMSI is defined using flexion/extension X rays (Figures 17-5 and 17-6 below). A diagnosis of AOMSI in the lumbar spine (L1-L5) by translation measurements requires greater than 8% anterior or greater than 9% posterior relative translation of one vertebra on another, on flexion or extension radiographs respectively. In the lumbosacral spine (L5-S1), it requires greater than 6% anterior or greater than 9% posterior relative translation at L5-S1 of L5 on S1 on flexion or extension radiographs, respectively. A diagnosis of AOMSI in the lumbosacral spine by angular motion measurements requires greater than 15° at L1-2, L2-3, and L3-4; greater than 20° at L4-5, or greater than 25° at L5-S1 (compared with adjacent level angular motion).

Alternatively, there may be complete or near-complete loss of motion of a motion segment due to developmental fusion or to successful or unsuccessful attempts at surgical arthrodesis; including dynamic stabilization; or preserved motion with disk arthroplasty.

Figure 17-6, Loss of Motion Segment Integrity, Angular Motion (Sagittal Rotation), Lumbar Spine
Lines are drawn along the superior border of the vertebral body of the lower vertebrae and the superior border of the body of the upper vertebrae and the lines extended until they join. The angles are measured and subtracted. Note that lordosis (extension) is represented by a negative angle and kyphosis (flexion) by a positive angle. Loss of motion segment integrity is defined as motion greater than 15° at L1-2, L2-3, and L3-4 and greater than 20° at L4 to L5. Loss of integrity of the lumbosacral joint is defined as angular motion between L5 and S1 that is greater than 25°. The flexion angle is +8° and the extension angle is -18°. In the illustration, the flexion angle is 8°. Therefore (8) - (-18) = 26° and would qualify for loss of structural integrity at any lumbar level.