Translation and Angular Motion: How Spinal Instability Is Measured on X-ray

By Dr. Justin M. Dick, DC, CNMT, ARRT(N)(CT) | Clear Life Scoliosis and Chiropractic Center, Charlotte, NC | Published October 16, 2026

Spinal instability on X-ray comes down to two measurements at each level: translation, how far one vertebra slides forward or backward on the one below, and angular motion, how much the two vertebrae tilt toward each other. Both are taken from lateral X-rays in flexion and extension. The numbers look precise on a report. How they were measured decides whether they hold up, and that is what I check first when I review motion studies in Charlotte.

How Translation Is Measured

Translation is measured between the back edges of two adjacent vertebral bodies. In a computer-assisted study of normal cervical motion, translation was defined as the distance from the posteroinferior corner of the upper vertebral body to the posterior border of the vertebra below (Lin, Spine 2001). Under the AMA Guides Fifth Edition, translation is measured on the flexion film and the extension film, and the difference between them is the translational excursion. Cervical translation greater than 3.5 mm meets the threshold. The Fifth Edition criterion is strictly greater than 3.5 mm, so 3.5 mm itself does not qualify.

Millimeters on an X-ray are affected by magnification, which depends on the distance from the spine to the film and from the tube to the film. The Sixth Edition expresses cervical translation as a percentage, greater than 20% of the vertebral body depth, which reduces the effect of magnification. One lumbar study shows the conversion: it treated 15% translation as equivalent to 5.3 mm, assuming a 35 mm vertebral body depth (Davis, Int J Spine Surg 2015).

How Angular Motion Is Measured

Angular motion is the angle between the endplates of two adjacent vertebrae, compared with the same angle at the levels above and below. Under the Fifth Edition, a cervical level qualifies when its angular motion is more than 11 degrees greater than an adjacent level. As the Fifth Edition is commonly applied, angular motion is rated on the flexion film, and the neutral film is not rated. That differs from the original White and Panjabi checklist. The Sixth Edition lumbar thresholds are more than 15 degrees at L1 through L4, 20 degrees at L4-L5, and 25 degrees at L5-S1.

For a sense of normal, the study of 75 normal subjects found nearly all cervical angular differences in flexion were under 7 degrees, measured against C2-C3 (Lin, Spine 2001).

Measurement Error

Every one of these measurements carries error. In a lumbar study of 509 symptomatic patients and 73 asymptomatic participants, the coefficients of repeatability for rotation and translation measurements ranged from 1.99 to 3.11, depending on the method and whether the same reader or different readers measured (Davis, Int J Spine Surg 2015). A coefficient of repeatability is the range within which two measurements of the same thing should fall 95% of the time. A measurement that sits within that range of a threshold is not a clear result in either direction.

The same study compared static flexion and extension films with a continuous fluoroscopic motion analysis system. The motion system detected radiographic instability at roughly twice the rate, 12.3% versus 6.1% for rotation and 11.9% versus 5.4% for translation. If that holds, static motion films underdetect instability. The study measured lumbar levels with a commercial system, and I could not determine from the abstract whether any authors had a financial interest in it.

Software Measurement

I digitize motion films in PostureRay, which calculates translation and angulation once the vertebral corners are marked. A reliability study of its computer-vision method on 254 lateral cervical radiographs reported excellent reliability for sagittal alignment measures, with coauthors affiliated with the developer (Hosseini, J Clin Med 2024). That study did not test flexion and extension instability measurements. The marking of vertebral corners is still a human judgment, and the measurement error above applies to it.

What a Defensible Report Includes

A motion study report should state which levels were measured, the translation and angular values at each, which edition of the AMA Guides and which threshold were applied, which adjacent levels were used for angular comparison, and whether translation was reported in millimeters or as a percentage. A value just over a threshold should be reported as such, not rounded into a conclusion. A report that says "instability present" without the numbers behind it cannot be checked by anyone else.

For an example of how post-collision imaging findings and outcomes are documented in the peer-reviewed literature, read Dr. Justin Dick's published case report on refractory lumbar pain following a motor vehicle accident in a geriatric patient.

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Frequently Asked Questions

What is translation on a spinal X-ray?

Translation is how far one vertebra slides forward or backward on the vertebra below, measured between the back edges of the vertebral bodies on flexion and extension X-rays. Under the AMA Guides Fifth Edition, cervical translation greater than 3.5 mm meets the instability threshold.

What is angular motion on a spinal X-ray?

Angular motion is the tilt between two adjacent vertebrae, compared with the levels above and below. Under the AMA Guides Fifth Edition, a cervical level qualifies when its angular motion is more than 11 degrees greater than an adjacent level.

Why does the AMA Guides Sixth Edition use percentages for translation?

A percentage of vertebral body depth reduces the effect of X-ray magnification, which changes millimeter measurements depending on the imaging setup. The Sixth Edition cervical threshold is greater than 20% translation.

How accurate are spinal motion measurements on X-ray?

They carry measurable error. In one lumbar study, coefficients of repeatability ranged from 1.99 to 3.11 (Davis, Int J Spine Surg 2015). A value close to a threshold is not a clear result.

Do flexion and extension X-rays miss instability?

Possibly. One lumbar study found continuous fluoroscopic motion analysis detected radiographic instability at roughly twice the rate of static flexion and extension films (Davis, Int J Spine Surg 2015).

Who measures spinal motion X-rays after a car accident in Charlotte, NC?

Dr. Justin Dick, DC, CNMT, ARRT(N)(CT), measures flexion and extension X-rays against AMA Guides thresholds at Clear Life Scoliosis and Chiropractic Center, 8814 Rachel Freeman Way, Suite 103, Charlotte, NC 28278, phone 980-368-0766. Personal injury patients are treated on a lien basis, with no out-of-pocket payment at the time of care.

Sources and Evidence Tiers

1. American Medical Association. Guides to the Evaluation of Permanent Impairment, Fourth, Fifth, and Sixth Editions.Evidence tier: Expert consensus (impairment rating guideline)
2. Lin RM, Tsai KH, Chu LP, Chang PQ. Characteristics of sagittal vertebral alignment in flexion determined by dynamic radiographs of the cervical spine. Spine. 2001;26(3):256-261. PMID 11224861. LinkEvidence tier: Cross-sectional normative study (75 subjects)
3. Davis RJ, Lee DC, Wade C, Cheng B. Measurement performance of a computer assisted vertebral motion analysis system. Int J Spine Surg. 2015;9:36. PMID 26273554. LinkEvidence tier: Retrospective diagnostic performance study
4. Hosseini MM, Mahoor MH, Haas JW, et al. Intra-examiner reliability and validity of sagittal cervical spine mensuration methods using deep convolutional neural networks. J Clin Med. 2024;13(9):2573. PMID 38731102. LinkEvidence tier: Reliability study; coauthors affiliated with the software developer

Service Area

Clear Life Scoliosis and Chiropractic Center sees personal injury and spinal imaging patients from Charlotte NC, Huntersville NC, Ballantyne NC, Matthews NC, Concord NC, Mooresville NC, Rock Hill SC, and Fort Mill SC.

Related Pages

• Personal injury and car accident spinal care in Charlotte
• Dr. Justin Dick's research
• Case report: refractory lumbar pain after a motor vehicle accident
• Research and evidence at Clear Life
• Questions and answers
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This article is educational and is not a diagnosis. Imaging decisions depend on an in-person examination.