What Is Spinal Instability? The Clinical Definition and the X-ray Definition
Spinal instability has two working definitions, and most confusion about the term comes from mixing them. The clinical definition describes what the spine fails to do. The radiographic definition describes a number on an X-ray that crosses a threshold. In personal injury cases I see in Charlotte, the second definition is the one that gets documented and rated, but the first is the one that explains why a patient hurts.
The Clinical Definition
The definition most often quoted comes from White and Panjabi's textbook, Clinical Biomechanics of the Spine (White and Panjabi, 1990). Clinical instability is the loss of the spine's ability, under physiologic loads, to maintain its pattern of displacement so that there is no initial or additional neurologic deficit, no major deformity, and no incapacitating pain. Under that definition, instability is a failure of function, judged by its consequences.
Panjabi later separated mechanical instability, the inability of the spinal column to carry loads, from clinical instability, which includes pain or neurologic dysfunction. He wrote that the definition remains controversial and that the most widely held view is that loss of the normal pattern of spinal motion causes pain, neurologic dysfunction, or both (Panjabi, J Electromyogr Kinesiol 2003).
The Stabilizing System
Panjabi described spinal stability as the product of three subsystems (Panjabi, J Spinal Disord 1992). The passive subsystem is the vertebrae, discs, and ligaments. The active subsystem is the muscles and tendons that apply force to the spine. The neural subsystem monitors signals from the other two and directs the muscles. When one component fails, the others may compensate immediately, adapt over the long term, or fail in turn, and that third outcome is where he placed conditions such as low back pain.
This model matters after a car accident because a ligament injury is a passive subsystem injury, and the model predicts the muscles will try to cover for it. A patient with an injured ligament and strong muscle guarding may show less abnormal motion on an X-ray than the ligament damage alone would produce. That prediction comes from the model. I am not aware of a clinical study that has measured it directly in collision patients.
Panjabi also proposed that the neutral zone, the small range around the resting posture where the spine moves with little resistance, is a better marker of instability than total range of motion. In laboratory studies it increased with injury and decreased with muscle force and fusion (Panjabi, J Spinal Disord 1992). That is a hypothesis built on cadaver, animal, and mathematical work.
The X-ray Definition
The AMA Guides to the Evaluation of Permanent Impairment use the term alteration of motion segment integrity and measure it on plain-film lateral flexion and extension radiographs. The Fifth Edition flags cervical translation greater than 3.5 mm or cervical angular motion more than 11 degrees greater than an adjacent level. Those cervical numbers trace back to the White and Panjabi checklist for the lower cervical spine. The Sixth Edition uses percentage-based translation in the cervical spine and lumbar angular thresholds of more than 15, 20, or 25 degrees depending on level.
For context on the cervical angle threshold, a study of 75 normal subjects found nearly all intervertebral angular differences in flexion were under 7 degrees, using C2-C3 as the baseline (Lin, Spine 2001).
Why the Two Definitions Do Not Always Agree
A patient can meet the clinical definition, with pain from abnormal motion, and still measure under the X-ray threshold, especially if muscle guarding limits motion on the day of the films. A patient can also measure over a threshold and have little pain. The thresholds themselves are not universal. A 2015 lumbar study described radiographic instability thresholds as not well defined and used 15% translation and 25 degrees of rotation, which differ from the AMA lumbar values (Davis, Int J Spine Surg 2015). When an X-ray report says a segment is unstable, the first thing to check is which definition and which edition it used.
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.
Frequently Asked Questions
What is spinal instability?
Spinal instability is the loss of the spine's ability, under normal loads, to maintain its pattern of motion without neurologic deficit, major deformity, or incapacitating pain. That clinical definition comes from White and Panjabi's Clinical Biomechanics of the Spine.
How is spinal instability measured on X-ray?
On lateral flexion and extension X-rays. Under the AMA Guides Fifth Edition, cervical translation greater than 3.5 mm or angular motion more than 11 degrees greater than an adjacent level meets the threshold for alteration of motion segment integrity.
What is the difference between mechanical and clinical instability?
Mechanical instability is the inability of the spinal column to carry loads. Clinical instability adds the consequences, meaning pain or neurologic dysfunction (Panjabi, J Electromyogr Kinesiol 2003).
Can you have spinal instability with a normal X-ray?
Possibly. Muscle guarding can limit motion on the day of the films, so a patient with pain from abnormal motion may measure under the threshold. The thresholds also differ between editions of the AMA Guides and between research studies.
What are the three parts of the spine's stabilizing system?
Panjabi described a passive subsystem of vertebrae, discs, and ligaments, an active subsystem of muscles and tendons, and a neural subsystem that monitors and directs the other two (Panjabi, J Spinal Disord 1992). Dr. Justin Dick uses this model when evaluating ligament injuries in Charlotte, NC.
Who evaluates spinal instability after a car accident in Charlotte, NC?
Dr. Justin Dick, DC, CNMT, ARRT(N)(CT), evaluates spinal instability with flexion and extension X-rays 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
- White AA, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Philadelphia: Lippincott; 1990.Evidence tier: Textbook (expert opinion)
- Panjabi MM. The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement. J Spinal Disord. 1992;5(4):383-389. PMID 1490034. LinkEvidence tier: Conceptual model (mechanistic reasoning)
- Panjabi MM. The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. J Spinal Disord. 1992;5(4):390-396. PMID 1490035. LinkEvidence tier: Hypothesis built on cadaveric, animal, and mathematical studies (mechanistic reasoning)
- Panjabi MM. Clinical spinal instability and low back pain. J Electromyogr Kinesiol. 2003;13(4):371-379. PMID 12832167. LinkEvidence tier: Narrative review (mechanistic reasoning)
- American Medical Association. Guides to the Evaluation of Permanent Impairment, Fourth, Fifth, and Sixth Editions.Evidence tier: Expert consensus (impairment rating guideline)
- 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
- 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)
About the author. Dr. Justin M. Dick, DC, is the owner and treating clinician at Clear Life Scoliosis and Chiropractic Center in Charlotte, NC. He holds dual imaging credentials as a Certified Nuclear Medicine Technologist (CNMT) and as an ARRT-registered technologist in nuclear medicine and CT, ARRT(N)(CT). He is a Senior Fellow of the CLEAR Institute, holds the ISICO World Masters credential, is a SOSORT provisional member, and is a certified DOT medical examiner on the FMCSA National Registry. He has multiple peer-reviewed publications indexed in PubMed and serves as adjunct faculty at Life, Sherman, and Palmer chiropractic colleges.
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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
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8814 Rachel Freeman Way, Suite 103
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Phone: 980-368-0766
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