Spinal Instability: Definition, theory and assessment of spinal column function and dysfunction
1. OVERVIEW AND DESCRIPTION
The spine is made up of segments consisting of two vertebrae and the interconnecting soft tissues. The integration of the biomechanical characteristics of these individual spinal components provides the overall strength and structure of the spine to protect the spinal cord and nerve roots. In addition to protecting the spinal cord and nerve roots, spine stability is necessary to perform the other basic biomechanical function of the spinal system, which is to allow movements between body parts and to carry loads1.
The objective of this topic is to summarize the current concepts of spinal instability. While spinal instability has been defined in various ways, no universally accepted definition currently exists.
In one instance, Panjabi et al.1 describe the stability system as consisting of three conceptually separate but interdependent components:
- Passive subsystem – spinal column (vertebrae, facet joints, intervertebral disc, spinal ligaments and joint capsules)
- Active subsystem – spinal muscles (muscles and tendons)
- Control subsystem – neural feedback (neural control centers and force transducers located in ligaments, tendons, muscles)
Excessive displacement of the spinal components would result in neurological deficit, deformity or pain. White and Panjabi2 define instability as “the loss of the ability of the spine under physiologic loads to maintain relationships between vertebrae in such a way that there is neither initial damage nor subsequent irritation to the spinal cord or nerve roots and, in addition, there is no development of incapacitating deformity or pain due to the structural changes.”
In another instance, Francis Denis3 proposed the “three-column” model. The spine has three load-bearing columns on the sagittal plane, anterior, middle, and posterior columns. The anterior column consists of the anterior longitudinal ligament, anterior two thirds of vertebral body, and anterior two thirds of annulus fibrosis or disk. The middle column is formed by the posterior third of the vertebral body, posterior third of annulus fibrosis, and posterior longitudinal ligament. The posterior column includes pedicles, facet joints, capsules, laminae, supraspinous and interspinous ligaments, joint capsule, and ligamentum flavum. When integrity of the middle and either the anterior or posterior column are compromised, the spine will likely be unstable.
Additionally, instability can also be described as acute versus chronic, as well as overt, anticipated, or covert. Overt instability is excessive motion that is supported by radiographic studies and results in pain, deformity, or neurological deficits. Overt instability generally requires stabilization either by external or internal means. Anticipated instability refers to instability that would be produced by a surgical procedure that is required for proper decompression of neural elements or resection of an offending lesion. Covert or microinstability refers to circumstances in which excessive motion cannot be grossly demonstrated but is presumed to exist based on the combination of clinical and radiographic findings4.
2. RELEVANCE TO CLINICAL PRACTICE
The pathophysiology of instability is highly variable depending on the etiology. The determination of stability may be made with an understanding of the biomechanics of the spine and the mechanism of injury (i.e. force vectors) with spine stability models. Mechanism of spinal injury associated with instability is summarized in the table below adapted from Marx J.A. Rosen’s emergency medicine: concepts and clinical practice, 6th edition5.
|Mechanism of Spinal Injury||Stability|
|Flexion||Anterior wedge fracture||Stable|
|Flexion teardrop fracture||Extremely|
|Clay shoveler’s fracture||Stable|
|Bilateral facet dislocation||Always|
|Anterior atlantoaxial dislocation with or without fracture||Unstable|
|Odontoid fracture with lateral displacement||Unstable|
|Fracture of transverse process||Stable|
|Flexion-rotation||Unilateral facet dislocation||Stable|
|Rotary atlantoaxial dislocation||Unstable|
|Extension||Posterior neural arch fracture (C1)||Unstable|
|Hangman’s fracture (C2)||Unstable|
|Extension teardrop fracture||Unstable|
|Posterior atlantoaxial dislocation with or without fracture||Unstable|
|Burst fracture of vertebral body||Stable|
|Jefferson fracture (C1)||Extremely|
|Isolated fractures of articular pillar and vertebral body||Stable|
As a general rule, conditions that result in acute overt instability require stabilization, either internally by fusion or externally by reduction and bracing. In traumatic injuries, if instability is due to a fracture rather than ligamentous rupture, if the fracture fragments are in contact and in near-anatomical alignment, and if there is no significant neural compression, an external brace is tried until the fracture heals. In all other circumstances and in cases where bracing has failed, fusion is indicated4.
Spinal instability may manifest in three distinct ways: neurologic deficit due to cord, nerve root or cauda equina compression, pain and incapacitating deformity. Many tools have been proposed to assess for spinal instability, though no true consensus has been reached. Evaluation for spinal instability differs in the acute traumatic setting versus the chronic degenerative setting.
Acute trauma setting:
- Canadian C-spine rule6: used to determine what patients need cervical spine imaging
- Age ≥65 years
- Paresthesias in extremities
- Dangerous mechanism:
- fall from an elevation ≥3 ft or 5 stairs
- axial load to the head (e.g. diving)
- motor vehicle collision at high speed (>100 km/hr) or with rollover or ejection
- bicycle collision
- collision involving a motorized recreational vehicle
- Nexus criteria7: relieves patients from c-spine immobilization and need for cervical spine imaging (no age cutoffs). Image only if one of the following are present
- Focal neurologic deficit
- Midline spinal tenderness
- Altered level of consciousness
- Distracting injury
Chronic progressive deformity:
- Focused history and physical exam with appropriate imaging
- History and symptoms of clinical instability8:
- Giving way or back giving out, feeling of instability
- Painful catching or locking during trunk motions
- Pain during transitional activities
- Worse with sustained postures
- Long-term chronic disorder
- Physical signs of instability9:
- Visible, palpable (spinous process) step (listhesis, not necessarily hypermobility or instability)
- Palpable mobility
- Visible band of hypertrophied muscle at the level of instability
- Observe (side of patient) active flexion for sudden shake or catch
- Observe (behind patient) active flexion for side bending, may represent facet hypermobility on the opposite side
- Palpate for step forward or rotation; if persistent in standing and prone (i.e. not positional), then potentially unstable
- Transient neurologic signs such as neurologic claudication
- Radiologic Diagnosis of Instability:
- Cervical radiographs2,11
- Segmental kyphosis greater than 11 degrees
- Anterolisthesis greater than 3.5 mm of one vertebral body on another
- Lateral neutral, flexion and extension xrays10, 12
- Forward displacement of one vertebrae on another: spondylolisthesis
- Backward displacement of one vertebrae on another: retrolisthesis
- Narrowing of the intervertebral foramen and loss of disc thickness
- An abrupt apparent change in pedicle length
- Anteroposterior xrays with lateral bending10,12
- Bending to one side or another
- Decreased bending to one side with loss of both vertebral rotation and tilt with actual opening of the disc on the side to which the patient is bending
- An abnormal degree of disc closure on the side to which the patient is bending
- Malalignment of spinous processes and pedicles
- Lateral translation of one vertebra on another due to an abnormal degree of rotation
- History and physical exam help guide the initial imaging modality, which will often be plain x-rays. However, more advanced detail imaging such as dynamic imaging, MRI, CT and studies with contrast, is often necessary to detect instability13
- Cervical radiographs2,11
3. CUTTING EDGE/UNIQUE CONCEPTS/EMERGING ISSUES
Stem cell technology has gathered steam over the past several years and is being researched in all aspects of medicine. The use of stem cells in spine surgery has now become widespread, but is not yet in routine clinical use in spine surgery. Mesenchymal stem cells possess the ability to regenerate bone, cartilage, and fibrous tissues. There are currently several studies on clinicaltrials.gov that are underway using mesenchymal stem cells to potentially repair intervertebral discs or regenerate new bone during spine surgery. More research is needed to validate the potential benefits of using stem cells to improve spine instability.
4. GAPS IN KNOWLEDGE/EVIDENCE BASE
The current definitions of spine instability are not uniformly accepted nor applied. Therefore, there is no consensus about the timing of conservative versus surgical treatment in spine diseases with covert instability. The understandings of spinal biomechanics need improvement to determine and differentiate the relationship and severity between radiographic instability and its clinical manifestation. This gap in knowledge creates a burden for all disciplines involved in the diagnosis and treatment of patients suffering from disorders of spine stability.
- Panjabi MM. The Stabilizing System of the Spine. Part 1. Function, Dysfunction, Adaptation and Enhancement. Journal of Spinal Disorders and Techniques 1992;5(4): 383-389.
- White AA, 3rd, Johnson RM, Panjabi MM, Southwick WO. Biomechanical analysis of clinical stability in the cervical spine.Clin Orthop Relat Res.1975;109: 85–96.
- Denis F. Spinal Instability as Defined by the Three-column spine concept in acute spinal trauma. Clinical Orthopedics and Related Research 1984;189: 65-76.
- Pakzaban P, Kopell BH. Spinal Instability and Spinal Fusion Surgery. Medscape Reference, updated 12/3/2013. http://emedicine.medscape.com/article/1343720-overview
- Marx, JA, Hockberber RS, Walls RM. Rosen’s emergency medicine: concepts and clinical practice, 6th edition. Mosby Inc., St Louis 2006. Copyright 2006 Elsevier.
- Stiell, Ian G., et al. The Canadian C-spine rule for radiography in alert and stable trauma patients. Jama. 2001; 286(15): 1841-1848.
- Hoffman JR, Wolfson AB, Todd K, Mower WR. Selective cervical spine radiography in blunt trauma: methodology of the National Emergency X-Radiography Utilization Study (NEXUS).Ann Emerg Med.1998;32(4): 461–9
- Biely S, Smith S, Silfies SP. Clinical instability of the lumbar spine: diagnosis and intervention. Ortho Phys Ther Pract. 2006;18(3):11–18.
- Paris S. Physical signs of instability. Spine.1985;10(3): 277-279.
- Kirkaldy-Willis WH, Farfan HF. Instability of the Lumbar Spine. Clinical Orthopedics and related research 165 (1982): 110-123
- Torretti, JA and Sengupta, DK. Cervical Spine Trauma. Indian J Orthop. 2007 Oct-Dec; 41(4): 255–267.
- Beazell JR, Mullins M, Grindstaff TL. Lumbar instability: an evolving and challenging concept. J Man Manip Ther. 2010 Mar; 18(1): 9–14.
- Davis PC, Wippold FJ, Brunberg JA, Cornelius RS, Robert L, Dormont PD, Turski PA. ACR Appropriateness Criteria® on low back pain. 2009 Journal of the American College of Radiology, 6(6), 401-407.
Vincent Huang, MD
Nothing to Disclose
Andrew Lederman, MD
Nothing to Disclose