High-yield overview
White and Panjabi | Dynamic Imaging | Fusion Decisions
4mmTranslation threshold
10-15°Angular motion threshold
L4-L5Most common level
50%Facetectomy fusion threshold
TYPES OF LUMBAR INSTABILITY
Degenerative
PatternDisc and facet degeneration
TreatmentMost common in adults
Isthmic
PatternPars defect with slip
TreatmentYoung athletes, L5-S1
Iatrogenic
PatternPost-laminectomy, facetectomy
TreatmentSurgical complication
Traumatic
PatternThree-column injury
TreatmentAcute fracture-dislocation
Critical Must-Knows
- White & Panjabi: loss of ability to maintain normal motion pattern
- Radiographic: greater than 4mm translation OR greater than 10-15° angular motion
- Clinical diagnosis supported by imaging, not purely radiographic
- Bilateral greater than 50% facetectomy OR complete unilateral = iatrogenic instability
- Add fusion if extensive decompression creates instability
Clinical Pearls
- "Instability is CLINICAL with radiographic support
- "Flexion-extension films must be standing/weight-bearing
- "Degenerative cascade: dysfunction → instability → restabilization
- "Three-column theory: 2+ columns = unstable
Clinical Imaging
Imaging Gallery
Critical Exam Concepts
White and Panjabi Definition
Loss of ability to maintain normal motion pattern under physiologic loads without neurological deficit, major deformity, or incapacitating pain.
Radiographic Criteria
Flexion-extension films. Greater than 4mm translation OR greater than 10-15° angular motion at single segment indicates instability.
Iatrogenic Instability
Facetectomy threshold. Bilateral greater than 50% OR complete unilateral facetectomy significantly increases instability risk. Add fusion!
Clinical vs Radiographic
Clinical diagnosis. Imaging supports but doesn't define instability. Symptoms and function guide treatment decisions.
Clinical vs Radiographic Instability
| Feature | Clinical Instability | Radiographic Instability |
|---|---|---|
| Definition | Symptomatic motion disorder | Excessive motion on imaging |
| Assessment | History, exam, function | Flexion-extension X-rays |
| Threshold | Pain with movement | Greater than 4mm or greater than 10-15° |
| Treatment trigger | Failed conservative care | If symptomatic |
| Key point | Clinical diagnosis primary | Supports clinical diagnosis |
Mnemonic
PANJABIWhite and Panjabi Instability Criteria
| P | Pain Mechanical back pain with activity |
| A | Abnormal motion Excessive segmental movement |
| N | Neural risk Potential for neurological compromise |
| J | Joint dysfunction Facet and disc degeneration |
| A | Angular deformity Progressive kyphosis or scoliosis |
| B | Biomechanical failure Loss of three-column support |
| I | Instability catch Sharp pain with movement transitions |
| P | Pain Mechanical back pain with activity | J | Joint dysfunction Facet and disc degeneration | I | Instability catch Sharp pain with movement transitions |
| A | Abnormal motion Excessive segmental movement | A | Angular deformity Progressive kyphosis or scoliosis | ||
| N | Neural risk Potential for neurological compromise | B | Biomechanical failure Loss of three-column support |
Hook:Remember PANJABI for the stability expert's criteria!
Mnemonic
AMPThree-Column Theory (Denis)
| A | Anterior column ALL, anterior VB/disc |
| M | Middle column PLL, posterior VB/disc (KEY) |
| P | Posterior column Pedicles, facets, lamina, ligaments |
| A | Anterior column ALL, anterior VB/disc |
| M | Middle column PLL, posterior VB/disc (KEY) |
| P | Posterior column Pedicles, facets, lamina, ligaments |
Hook:AMP up your spine stability knowledge - 2+ columns = unstable!
Mnemonic
PANPanjabi's Three Subsystems
| P | Passive Osteoligamentous structures |
| A | Active Musculature (dynamic stabilizers) |
| N | Neural Proprioceptive feedback and control |
| P | Passive Osteoligamentous structures |
| A | Active Musculature (dynamic stabilizers) |
| N | Neural Proprioceptive feedback and control |
Hook:PAN-jabi's three subsystems for spinal stability!
Mnemonic
DIRDegenerative Cascade Phases
| D | Dysfunction 15-45 years: disc tears, minor height loss |
| I | Instability 35-70 years: abnormal motion, mechanical pain |
| R | Restabilization 60+ years: osteophytes, stenosis, less motion |
| D | Dysfunction 15-45 years: disc tears, minor height loss |
| I | Instability 35-70 years: abnormal motion, mechanical pain |
| R | Restabilization 60+ years: osteophytes, stenosis, less motion |
Hook:DIR-ect path of degenerative cascade from dysfunction to restabilization!
Overview and Epidemiology
Definition Core Concept
White and Panjabi's definition is exam gold. Instability = loss of spine's ability to maintain normal motion pattern under physiologic loads without neurological deficit, major deformity, or incapacitating pain. This is CLINICAL!
Epidemiology Key Points
Prevalence and incidence:
- Exact prevalence uncertain due to diagnostic variability
- Estimated 10-25% of chronic low back pain cases have instability component
- Increases with age due to degenerative cascade progression
Demographics:
- Age: Degenerative instability peaks at 40-65 years
- Gender: Female 2:1 higher incidence (ligamentous laxity, hormonal factors)
- Occupation: Heavy manual labor increases risk
- Genetics: Familial clustering observed in degenerative spine disease
Level distribution:
- L4-L5 most commonly affected (50-60% of cases)
- L5-S1 second most common (20-30%)
- Multilevel involvement in approximately 20%
- Upper lumbar levels less common but occur with trauma or iatrogenic causes
Healthcare impact:
- Significant contributor to chronic disability and lost work days
- Fusion surgery for degenerative instability among most common spine procedures
- Substantial economic burden from conservative and surgical care
Pathophysiology and Mechanisms
Spinal Stability Biomechanics
Three-Column Theory (Denis):
- Anterior column: ALL, anterior VB and disc (compression loads)
- Middle column: PLL, posterior VB and disc (critical for stability)
- Posterior column: Pedicles, facets, lamina, ligaments (tension and rotation)
Instability rule: Injury to TWO or more columns indicates mechanical instability requiring stabilization.
Facet joint contribution:
- Resist 20% of axial load in neutral position
- Resist 40-50% in extension
- Greater than 50% bilateral facetectomy significantly increases flexion-extension motion
- Complete unilateral facetectomy increases axial rotation and lateral bending
Disc stabilizing function:
- Intact annulus provides torsional stiffness
- Discectomy reduces torsional stiffness by approximately 30%
- Large annular defects increase instability risk
Understanding biomechanics guides surgical decision-making about fusion necessity.
Iatrogenic Instability
Surgical decompression can CREATE instability. Bilateral facetectomy greater than 50% OR complete unilateral facetectomy significantly increases risk. If extensive decompression needed, plan for fusion to prevent postoperative instability and progressive deformity.
Classification Systems
Classification by Etiology
| Type | Mechanism | Typical Age | Key Features |
|---|---|---|---|
| Degenerative | Disc and facet degeneration | 40-65 years | Most common, L4-L5 |
| Isthmic | Pars defect with spondylolisthesis | 15-35 years | Athletes, L5-S1 |
| Iatrogenic | Post-laminectomy, facetectomy | Post-surgery | Excessive bone removal |
| Traumatic | Fracture-dislocation | Any age | Three-column injury |
| Pathological | Tumor, infection | Any age | Bone destruction |
Etiological classification guides treatment approach and prognosis.
Clinical Assessment
History Red Flags
- Instability catch: Sharp pain with movement transitions
- Giving way sensation: Back feels unstable
- Positional relief: Better sitting than standing
- Activity limitation: Avoids bending, twisting
- Hand support: Needs to support back with hands
Pain Characteristics
- Mechanical pattern: Worse with activity
- Positional: Worse prolonged standing
- Relief: Better with sitting or lying
- Morning stiffness: After inactivity
- No night pain: Unless severe degeneration
Physical Examination
Inspection:
- Loss of lumbar lordosis (muscle spasm)
- Forward-flexed posture
- Palpable step-off if spondylolisthesis
- Asymmetric paraspinal muscle bulk
Palpation:
- Paraspinal muscle spasm
- Midline tenderness over affected level
- Step-off palpable in spondylolisthesis
Range of motion:
- Guarded movements
- Limited flexion and extension
- Instability catch during flexion-to-extension transition
Neurological examination:
- Often NORMAL in pure mechanical instability
- May have radiculopathy if associated stenosis or foraminal narrowing
- Lower extremity strength, sensation, reflexes
Special Tests
| Test | Technique | Positive Finding |
|---|---|---|
| Instability catch | Flex then extend spine | Sharp catch pain during transition |
| Prone instability test | Prone, legs off table, press spinous process | Pain relieved with leg lift |
| Posterior shear test | Prone, PA pressure on spinous process | Excessive motion or pain |
| Standing flexion test | Forward bend while palpating | Excessive segmental motion felt |
Prone Instability Test
High specificity for lumbar instability. Patient prone with legs off table. Examiner applies PA pressure to spinous process - pain indicates instability. Patient lifts legs (activates paraspinals) - pain relief with muscle activation confirms dynamic instability. Positive likelihood ratio approximately 4.0.
Differential Diagnosis
Differentiating Lumbar Instability from Mimics
| Condition | Pain pattern | Discriminating feature | Key test |
|---|---|---|---|
| Lumbar instability | Mechanical, worse with activity/transition, relief with sitting | Instability catch, giving-way; positive prone instability test | Standing flexion-extension radiographs (greater than 4mm / greater than 10-15°) |
| Lumbar spinal stenosis | Neurogenic claudication, worse standing/walking, relief with flexion | Symptoms positional and neurogenic, not motion-triggered | MRI showing canal/lateral recess stenosis |
| Discogenic LBP | Axial pain worse with flexion/sitting | Provoked by sustained flexion, not by motion transition | MRI HIZ/Modic changes; no abnormal dynamic motion |
| Facet (zygapophyseal) pain | Axial pain worse with extension/rotation | Relief with medial branch block | Diagnostic facet/medial branch block (greater than 50% relief) |
| Isthmic spondylolisthesis | Mechanical back pain ± radiculopathy in young athlete | Pars defect; step-off; commonest L5-S1 | Oblique radiograph / CT pars; lateral standing film |
| Sacroiliac joint dysfunction | Buttock pain below L5, often unilateral | Provocation cluster positive; pain over PSIS | SIJ provocation tests / image-guided block |
Investigations
Plain Radiographs - KEY Investigation
Standing AP and lateral:
- MUST be weight-bearing (supine misses instability)
- Assess disc height, alignment, osteophytes
- Measure slip if spondylolisthesis present
Flexion-extension lateral radiographs:
- GOLD STANDARD for diagnosing instability
- Patient performs maximal safe flexion and extension
- Measure translation and angular motion
Radiographic instability criteria:
- Translation: greater than 4mm sagittal plane shift
- Angular: greater than 10-15 degrees segmental motion
- Traction spur: horizontal osteophyte indicates chronic instability
Measurement technique:
- Translation: measure posterior body offset between adjacent vertebrae
- Angular: measure change in segmental Cobb angle from flexion to extension
- Use same anatomic landmarks on both views
Dynamic radiographs essential - static films miss functional instability.
Imaging Gallery - Diagnostic Assessment
Management Algorithm
Algorithm
Non-Operative Management - First Line
Indications for conservative care:
- Mild to moderate symptoms
- No progressive deformity
- No neurological deficit
- Patient preference
- Medical comorbidities precluding surgery
Core stabilization physiotherapy:
- Target transversus abdominis and multifidus
- Proprioceptive training
- Postural education
- Goal: enhance active subsystem compensation
- 6-12 weeks structured program
- Most important non-surgical intervention
Activity modification:
- Avoid repetitive bending and twisting
- Proper lifting mechanics
- Ergonomic workplace assessment
- Weight loss if obese
Bracing:
- Short-term use for acute flares (2-4 weeks maximum)
- Lumbosacral corset provides external support
- Prolonged use causes muscle deconditioning
- Wean as core strength improves
Medications:
- NSAIDs for inflammation and pain
- Muscle relaxants for spasm (short-term)
- Neuropathic agents if radicular component
- Avoid opioids for chronic mechanical pain
Interventional procedures:
- Epidural steroid injections if radicular symptoms
- Facet joint injections (diagnostic and therapeutic)
- Medial branch blocks
- Radiofrequency ablation for facet-mediated pain
Success rates:
- Approximately 30-40% adequate improvement with conservative care
- Better outcomes with structured physiotherapy program
- Predictors of success: mild symptoms, good compliance, no significant deformity
Minimum 6 months conservative trial before considering surgery (unless progressive neurology).
Smoking and Fusion
Smoking significantly increases pseudarthrosis risk. Nicotine impairs bone healing and fusion rates. Non-smokers: 90-95% fusion. Smokers: 70-80% fusion. Strongly encourage smoking cessation minimum 4 weeks pre-op and throughout healing. Consider bone morphogenetic protein (BMP) in smokers.
Complications
Early Complications (under 6 weeks)
Intraoperative:
- Dural tear (5-10%): Primary repair, bed rest, avoid Valsalva
- Neural injury (1-2%): Nerve root or cauda equina from retraction or instrumentation
- Vascular injury (under 1%): Aorta, vena cava, iliac vessels (ALIF higher risk)
- Excessive bleeding: Epidural venous plexus, bone bleeding
Immediate post-operative:
- Wound infection (2-5%): Superficial or deep, higher with multilevel, obesity
- Hematoma: Epidural or wound, may cause neurological compression
- CSF leak: From unrecognized or inadequately repaired dural tear
- Medical: DVT/PE, MI, pneumonia, UTI
Prevention strategies:
- Meticulous hemostasis
- Prophylactic antibiotics
- DVT prophylaxis
- Careful retraction and neural handling
Early recognition and management critical for optimal outcomes.
Evidence Base
Panjabi: Clinical Spinal Instability and Low Back Pain
Key Findings:
- Clinical instability defined as loss of the normal pattern of spinal motion causing pain and/or neurological dysfunction
- Stabilising system comprises three subsystems: spinal column, spinal muscles, and the neural control unit
- The neutral zone is a more sensitive parameter than range of motion for detecting destabilisation and restabilisation
- Reduction of an enlarged neutral zone (by fusion, osteophytes, or muscle activation) correlates with pain relief
Clinical Implication: Must know this definition verbatim for exams; examiners expect the clinical versus radiographic instability distinction and the neutral-zone concept.
Panjabi Three Subsystem Model
Key Findings:
- Described spinal stability as interaction of passive (osteoligamentous), active (muscular), and neural control subsystems
- Explained how dysfunction of any subsystem can be compensated, adapted to, or lead to injury and clinical instability
- Provided framework for understanding why structural findings do not always correlate with symptoms
- Supported role of rehabilitation targeting the active and neural subsystems
Clinical Implication: High-yield for explaining instability pathophysiology; examiners appreciate discussion of subsystem interactions.
Kirkaldy-Willis Degenerative Cascade
Key Findings:
- Described three phases: dysfunction, instability, and restabilisation
- Instability phase characterised by abnormal motion and mechanical pain
- Restabilisation phase shows decreased motion but stenosis develops
- Natural history shows transition from mechanical to neurogenic symptoms
Clinical Implication: Classic framework for discussing degenerative spine disease; examiners expect knowledge of the three phases and their clinical implications.
Abumi: Graded Facetectomies and Lumbar Stability
Key Findings:
- In vitro study of fresh human functional spinal units under six load modes (range of motion and neutral zone measured)
- Medial facetectomy (partial) and division of supraspinous/interspinous ligaments did NOT significantly affect stability
- Total facetectomy, even when created unilaterally, rendered the lumbar segment unstable (increased flexion and axial rotation)
- Provides the biomechanical basis for preserving at least the lateral facet and for adding fusion when total facetectomy is required
Clinical Implication: Critical for discussing iatrogenic instability; examiners test the threshold of facet resection that mandates fusion.
Martin: Reoperation Rates After Lumbar Surgery
Key Findings:
- Washington State discharge registry; adults having lumbar surgery for degenerative disease in 1990-1993 (n=24,882)
- Cumulative incidence of reoperation was 19% over the subsequent 11 years
- For spondylolisthesis, reoperation was LESS likely after fusion than after decompression alone (17.1% vs 28.0%, P=0.002)
- For other degenerative diagnoses, reoperation was HIGHER after fusion than decompression alone (21.5% vs 18.8%, P=0.008); 62.5% of post-fusion reoperations involved device complication or pseudarthrosis
Clinical Implication: Examiners expect a balanced, indication-specific view of fusion; quote the spondylolisthesis benefit and the device/pseudarthrosis-driven reoperation burden.
Herkowitz & Kurz: Decompression vs Decompression + Arthrodesis
Key Findings:
- 50 patients with spinal stenosis and degenerative spondylolisthesis prospectively studied (mean follow-up 3 years)
- Concomitant intertransverse-process arthrodesis gave significantly better relief of back and leg pain than decompression alone
- Established that an unstable spondylolisthetic segment should be fused at the time of decompression
- Landmark evidence behind fusing the listhetic level during decompression
Clinical Implication: High-yield exam answer: when stenosis coexists with degenerative spondylolisthesis, decompression plus arthrodesis outperforms decompression alone.
Fischgrund (1997 Volvo Award): Instrumentation and Fusion
Key Findings:
- 76 patients randomised to instrumented versus non-instrumented posterolateral fusion after decompression
- Successful arthrodesis: 82% instrumented vs 45% non-instrumented (P=0.0015)
- Clinical outcome excellent/good in 76% instrumented vs 85% non-instrumented (P=0.45, not significant)
- Pedicle-screw instrumentation improves fusion rate but did not improve 2-year clinical pain outcome
Clinical Implication: Examiners expect the dissociation between radiographic fusion and clinical outcome; quote the 82% vs 45% fusion difference.
SPORT: Surgical vs Nonsurgical for Degenerative Spondylolisthesis
Key Findings:
- 607 patients across 13 US centres (randomised + observational cohorts); high crossover (~40% each way) in the randomised arm
- As-treated analysis showed a significant advantage for surgery at 2 years: bodily pain effect 18.1, physical function 18.3, Oswestry -16.7
- Surgically treated patients had substantially greater improvement in pain and function than nonsurgical care over 2 years
- Little evidence of harm from either treatment
Clinical Implication: Examiners expect the as-treated benefit of surgery and the caveat of high crossover undermining the intention-to-treat result.
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
CLINICAL SCENARIOStandard
Scenario 1: Degenerative Instability After Decompression
CLINICAL PROMPT
"A 58-year-old woman underwent L4-L5 laminectomy for stenosis 18 months ago with initial good relief of leg symptoms. She now presents with mechanical back pain worse with activity. Flexion-extension X-rays show 6mm translation at L4-L5. How would you manage this?"
PRACTICAL APPROACH
This is iatrogenic instability following decompression for lumbar stenosis. I would take a systematic approach. First, I would obtain detailed history about pain characteristics - mechanical back pain worsening with activity suggests instability rather than recurrent stenosis. Second, I would examine for neurological deficit and assess for instability catch sign. Third, I would review her flexion-extension radiographs confirming greater than 4mm translation indicating instability, and MRI to rule out recurrent stenosis. My initial management would be conservative with core stabilization physiotherapy for 6 months. If symptoms persist despite optimal non-operative care, I would discuss surgical fusion using posterolateral fusion with pedicle screw instrumentation. I would counsel about 85-95% fusion rate with instrumentation, risk of pseudarthrosis especially if she smokes, and adjacent segment disease risk of 2-3% per year long-term.
KEY CLINICAL POINTS
Recognize iatrogenic instability from prior decompression
Differentiate mechanical instability pain from neurogenic stenosis symptoms
Confirm radiographic instability with flexion-extension films
Conservative trial first unless progressive neurology
Instrumented fusion if failed conservative care
COMMON PITFALLS
Rushing to surgery without conservative trial
Missing recurrent stenosis requiring additional decompression
Not counseling about smoking cessation if applicable
Forgetting to mention adjacent segment disease risk
FURTHER QUESTIONS
"What if she had only 3mm translation on flexion-extension films?"
"How would you decide between PLIF vs TLIF vs posterolateral fusion?"
"What factors increase pseudarthrosis risk in this patient?"
"How would you manage her if she developed adjacent segment disease at L3-L4 five years later?"
CLINICAL SCENARIOChallenging
Scenario 2: Extensive Decompression Planning
CLINICAL PROMPT
"You are planning L4-L5 decompression for severe central and lateral recess stenosis. Pre-operative flexion-extension films show no instability. Intraoperatively, you find you need bilateral 60% facetectomy to adequately decompress. What do you do?"
PRACTICAL APPROACH
This scenario involves intraoperative decision-making about adding fusion during extensive decompression. Based on biomechanical studies by Abumi and colleagues, bilateral facetectomy greater than 50% significantly increases flexion-extension motion and creates iatrogenic instability. Even though pre-operative films showed no instability, the extensive bone removal I need for adequate decompression will CREATE instability. I would proceed with the necessary bilateral facetectomy for neural decompression, then add posterolateral fusion with pedicle screw instrumentation to prevent post-operative instability and progressive deformity. I would place pedicle screws at L4 and L5, perform meticulous facet decortication, and place bone graft over the transverse processes and remaining facets. If the patient had significant disc degeneration or foraminal stenosis, I would also consider TLIF to restore disc height and foraminal volume. Post-operatively, I would counsel about the decision to add fusion to prevent instability, expected fusion rate of 85-95% with instrumentation, and importance of smoking cessation if applicable.
KEY CLINICAL POINTS
Recognize 50% bilateral facetectomy threshold from biomechanical literature
Understand extensive decompression creates iatrogenic instability
Plan fusion at time of index surgery rather than revision
Consider interbody fusion if disc degeneration or foraminal stenosis
Counsel patient about intraoperative decision and rationale
COMMON PITFALLS
Performing extensive facetectomy without fusion leading to instability
Not knowing the 50% bilateral facetectomy threshold
Inadequate decompression to avoid fusion (compromises neural outcome)
Adding fusion without counseling patient about decision
FURTHER QUESTIONS
"What is the evidence for the 50% facetectomy threshold?"
"Would your decision change if only unilateral complete facetectomy was needed?"
"How do you counsel a patient pre-operatively about possible fusion?"
"What if the patient refuses fusion - do you proceed with decompression alone?"
CLINICAL SCENARIOChallenging
Scenario 3: Clinical vs Radiographic Instability
CLINICAL PROMPT
"A 52-year-old manual laborer has chronic mechanical low back pain worse with activity. MRI shows severe L4-L5 disc degeneration with facet arthropathy. Flexion-extension films show 3mm translation and 8 degrees angular motion - below instability thresholds. He has failed 12 months of physiotherapy. Does he have instability? Would you offer surgery?"
PRACTICAL APPROACH
This case highlights the crucial distinction between clinical and radiographic instability. By White and Panjabi's definition, instability is the loss of the spine's ability to maintain normal motion pattern under physiologic loads resulting in pain and functional impairment. This is fundamentally a CLINICAL diagnosis, supported but not defined by radiographic findings. This patient has clinical instability despite radiographs not meeting traditional thresholds - he has mechanical back pain correlating with degenerative segment, functional impairment, and failed optimal conservative management. The radiographic thresholds of greater than 4mm and greater than 10-15 degrees are guidelines, not absolute requirements. His 3mm translation and 8 degrees motion, combined with severe disc degeneration and facet arthropathy on MRI, support the clinical diagnosis. Having failed 12 months of appropriate physiotherapy, I would discuss surgical fusion as a treatment option. I would use shared decision-making, explaining that fusion has 70-80% good outcomes but also risks including pseudarthrosis especially given his manual labor occupation, adjacent segment disease long-term, and that approximately 10-20% have persistent pain despite solid fusion. I would optimize modifiable factors including smoking cessation if applicable, weight loss if obese, and ensure realistic expectations before proceeding.
KEY CLINICAL POINTS
Instability is CLINICAL diagnosis, not purely radiographic
White and Panjabi definition emphasizes clinical presentation
Radiographic thresholds are guidelines, not absolute requirements
Failed conservative care is key surgical indication
Shared decision-making with realistic outcome expectations
COMMON PITFALLS
Denying surgery solely based on radiographic thresholds
Not recognizing clinical instability concept
Offering surgery without discussing risks and realistic expectations
Not addressing modifiable risk factors before surgery
FURTHER QUESTIONS
"What is the difference between clinical and radiographic instability?"
"How would you counsel this patient about fusion outcomes in manual laborers?"
"Would disc replacement be appropriate for this patient?"
"What factors predict poor outcome after fusion for mechanical back pain?"
MCQ Practice Points
Clinical Pearl
Q: What radiographic findings on flexion-extension X-rays define lumbar instability? A: Translation greater than 4mm (or greater than 10% of vertebral body width) OR angular motion greater than 10-15 degrees between adjacent segments. These are the classic White-Panjabi criteria for clinical instability.
Clinical Pearl
Q: How much facet resection causes iatrogenic instability requiring fusion? A: Greater than 50% bilateral facetectomy or complete unilateral facetectomy. The facet joints contribute 40-50% of torsional stability. Partial medial facetectomy (less than 50% per side) typically preserves stability.
Clinical Pearl
Q: What is the difference between clinical and mechanical instability? A: Clinical instability produces symptoms (pain, neurological signs) with motion; mechanical instability is radiographic abnormal motion that may be asymptomatic. Surgical fusion addresses mechanical instability but is only indicated when clinically symptomatic.
Clinical Pearl
Q: When should fusion be added to decompression for degenerative conditions? A: When pre-existing instability exists (greater than 4mm translation), when decompression creates iatrogenic instability (extensive facetectomy), or when deformity correction is required. SPORT trial showed no benefit of routine fusion for stable stenosis.
Guidelines, Registries & Global Practice
Global Epidemiology
Lumbar instability is most often a manifestation of degenerative disc and facet disease, the leading structural driver of low back pain (LBP). According to PubMed, a systematic review of 165 studies from 54 countries estimated the global mean point prevalence of LBP at approximately 11.9% and the 1-month prevalence at approximately 23.2%, with the highest burden in women and those aged 40-80 years (Hoy et al. 2012, DOI). LBP is consistently ranked the single leading cause of years lived with disability worldwide, and the absolute number affected is rising with population ageing.
- Most affected segment: L4-L5 (degenerative spondylolisthesis), then L5-S1
- Sex: Degenerative spondylolisthesis is markedly more common in women
- Trend: Lumbar fusion volumes have risen faster than decompression in high-income systems, without a fall in reoperation rates (Martin et al. 2007, DOI)
Guideline & Society Positions
Side-by-side Guidance on Instability and Fusion
| Body / Region | Position on fusion for instability | Evidence basis |
|---|---|---|
| NICE NG59 (UK) | Do NOT offer fusion for non-specific LBP outside a trial; reserve for deformity/instability with clear indication | RCT-informed, conservative |
| NASS / AAOS (North America) | Decompression PLUS fusion supported for symptomatic degenerative spondylolisthesis with instability | Level I/II (SPORT, Herkowitz, Fischgrund) |
| BOA / British spine practice | MDT assessment; fusion for documented instability or iatrogenic destabilisation after wide decompression | Consensus + RCT |
| AOSpine / EFORT (Europe) | Reserve fusion for true instability, deformity, or post-decompression destabilisation; favour selective single-level constructs | Consensus + biomechanical |
Registry & Trial Evidence
- SPORT (NEJM 2007): as-treated benefit of surgery over non-operative care for degenerative spondylolisthesis at 2 years (Weinstein et al., DOI).
- Fischgrund 1997 Volvo Award: instrumentation raised fusion rate (82% vs 45%) without improving 2-year clinical outcome (DOI) — a recurring theme in registry analyses.
- National spine registries (e.g. Swespine, the British Spine Registry, and the Norwegian/Danish spine registries) collect PROMs (Oswestry Disability Index, EQ-5D) to benchmark fusion outcomes; routine reporting of reoperation and adjacent-segment surgery is now standard.
Practice Variation
Marked international and intra-national variation exists in fusion rates for degenerative lumbar disease, driven by reimbursement, surgeon training, and differing thresholds for diagnosing "instability." Conservative-first pathways (physiotherapy-led core stabilisation, activity modification, judicious injections) are universally endorsed before elective fusion, except for progressive neurological deficit, cauda equina syndrome, or progressive deformity.
Australian Context
In Australia, physiotherapy-based core stabilisation is first-line, with PBS-subsidised analgesia for symptomatic relief during rehabilitation. Spinal fusion (commonly TLIF or PLIF, with minimally invasive and navigation/robotic options at tertiary centres) is reserved for symptomatic instability refractory to conservative care. Post-operative rehabilitation is delivered through public hospital outpatient services and private practice, with return-to-work coordination via occupational therapy and vocational rehabilitation.
LUMBAR INSTABILITY - EXAM ESSENTIALS
Clinical summary
Key Definitions
- •White & Panjabi: Loss of spine's ability to maintain normal motion pattern under physiologic loads without neurological deficit, major deformity, or incapacitating pain
- •Clinical instability: CLINICAL diagnosis supported by imaging, not purely radiographic
- •Radiographic thresholds: Greater than 4mm translation OR greater than 10-15° angular motion on flexion-extension films
- •Panjabi subsystems: Passive (osteoligamentous), Active (muscular), Neural (proprioceptive control)
Classification
- •Etiological: Degenerative (most common), isthmic (pars defect), iatrogenic (post-laminectomy), traumatic, pathological
- •Three columns (Denis): Anterior (ALL, anterior VB/disc), Middle (PLL, posterior VB/disc - KEY), Posterior (facets, ligaments)
- •Degenerative cascade: Dysfunction (15-45y) → Instability (35-70y) → Restabilization (60+y)
- •Instability rule: Injury to 2 or more columns = mechanical instability
Clinical Assessment
- •History: Mechanical back pain worse with activity, instability catch, giving way sensation, relief with sitting
- •Prone instability test: PA pressure on spinous process causes pain, relieved with leg lift (muscle activation)
- •Flexion-extension radiographs: GOLD STANDARD - must be weight-bearing
- •MRI findings: Disc degeneration, facet effusion, Modic Type II changes, high-intensity zone in annulus
Surgical Indications
- •Failed conservative management minimum 6 months (unless progressive neurology)
- •Documented instability on imaging with symptom correlation
- •Iatrogenic: Bilateral greater than 50% facetectomy OR complete unilateral facetectomy
- •Progressive deformity with functional impairment
- •Add fusion to decompression if creating instability
Surgical Techniques
- •Posterolateral fusion with instrumentation: Gold standard, 85-95% fusion rate
- •TLIF: Unilateral approach, less neural retraction, excellent foraminal decompression
- •PLIF: Bilateral approach, direct neural decompression, higher dural tear risk
- •ALIF: Anterior approach, large graft area, useful for spondylolisthesis reduction
Complications
- •Pseudarthrosis: 5-10% instrumented PLF. Risk factors: smoking, obesity, multilevel, diabetes
- •Adjacent segment disease: 2-3% per year, 15-20% require surgery at 10 years
- •Dural tear: 5-10%. Management: primary repair, bed rest, avoid Valsalva
- •Smoking increases pseudarthrosis risk: 90-95% fusion non-smokers vs 70-80% smokers
Evidence Pearls
- •White & Panjabi (1990): Foundation definition and radiographic thresholds
- •Panjabi three subsystem model: Explains variable clinical presentation
- •Abumi facetectomy study: Bilateral greater than 50% = significant motion increase
- •Kirkaldy-Willis cascade: Dysfunction → Instability → Restabilization phases
- •Conservative success 30-40%, surgical 70-80% good/excellent outcomes
References
- White AA, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Philadelphia: Lippincott; 1990.
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