ISTHMIC SPONDYLOLISTHESIS
Pars Defect | Spondylolysis | L5-S1 Slip
WILTSE & MEYERDING
Critical Must-Knows
- PARS DEFECT (spondylolysis) distinguishes from degenerative type
- L5-S1 most common level (90%) vs L4-5 for degenerative
- Scottie dog collar sign on oblique X-ray shows pars defect
- 80% asymptomatic - most patients never need surgery
- CT is gold standard for pars defect visualization
Examiner's Pearls
- "Young athlete with extension-related back pain = think pars stress reaction
- "Hamstring tightness common in adolescents with spondylolisthesis
- "High-grade slips may have waddling gait and heart-shaped sacrum
- "SPECT-CT shows metabolic activity in acute/healing pars defect
Critical Isthmic Spondylolisthesis Exam Points
Isthmic vs Degenerative
Isthmic has PARS DEFECT - spondylolysis present on imaging. Occurs at L5-S1 (90%). Young patients (adolescence). Degenerative has NO pars defect, occurs at L4-L5 (70%), older patients.
Scottie Dog Sign
On oblique X-ray, the posterior elements form a "Scottie dog". The pars defect appears as a "collar" across the dog's neck. CT is more sensitive and specific for pars visualization.
Meyerding Grading
Grade I: Less than 25% slip. Grade II: 25-50%. Grade III: 50-75%. Grade IV: 75-100%. Grade V (Spondyloptosis): Greater than 100% slip. Low-grade (I-II), High-grade (III-V).
Treatment Principles
Most are asymptomatic - observation only. Conservative first for symptomatic (physio, activity mod). Surgery if: failed conservative, progressive slip, neurological deficit, high-grade slip.
Isthmic vs Degenerative Spondylolisthesis
| Feature | Isthmic | Degenerative |
|---|---|---|
| Pars status | DEFECT present (spondylolysis) | Intact (no defect) |
| Most common level | L5-S1 (90%) | L4-L5 (70%) |
| Age of onset | Adolescence/young adult | Over 50 years |
| Gender | Male predominant (3:1) | Female predominant (6:1) |
| Mechanism | Stress fracture of pars | Facet and disc degeneration |
| Maximum grade | Any grade (up to spondyloptosis) | Usually Grade I-II only |
| Surgery if symptomatic | Pars repair or fusion | Decompression with or without fusion |
At a Glance
Isthmic spondylolisthesis is defined by a pars interarticularis defect (spondylolysis), distinguishing it from degenerative type which has an intact pars. It occurs at L5-S1 in 90% of cases (versus L4-5 for degenerative) with a 6% population prevalence but 80% remain asymptomatic. The classic "Scottie dog collar" sign on oblique X-ray indicates the pars defect, though CT is the gold standard for visualization. Meyerding grading quantifies slip severity (I: less than 25%, II: 25-50%, III: 50-75%, IV: 75-100%, V: spondyloptosis). Wiltse Type IIA (lytic/stress fracture) is most common. Young athletes with extension-related back pain should raise suspicion for pars stress reaction.
PARS - Pars Defect Features
Memory Hook:PARS defect defines isthmic spondylolisthesis
SLIP - Meyerding Grading
Memory Hook:SLIP grades progress from Slight to Profound
WILT - Wiltse Classification
Memory Hook:WILT classification covers all spondylolisthesis types
Overview and Epidemiology
Isthmic Spondylolisthesis is forward slippage of a vertebra due to a defect (spondylolysis) or elongation of the pars interarticularis. It is the most common type of spondylolisthesis in patients under 50 years.
Definition:
Spondylolysis refers to a defect in the pars interarticularis (the bony bridge between the superior and inferior articular processes). Spondylolisthesis is forward displacement of one vertebra on another. Isthmic spondylolisthesis occurs when spondylolysis allows this slip.
Epidemiology:
| Factor | Details |
|---|---|
| Prevalence | 6% of general population |
| Symptomatic rate | Only 20% become symptomatic |
| Most common level | L5-S1 (90%), L4-L5 (5-10%) |
| Peak age | Adolescence (15-25 years) |
| Gender | Male predominant 3:1 |
| High-risk sports | Gymnastics, football linemen, weightlifting, cricket fast bowlers |
Natural History:
Most patients with pars defects remain asymptomatic throughout life. Progression of slip is most likely in childhood/adolescence and typically stabilizes after skeletal maturity.
Anatomy and Biomechanics
Pars Interarticularis
Anatomical Location:
The pars interarticularis is the portion of the lamina between the superior and inferior articular processes. It is the thinnest and weakest part of the neural arch.
| Structure | Relationship |
|---|---|
| Superior | Inferior margin of superior articular facet |
| Inferior | Superior margin of inferior articular facet |
| Medial | Lamina |
| Lateral | Transverse process |
Why L5?
L5 is predisposed because:
- Maximum lordosis and shear stress at L5-S1
- Orientation of L5 facets resists forward slip less effectively
- Highest compressive and shear loads during extension
- Pars at L5 is anatomically thinner
Biomechanics of Slip
Load Distribution:
The posterior elements (facets, pars) normally resist 25-30% of axial load. When the pars is deficient, this load transfers to the disc, leading to degeneration and progressive slip.
Slip Angle:
The slip angle (lumbosacral kyphosis) is the angle between L5 and S1. High slip angles indicate more kyphosis at the lumbosacral junction and correlate with worse outcomes.
Pathophysiology
Mechanism of Pars Defect
Type IIA - Lytic/Fatigue Fracture
Mechanism: Repetitive hyperextension causes cyclic loading of the pars. The pars experiences tensile stress on the inferior surface and compressive stress superiorly. Fatigue failure occurs when bone remodeling cannot keep pace with microdamage.
Stages: Stage 1 is pars stress reaction with bone edema on MRI. Stage 2 is incomplete fracture with a hairline on CT. Stage 3 is complete fracture with visible defect. Stage 4 is established non-union with sclerotic margins.
Risk Activities: Gymnastics, cricket fast bowling, diving, weightlifting, football (linemen).
Progression of Slip
Factors Promoting Progression:
| Factor | Impact |
|---|---|
| Young age | Growth remaining allows progression |
| Female | Higher progression rates |
| High-grade | More likely to progress further |
| Disc degeneration | Loss of disc height facilitates slip |
| High slip angle | Indicates unstable mechanics |
Neurological Involvement:
L5 radiculopathy can occur by two mechanisms. First is fibrous tissue or callus at the pars defect compressing the L5 root. Second is foraminal narrowing as the L5 vertebra slides forward and the sacral dome rises posteriorly.
Classification
Wiltse-Newman Classification
Congenital Facet Deficiency
Definition: Congenital abnormality of the upper sacrum or L5 arch with hypoplastic facets that allow forward slip without pars defect.
Features: No true pars defect. Facets are hypoplastic or abnormally oriented. Rare, accounts for 5-10% of spondylolisthesis.
Neurological Risk: Higher risk of cauda equina compression because the posterior elements remain attached and can impinge on the canal.
Meyerding Grading
| Grade | Slip Percentage | Description |
|---|---|---|
| I | Less than 25% | Low-grade, usually asymptomatic |
| II | 25-50% | Low-grade, may be symptomatic |
| III | 50-75% | High-grade, often symptomatic |
| IV | 75-100% | High-grade, typically symptomatic |
| V | Greater than 100% | Spondyloptosis (L5 anterior to S1) |
Clinical Presentation
History
Pain Characteristics:
| Feature | Pattern |
|---|---|
| Location | Low back, may radiate to buttocks |
| Character | Aching, mechanical |
| Aggravating | Extension, standing, sports |
| Relieving | Flexion, rest |
| Radicular | L5 if nerve compression |
Red Flags:
| Finding | Concern |
|---|---|
| Bowel/bladder dysfunction | Cauda equina (rare) |
| Progressive weakness | Neurological compromise |
| Rapidly progressive slip | High-grade instability |
Physical Examination
Observation:
- Step-off palpable at lumbosacral junction in high-grade
- Increased lumbar lordosis
- Vertical sacrum (spondyloptotic crisis)
- Waddling gait in severe cases
Palpation:
- Tenderness over L5-S1
- Hamstring tightness (common finding)
Range of Motion:
- Limited lumbar flexion
- Pain with extension
- Stiffness from hamstring spasm
Neurological Examination:
| Finding | Root | Interpretation |
|---|---|---|
| EHL weakness | L5 | Stretched over sacral dome |
| Ankle reflex absent | S1 | Less common |
| Saddle anesthesia | Cauda equina | Emergency |
Special Tests:
| Test | Technique | Positive if |
|---|---|---|
| Single leg hyperextension | Stand on one leg, extend | Reproduces ipsilateral LBP |
| Hamstring tightness | Popliteal angle, SLR | Tight hamstrings correlate with slip |
| Neurological exam | Standard testing | L5 radiculopathy |
Hamstring Tightness
Tight hamstrings are a classic finding in adolescents with isthmic spondylolisthesis. The mechanism is debated but may relate to postural compensation for anterior pelvic tilt caused by the slip.
Investigations
Imaging Protocol
X-ray (First Line):
| View | Purpose |
|---|---|
| AP | Overall alignment, transitional vertebra |
| Lateral | Slip percentage (Meyerding grade), slip angle |
| Oblique | Scottie dog - pars defect (collar sign) |
| Flexion-extension | Instability assessment |
Measuring Slip:
Meyerding grading divides the sacral endplate into quarters. Slip percentage is how far the posterior corner of L5 has slipped forward.
CT Scan:
Gold standard for pars defect visualization.
| Assessment | Finding |
|---|---|
| Pars status | Defect, elongation, sclerosis |
| Healing potential | Sclerotic margins = low healing potential |
| Bony anatomy | Foraminal stenosis, facet arthrosis |
MRI:
| Sequence | Assessment |
|---|---|
| T2 sagittal | Disc degeneration, canal stenosis |
| T1/T2 axial | Neural compression, foraminal narrowing |
| STIR | Bone marrow edema (acute pars stress) |
SPECT-CT:
Combines CT anatomy with SPECT metabolic activity. Hot spot at pars indicates acute/healing lesion with potential for conservative healing. Cold defect indicates established non-union.
Laboratory Studies
Not routinely required. If concern for pathologic etiology (Type V), check CBC, ESR, CRP, calcium, phosphate, ALP, and tumor markers if indicated.
Management
Treatment Algorithm
Asymptomatic Spondylolysis/Spondylolisthesis:
Most patients (80%) remain asymptomatic. No treatment required. Activity restrictions not routinely indicated for low-grade slips.
Conservative Management (First Line for Symptomatic):
| Intervention | Details |
|---|---|
| Activity modification | Avoid aggravating extension activities |
| Physical therapy | Core strengthening, hamstring stretching |
| Bracing | Anti-lordotic brace for acute pars stress (healing potential) |
| NSAIDs | Symptom control |
Duration: 6-12 weeks trial before considering surgery.
Indications for Surgery:
| Indication | Comment |
|---|---|
| Failed conservative (6+ months) | Most common indication |
| Progressive slip | Especially in skeletally immature |
| Neurological deficit | L5 radiculopathy |
| High-grade slip (III-V) | Often require surgery |
| Severe pain impacting function | Quality of life indication |
Surgical Options
Direct Pars Repair (Scott, Buck)
Indication: Young patient (under 25), low-grade slip, minimal or no disc degeneration, single-level defect.
Technique: Scott wiring uses a figure-of-8 wire around transverse process and spinous process. Buck's screw is a lag screw across pars defect.
Outcomes: 80-90% healing rate in appropriately selected patients. Motion-preserving. Avoids fusion.
Contraindications: Significant disc degeneration, high-grade slip, multilevel disease, sclerotic pars margins.
In Situ vs Reduction
For high-grade slips, the debate between in situ fusion and reduction continues. In situ is safer but may not correct sagittal imbalance. Reduction improves alignment but carries 10-25% risk of L5 radiculopathy from nerve stretch. Consider patient factors, surgeon experience, and use neuromonitoring.
Complications
Conservative Management Complications
Progression of Slip:
- Higher risk in young, female, high-grade
- Monitor with standing lateral X-rays
- Skeletal maturity typically stabilizes slip
Chronic Pain:
- May develop despite conservative measures
- Consider surgery if refractory
Surgical Complications
Intraoperative:
| Complication | Incidence | Prevention |
|---|---|---|
| L5 radiculopathy (reduction) | 10-25% | Neuromonitoring, cautious reduction |
| Dural tear | 1-5% | Careful dissection |
| Vascular injury | Rare | Avoid anterior to sacrum |
Postoperative:
| Complication | Incidence | Management |
|---|---|---|
| Pseudarthrosis | 5-15% | Revision fusion, bone graft |
| Hardware failure | 2-5% | Revision fixation |
| Adjacent segment disease | 5-10% long-term | May need extension of fusion |
| Infection | 1-3% | Antibiotics, possible washout |
High-Grade Specific Complications
Reduction-Related:
- L5 nerve stretch injury (most common)
- Cauda equina injury
- Vascular injury
Prevention: Staged reduction, intraoperative neuromonitoring, and accepting partial reduction if significant EMG changes are all important strategies.
Evidence Base
SLIP II Trial
- Randomized trial comparing reduction vs in situ fusion for high-grade slips
- Both techniques showed significant improvement in outcomes
- Reduction improved sagittal balance but higher complication rate
- No significant difference in patient-reported outcomes at 2 years
Pars Repair Outcomes
- Buck's screw technique for pars repair
- 87% healing rate in select patients
- Young age and single level = best results
- Disc degeneration = poor healing
Natural History
- Prospective study of spondylolysis development
- 5.8% incidence in children by age 6
- Progression rare after skeletal maturity
- Most remain asymptomatic
SPORT - Degenerative Spondylolisthesis
- Fusion superior to decompression alone for degenerative spondylolisthesis
- Surgery superior to conservative at 2 years
- Applies to degenerative, not isthmic type
Viva Scenarios
Practice these scenarios to excel in your viva examination
Adolescent with Back Pain
"A 15-year-old male gymnast presents with 6 months of low back pain worse with extension. Lateral X-ray shows Grade I L5-S1 slip. Oblique view shows bilateral pars defects (collar on Scottie dog)."
High-Grade Spondylolisthesis
"An 18-year-old female presents with severe back pain and bilateral L5 radiculopathy. Standing lateral X-ray shows Grade III L5-S1 slip with high slip angle. She has tight hamstrings and a waddling gait."
Young Athlete - Acute Pars Stress
"A 16-year-old cricket fast bowler presents with 3 weeks of acute low back pain. No radiculopathy. X-rays are normal. MRI shows T2 hyperintensity in the left L5 pars. SPECT-CT shows increased uptake at L5."
MCQ Practice Points
Exam Pearl
Q: What is the pathological lesion in isthmic spondylolisthesis? A: Defect in the pars interarticularis (spondylolysis). This stress fracture typically occurs at L5 due to the oblique orientation of the pars at this level which concentrates shear forces during extension and rotation.
Exam Pearl
Q: What is the characteristic radiographic finding on oblique lumbar X-ray? A: "Collar on the Scotty dog" sign - the lucency through the pars appears as a collar on the dog-shaped vertebra on oblique views. The dog's nose is the transverse process, eye is the pedicle, ear is the superior facet, front leg is the inferior facet.
Exam Pearl
Q: What is the most common level for isthmic spondylolisthesis and why? A: L5-S1 (85-95% of cases). The L5 pars is thinner and more obliquely oriented, concentrating stress at this level. Additionally, L5 bears the maximum shear force at the lumbosacral junction due to sacral inclination.
Exam Pearl
Q: Which sports have the highest risk for developing spondylolysis? A: Gymnastics, cricket fast bowling, diving, and American football linemen. These activities involve repetitive hyperextension and rotation which concentrate stress at the pars interarticularis. Up to 40% of adolescent gymnasts have pars defects.
Australian Context
Epidemiology: Isthmic spondylolisthesis affects approximately 5-6% of the Australian population. Higher prevalence is seen in young athletes involved in repetitive extension sports (cricket fast bowlers, gymnasts, football players). Screening programs are implemented in elite sports pathways.
Clinical Management: Initial management is conservative with physiotherapy focusing on core stabilisation and activity modification. PBS-subsidised analgesia (paracetamol, NSAIDs) is used for symptom control. Referral to paediatric or spinal orthopaedic surgeons is indicated for progressive slip or failed conservative management.
Surgical Care: High-grade slips requiring reduction and fusion are managed at tertiary paediatric or spinal surgery centres. Intraoperative neuromonitoring is standard practice for high-grade slip reduction.
Sports Medicine Integration: Australian sports medicine physicians play a key role in early diagnosis and management of young athletes with spondylolysis and low-grade spondylolisthesis.
ISTHMIC SPONDYLOLISTHESIS
High-Yield Exam Summary
KEY DIFFERENTIATORS
- •PARS DEFECT present = isthmic type
- •L5-S1 level (90%) vs L4-5 for degenerative
- •Young patients (adolescence) vs old (degenerative)
- •Scottie dog collar sign on oblique X-ray
CLASSIFICATIONS
- •Wiltse: Type II = isthmic (IIA lytic, IIB elongated, IIC acute)
- •Meyerding: I (less than 25%) to V (spondyloptosis)
- •Low-grade: I-II; High-grade: III-V
- •CT is gold standard for pars visualization
CLINICAL FEATURES
- •Extension-related back pain
- •Hamstring tightness (classic finding)
- •Step-off palpable in high-grade
- •L5 radiculopathy if nerve stretch
MANAGEMENT PRINCIPLES
- •80% remain asymptomatic - observation
- •Conservative first: PT, activity modification
- •Pars repair: Young, no disc disease, single level
- •Fusion: Failed conservative, high-grade, neurological
SURGICAL PEARLS
- •In situ fusion is safer for high-grade
- •Reduction improves alignment but 10-25% L5 neuropathy risk
- •Always use neuromonitoring for reduction
- •Pars repair: 80-90% healing in select patients