High Yield Overview
SACROILIAC JOINT DYSFUNCTION
Low Back Pain Source | Provocative Tests | Injection
15-30%Of low back pain
3+ testsPositive for diagnosis
InjectionDiagnostic gold standard
FusionIf conservative fails
Causes
Mechanical
PatternHypermobility, leg length
TreatmentCommon
Inflammatory
PatternAnkylosing spondylitis
TreatmentSpondyloarthropathy
Traumatic
PatternPost-pelvic fracture
TreatmentInstability
Peripartum
PatternHormonal relaxin
TreatmentUsually self-limiting
Critical Must-Knows
- 15-30% of low back pain may originate from SI joint
- Cluster of 3+ positive provocative tests supports diagnosis
- Fluoroscopic-guided injection (greater than 75% relief) confirms diagnosis
- Consider spondyloarthropathy in young patients (HLA-B27)
- SI joint fusion for refractory cases after failed conservative treatment
Examiner's Pearls
- "FABER, Gaenslen's, thigh thrust, sacral compression, distraction tests
- "Pain typically over PSIS, may radiate to buttock/posterior thigh
- "MRI for inflammatory causes, plain film for degenerative
- "Injection both diagnostic and therapeutic
Clinical Imaging
Imaging Gallery
Click to expand
Click to expand
Critical SI Joint Exam Points
Diagnosis Challenge
SI joint pain is difficult to diagnose. No single test is diagnostic. Cluster of 3+ positive provocative tests increases likelihood. Injection is gold standard.
Red Flags
Consider spondyloarthropathy (ankylosing spondylitis, reactive arthritis) in young patients with inflammatory pattern pain (morning stiffness, improves with activity). Check HLA-B27, inflammatory markers.
Provocative Tests
FABER (Patrick's), Gaenslen's, Thigh Thrust, Sacral Compression, Distraction. 3+ positive = increased likelihood of SI joint origin.
Treatment
Conservative: PT, injection, activity modification. Surgical: SI joint fusion for refractory cases. Minimally invasive techniques available.
At a Glance
Sacroiliac joint dysfunction is responsible for 15-30% of low back pain cases and is notoriously difficult to diagnose—no single test is definitive. A cluster of 3+ positive provocative tests (FABER, Gaenslen's, thigh thrust, sacral compression, distraction) increases diagnostic probability. Fluoroscopic-guided SI joint injection with over 75% pain relief is the gold standard for confirming the diagnosis. Pain typically localizes over the PSIS with radiation to the buttock or posterior thigh. In young patients with inflammatory pattern symptoms (morning stiffness, improvement with activity), consider spondyloarthropathy and check HLA-B27. Treatment progresses from conservative measures (physiotherapy, injection) to SI joint fusion for refractory cases.
Mnemonic
FGTCDSI Joint Provocative Tests
F
FABER (Patrick's)
Flexion, Abduction, External Rotation
G
Gaenslen's
Hip extended off table, other flexed
T
Thigh Thrust
Axial load through flexed hip
C
Compression
Lateral compression of pelvis
D
Distraction
Press anterior pelvis apart
Memory Hook:FGTCD = FABER, Gaenslen's, Thigh thrust, Compression, Distraction!
Overview and Epidemiology
The sacroiliac (SI) joint is a diarthrodial synovial joint between the sacrum and ilium. It is a significant source of low back and buttock pain, estimated to cause 15-30% of non-radicular low back pain.
Anatomy
The SI joint has both synovial and ligamentous components. The posterior ligamentous complex is the primary stabilizer. Innervation is complex, with contributions from L4-S3 (posterior primarily S1-3). This makes diagnosis challenging.
Etiology
Mechanical: Hypermobility, asymmetric loading, leg length discrepancy.
Inflammatory: Ankylosing spondylitis and other spondyloarthropathies.
Traumatic: Post-pelvic fracture instability.
Degenerative: Osteoarthritis of SI joint.
Peripartum: Hormonal relaxation (relaxin), usually self-limiting.
Anatomy
SI Joint Anatomy
Joint Structure
- Diarthrodial synovial joint between sacrum and ilium
- Auricular (ear-shaped) surfaces interlock for stability
- Irregular surface topography increases friction and stability
- Small range of motion (1-4° rotation, 1-2mm translation)
Ligamentous Stabilizers
- Anterior sacroiliac ligament: Thin, covers anterior joint
- Posterior sacroiliac ligaments: Short and long posterior ligaments, primary stabilizers
- Interosseous sacroiliac ligament: Strongest, fills posterior joint space
- Sacrotuberous and sacrospinous ligaments: Contribute to pelvic stability
Innervation
- Complex and variable
- Posterior innervation: L5 dorsal ramus, S1-S4 lateral branches
- Anterior innervation: L2-S2 contributions
- Explains referred pain patterns to buttock, posterior thigh, groin
Classification
Classification by Etiology
Mechanical SI Joint Dysfunction
- Hypermobility or hypomobility
- Leg length discrepancy
- Asymmetric loading patterns
- Post-trauma (falls, motor vehicle accidents)
Inflammatory SI Joint Disease
- Ankylosing spondylitis
- Reactive arthritis
- Psoriatic arthritis
- Inflammatory bowel disease-associated arthritis
- Characterized by sacroiliitis on imaging
Degenerative SI Joint Arthritis
- Osteoarthritis of SI joint
- More common in older patients
- Sclerosis and osteophyte formation
Peripartum-Related
- Hormonal effects (relaxin) causing ligament laxity
- Usually self-limiting postpartum
- May require treatment if persistent
Clinical Presentation
History
Pain typically located over the posterior superior iliac spine (PSIS) or "Fortin's area." May radiate to buttock, posterior thigh, or groin. Rarely below the knee. Worse with sitting, transitional movements, or prolonged standing. May note stiffness.
Inflammatory Pattern (consider spondyloarthropathy): Young patient (less than 40 years), insidious onset, morning stiffness greater than 30 minutes, improves with activity, not relieved by rest.
Examination
Provocative Tests: 3 or more positive tests increase likelihood of SI joint origin.
FABER (Patrick's Test): Flexion, Abduction, External Rotation of hip. Pain at SI joint suggests SI pathology (also stresses hip).
Gaenslen's Test: Patient supine. One hip flexed to chest, other hip extended off table edge. Stresses SI joint.
Thigh Thrust (Posterior Shear Test): Supine, hip flexed 90°. Axial load through femur toward SI joint.
Sacral Compression Test: Side-lying. Downward pressure on ilium.
Distraction Test: Supine. Posterior force on ASISs to distract anterior pelvis.
Diagnosis
Gold Standard: Fluoroscopic or CT-guided injection of local anesthetic into the SI joint.
Diagnostic Criteria: Greater than 75% pain relief following injection confirms SI joint as pain source.
Therapeutic: Corticosteroid can be included for therapeutic effect.
This is the most reliable method to confirm SI joint origin given the limitations of clinical testing.
Investigations
Imaging
Plain Radiographs
- AP pelvis: Assess joint margins, sclerosis, osteophytes
- Ferguson view: Angled for better SI joint visualization
- May show degenerative changes or advanced sacroiliitis
MRI
- Gold standard for detecting sacroiliitis
- Shows bone marrow edema (active inflammation)
- Detects erosions, sclerosis, fatty replacement
- STIR or fat-saturated T2 sequences best for edema
CT Scan
- Better bony detail than MRI
- Shows erosions, sclerosis, ankylosis
- Useful for surgical planning
Laboratory
- Inflammatory markers: ESR, CRP (elevated in sacroiliitis)
- HLA-B27: Positive in most ankylosing spondylitis patients
- Rheumatoid factor: Usually negative in spondyloarthropathy
Management
📊 Management Algorithm
Click to expand
Physiotherapy: Core strengthening, pelvic stabilization, stretching.
Activity Modification: Avoid aggravating activities.
SI Belt: External support for hypermobility.
Injection: Intra-articular corticosteroid. May provide lasting relief. Can repeat.
Pharmacotherapy: NSAIDs (especially for inflammatory pain). DMARDs/biologics for spondyloarthropathy.
Surgical Technique
SI Joint Fusion Techniques
Minimally Invasive Lateral Transiliac Approach
- Most common modern technique
- Small lateral incision over ilium
- Fluoroscopic guidance essential
- Triangular titanium implants placed across joint
- Typically 3 implants in triangular configuration
Key Steps
- Patient positioning: Prone or lateral
- Fluoroscopic setup: AP and lateral views
- Small incision over lateral ilium
- Guidewire placement across SI joint
- Sequential drilling and implant placement
- Confirm position on multiple views
Open SI Joint Fusion
- Indicated for traumatic instability
- Anterior or posterior approach
- Cartilage debridement and bone grafting
- Screw or plate fixation
Complications
Complications
Nerve Injury
- L5 nerve root: Lateral approach risk
- S1 nerve root: If implants too superior
- Sciatic nerve: Rare, from malpositioning
- Prevention: Fluoroscopic guidance, stay below S1 foramen
Hardware-Related
- Implant malposition
- Implant prominence (buttock pain)
- Implant loosening or migration
- Pseudarthrosis (failed fusion)
Persistent Pain
- Most common "complication"
- May indicate incorrect diagnosis or adjacent pathology
- Rate: 15-30% of patients
Other Complications
- Wound infection
- Hematoma
- DVT/PE
- Adjacent segment degeneration
Postoperative Care
Postoperative Protocol
Immediate Postoperative
- Pain control: Multimodal analgesia
- Wound care: Keep dry, monitor for infection
- DVT prophylaxis: Mechanical and pharmacological
Weight-Bearing
- Variable protocols depending on technique
- MIS with triangular implants: Often weight-bearing as tolerated
- Open fusion: May require protected weight-bearing 6-8 weeks
Activity Restrictions
- Limit twisting and bending initially
- Avoid prolonged sitting (aggravates SI joint)
- Gradual return to normal activities over 6-12 weeks
Physiotherapy
- Begin after initial healing phase (2-4 weeks)
- Core strengthening and pelvic stabilization
- Gait training if needed
Outcomes
Treatment Outcomes
Conservative Treatment
- Physiotherapy: Variable success, better for mild cases
- SI joint injection: 50-70% get temporary relief
- Repeat injections: May provide ongoing benefit
Radiofrequency Ablation
- 60-70% significant pain reduction
- Duration: 6-12 months typically
- Can be repeated if symptoms recur
Surgical Fusion
- Success rate: 70-85% significant improvement
- Fusion rate: 85-95% radiographic fusion
- Time to maximum improvement: 6-12 months
Factors Affecting Outcomes
- Positive diagnostic injection response (most important)
- Patient selection and realistic expectations
- Technique and implant choice
- Presence of other pain generators
Evidence Base
II
📚 Laslett et al
Key Findings:
- Clinical examination for SI joint pain
- Cluster of 3+ positive tests increases likelihood
- Individual tests have limited reliability
- Validates clinical examination approach
Clinical Implication: Use cluster of provocative tests for clinical diagnosis.
Source: Spine 2005
III
📚 Whang et al (iFuse)
Key Findings:
- Minimally invasive SI joint fusion outcomes
- Good pain reduction in selected patients
- Device and technique specific
- Alternative to open fusion
Clinical Implication: MIS SI joint fusion is viable for refractory cases.
Source: Med Devices (Auckl) 2019
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
Scenario 1: Buttock Pain
EXAMINER
"A 45-year-old woman has low back and right buttock pain for 6 months. MRI spine is unremarkable. How do you assess for SI joint dysfunction?"
EXCEPTIONAL ANSWER
With unremarkable lumbar MRI and persisting buttock pain, SI joint dysfunction should be considered as the SI joint is responsible for 15-30% of non-radicular low back pain. My history would focus on pain location (over PSIS is classic), radiation pattern (buttock, posterior thigh, rarely below knee), aggravating factors (sitting, transitional movements), and nature of pain. I would ask about inflammatory features (young age, morning stiffness, improves with activity) which would suggest spondyloarthropathy. On examination, I would perform a cluster of SI joint provocative tests. A positive cluster is 3 or more positive tests including FABER (Patrick's), Gaenslen's, thigh thrust, sacral compression, and distraction. I would also examine the hip to exclude it as a source. If clinical suspicion is high based on positive test cluster, the gold standard for diagnosis is fluoroscopic-guided SI joint injection. Greater than 75% pain relief confirms the SI joint as the pain source. This is also therapeutic if corticosteroid is injected. Imaging with plain radiographs can show degenerative changes. MRI of SI joints specifically would show sacroiliitis if inflammatory. For management, I would start with physiotherapy for core strengthening and pelvic stabilization, possibly with an SI belt. If injection provides relief but is temporary, repeat injections are an option. For refractory cases, minimally invasive SI joint fusion may be considered.
KEY POINTS TO SCORE
SI joint causes 15-30% of low back pain
Cluster of 3+ positive tests supports diagnosis
Injection (greater than 75% relief) is gold standard
Fusion for refractory cases after failed conservative
COMMON TRAPS
✗Not knowing the provocative tests
✗Missing inflammatory causes (spondyloarthropathy)
✗Not mentioning injection as diagnostic gold standard
LIKELY FOLLOW-UPS
"What are the SI joint provocative tests?"
"When would you suspect spondyloarthropathy?"
VIVA SCENARIOChallenging
Scenario 2: Equivocal Provocative Tests - Diagnostic Dilemma
EXAMINER
"A 52-year-old male office worker presents with 12 months of right-sided low back and buttock pain. He describes the pain as localized over the right PSIS with occasional radiation to the posterior thigh, never below the knee. The pain is worse with sitting for prolonged periods and with transitional movements from sitting to standing. He has tried physiotherapy for 6 months with minimal benefit. MRI lumbar spine shows mild L4-5 and L5-S1 facet arthropathy bilaterally, but no significant disc pathology, stenosis, or nerve root compression. On examination, FABER test is positive on the right (reproduces his typical pain), Gaenslen's test is equivocal (some discomfort but not clearly positive), thigh thrust is negative, sacral compression is positive, and distraction is negative. So you have 2 clearly positive tests, 1 equivocal, and 2 negative. Hip examination shows full range of motion with no pain. There is no neurological deficit. The patient is frustrated and asks for a definitive diagnosis and treatment plan. How do you approach this diagnostic dilemma, and what is your management strategy?"
EXCEPTIONAL ANSWER
This is a **challenging diagnostic scenario** where the clinical picture suggests SI joint dysfunction but the provocative test cluster is **equivocal** (only 2 clearly positive tests out of 5), falling short of the typical diagnostic threshold of 3 or more positive tests. Additionally, the MRI shows **facet arthropathy** which can mimic SI joint pain. This requires a systematic approach to differentiate between **SI joint dysfunction**, **facet-mediated pain**, and **mixed pathology**.
### Differential Diagnosis
**Primary Considerations:**
(1) **SI Joint Dysfunction (Most Likely)**: Pain localized over PSIS (classic Fortin's area), radiation to buttock/posterior thigh (SI joint referral pattern), worse with sitting and transitional movements (classic aggravating factors), 2 positive provocative tests (FABER, sacral compression). However, the test cluster is not conclusive (less than 3 positive).
(2) **Facet-Mediated Pain**: MRI shows L4-5 and L5-S1 facet arthropathy, facet pain also presents with low back pain worse with extension and rotation, radiation to buttock is possible, but facet pain typically does not localize to PSIS as specifically as this patient's pain does.
(3) **Combined SI Joint and Facet Pathology**: Not uncommon - patients can have multiple pain generators simultaneously. The facet arthropathy may be an incidental finding (MRI changes don't always correlate with symptoms) or may be a contributing source.
(4) **Hip Pathology (Less Likely)**: FABER stresses both SI joint and hip, but full pain-free hip ROM makes intra-articular hip pathology unlikely. However, extra-articular hip pathology (greater trochanteric pain syndrome, labral tear) should be considered.
### Diagnostic Gold Standard - Fluoroscopic-Guided Injection
Given the **equivocal clinical picture**, I would proceed with **diagnostic injection** to definitively identify the pain source:
**Fluoroscopic-Guided SI Joint Injection**:
(1) **Technique**: Under fluoroscopy, inject 1.5-2mL of local anesthetic (bupivacaine 0.25% or lidocaine 1%) into the SI joint. Confirm intra-articular placement with contrast (arthrogram shows characteristic joint outline).
(2) **Diagnostic Criteria**: Patient records pain level before injection (VAS 0-10). Re-assess pain at 30 minutes (lidocaine) or 1-2 hours (bupivacaine). **Greater than 75% pain relief confirms SI joint as the dominant pain source**. 50-75% relief suggests SI joint is a contributor but not the only source. Less than 50% relief suggests SI joint is NOT the primary source.
(3) **Therapeutic Component**: Add corticosteroid (triamcinolone 40mg or methylprednisolone 40mg) for therapeutic effect. Duration of relief varies (weeks to months). Some patients get lasting relief from a single injection.
(4) **Comparative Block (Optional)**: If SI joint injection provides less than 75% relief but still significant improvement (50-75%), consider **facet medial branch block** at L4-5 and L5-S1 to determine if facets are contributing. This helps identify dual pathology.
### Management Strategy
**Conservative Management (Continue During Diagnostic Workup):**
(1) **Physiotherapy**: Continue targeted PT focusing on **pelvic stabilization**, **core strengthening** (transversus abdominis, multifidus), and **gluteal strengthening**. Address any muscle imbalances or asymmetry.
(2) **SI Belt**: Consider SI joint belt for external stabilization, especially during aggravating activities (sitting, transitional movements). Worn around pelvis at level of greater trochanters.
(3) **Activity Modification**: Avoid prolonged sitting (use standing desk, frequent position changes), avoid asymmetric loading. Ergonomic assessment for office work.
(4) **NSAIDs**: Trial of regular NSAIDs (if not contraindicated) for 2-4 weeks to reduce inflammation.
**Based on Injection Results:**
**If Greater Than 75% Relief (SI Joint Confirmed)**:
(1) **Repeat Injection**: If initial relief wears off after weeks/months, repeat injection (with corticosteroid) is reasonable. Some patients do well with periodic injections (every 3-6 months).
(2) **Radiofrequency Ablation (RFA)**: For patients who get good but temporary relief from injections, consider **lateral branch radiofrequency neurotomy** (ablate L5 dorsal ramus and S1-3 lateral branches that innervate SI joint). Can provide 6-12 months of relief.
(3) **SI Joint Fusion**: Reserved for **refractory cases** who have failed at least **6 months of aggressive conservative treatment**, had a positive diagnostic injection response, and have significant functional impairment. Counsel about variable outcomes. Minimally invasive techniques available (lateral transiliac approach with triangular titanium implants).
**If 50-75% Relief (Mixed Pathology)**:
(1) Consider **facet medial branch blocks** at L4-5 and L5-S1 to assess facet contribution. If positive, may need to address both SI joint and facets (dual injection therapy or RFA).
**If Less Than 50% Relief (SI Joint Not Primary Source)**:
(1) Re-evaluate for other sources: facet-mediated pain (medial branch blocks), hip pathology (MRI hip, intra-articular hip injection), myofascial pain, piriformis syndrome.
### Key Teaching Points
(1) **SI joint dysfunction diagnosis is clinical**: Cluster of 3+ positive provocative tests increases likelihood, but no single test or cluster is 100% sensitive/specific. **Injection is the gold standard** when clinical picture is unclear.
(2) **Equivocal test results require diagnostic injection**: This patient has 2/5 positive tests, which is not conclusive. Injection provides definitive answer.
(3) **MRI findings don't always correlate with symptoms**: The facet arthropathy may be incidental (asymptomatic degenerative changes). Diagnostic injections help differentiate.
(4) **SI joint pain pattern**: Pain over PSIS (Fortin's area), radiation to buttock/posterior thigh (rarely below knee), worse with sitting and transitional movements are classic. This patient's presentation fits SI joint pattern despite equivocal tests.
(5) **Multiple pain generators are common**: Patients can have SI joint dysfunction AND facet arthropathy AND myofascial pain simultaneously. Systematic diagnostic approach identifies dominant source.
KEY POINTS TO SCORE
Equivocal provocative tests (2/5 positive, need 3+): Injection is gold standard to confirm diagnosis when clinical picture unclear, greater than 75% relief confirms SI joint as dominant source, 50-75% relief suggests mixed pathology (SI + facets)
Differential diagnosis: SI joint dysfunction (pain over PSIS, worse sitting/transitional movements), facet-mediated pain (L4-5/L5-S1 arthropathy on MRI), mixed pathology (both contributors), MRI findings don't always correlate with symptoms (may be incidental)
Fluoroscopic SI joint injection technique: 1.5-2mL local anesthetic (bupivacaine 0.25%), confirm intra-articular with contrast arthrogram, assess pain relief at 30min-2h, add corticosteroid for therapeutic effect (triamcinolone 40mg)
Management if SI joint confirmed (greater than 75% relief): Repeat injections if temporary relief (every 3-6 months), radiofrequency ablation (RFA) of lateral branches L5-S3 for longer-lasting relief (6-12 months), SI joint fusion for refractory cases after 6+ months failed conservative (variable outcomes)
Comparative blocks for mixed pathology: If SI injection gives 50-75% relief, perform facet medial branch blocks L4-5/L5-S1 to assess dual pathology, may need to treat both pain generators (dual injection therapy or RFA)
COMMON TRAPS
✗Assuming 2 positive tests is enough for diagnosis (need 3+ for clinical diagnosis, or use injection)
✗Attributing all symptoms to MRI facet arthropathy findings (MRI changes don't always correlate with symptoms, may be incidental degenerative changes)
✗Not knowing injection diagnostic threshold (greater than 75% relief confirms SI joint as dominant source)
✗Recommending fusion without proper diagnostic workup (must have positive injection response confirming SI joint as source before considering fusion)
✗Not considering mixed pathology (patients can have multiple concurrent pain generators: SI joint, facets, myofascial)
LIKELY FOLLOW-UPS
"What is the sensitivity and specificity of individual provocative tests? (Low - that's why we use a cluster)"
"Describe the fluoroscopic technique for SI joint injection. (Posterior approach, oblique view, contrast confirms intra-articular)"
"What are the lateral branches that innervate the SI joint? (L5 dorsal ramus, S1-3 lateral branches - targets for RFA)"
"What is the evidence for SI joint fusion? (Mixed - better outcomes in well-selected patients with positive injection response and failed conservative treatment)"
"How do you differentiate facet pain from SI joint pain clinically? (Facet: worse with extension/rotation, SI: worse with sitting/transitional movements, pain over PSIS, but significant overlap)"
VIVA SCENARIOCritical
Scenario 3: Failed SI Joint Fusion - Persistent Pain After Surgery
EXAMINER
"A 48-year-old woman underwent minimally invasive SI joint fusion 9 months ago for chronic right SI joint pain that had been refractory to 18 months of conservative treatment including physiotherapy, SI belt, and multiple corticosteroid injections. Pre-operatively, she had a cluster of 4 positive provocative tests and 85% pain relief with diagnostic SI joint injection, confirming the SI joint as the pain source. She underwent lateral transiliac approach with placement of three triangular titanium implants across the right SI joint. Initial post-operative X-rays showed good implant position. However, she now presents to your clinic complaining that her pain has never improved after surgery - she rates it as 7/10 (same as pre-operative). The pain remains localized over the right PSIS and buttock. She is frustrated, has been off work for 9 months, and is demanding answers. On examination, she has tenderness over the right SI joint, 3 positive provocative tests on the right (same as pre-op), and no neurological deficit. CT scan shows the three implants in good position crossing the SI joint with some bridging bone formation around the implants, but no complete fusion visible. There is no implant loosening, fracture, or migration. Inflammatory markers are normal (CRP less than 5, ESR 12). What is your differential diagnosis for persistent pain after SI joint fusion, how do you investigate this patient, and what are your management options?"
EXCEPTIONAL ANSWER
This is a **challenging and frustrating scenario** of **failed SI joint fusion** - defined as **persistent or recurrent pain after SI joint fusion surgery** despite technically successful implant placement. This occurs in **15-30% of cases** and requires a systematic approach to identify the cause and guide management. The patient had appropriate indications for surgery (refractory pain, positive diagnostic injection 85% relief, failed conservative treatment) but has not achieved pain relief despite good implant position.
### Differential Diagnosis for Persistent Pain
I would approach this with a broad differential, categorizing causes into **mechanical**, **biological**, **incorrect diagnosis**, and **other pain generators**:
**1. Incomplete Fusion (Pseudarthrosis) - Most Common**
The CT shows bridging bone but no complete fusion at 9 months. SI joint fusion can take **12-18 months to achieve solid arthrodesis**. Some patients have persistent pain until solid fusion occurs. However, some series show that radiographic fusion does not always correlate with clinical outcome - some patients with incomplete fusion on imaging are pain-free, while others with solid fusion continue to have pain.
**2. Hardware-Related Pain**
(a) **Implant Prominence/Irritation**: Triangular titanium implants can cause local irritation if they protrude or abut adjacent structures. Examine for focal tenderness over implant trajectory.
(b) **Stress Shielding or Adjacent Bone Reaction**: Periosteal reaction or stress concentration around implants, though this usually manifests as implant loosening (not seen here).
**3. Incorrect Initial Diagnosis (Misidentified Pain Source)**
This is critical to reconsider. Despite pre-operative diagnostic workup showing 85% relief with injection and positive provocative tests, it's possible the SI joint was **not the sole or dominant pain generator**:
(a) **Lumbar Facet Arthropathy**: Facet-mediated pain can mimic SI joint pain (buttock pain, positive FABER test which also stresses lumbar spine). The patient may have had **dual pathology** (SI joint AND facets), and fusing the SI joint unmasked persistent facet pain.
(b) **Lumbar Disc or Radiculopathy**: L5-S1 pathology can refer pain to buttock and PSIS area. Review pre-operative MRI lumbar spine.
(c) **Hip Pathology**: Intra-articular hip pathology (labral tear, femoroacetabular impingement, early arthritis) or extra-articular hip pathology (greater trochanteric pain syndrome, gluteal tendinopathy). Hip pathology can present as buttock pain and give positive FABER test (which stresses hip AND SI joint).
(d) **Myofascial Pain or Piriformis Syndrome**: Gluteal muscle or piriformis pain can localize to buttock and PSIS area, positive provocative tests may reflect muscle guarding rather than true SI joint pathology.
**4. Adjacent Segment Pathology**
Fusion of the SI joint alters biomechanics, potentially increasing stress on adjacent structures:
(a) **Contralateral SI Joint**: Increased loading on left SI joint after right fusion may cause left SI joint pain. Examine for left SI joint tenderness and provocative tests.
(b) **Lumbar Spine (Especially L5-S1)**: Increased stress on lumbosacral junction after SI fusion. Check for new lumbar symptoms.
**5. Chronic Pain Syndrome or Central Sensitization**
After prolonged pre-operative pain (18 months failed conservative treatment) and now 9 months post-operative pain, the patient may have developed **chronic pain syndrome** with central sensitization. Pain persists despite removing peripheral nociceptive source. Psychosocial factors (catastrophizing, kinesiophobia, secondary gain from disability) may perpetuate symptoms.
**6. Infection (Low-Grade)**
Less likely given normal inflammatory markers (CRP less than 5, ESR 12), no systemic symptoms, and no implant loosening. However, **low-grade infection** (Cutibacterium acnes, Staphylococcus epidermidis) can present with persistent pain and normal inflammatory markers. Would expect more local signs (drainage, warmth, erythema).
### Investigations
**1. Detailed History and Re-Examination**
(a) **Pain Mapping**: Ask patient to point with one finger exactly where her pain is. Is it still precisely over right PSIS (SI joint), or has it shifted to lumbar spine, contralateral SI joint, hip, or become diffuse?
(b) **Compare to Pre-Operative Pain**: Is the pain **identical** to pre-operative pain, or has the character/location changed? If identical, suggests persistent SI joint pathology (incomplete fusion) or wrong diagnosis. If changed, suggests new pathology (adjacent segment, hardware irritation).
(c) **Functional Impact**: Activities that worsen pain (sitting? walking? stairs?). Pre-operative activities that were most painful - are those still problematic?
(d) **Provocative Tests**: Repeat cluster. Are they positive on the **same side** (right - suggests persistent SI joint pathology), **opposite side** (left - suggests contralateral SI joint overload), or **lumbar** (suggests facet or discogenic pain)?
(e) **Hip Examination**: Full hip ROM, FABER test (distinguishing hip vs SI pain), impingement tests (FADIR), Stinchfield test (hip flexion against resistance).
(f) **Neurological Examination**: L5-S1 radiculopathy? Straight leg raise? Sensory/motor deficits?
**2. Advanced Imaging**
(a) **CT Scan (Already Done)**: Shows implant position (good), no loosening/fracture/migration, some bridging bone but incomplete fusion. This is not unexpected at 9 months. Reassess at 12-18 months for solid fusion.
(b) **MRI Lumbar Spine with Metal Artifact Reduction Sequences (MARS)**: Assess for facet arthropathy, disc pathology, nerve root compression that may have been overlooked pre-operatively or developed post-operatively. MRI also shows soft tissue changes, inflammation, or edema.
(c) **Nuclear Medicine Bone Scan or SPECT-CT**: Can localize increased metabolic activity (bone remodeling, inflammation, stress). Increased uptake at **fusion site** suggests ongoing bone remodeling (normal up to 12-18 months) or incomplete fusion. Increased uptake at **adjacent sites** (contralateral SI joint, lumbar facets, hip) suggests adjacent segment pathology.
(d) **MRI Pelvis/Hip**: If hip pathology suspected, MRI hip to assess for labral tear, FAI, AVN, gluteal tendon pathology.
**3. Diagnostic Injections (Critical for Management Decision)**
(a) **Repeat SI Joint Injection (Right)**: Fluoroscopic-guided injection of local anesthetic into the **fused right SI joint**. If greater than 75% relief, suggests the fusion is incomplete and the SI joint remains a pain source. If less than 50% relief, suggests the SI joint is no longer the source (fusion successful biomechanically, or was never the source).
(b) **Contralateral SI Joint Injection (Left)**: If right SI joint injection negative but left SI joint tenderness/provocative tests positive, inject left SI joint. Relief confirms contralateral overload.
(c) **Lumbar Facet Medial Branch Blocks**: If lumbar facet arthropathy seen on MRI and facet provocation on examination, perform diagnostic medial branch blocks at L4-5 and L5-S1. Relief confirms facet-mediated pain.
(d) **Hip Intra-Articular Injection**: If hip examination suggestive, fluoroscopic-guided hip injection with local anesthetic. Relief confirms intra-articular hip pathology.
### Management Options
Management depends on findings from investigations and diagnostic injections:
**Scenario 1: Incomplete Fusion (Positive Right SI Joint Injection)**
(a) **Conservative Management (First-Line)**: Continue physiotherapy, activity modification, NSAIDs. Many fusions mature between 12-18 months. Repeat imaging at 12 months to assess fusion progression.
(b) **Revision Fusion**: If fusion fails to progress or patient has severe symptoms, consider **revision SI joint fusion** with bone grafting or biologics (BMP) to promote fusion. Open approach may be needed to debride pseudarthrosis and achieve solid fusion. This is a salvage option with uncertain outcomes.
**Scenario 2: Adjacent Segment Pathology (Positive Contralateral SI Joint or Facet Injection)**
(a) **Conservative Management**: Physiotherapy targeting new pain source, injections for symptom relief (repeat SI joint injection contralateral side, or facet injections/RFA for facets).
(b) **Contralateral SI Joint Fusion**: Rarely indicated, but if severe symptoms and failed conservative treatment, may consider. Risk of bilateral SI joint fusion includes loss of pelvic mobility and increased stress on lumbar spine.
(c) **Lumbar Fusion**: If facet pain severe and refractory, may need lumbar fusion (L5-S1 or above depending on pathology). This extends the fusion but carries significant morbidity.
**Scenario 3: Incorrect Initial Diagnosis (Negative SI Joint Injection, Positive Alternative Source)**
(a) **Treat the Correct Source**: Hip arthroscopy for labral tear/FAI, lumbar facet RFA or fusion for facet arthropathy, piriformis injection or release for piriformis syndrome, etc.
(b) **Accept Failed Surgery**: Counsel patient that SI joint was likely not the source, surgery addressed a non-painful joint. Focus on identifying and treating actual pain generator.
**Scenario 4: Hardware Irritation (Focal Tenderness Over Implant Trajectory)**
(a) **Hardware Removal**: If fusion solid on CT (bridging bone visible) and symptoms localized to hardware, can remove implants. However, risk of recurrent instability if fusion incomplete. Usually wait 12-18 months to ensure fusion solid before hardware removal.
**Scenario 5: Chronic Pain Syndrome/Central Sensitization (Negative All Injections, No Identifiable Source)**
(a) **Multidisciplinary Pain Management**: Refer to chronic pain clinic for comprehensive approach including psychology (CBT for pain, address catastrophizing), physical therapy (graded functional restoration), medications (neuropathic pain agents like gabapentin, duloxetine, NOT opioids), and interventional pain techniques (spinal cord stimulation if appropriate).
(b) **Counsel About Realistic Expectations**: Some patients do not achieve complete pain relief despite technically successful surgery. Goal is functional improvement, not complete pain elimination.
### Key Teaching Points
(1) **Failed SI joint fusion occurs in 15-30% of cases**: Despite appropriate patient selection and technically successful surgery, some patients do not improve. Causes include incomplete fusion, incorrect initial diagnosis, adjacent segment pathology, hardware issues, or chronic pain syndrome.
(2) **Diagnostic injections are critical for failed fusion workup**: Repeat SI joint injection, contralateral SI joint injection, lumbar facet blocks, and hip injection help identify persistent or new pain sources. Guide management decisions.
(3) **Pre-operative diagnostic injection response doesn't guarantee surgical success**: Even with 85% relief from diagnostic injection, the patient may have had dual pathology, or the pain may have evolved post-operatively.
(4) **Incomplete fusion at 9 months is not necessarily a failure**: SI joint fusion can take 12-18 months to achieve solid arthrodesis. Repeat imaging at 12-18 months before declaring pseudarthrosis.
(5) **Adjacent segment pathology after SI fusion is recognized**: Contralateral SI joint overload and increased lumbar stress (especially L5-S1) can develop after SI fusion. Biomechanical trade-off.
(6) **Revision surgery has uncertain outcomes**: Revision SI joint fusion, hardware removal, or extension of fusion to adjacent structures (contralateral SI joint, lumbar spine) should be approached with caution. Ensure correct diagnosis with diagnostic injections before revision.
(7) **Chronic pain psychology matters**: After 27 months of pain (18 months pre-op + 9 months post-op), central sensitization and psychological factors may perpetuate symptoms. Multidisciplinary approach needed.
KEY POINTS TO SCORE
Failed SI joint fusion (15-30% incidence): Persistent pain despite technically successful surgery, causes include incomplete fusion/pseudarthrosis (fusion takes 12-18 months), incorrect initial diagnosis (SI joint not sole pain source), adjacent segment pathology (contralateral SI joint, lumbar facets), hardware irritation, chronic pain syndrome
Differential diagnosis: Incomplete fusion (most common, CT shows bridging bone but not solid fusion at 9 months), dual pathology pre-operatively (SI joint + facets/hip, fusing SI joint unmasked other source), adjacent segment overload (contralateral SI joint, L5-S1 increased stress), hardware prominence/irritation, chronic pain/central sensitization
Investigation algorithm: Detailed history (pain mapping - identical vs changed?), re-examination (provocative tests right vs left vs lumbar), MRI lumbar/hip with MARS protocol (facet arthropathy, disc, labral tear), SPECT-CT (localize metabolic activity), diagnostic injections (repeat right SI joint, contralateral SI joint, facet blocks, hip injection)
Diagnostic injections guide management: Positive right SI joint injection (incomplete fusion - continue conservative, consider revision fusion with bone graft/BMP if no progression at 12-18 months), positive contralateral or facet injection (adjacent segment - PT, injections/RFA, rarely contralateral fusion or lumbar fusion), negative all injections (chronic pain syndrome - multidisciplinary pain management, CBT, avoid further surgery)
Key teaching: Pre-operative 85% relief from injection doesn't guarantee surgical success (may have dual pathology), incomplete fusion at 9 months not failure (reassess 12-18 months), adjacent segment pathology recognized complication (contralateral SI joint overload, L5-S1 stress), revision surgery uncertain outcomes (ensure correct diagnosis first), chronic pain psychology (after 27 months pain, central sensitization likely)
COMMON TRAPS
✗Assuming incomplete fusion at 9 months is a surgical failure (fusion takes 12-18 months, repeat imaging before declaring pseudarthrosis)
✗Rushing to revision surgery without diagnostic injections (must identify pain source - persistent SI joint, adjacent segment, or other - before deciding on revision)
✗Not considering incorrect initial diagnosis (pre-operative diagnostic injection had 85% relief, but patient may have had dual pathology - SI + facets/hip - fusing SI joint unmasked other source)
✗Attributing all symptoms to 'incomplete fusion' on CT without considering adjacent segment pathology (contralateral SI joint, lumbar facets)
✗Not recognizing chronic pain syndrome after 27 months of pain (18 months pre-op + 9 months post-op leads to central sensitization, psychological factors - needs multidisciplinary approach, not more surgery)
✗Offering bilateral SI joint fusion for contralateral pain without exhausting conservative options (bilateral SI fusion = loss of pelvic mobility, increased lumbar stress)
LIKELY FOLLOW-UPS
"What is the reported success rate of SI joint fusion? (Variable - 60-80% good outcomes in well-selected patients with positive injection response)"
"How long does SI joint fusion typically take to achieve solid arthrodesis? (12-18 months - incomplete fusion at 9 months is not necessarily a failure)"
"What are the biomechanical consequences of SI joint fusion? (Loss of pelvic mobility, increased stress on contralateral SI joint, increased lumbosacral stress especially L5-S1)"
"What is your threshold for revision SI joint fusion? (Confirmed incomplete fusion on CT at 12-18 months, positive repeat SI joint injection confirming SI joint as pain source, failed conservative management, patient understands uncertain outcomes)"
"How do you differentiate mechanical failure from chronic pain syndrome? (Diagnostic injections - if positive suggest mechanical source, if all negative suggests central sensitization/chronic pain, SPECT-CT can show increased metabolic activity at mechanical failure site)"
"What is the role of bone morphogenetic protein (BMP) in revision SI fusion? (May promote fusion in pseudarthrosis cases, but evidence limited, off-label use, risk of ectopic bone formation)"
MCQ Practice Points
Diagnosis
Q: What is the gold standard for diagnosing SI joint as pain source? A: Fluoroscopic-guided injection of local anesthetic. Greater than 75% pain relief confirms SI joint origin.
Clinical Tests
Q: How many positive provocative tests support SI joint dysfunction? A: 3 or more positive tests (from FABER, Gaenslen's, thigh thrust, compression, distraction) increases likelihood of SI joint origin.
Australian Context
Australian Practice Considerations
Epidemiology
- Common cause of low back pain in Australian population
- Manual workers and athletes at higher risk
- Peripartum dysfunction common in Australian women
Healthcare Pathway
- Initial management by GP or physiotherapist
- Specialist referral to rheumatology (inflammatory) or spine surgery (mechanical)
- Pain medicine specialists for injection therapy
Funding Considerations
- Conservative treatment: Medicare rebates for physiotherapy (with GP referral)
- Diagnostic injections: Covered under Medicare with specialist referral
- MIS fusion: Available in public and private systems
SI JOINT DYSFUNCTION
High-Yield Exam Summary
Key Facts
- •15-30% of low back pain
- •Pain over PSIS (Fortin's area)
- •Radiates to buttock, posterior thigh
- •Cluster of 3+ tests positive
Provocative Tests (FGTCD)
- •FABER (Patrick's)
- •Gaenslen's
- •Thigh Thrust
- •Compression
- •Distraction
Diagnosis
- •Injection is gold standard
- •Greater than 75% relief confirms
- •MRI for inflammatory causes
- •XR for degenerative
Red Flags
- •Young patient (less than 40)
- •Morning stiffness greater than 30 min
- •Improves with activity
- •Consider spondyloarthropathy