High Yield Overview

ANKYLOSING SPONDYLITIS

Seronegative Spondyloarthropathy | HLA-B27 | Axial Skeleton Involvement

HLA-B27Positive in 90-95% of patients

SI JointsSacroiliitis - earliest manifestation

M:F 2-3:1Male predominance

20-30Peak age of onset (years)

DISEASE STAGES

Prodromal

PatternInflammatory back pain, SI joint involvement

TreatmentNSAIDs, exercise

Early

PatternProgressive spinal stiffness

TreatmentBiologics if NSAID failure

Established

PatternFixed kyphosis, syndesmophytes

TreatmentBiologics, physiotherapy

Advanced/Bamboo Spine

PatternComplete fusion, high fracture risk

TreatmentFall prevention, fracture care

Critical Must-Knows

HLA-B27 positive in 90-95% of patients - but not diagnostic alone
Sacroiliitis is the hallmark - bilateral symmetric involvement
Inflammatory back pain: Morning stiffness greater than 30 min, improves with exercise, worse with rest
Bamboo spine - complete fusion creates high fracture risk from minor trauma
Spinal fractures - all 3 columns at risk, treat as unstable, high neurological risk

Examiner's Pearls

"
Schober test measures lumbar flexion - less than 5cm increase is abnormal
"
Romanus lesion (shiny corner sign) = vertebral body corner erosion
"
Fractures: Assume unstable, image entire spine, CT is gold standard
"
Cervical osteotomy: C7-T1 preferred level for kyphosis correction

Clinical Imaging

Imaging Gallery

X-ray of lumbosacral spine showing bilateral sacroiliitis, calcification of anterior and posterior longitudinal ligaments with syndesmophytes and bamboo spine. Needle insertion point and direction is — X-ray of lumbosacral spine showing bilateral sacroiliitis, calcification of anterior and posterior longitudinal ligaments with syndesmophytes and bambCredit: Shrestha GS et al. via Indian J Crit Care Med via Open-i (NIH) (Open Access (CC BY))

Patient lying in left lateral position showing loss of lumbar lordosis. Posterior superior iliac spine is marked by white arrow and the site of skin puncture for lumbar puncture by Taylor's apporach i — Patient lying in left lateral position showing loss of lumbar lordosis. Posterior superior iliac spine is marked by white arrow and the site of skin pCredit: Shrestha GS et al. via Indian J Crit Care Med via Open-i (NIH) (Open Access (CC BY))

AP and lateral radiographs showing bamboo spine in ankylosing spondylitis — AP (A) and lateral (B) radiographs of the thoracolumbar spine demonstrating classic 'bamboo spine' appearance. Note the flowing syndesmophytes bridging vertebral bodies with complete anterior longitudinal ligament ossification. The spine appears as a single ossified column - this creates high fracture risk from even minor trauma.Credit: PMC - CC BY 4.0

Critical AS Exam Points

Fracture Management

ALL spinal fractures in AS are UNSTABLE - fused spine fractures through all 3 columns. Assume unstable even with minor trauma. Image ENTIRE spine (high rate of non-contiguous fractures). CT is the imaging of choice - X-rays often miss fractures.

HLA-B27 Interpretation

HLA-B27 is NOT diagnostic - present in 8% of general population. 90-95% of AS patients are positive. A negative HLA-B27 does not exclude AS. Diagnosis is clinical + imaging (sacroiliitis on MRI or X-ray).

Surgical Considerations

Airway management critical - fixed cervical kyphosis limits intubation options. Fibreoptic intubation often required. Position carefully - avoid neck extension. Consider awake positioning before induction.

Arthroplasty Outcomes

THA commonly required - hip involvement in 30-50% of patients. Outcomes good but higher heterotopic ossification risk. Consider prophylaxis. Stiff spine increases mechanical demands on hip.

Seronegative Spondyloarthropathies Comparison

Feature	Ankylosing Spondylitis	Reactive Arthritis	Psoriatic Arthritis
HLA-B27 association	90-95%	60-80%	40-50%
Axial involvement	Always (defining feature)	Common	40% have spondylitis
Peripheral arthritis	Uncommon	Predominant	Common - DIP, dactylitis
Sacroiliitis pattern	Bilateral symmetric	Asymmetric	Asymmetric
Extra-articular	Uveitis, aortitis	Conjunctivitis, urethritis	Skin, nail changes
Disease course	Chronic progressive	Often self-limiting	Variable

Mnemonic

NIGHTInflammatory Back Pain Features

Night pain and stiffness

Wakes patient in second half of night

Insidious onset

Gradual over months, age less than 40

Gets better with exercise

Improves with activity, worse with rest

Half hour morning stiffness

Greater than 30 minutes of AM stiffness

Three months duration

Chronic low back pain over 3 months

Memory Hook:NIGHT pain keeps AS patients awake but MOVEMENT makes it better!

Mnemonic

APICALExtra-Articular Manifestations

Aortitis and cardiac

Aortic regurgitation, conduction defects

Pulmonary fibrosis

Apical lung fibrosis - rare

Iritis/uveitis

Acute anterior uveitis - 25-40% of patients

Cauda equina syndrome

Rare late complication from arachnoiditis

Amyloidosis

Secondary amyloidosis - rare, late

Lung restriction

Chest wall restriction from costovertebral fusion

Memory Hook:APICAL manifestations affect areas beyond the spine!

Mnemonic

FUSEDSpinal Fracture Principles in AS

Full spine imaging required

Non-contiguous fractures common

Unstable assumed

All 3 columns fracture through ankylosed spine

Stabilization urgent

High neurological risk - early fixation

Extension injury pattern

Hyperextension mechanism common

Delayed presentation possible

Minor trauma, occult fractures

Memory Hook:FUSED spines fracture like long bones - through everything!

Overview and Epidemiology

Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton, leading to progressive spinal fusion. It is the prototypical seronegative spondyloarthropathy, characterized by sacroiliitis and enthesitis.

Epidemiology:

Prevalence: 0.1-0.5% in Caucasian populations
Male to female ratio: 2-3:1 (historically higher ratios due to underdiagnosis in women)
Peak age of onset: 20-30 years (rarely presents after age 45)
Strong HLA-B27 association: 90-95% of patients positive
Family history: 10-20 times increased risk in first-degree relatives

Risk Factors:

HLA-B27 positivity: Major genetic risk factor
Family history: Strong familial aggregation
Male sex: Higher prevalence and more severe disease
Smoking: Associated with worse outcomes and progression

HLA-B27 Population Data

HLA-B27 is present in approximately 8% of the general Caucasian population but only 5-10% of HLA-B27 positive individuals develop AS. It is a risk factor, not a diagnostic test. Approximately 5-10% of AS patients are HLA-B27 negative.

Pathophysiology

Understanding the pathophysiology of AS is essential for both diagnosis and management. The disease involves aberrant immune responses at entheses leading to inflammation and subsequent new bone formation.

Genetic Basis

HLA-B27 association:

MHC Class I molecule expressed on cell surfaces
Mechanism of disease association not fully understood
Theories include: arthritogenic peptide presentation, protein misfolding, cell surface homodimer formation
Over 100 HLA-B27 subtypes - B27:05 and B27:02 most associated with AS

Non-HLA genes:

ERAP1 (aminopeptidase) - involved in peptide processing
IL-23R - IL-23/IL-17 axis important in pathogenesis
Multiple other loci identified by GWAS

Inflammation and New Bone Formation

Enthesitis:

Primary site of inflammation - where tendons/ligaments attach to bone
Entheses at sacroiliac joints and spine particularly affected
Inflammatory infiltrate with TNF-alpha, IL-17, IL-23

Paradox of inflammation and ossification:

Initial erosive inflammation at entheses
Bone marrow edema visible on MRI
TNF and IL-17 drive inflammation
Repair response involves new bone formation (syndesmophytes)
Eventually leads to complete fusion

TNF Paradox

Anti-TNF therapy effectively controls inflammation but may not prevent radiographic progression (new bone formation). This disconnect between inflammation and ossification is important for patient counseling and is a common exam discussion point.

Pathological Features

Sacroiliac joints:

Earliest site of involvement
Subchondral bone erosions
Sclerosis and widening initially, then fusion

Spine:

Romanus lesion: Corner erosion at vertebral body margins
Syndesmophytes: Vertical bony bridges between vertebrae
Squaring of vertebral bodies
Complete fusion: Bamboo spine appearance

Clinical Presentation

Axial Symptoms

Inflammatory back pain (cardinal feature):

Insidious onset over months
Age of onset typically less than 40 years
Morning stiffness greater than 30 minutes
Improves with exercise, worse with rest
Night pain - wakes in second half of night
Alternating buttock pain (sacroiliitis)

Progressive spinal restriction:

Loss of lumbar lordosis
Thoracic kyphosis increases
Reduced chest expansion (less than 2.5cm abnormal)
Cervical involvement leads to fixed forward gaze

Peripheral Manifestations

Hip involvement:

Present in 30-50% of patients
Often bilateral
Major cause of disability
Early onset (less than 10 years disease duration) = worse prognosis

Other joints:

Shoulder involvement common
Knee, ankle less frequent
Asymmetric oligoarthritis pattern

Extra-Articular Manifestations

Acute anterior uveitis:

Most common extra-articular feature (25-40%)
Unilateral, recurrent
Presents with painful red eye, photophobia
Ophthalmology emergency - can cause vision loss

Cardiovascular:

Aortitis and aortic root dilatation
Aortic regurgitation (1-10%)
Conduction defects

Pulmonary:

Apical pulmonary fibrosis (rare)
Restrictive lung disease from chest wall fusion

Physical Examination

Inspection:

Loss of lumbar lordosis
Increased thoracic kyphosis
Fixed cervical flexion (chin-on-chest deformity in severe cases)
Question mark posture (global kyphosis)

Specific tests:

Schober test: Mark L5 and 10cm above. On forward flexion, less than 5cm increase is abnormal
Modified Schober: Mark 5cm below and 10cm above PSIS
Chest expansion: Less than 2.5cm at nipple line is abnormal
Occiput-to-wall distance: Should be zero - increased indicates cervical kyphosis
Tragus-to-wall distance: Alternative cervical mobility measure

Investigations

Laboratory Studies

Inflammatory markers:

ESR and CRP elevated in 50-70%
Normal inflammatory markers do not exclude AS
Useful for monitoring disease activity

HLA-B27:

Positive in 90-95% of AS patients
Not diagnostic alone - supports clinical diagnosis
Useful in early disease when imaging inconclusive

Rheumatoid factor and anti-CCP:

Negative (seronegative spondyloarthropathy)
Helps distinguish from RA

Imaging

Plain Radiographs:

Sacroiliac joints:

Bilateral symmetric sacroiliitis
Grading 0-4 (modified New York criteria)
Erosions, sclerosis, joint space widening/narrowing, fusion

Spine:

Squaring of vertebral bodies
Syndesmophytes (marginal, vertical)
Bamboo spine (complete fusion)
Romanus lesion (shiny corner sign)
Anderson lesion (discovertebral destruction)

MRI (Critical for early diagnosis):

Bone marrow edema at SI joints - active sacroiliitis
Can detect inflammation before radiographic changes
STIR/T2 fat-sat sequences best for edema
Enables diagnosis 5-10 years earlier than X-ray

CT:

Best for fracture detection in AS spine
Superior to X-ray for visualizing fractures through fused segments
Image entire spine when fracture suspected

Modified New York Criteria

Definite AS requires: Radiographic sacroiliitis (bilateral grade 2-4 OR unilateral grade 3-4) PLUS at least one clinical criterion (inflammatory back pain, limited lumbar motion, reduced chest expansion). These criteria miss early disease - ASAS criteria incorporate MRI.

Management

Medical Management

First-line: NSAIDs:

Continuous NSAIDs more effective than on-demand
Indomethacin, naproxen, etoricoxib all effective
May slow radiographic progression (controversial)
Continue if effective and tolerated

Biologic therapy (TNF inhibitors):

Indicated when NSAIDs fail (2 agents over 4 weeks)
Adalimumab, etanercept, infliximab, golimumab, certolizumab
Highly effective for symptoms and inflammation
PBS listed with specific criteria in Australia

IL-17 inhibitors:

Secukinumab, ixekizumab
Alternative to TNF inhibitors
Particularly useful if TNF failure

Other agents:

Sulfasalazine: May help peripheral arthritis, limited axial benefit
Methotrexate: Not effective for axial disease
Corticosteroids: Local injections useful; avoid long-term systemic

Physiotherapy (Essential):

Maintain posture and mobility
Regular stretching and strengthening
Hydrotherapy beneficial
Lifelong commitment required

This section covers the medical management approach.

Surgical Management

Indications for Spinal Surgery

Spinal fractures: Most common surgical indication
Kyphosis correction: For fixed forward gaze, inability to see horizon
Spinal stenosis: Rare, cauda equina syndrome
Pseudoarthrosis: Anderson lesion causing instability

Spinal Fracture Management in AS

ALL AS Spinal Fractures Are Unstable

The ankylosed spine functions as a long bone. Fractures extend through all three columns. Even minor trauma can cause fractures. Neurological injury occurs in 50-70% of cases. Treat all fractures as unstable until proven otherwise.

Cervical spine fracture in AS showing MRI and surgical fixation — Cervical spine fracture in ankylosing spondylitis: (A) Sagittal T2 MRI showing hyperextension fracture through the ankylosed cervical spine with spinal cord signal change indicating cord injury, (B) Lateral X-ray demonstrating the fracture through the fused segments, (C) Post-operative lateral X-ray showing posterior cervical instrumentation spanning multiple levels above and below the fracture. CT is essential as X-rays miss 30% of AS spine fractures.Credit: PMC - CC BY 4.0

Comprehensive cervical fracture case in AS with multimodal imaging and fixation — Comprehensive cervical fracture management in AS: (A) Lateral X-ray showing severe cervical kyphosis, (B) CT sagittal showing fracture through ankylosed spine, (C) MRI demonstrating cord compression and signal change, (D) Post-op AP showing instrumentation, (E-H) CT reconstructions confirming posterior fixation spanning 3+ levels above and below fracture. Long segment fixation is essential due to osteoporotic bone and lever arm forces.Credit: PMC - CC BY 4.0

Principles:

Assume unstable - all 3 columns involved
Image entire spine - non-contiguous fractures in 5-10%
CT is essential - X-rays miss 30% of fractures
MRI for cord assessment - if neurological deficit
Immobilize in position of deformity - do not attempt correction

Surgical Management (preferred):

Early stabilization reduces complications
Long posterior instrumentation (3+ levels above and below)
Consider anterior support if significant kyphosis
Cement augmentation for osteoporotic bone

Conservative management:

Reserved for non-displaced, stable fractures without neurological deficit
Halo vest or Minerva cast problematic due to rigid spine
Higher complication rates than surgical treatment

Outcomes:

Mortality 5-15% (higher than general population)
Neurological injury in up to 70%
High rate of epidural hematoma
Delayed union and pseudoarthrosis risk

This section covers fracture management in AS.

Complications

Disease Complications

Spinal fractures: High risk with minor trauma
Cauda equina syndrome: Late complication from arachnoiditis
Atlanto-axial subluxation: Rare but serious
Restrictive lung disease: Costovertebral fusion
Aortic regurgitation: From aortitis
Uveitis complications: Vision loss if untreated
Amyloidosis: Rare, late complication

Surgical Complications

Neurological injury: High risk with fractures, osteotomy
Epidural hematoma: Common with AS fractures
Pseudoarthrosis: Difficult healing, especially at osteotomy sites
Implant failure: Osteoporotic bone, long lever arms
Dural tear: Ossified dura may be encountered
Heterotopic ossification: After hip surgery

Evidence Base

ATLAS Trial - Secukinumab in AS

Baraliakos X et al. • Lancet (2019)

Key Findings:

ASAS40 response maintained in 70% at 5 years
Low radiographic progression in majority
Good safety profile with long-term use

Surgical Outcomes of AS Spine Fractures

Westerveld LA et al. • Spine (2009)

Key Findings:

Surgical treatment associated with better outcomes
Conservative management has higher complication rates
Early stabilization recommended

THA Outcomes in Ankylosing Spondylitis

III

Bhan S et al. • J Arthroplasty (2008)

Key Findings:

Good pain relief and functional improvement
HO rates up to 50% without prophylaxis
NSAID or radiation prophylaxis recommended

MRI in Early Diagnosis of AS

Rudwaleit M et al. • Arthritis Rheum (2009)

Key Findings:

MRI detects sacroiliitis 5-10 years before X-ray
Bone marrow edema highly specific for active inflammation
Enables earlier treatment initiation

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Spinal Fracture in AS

EXAMINER

"A 55-year-old man with known ankylosing spondylitis presents after a ground-level fall. He has neck pain and bilateral arm numbness. X-rays are reported as normal."

EXCEPTIONAL ANSWER

Thank you. This is a high-risk scenario. Any spinal fracture in AS must be assumed unstable, and X-rays miss up to 30% of fractures in ankylosed spines. I would immediately immobilize the cervical spine in the position of deformity, not forcing neutral position. I would urgently arrange CT of the entire spine - fractures can be non-contiguous. Given the neurological symptoms, MRI is also indicated to assess the cord. I would expect to find a fracture extending through all three columns, often with associated epidural hematoma. Management would be surgical stabilization with long-segment posterior instrumentation, typically 3 or more levels above and below the fracture. I would ensure the anaesthetic team is aware of the airway challenges with AS.

KEY POINTS TO SCORE

X-rays miss 30% of fractures in AS

CT entire spine is mandatory

Assume all fractures are unstable (3-column)

Immobilize in position of deformity

COMMON TRAPS

✗Accepting normal X-ray as excluding fracture

✗Imaging only the painful level

✗Attempting to straighten the neck

LIKELY FOLLOW-UPS

"What if CT shows a C5-6 fracture with epidural hematoma?"

"How would you manage the airway?"

"What are the mortality rates for AS spine fractures?"

VIVA SCENARIOStandard

Scenario 2: Young Man with Inflammatory Back Pain

EXAMINER

"A 28-year-old man presents with 18 months of low back pain. He reports morning stiffness lasting over an hour that improves with exercise. His pain wakes him at night. Examination shows reduced lumbar flexion."

EXCEPTIONAL ANSWER

Thank you. This presentation is highly suggestive of inflammatory back pain, raising concern for ankylosing spondylitis or axial spondyloarthropathy. The key features are: young age, insidious onset, morning stiffness greater than 30 minutes improving with exercise, night pain, and reduced spinal mobility. I would perform a Schober test to quantify lumbar restriction. Investigations would include HLA-B27, inflammatory markers (ESR, CRP), and imaging. I would request pelvic X-ray for sacroiliac joints, but given his age and symptom duration, MRI of SI joints is more sensitive for early disease showing bone marrow edema. If sacroiliitis is confirmed, this meets criteria for axial spondyloarthropathy. Management would begin with NSAIDs and physiotherapy, with referral to rheumatology for consideration of biologics if inadequate response.

KEY POINTS TO SCORE

Inflammatory back pain features: age less than 40, insidious, AM stiffness, improves with exercise

Schober test for lumbar flexion

MRI SI joints for early sacroiliitis

NSAIDs first-line, biologics if failure

COMMON TRAPS

✗Attributing to mechanical back pain

✗Only requesting lumbar spine X-ray

✗Delaying rheumatology referral

LIKELY FOLLOW-UPS

"What are the modified New York criteria?"

"What is the role of HLA-B27 testing?"

"What are the extra-articular manifestations?"

VIVA SCENARIOAdvanced

Scenario 3: Severe Kyphosis in AS

EXAMINER

"A 60-year-old man with longstanding AS has progressive difficulty seeing ahead when walking. He cannot see the horizon and has trouble eating. He has a fixed chin-on-chest deformity."

EXCEPTIONAL ANSWER

Thank you. This patient has severe fixed cervical kyphosis from advanced AS affecting his horizontal gaze and quality of life. Surgical correction with cervical osteotomy is indicated when conservative measures fail and the deformity significantly impacts function. My workup would include full-length standing radiographs to assess global sagittal balance, CT for surgical planning, and MRI to assess the cord. Pulmonary function tests are important given chest wall restriction. The preferred osteotomy level is C7-T1 as it is below the spinal cord (cord ends at T1-T2), allowing safer correction. A closing wedge osteotomy can achieve 30-40 degrees of correction. This is a high-risk procedure requiring an experienced multidisciplinary team including spine surgeons, anaesthetists familiar with fibreoptic intubation, and neurophysiology monitoring. I would discuss risks thoroughly including neurological injury, vertebral artery damage, and need for potential staged procedures.

KEY POINTS TO SCORE

Chin-on-chest with loss of horizontal gaze is surgical indication

C7-T1 is preferred osteotomy level (below cord)

30-40 degrees correction achievable

High-risk procedure requiring expert team

COMMON TRAPS

✗Operating at higher cervical level (cord at risk)

✗Underestimating airway challenges

✗Not assessing pulmonary function

LIKELY FOLLOW-UPS

"What anaesthetic considerations are important?"

"What is a pedicle subtraction osteotomy?"

"What complications would you discuss?"

VIVA SCENARIOStandard

Scenario 4: THA Planning in AS

EXAMINER

"A 45-year-old man with AS has bilateral hip pain limiting walking to 100 metres. X-rays show severe bilateral hip arthritis. He has 30 degrees of fixed thoracolumbar kyphosis."

EXCEPTIONAL ANSWER

Thank you. This patient requires bilateral THA for end-stage hip arthritis secondary to AS. Several AS-specific considerations are important. First, his fixed spinal kyphosis affects spinopelvic mechanics - the stiff spine means the hip must accommodate all pelvic tilt changes between sitting and standing. Standard cup positioning may not be appropriate; I would consider this in my templating. Second, he is at high risk of heterotopic ossification - I would plan prophylaxis with indomethacin 75mg daily for 6 weeks or single-dose radiation if NSAIDs contraindicated. Third, positioning may be challenging with fixed kyphosis - I would plan for appropriate supports and avoid forced positions. Fourth, anaesthetic assessment is essential for airway management. I would counsel him about the higher complication rates in AS including HO, and discuss staging versus simultaneous bilateral procedures.

KEY POINTS TO SCORE

Stiff spine affects hip biomechanics and cup positioning

High HO risk - prophylaxis essential

Positioning challenges with fixed kyphosis

Airway considerations for anaesthesia

COMMON TRAPS

✗Standard cup positioning without considering spine

✗Not planning HO prophylaxis

✗Not addressing anaesthetic concerns

LIKELY FOLLOW-UPS

"How does spine-hip relationship affect cup position?"

"What HO prophylaxis options exist?"

"Would you do staged or simultaneous bilateral THA?"

Australian Context

In Australia, ankylosing spondylitis affects approximately 0.5% of the population, with higher prevalence in certain Indigenous communities. The condition is managed through a multidisciplinary approach involving rheumatologists, orthopaedic surgeons, physiotherapists, and ophthalmologists.

PBS-listed medications:

NSAIDs: Various PBS listed for inflammatory conditions
TNF inhibitors: PBS listed under Rheumatology Authority for AS with specific criteria (BASDAI greater than 4, failure of 2 NSAIDs, radiographic evidence)
IL-17 inhibitors: Secukinumab PBS listed for AS after TNF failure or contraindication

Australian Rheumatology Association guidelines emphasize early diagnosis using MRI, prompt initiation of biologic therapy when indicated, and the importance of physiotherapy. Orthopaedic involvement is critical for managing spinal fractures and performing hip arthroplasty, both common in the AS population. Coordination between rheumatology and orthopaedics is essential for optimal patient outcomes.