What is the differential diagnosis for a young adult with sacroiliac joint pain one month after a lifting and carrying injury?

Differential Diagnosis for Sacroiliac Joint Pain After Lifting Injury in a Young Adult

Immediate Red Flags to Exclude First

Before considering mechanical sacroiliac joint dysfunction, you must systematically exclude serious pathology that can present with similar pain patterns. 1

• Stress fractures (sacrum, ilium, or pubic ramus) present with insidious onset, night pain, inability to bear weight, and focal bone tenderness—critical to exclude in the setting of recent trauma 1, 2
• Infection (septic sacroiliitis, osteomyelitis) manifests with acute onset, fever, severe pain at rest, and inability to bear weight 1, 2
• Tumors demonstrate night pain, constitutional symptoms, and progressive worsening unrelated to activity 1, 2
• Inflammatory sacroiliitis from axial spondyloarthropathy presents with inflammatory back pain characteristics: insidious onset, improvement with exercise (not rest), pain at night/early morning, morning stiffness >30 minutes, and age of onset <45 years 1

Primary Differential Diagnoses

Mechanical Sacroiliac Joint Dysfunction (Most Likely Given Mechanism)

Mechanical SI joint pain accounts for 15-30% of chronic mechanical low back pain cases and is the most probable diagnosis given the acute lifting/carrying mechanism in a young adult. 3, 4, 5

• Pain pattern: Predominantly gluteal region pain (94% of cases), with frequent radiation to lower lumbar region (72%), groin (14%), and lower extremity (50%)—pain can even extend below the knee (28%) or to the foot (14%) 6
• Mechanism: Lifting and carrying creates repetitive stress and potential misalignment of the SI joint, particularly common in young athletes and active individuals 7
• Clinical presentation: Pain one month post-injury suggests subacute mechanical dysfunction rather than acute inflammation 5

Hip-Related Pain (Must Be Distinguished)

Hip pathology frequently refers pain to the SI region and must be systematically excluded, as clinical examination and imaging each have limited diagnostic utility in isolation. 1

• Femoroacetabular impingement (FAI) syndrome: Groin pain with hip flexion, adduction, and internal rotation; positive FADIR test; cam or pincer morphology on radiographs 1, 2
• Acetabular labral tears: Sharp, catching pain in groin/medial hip with mechanical symptoms; often coexist with FAI or dysplasia 1, 2
• Acetabular dysplasia/hip instability: Medial groin pain with instability sensation and misalignment between femoral head and acetabulum 1

Lumbar Spine Pathology (Competing Musculoskeletal Source)

Lumbar spine screening is mandatory in all cases of suspected SI joint pain, as referred pain patterns overlap extensively. 1, 2, 8

• Pain characteristics: Sharp lancinating pain radiating to buttock/leg; induced by sitting, standing, or walking; often present at rest; improved by position change 2, 9
• Key differentiator: History of back problems, worse with sitting, relief when supine or standing 9

Inflammatory Sacroiliitis (Axial Spondyloarthropathy)

While less likely given the acute traumatic mechanism, inflammatory sacroiliitis must be considered in young adults with persistent SI pain, especially if inflammatory features are present. 1

• Clinical features: Inflammatory back pain (insidious onset, improvement with exercise, no improvement with rest, night pain, age <40 years) present in 70-80% of axSpA patients 1
• HLA-B27 testing: Has 90% sensitivity and post-test probability of 32% when positive—ideal screening test for axSpA 1
• Imaging: MRI of SI joints shows inflammatory changes (bone marrow edema) before radiographic changes develop; radiographs may be normal early in disease 1

Diagnostic Algorithm

Step 1: Clinical Assessment to Exclude Red Flags

• History: Determine if pain has inflammatory characteristics (night pain, morning stiffness >30 minutes, improvement with exercise) versus mechanical pattern (worse with activity, better with rest) 1
• Constitutional symptoms: Fever, weight loss, or night sweats suggest infection or tumor 1, 2
• Ability to bear weight: Inability suggests fracture or infection 1, 2

Step 2: Physical Examination

No single physical examination test has adequate diagnostic utility; a comprehensive approach combining multiple findings is essential. 1

• SI joint provocative tests: Specificity and sensitivity are relatively high when ≥3 tests are positive, though recent studies question their predictive value 5
• FADIR test (flexion-adduction-internal rotation): A negative test helps rule out hip-related pain, though clinical utility is limited 1
• Hip range of motion: Assess internal rotation and overall mobility to evaluate for hip pathology 2, 9
• Lumbar spine screening: Mandatory in all cases—assess for pain with sitting, relief with position change, and neurological signs 1, 2, 8

Step 3: Laboratory Testing (If Inflammatory Features Present)

• HLA-B27: Order if inflammatory back pain features are present—90% sensitivity for axSpA with post-test probability of 32% when positive 1
• ESR/CRP: Only 50% sensitivity in axSpA, so normal values do not exclude inflammatory disease 1

Step 4: Imaging Protocol

Imaging is essential to exclude red flags but contributes little to diagnosing mechanical SI joint pain. 3, 4, 5

• First-line: AP pelvis radiographs to exclude fractures, tumors, and assess for radiographic sacroiliitis (though sensitivity is only 80% for axSpA) 1
• MRI of SI joints: Indicated if inflammatory sacroiliitis suspected (inflammatory back pain features, HLA-B27 positive, or high clinical suspicion)—demonstrates bone marrow edema before radiographic changes 1
• MRI of lumbar spine: If radicular symptoms or lumbar pathology suspected 1, 9
• Advanced hip imaging (MRI/MRA): If hip pathology suspected based on positive FADIR test or limited hip range of motion 1, 2

Step 5: Diagnostic SI Joint Block (If Mechanical Dysfunction Suspected)

Fluoroscopically guided SI joint injection with local anesthetic is the diagnostic gold standard for mechanical SI joint pain, though false-positive and false-negative results occur frequently. 3, 4, 5

• Interpretation: Significant pain relief after injection confirms SI joint as pain generator 3, 4
• Caution: Must be interpreted carefully due to potential for false results 3, 5

Critical Clinical Pitfalls

• Referred pain patterns are highly variable: SI joint pain is not limited to the lumbar region and buttock—50% of patients have lower extremity pain, 28% have pain below the knee, and 14% have foot pain, which can mimic radiculopathy 6
• Younger patients more likely to have distal pain: Statistically significant relationship exists between age and pain location, with younger patients more likely to describe pain distal to the knee 6
• Imaging findings must correlate clinically: Incidental findings are common in asymptomatic individuals—never diagnose based on imaging alone 1
• Multiple pathologies often coexist: Labral tears frequently coexist with FAI or dysplasia; SI joint dysfunction can coexist with lumbar pathology 1, 2
• Acute trauma can unmask inflammatory disease: While mechanical dysfunction is most likely given the lifting mechanism, the trauma may have unmasked underlying inflammatory sacroiliitis—maintain high suspicion if inflammatory features develop 1