What is the differential diagnosis and initial work‑up for a patient presenting with mono‑articular hip pain?

Differential Diagnosis of Monoarticular Hip Pain

The differential diagnosis of monoarticular hip pain is anatomically organized by pain location (anterior/groin, lateral, or posterior), with septic arthritis being the most critical diagnosis to exclude emergently, followed by systematic evaluation for intra-articular pathology, periarticular soft tissue disorders, and referred pain from the spine or pelvis. 1, 2

Critical Emergency Diagnoses (Rule Out First)

Septic Arthritis

• Most common cause of acute severe monoarticular pain in children and a medical emergency in all ages requiring immediate diagnosis and intervention to prevent permanent joint destruction 1
• Typically presents with fever, severe pain, joint effusion, warmth, and erythema 3
• Hematogenous spread of Staphylococcus aureus is the most common etiology, with the hip being the most frequent site 1
• Immediate arthrocentesis is mandatory when infection is suspected—do not delay by administering antibiotics before joint aspiration 3
• Synovial fluid leukocyte count, Gram stain, and culture are diagnostic 4, 3

Fracture/Trauma

• Acute fracture, stress fracture, or occult traumatic fracture must be excluded with focal bone pain or recent trauma 2, 3
• Particularly important in elderly, diabetic, or osteoporotic patients 5

Osteonecrosis (Avascular Necrosis)

• Critical diagnosis in diabetic patients, presenting with insidious onset hip/groin pain without trauma 5
• Risk factors include diabetes, corticosteroid use, alcohol abuse, and sickle cell disease 5
• Early detection is crucial as progression leads to femoral head collapse 5

Anatomic Approach to Differential Diagnosis

Anterior Hip/Groin Pain (Intra-articular Pathology)

Osteoarthritis

• Most common cause in obese and aging patients, presenting with anterior hip/groin pain worsened by activity 1, 5, 2
• Physical examination and radiography have reasonable sensitivity and specificity 1

Femoroacetabular Impingement (FAI) Syndrome

• Anterior hip/groin pain in young to middle-aged active adults 1, 5
• Associated with labral tears and cartilage damage 1
• Requires both clinical findings AND imaging—never diagnose on imaging alone 1

Labral Tears

• Frequently associated with FAI syndrome, presenting with anterior hip/groin pain 1, 2
• May cause mechanical symptoms (clicking, catching, locking) 1

Acetabular Dysplasia/Hip Instability

• Can present with hip-related pain in young and middle-aged adults 1
• Requires assessment of bony morphology on imaging 1

Inflammatory Arthritis

• Rheumatoid arthritis, reactive arthritis, or crystalline arthropathy (gout, pseudogout) 6, 7
• Synovial fluid analysis with crystal examination is diagnostic 4, 3

Pigmented Villonodular Synovitis

• Rare proliferative synovial process causing monoarticular pain 1, 6
• Brown or bloody joint aspirate is characteristic 1

Lateral Hip Pain (Periarticular Pathology)

Greater Trochanteric Pain Syndrome (GTPS)

• Lateral hip pain is the hallmark, distinguishing it from intra-articular pathology which causes groin pain 8
• Includes trochanteric bursitis and abductor tendon pathology (gluteus medius/minimus tears) 1, 8
• Trendelenburg gait suggests abductor tendon tear 8

Posterior Hip/Buttock Pain (Extra-articular/Referred Pain)

Lumbar Radiculopathy (L3)

• Dermatomal sensory loss along medial lower leg distinguishes L3 radiculopathy from hip pathology 9
• Pain distribution includes buttock, lateral hip, and anterior-lateral thigh following L3 dermatome 9
• Negative hip-specific tests (FABER, FADIR) effectively exclude intra-articular hip pathology 9

Piriformis Syndrome

• Posterior hip and buttock pain 2

Sacroiliac Joint Dysfunction

• Posterior hip and buttock pain 2

Ischiofemoral Impingement

• Less common cause of posterior hip pain 2

Initial Diagnostic Work-Up

Step 1: History and Physical Examination

• Pain location is critical: anterior/groin (intra-articular), lateral (periarticular), posterior (referred) 2
• Presence of fever, erythema, warmth suggests infection requiring immediate arthrocentesis 1, 3
• Assess risk factors: diabetes, obesity, corticosteroid use, trauma history 5
• Screen for spine and pelvis pathology as hip pain may be referred 1

Step 2: Initial Imaging

Plain Radiographs (First-Line for All Patients)

• AP pelvis and lateral femoral head-neck views (frog-leg, Dunn, or cross-table) are mandatory initial imaging 1, 5, 8, 2
• Identifies: joint space narrowing, cam/pincer morphology, femoral head collapse, acetabular dysplasia, fractures, tumors 1, 5
• Radiographs should be obtained in most, if not all, cases as an excellent screening tool 1

Step 3: Arthrocentesis (When Indicated)

Perform immediately if:

• Signs of infection present (fever, erythema, warmth) AND joint effusion without recent trauma/surgery 3
• Septic arthritis must be excluded—do not delay with antibiotics before aspiration 1, 3

Synovial fluid analysis includes:

• Leukocyte count with differential 4, 3
• Gram stain and culture for bacterial pathogens 4, 3
• Crystal examination (monosodium urate for gout, calcium pyrophosphate for pseudogout) 4, 3

Interpretation:

• Inflammatory fluid with monosodium urate crystals = gout 3
• Noninflammatory fluid = osteoarthritis or internal derangement 3
• Brown/bloody aspirate = pigmented villonodular synovitis 1

Step 4: Advanced Imaging (When Radiographs Negative/Equivocal)

MRI Hip Without Contrast

• Superior for detecting osteonecrosis, occult fractures, bone marrow edema, soft tissue pathology 1, 5, 2
• Should be the first imaging technique after radiographs for most conditions 1
• Evaluates: abductor tendon tears, bursitis, muscle injuries, tumors, inflammation 1
• Particularly important in diabetic patients to detect osteonecrosis early 5

MR Arthrography (Direct)

• Indicated when labral tear or FAI syndrome strongly suspected clinically 1, 5, 2
• Superior to standard MRI for labral tears and cartilage delamination 1
• Requires intra-articular injection of dilute gadolinium 1

Ultrasound

• Rapid diagnosis of joint effusion and can guide aspiration 1
• Evaluates periarticular soft tissues: trochanteric bursitis, abductor tendons, snapping hip 1
• Limited for intra-articular structures due to inability to visualize acetabular or femoral head cartilage 1

Lumbar Spine MRI Without Contrast

• First-line imaging for suspected L3 radiculopathy to evaluate disc herniation or foraminal stenosis 9
• Obtain when dermatomal sensory loss present or negative hip-specific tests 9

Step 5: Diagnostic/Therapeutic Injections

• Image-guided intra-articular hip injection with anesthetic ± corticosteroid provides both diagnostic confirmation and therapeutic benefit 1, 5
• Pain relief with injection confirms intra-articular source 1
• Trochanteric or iliopsoas bursal injections help localize periarticular pain 1

Critical Pitfalls to Avoid

1. Never diagnose hip-related pain on imaging alone—must combine with symptoms and clinical signs 1
2. Incidental imaging findings are common in asymptomatic individuals; manage with clinical sensitivity 1, 8
3. Failure to perform arthrocentesis when infection suspected is catastrophic 3
4. Administering antibiotics before joint aspiration in suspected septic arthritis obscures diagnosis 3
5. Starting treatment based solely on elevated uric acid level without synovial fluid analysis 3
6. Multiple hip conditions can coexist (e.g., labral tear with FAI syndrome) 8
7. Straight leg raise test is insensitive for upper lumbar radiculopathy (L3) 9
8. Clinical examination tests for hip pathology have limited sensitivity and specificity 1