What is the appropriate diagnosis and treatment for sudden onset of pain along the elbow with a hard knot?

Differential Diagnosis: Sudden Elbow Pain with Hard Knot

The most likely diagnosis is olecranon bursitis, which presents as a palpable hard knot (fluid-filled swelling) over the posterior elbow with acute onset pain, though medial epicondylitis with calcification, heterotopic ossification, or an intra-articular loose body must also be considered. 1

Immediate Diagnostic Approach

Obtain plain radiographs (AP, lateral, and oblique views) as the first-line imaging study to differentiate between osseous pathology, soft tissue calcification, heterotopic ossification, intra-articular bodies, and soft tissue swelling. 2, 3

Key Physical Examination Findings to Establish Diagnosis

• Posterior elbow location with fluctuant or firm swelling directly over the olecranon suggests olecranon bursitis (septic vs. aseptic differentiation requires bursal fluid aspiration and analysis) 1
• Medial epicondyle tenderness with pain on resisted wrist flexion/forearm pronation indicates medial epicondylitis (may have associated calcification appearing as a "knot") 4, 5
• Mechanical symptoms (locking, clicking, catching) with palpable mass suggest intra-articular loose body or osteochondral lesion 2, 4
• Limited range of motion with joint effusion points toward intra-articular pathology 4

Algorithmic Diagnostic Pathway

Step 1: Localize the "Knot"

• Posterior elbow (over olecranon) → Olecranon bursitis most likely; aspirate if septic bursitis suspected (fever, erythema, warmth) 1
• Medial epicondyle region → Medial epicondylitis with calcific tendinopathy or UCL injury with heterotopic ossification 2, 4
• Lateral aspect → Lateral epicondylitis with calcification or radial head pathology 3
• Mobile within joint → Intra-articular loose body 2, 4

Step 2: Radiographic Interpretation

• Soft tissue swelling without calcification → Olecranon bursitis (aseptic); treat conservatively with rest, ice, compression, NSAIDs 1
• Soft tissue calcification at tendon insertion → Calcific tendinopathy (medial or lateral epicondylitis); treat with activity modification, NSAIDs, physical therapy 2, 5
• Heterotopic ossification → Post-traumatic or neurogenic; may require surgical excision if symptomatic 2
• Intra-articular loose body → Requires MRI or CT arthrography for surgical planning 2, 3
• Normal radiographs → Proceed to Step 3 2, 3

Step 3: Advanced Imaging When Radiographs Normal or Indeterminate

MRI without contrast is the next appropriate study for suspected soft tissue pathology (tendon tear, nerve entrapment) with normal radiographs. 3, 4

• MR arthrography (3T) provides 81% sensitivity and 91% specificity for UCL tears and is superior for intra-articular pathology evaluation 4
• CT arthrography offers 93% sensitivity for loose body detection and excellent assessment of heterotopic ossification 2, 3
• Ultrasound with dynamic evaluation has 94% sensitivity and 98% specificity for common extensor tendon tears and allows point-of-care assessment 3, 6

Red Flags Requiring Urgent Evaluation

• Night pain or pain at rest suggests inflammatory, infectious, or neoplastic process (requires immediate workup including ESR, CRP, and consideration of MRI) 4
• Fever, erythema, warmth over swelling indicates septic olecranon bursitis (requires aspiration with cell count, Gram stain, culture before antibiotics) 1
• Neurologic symptoms (paresthesias, weakness, numbness) require nerve evaluation with EMG/nerve conduction studies and MR neurography 4
• Mechanical symptoms mandate evaluation for intra-articular pathology (loose bodies, osteochondral lesions) 2, 4

Treatment Based on Diagnosis

Olecranon Bursitis (Most Common for "Hard Knot")

• Aseptic bursitis: Rest, ice, compression, NSAIDs, avoid direct pressure; aspiration if tense and painful 1
• Septic bursitis: Aspiration for diagnosis, followed by antibiotics (cover Staphylococcus aureus); consider incision and drainage if refractory 1

Medial Epicondylitis with Calcification

• First-line: Activity modification (avoid repetitive wrist flexion/forearm pronation), NSAIDs, eccentric strengthening exercises, deep transverse friction massage 5
• Second-line: Corticosteroid injection (40-80 mg triamcinolone) into area of greatest tenderness if conservative measures fail after 6-12 weeks 5, 7
• Avoid injecting corticosteroid into tendon substance (risk of rupture); infiltrate around tendon sheath 7

Intra-articular Loose Body

• Surgical removal via arthroscopy if symptomatic with mechanical symptoms 2

Critical Pitfalls to Avoid

• Failing to obtain initial radiographs before advanced imaging wastes resources and delays diagnosis of osseous pathology 2, 3, 4
• Overreliance on corticosteroid injections without addressing underlying biomechanics leads to recurrence and potential tendon rupture 4, 5
• Misinterpreting olecranon bursitis as a solid mass rather than fluid-filled structure (aspiration confirms diagnosis) 1
• Missing septic bursitis by not aspirating when infection is possible (delay in treatment increases morbidity) 1
• Injecting corticosteroid directly into tendon substance rather than peritendinous tissue causes tendon weakening and rupture risk 7