Unilateral Knee Pain — The Complete OPD Framework
A practical clinical reasoning guide for general practitioners and junior doctors — built around a real OPD-style case of unilateral knee pain, where the first job is not to write a painkiller but to decide whether the knee is safe, inflammatory, mechanical, articular, periarticular, or dangerous.
Why This Guide Matters
Knee pain is one of the commonest OPD presentations, and the classic mistake is to label every middle-aged patient as “osteoarthritis” and prescribe an analgesic. This guide trains the opposite habit: pause first, classify the pain, search for red flags, and then decide the most likely diagnosis.
The case begins with a 45-year-old female with right-sided knee pain for 14 days. From there, the guide builds a complete OPD framework: articular versus periarticular pain, mechanical versus inflammatory pattern, acute monoarthritis red flags, knee examination, investigations, initial management, referral thresholds, and patient safety-netting.
What You’ll Learn
- The first OPD decision — danger first, then effusion, then mechanical versus inflammatory pattern.
- Articular versus periarticular pain — how to separate true joint disease from bursitis, tendon, ligament, IT band, or patellofemoral pain.
- Mechanical versus inflammatory pain — activity-related pain, morning stiffness duration, warmth, swelling, night pain, and systemic clues.
- Red flags in monoarthritis — septic arthritis, crystal arthritis, hot swollen knee, inability to bear weight, trauma, fever, immunosuppression, and night pain.
- Differential diagnosis framework — osteoarthritis, anserine bursitis, patellofemoral pain, IT band syndrome, meniscal tear, inflammatory arthritis, gout, CPPD, septic arthritis, ligament injury, referred pain, and bone pathology.
- Focused history — pain site, onset, swelling, locking, giving way, morning stiffness, systemic symptoms, trauma, occupation, BMI, menopause, drugs, diabetes, CKD, and gout risk.
- Physical examination — look, feel, move, effusion tests, joint line tenderness, pes anserine tenderness, ROM, crepitus, McMurray, Apley, ligament tests, hip and lumbar screening.
- Investigations — CBC, ESR, CRP, uric acid, HbA1c, X-ray views, RF, anti-CCP, ANA when indicated, vitamin D and bone profile, joint aspiration, ultrasound, and MRI indications.
- Initial OPD management — urgent referral for hot swollen knee, conservative care for mechanical pain, safe NSAID logic, physiotherapy, exercise, weight reduction, and follow-up triggers.
- Referral and safety-netting — when to refer same-day, when to refer rheumatology, when to refer orthopaedics, and what return warnings to give the patient.
A hot, swollen, severely painful knee with restricted movement is not “simple inflammation” until septic arthritis and crystal arthritis have been excluded. Do not inject steroid into an undiagnosed hot swollen joint.