Musculoskeletal Assessment and Diagnosis
Assessment as hypothesis testing
Assessment is not a test-collection ritual but a sequence of hypothesis tests. Begin with the clinical picture - age, onset, loading history, irritability, night pain, systemic features - and let it set a pre-test probability that you then refine. A special test does not create a diagnosis; it shifts probability. Understand sensitivity (a highly sensitive test that is negative helps rule out - SnNOut) and specificity (a highly specific test that is positive helps rule in - SpPIN), plus likelihood ratios, which are far more informative than a bare sensitivity figure.
What to know cold
- Red flags: cauda equina (saddle anaesthesia, retention, bilateral sciatica), fracture (trauma, osteoporosis, long-term steroids), infection (fever, injecting drug use, immunosuppression), malignancy (night pain, weight loss, cancer history), inflammatory arthropathy (morning stiffness over 30–45 minutes, improvement with exercise).
- Neurological examination: dermatomes, myotomes, reflexes, and a documented baseline.
- Named tests: Lachman (ACL), Thompson (Achilles rupture), Thessaly (meniscus), drop arm (rotator cuff), Ottawa ankle and knee rules.
- Imaging: MRI in non-specific low back pain without red flags rarely helps and increases intervention rates; disc and labral findings are common in asymptomatic people.
Traps
Treating a positive test as a diagnosis; ignoring pre-test probability; forgetting that clusters outperform single tests; missing red flags because the patient looks well; and interpreting age-related imaging findings as the pain generator.
Rule of thumb
The Ottawa Ankle Rules are near 100% sensitive for fracture - excellent for ruling out, useless for ruling in. Before ordering any test, ask whether a positive result would change management. If not, do not order it.