Ep 175 Emergency Orthopedics Differential: SCARED OF Mnemonic – When X-rays Lie

In this main episode podcast, Dr. Arun Sayal creator of the CASTED course and Dr. Yatin Chadha a radiologist with a fellowship in MSK radiology, join Anton for Part 1 of 2 podcasts on Emergency Orthopedic Injuries. This episode focuses on a differential diagnosis of MSK injuries that are occult to X-ray with the help of the SCARED OF mnemonic. It ensures we pick up all the “can’t miss” diagnoses that can be easily overlooked when we do not integrate a pointed history and physical exam with the X-ray in front of us. Essentially, we discuss ‘when X-rays lie’ and offer up a variety of clinical pearls and pitfalls in assessing patients in the ED with MSK presentations…

Podcast production, sound design & editing by Anton Helman; voice editing by Braedon Paul

Written Summary and blog post by Kate Dillon & Ian Beamish, edited by Anton Helman October, 2022

Cite this podcast as: Helman, A. Chadha, Y. Sayal, A. Emergency Orthopedics Differential: SCARED OF Mnemonic. Emergency Medicine Cases. October, 2022. https://emergencymedicinecases.com/orthopedics-differential-scared-of-mnemonic. Accessed May 30, 2024

Résumés EM CasesThe occult MSK X-ray: A normal X-ray does not preclude serious pathology

Just as a normal ECG does not rule out ACS, a normal x-ray does not rule out all serious MSK pathology. We tend to over-rely on x-rays to diagnose orthopedic injuries, sometimes overlooking key historical and physical exam data points that should inform our pre-test probability. As in all ED presentations, we should apply Bayesian analysis to orthopedic cases.

Using history to inform our pre-test probability for MSK pathology

Important aspects in history taking for MSK presentations that may influence pre-test probability:

* Age (certain fractures tend to occur in specific age ranges e.g. scaphoid fractures have peak incidence age 20-29)

* Traumatic/atraumatic mechanism

* Previous injuries

* Past medical history (e.g. cancer/autoimmune disease/other immunocompromising disease, osteoporosis)

* Medications (e.g. steroids/immunosuppressants - risk factors for septic arthritis, medications for osteoporosis)

Dr. Sayal's SCARED OF mnemonic: An approach to orthopedic differential diagnosis

In the presence of a normal (or near-normal) X-ray, what should we be “scared of”?

1. Septic arthritis

For patients who present with non-traumatic MSK pain, infection including septic arthritis/bursitis, osteomyelitis and discitis and should be near the top of the differential diagnosis.

The accuracy (or inaccuracy) of lab tests for septic arthritis

The likelihood ratios for serum tests are variable across studies and do not appear to be very predictive, especially in immunocompromised patients. In one review "no cutoff for ESR or CRP significantly increases or decreases the post-test probability of septic arthritis."

* ESR > 20mm/hr +LR 0.84, -LR 2.4

* ESR > 30mm/hr +LR 1.3, -LR 0.17

* ESR > 50mm/hr +LR 1.4, -LR 0.4

* ESR >100mm/hr +LR 7.0, -LR 0.6

* CRP >10 mg/L +LR 1.1-1.4, -LR 0.3-0.6

* CRP >100mg/L +LR 1.1-2.8, -LR 0.3-0.6

Synovial fluid tests likelihood ratios

Without testing synovial lactate levels, synovial fluid analysis had an overall  sensitivity of only 70-80% which drops to 50% when antibiotics are administered prior...