An immediate threat to any limb gets you there.
Septic joint, acute infection, compartment syndrome (acute, not exertional), lupus/RA exacerbation, acute open fracture that walks in your door instead of ER, herniated disc with inability to walk or acute weakness. Sarcoma. There are dozens of examples.
In ortho, these should be uncommon, but not rare. I generally only code 5's by time, or if I'm basically sending the patient directly to the ER.
But frankly, it is at the discretion of the surgeon - they need to understand that, if they're inflating it, it may be audited, clawed back, or worse. The ultimate responsibility is on their shoulders, not yours. The entire E&M guideline revisions were meant to take these determinations out of the hands of coders, auditors, EMR authors, and payors, and make them essentially at the discretion of the practitioner, based on their specialty's understanding of complexity.