Synopsis
Distinguish acute repairable meniscal injury and a genuinely locked knee from common degenerative tears, use radiographs and MRI selectively, begin rehabilitation early, and reserve arthroscopy for a defined mechanical target.
- Clarify mechanism, timing of swelling, joint-line pain, catching and whether the knee is truly fixed or merely painful to extend.
- Examine effusion, range, joint lines, McMurray or Thessaly only when tolerated, ligament stability, patella, hip and distal neurovascular status; no single manoeuvre is definitive.
- Obtain plain radiographs after trauma or when osteoarthritis is plausible. MRI is the preferred soft-tissue study when a repairable acute tear or locked knee will change management.
Key red flags
A fixed inability to extend rather than pain-limited movement after a twist suggests a displaced tear and requires urgent specialist review.
Investigation priorities
Separate fixed locking, traumatic target and degenerative pain.
Management branches
The knee has a persistent fixed extension block after a plausible injury.
- Protect the knee, document range, effusion, ligament and neurovascular findings and obtain trauma radiographs.
- Refer urgently to a soft-tissue knee clinician and arrange expedited MRI if it does not delay definitive care.
Key medicines
Short-course NSAID when suitableUse one licensed oral NSAID at the lowest effective adult dose for the shortest period needed to support movement and rehabilitation, or a licensed topical NSAID when local treatment is sufficient.
Paracetamol for brief rescueIf suitable, use paracetamol 500–1,000 mg orally up to every 4–6 hours as needed, maximum 4 g in 24 hours for a healthy adult, with a lower maximum in low body weight, frailty, liver disease, malnutrition or hazardous alcohol use.