Knee injuries
A pop, swelling within 2 hours and giving way is an ACL tear until proven otherwise. Use the Ottawa knee rule for X-rays.
A 24-year-old footballer twisted his right knee with his foot planted and felt a "pop". Within an hour the knee was hugely swollen. He can't bear weight. Lachman's test is hard to do because of the swelling and pain.
A 68-year-old man fell onto his bent knee. He can't lift his straight leg off the bed, and there's a gap you can feel above the kneecap.
What does each likely have, and who needs urgent orthopaedics?
- Neurovascular checkFoot pulses and sensation. After a knee dislocation the popliteal artery is often injured.
- Can they straight-leg raise?No: extensor mechanism rupture or patellar fracture.
- Ottawa knee ruleDecides the X-ray.
- Timing of swellingWithin 2 hours means blood (ACL, fracture, patellar dislocation). Overnight means fluid (meniscus).
- Ligament testsLachman (ACL), posterior drawer (PCL), valgus and varus stress (collaterals).
- Rest, ice, compression, elevationAnalgesia, crutches, walk as able.
- Early quadriceps exercisesPrevent wasting.
- Knee clinicSuspected ACL or meniscus injury within 1–2 weeks.
- Knee dislocation or absent foot pulses
- Can't straight-leg raise
- Locked knee
- Open injury or fracture
- Hot swollen knee with fever (septic arthritis)
- Tibial plateau fracture
| Ottawa knee X-ray | Age 55 or over, isolated patella tenderness, fibular head tenderness, can't flex to 90°, or can't walk 4 steps immediately and in the department |
| Haemarthrosis | Swelling within 2 hours: ACL tear in most cases |
| Effusion overnight | Meniscal tear more likely |
| Locked knee | Can't fully straighten: bucket-handle meniscal tear, urgent orthopaedics |
| Knee dislocation | Check pulses and ABPI, vascular injury is common even if pulses are present |
| Paracetamol | 1 g 4 times daily |
| Ibuprofen | 400 mg 3 times daily with food |
Check every dose against a current drug reference and your hospital protocol.
On this page
History
- Mechanism: twisting with the foot planted (ACL, meniscus), direct blow to the side (collateral), dashboard injury (PCL), fall onto a bent knee (patella, extensor), high energy (dislocation, plateau fracture).
- "Pop" at the time of injury: ACL.
- Swelling timing: within 2 hours (blood) or overnight (fluid).
- Locking (can't straighten), catching, giving way.
- Previous injuries, age, sport, job.
- No injury but hot swollen knee: septic arthritis, gout.
Examination
- LookEffusion, bruising, deformity, walk if able.
- FeelPatella, fibular head, joint lines, tibial tuberosity, quadriceps and patellar tendons (gap?).
- MoveFlexion to 90°? Full extension (locked)? Straight-leg raise.
- LigamentsLachman, anterior and posterior drawer, valgus and varus at 0° and 30°.
- Nerves and pulsesDorsalis pedis, posterior tibial, common peroneal nerve (foot dorsiflexion, sensation on the top of the foot).
Ottawa knee rule
X-ray AP and lateral
- No X-ray
- treat the soft-tissue injury
What is the injury?
| Injury | Story | Finding |
|---|---|---|
| ACL tear | Twist with planted foot, pop, rapid swelling, giving way | Positive Lachman, haemarthrosis |
| Meniscal tear | Twist while weight-bearing, slower swelling | Joint line tenderness, locking or catching |
| MCL sprain | Blow to the outer knee | Pain and laxity on valgus stress |
| LCL sprain | Blow to the inner knee | Pain on varus stress; check the peroneal nerve |
| PCL tear | Dashboard injury, fall onto the tibia | Posterior sag, posterior drawer |
| Patellar dislocation | Twist, kneecap moved out (often back in by arrival) | Medial tenderness, apprehension |
| Extensor rupture | Fall onto bent knee, older or on steroids | Can't straight-leg raise, palpable gap |
| Tibial plateau fracture | High energy or older with osteoporosis | Haemarthrosis, X-ray |
| Knee dislocation | High energy, grossly unstable | Vascular and nerve injury risk |
The timing of swelling often tells you more than the examination: within 2 hours means blood (ACL tear, fracture, patellar dislocation); overnight means fluid (meniscus).
Knee dislocation
A true knee dislocation (tibia on femur, not the kneecap) is a limb-threatening emergency. The popliteal artery is injured in many, and the knee may have reduced before arrival. Check pulses, measure the ankle-brachial pressure index, and get vascular and orthopaedic review urgently. Check the common peroneal nerve.
Treatment
- Soft tissue injury: rest, ice, compression, elevation for 48–72 hours, crutches, walk as able. Start quadriceps exercises early (straight-leg raises, static quads).
- Suspected ACL or meniscal injury: knee or fracture clinic within 1–2 weeks; MRI.
- Locked knee: urgent orthopaedics.
- Extensor mechanism rupture: knee splint in extension, orthopaedics (surgical repair, ideally early).
- Patellar dislocation: reduce by straightening the knee with gentle pressure; X-ray after; splint and physiotherapy.
- Fractures: orthopaedics or fracture clinic.
Common mistakes
- Not checking foot pulses after a high-energy knee injury.
- Missing an extensor rupture because nobody asked the patient to lift the straight leg.
- Calling a locked knee a sprain.
- X-raying every knee instead of using the Ottawa rule (or skipping it when the rule says yes).
- Forgetting the hip in a child with knee pain.
- Treating a hot swollen knee without injury as trauma.
Frequently asked questions
When does a knee injury need an X-ray?
Using the Ottawa knee rule: age 55 or over, isolated tenderness of the patella, fibular head tenderness, inability to flex to 90°, or inability to walk 4 steps immediately and in the department.
What does rapid knee swelling after injury mean?
Bleeding into the joint (haemarthrosis), most often from an ACL tear, a fracture or a patellar dislocation.
What is a locked knee?
A knee that can't fully straighten after injury, usually from a displaced (bucket-handle) meniscal tear. It needs urgent orthopaedic review.
Why is a knee dislocation dangerous?
The popliteal artery is often injured even when pulses seem present. It needs urgent vascular assessment.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A footballer's knee swells hugely within an hour after a twisting injury. What is the most likely cause?
Answer: ACL tear. Rapid haemarthrosis is most often from an ACL tear.
- Osgood-Schlatter: No.
- Patellar tendinitis: No.
- Meniscal tear: Swelling is slower.
- MCL sprain: Little effusion.
2. A patient cannot straight-leg raise after a fall onto a bent knee. What is suspected?
Answer: Extensor rupture. Inability to straight-leg raise suggests quadriceps or patellar tendon rupture or patellar fracture.
- Baker's cyst: No.
- ACL tear: Can usually straight-leg raise.
- Osteoarthritis: No.
- Meniscal tear: Can usually straight-leg raise.
3. Which is part of the Ottawa knee rule for X-ray?
Answer: Can't flex to 90°. Criteria: age 55 or over, isolated patella tenderness, fibular head tenderness, can't flex to 90°, can't walk 4 steps.
- Clicking: No.
- Pain at the joint line: Not a criterion.
- Age over 40: It is 55 or over.
- Any swelling: Not a criterion.
4. In the popliteal fossa, which structure lies deepest?
Answer: Popliteal artery. From superficial to deep in the midline: tibial nerve, popliteal vein, popliteal artery (deepest, lying on the joint capsule). That is why the pulse is hard to feel and why the artery is injured in knee dislocation and supracondylar femur fractures. The common fibular nerve runs laterally along biceps femoris.
- Popliteal vein: Middle layer.
- Tibial nerve: Most superficial of the three.
- Common fibular nerve: Lateral, along biceps femoris.
- Small saphenous vein: Superficial; joins the popliteal vein.
5. What forms the boundaries of the popliteal fossa, and what is its deepest content?
Answer: Hamstrings, gastrocnemius; artery deepest. Popliteal fossa: superolateral biceps femoris, superomedial semimembranosus and semitendinosus, inferior the two heads of gastrocnemius. Contents superficial to deep: tibial and common fibular nerves, popliteal vein, popliteal artery (deepest, closest to the femur — injured in supracondylar fractures and knee dislocation). Popliteal aneurysm is the commonest peripheral aneurysm.
- Sartorius, adductor longus; nerve deepest: Femoral triangle.
- Gluteus maximus, piriformis; sciatic deepest: Gluteal region.
- Tibialis anterior, EDL; artery deepest: Anterior leg.
- Soleus and plantaris; tibial nerve deepest: Not the boundaries.
References and dose sources
- NICE CKS — Knee pain: assessment
- Ottawa knee rule
- Paracetamol dosing: Drugs.com: Acetaminophen (paracetamol) dosage (see also our paracetamol dose page)
- Ibuprofen dosing: Drugs.com: Ibuprofen dosage (see also our ibuprofen dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.