Acute knee injury
Background
Knee ligaments
- Anterior Cruciate Ligament
- Limits anterior translation of tibia
- 75% of all hemarthroses are caused by disruption of ACL
- Posterior Cruciate Ligament
- Limits posterior translation of tibia
- Isolated injuries are rare
- Medial Collateral Ligament
- Provide restraint against valgus (outward) stress
- Lateral Collateral Ligament
- Provide restraint against varus (inward) stress
- Knee injuries are among the most common musculoskeletal complaints in the ED
- Mechanism of injury guides the differential (valgus stress, hyperextension, twisting, direct impact)
Clinical Features
- Acute pain, swelling, inability to bear weight
- Immediate large effusion (<2 hours) suggests hemarthrosis → ACL tear, fracture, or peripheral meniscal tear
- Locked knee (inability to fully extend) → meniscal tear with displaced fragment
- Giving way/instability → ligamentous injury
- Patellar apprehension → patellar subluxation/dislocation
Key Exam Maneuvers
- Lachman test: Most sensitive for ACL tear (anterior tibial translation at 20-30° flexion)
- Posterior drawer: PCL integrity
- Valgus stress (30°): MCL integrity
- Varus stress (30°): LCL integrity
- McMurray test: Meniscal tear (joint line tenderness is more sensitive)
Differential Diagnosis
Knee diagnoses
Acute knee injury
- Knee dislocation
- Knee fractures
- Meniscus and ligament knee injuries
- Patella dislocation
- Patellar tendonitis
- Patellar tendon rupture
- Quadriceps tendon rupture
Nontraumatic/Subacute
- Arthritis
- Gout and Pseudogout
- Osgood-Schlatter disease
- Patellofemoral syndrome (Runner's Knee)
- Patellar tendonitis (Jumper's knee)
- Pes anserine bursitis
- Popliteal cyst (Bakers cyst)
- Prepatellar bursitis (nonseptic)
- Septic bursitis
- Septic joint
- DVT
Evaluation
Ottawa knee rules
X-ray is only required in patients who have an acute injury and one or more of the following:
- Age >55
- Isolated tenderness of the patella
- Tenderness at the fibular head
- Inability flex to 90 degrees
- Inability to walk 4 steps BOTH immediately after the injury and in the ED
Knee x-rays
- Anteroposterior and lateral views
- Consider sunrise if pain over patella
- X-ray if Ottawa knee rules positive
- Lipohemarthrosis on lateral view (fat-fluid level in suprapatellar bursa) = occult fracture until proven otherwise
- MRI for suspected ligamentous or meniscal injury (usually outpatient)
- Aspiration if large tense effusion causing significant pain: send cell count, culture, crystals
Management
- If x-rays positive (fracture): treat underlying condition, splint, orthopedic consult
- If x-rays negative or not indicated per Ottawa knee rules:
- Perform full ligamentous exam
- Stable exam: RICE (rest, ice, compression, elevation), crutches if unable to bear weight, NSAIDs
- Unstable exam or unable to evaluate (pain/swelling): knee immobilizer + RICE, weight-bearing as tolerated with crutches
Disposition
- Most acute knee injuries are managed as outpatient with orthopedics referral
- ED consult/admit: Knee dislocation (check vascular status — popliteal artery injury), open fractures, septic arthritis
- Urgent orthopedics follow-up (within 1 week) for suspected ACL/meniscal tears, locked knee