Knee injuries

1. The 2-Minute Kinematics

• Knee Dislocations: Require tremendous ligamentous disruption resulting from high-velocity mechanisms (motor vehicle crashes, pedestrian struck) or low-velocity mechanisms (martial arts, trampoline falls, or spontaneously in morbidly obese patients). Anterior dislocations are most common (40%) and typically follow high-energy hyperextension. Posterior dislocations account for 33% of cases.
• Tibial Plateau Fractures: Result from valgus or varus forces combined with an axial load that drives the femoral condyle into the tibia, classically seen when a pedestrian's leg is struck by a car bumper.
• Patellar and Extensor Mechanism Injuries: Patellar fractures arise from a direct blow (e.g., dashboard injury, fall) or a forceful contraction of the quadriceps muscle. Sudden forceful quad contraction against a flexed knee can also cause complete avulsion of the tibial tuberosity.

2. The Bedside Action Plan

• Knee Dislocation Reduction: This is an emergent priority. Under procedural sedation, apply longitudinal traction-countertraction. Lateral pressure may be required, and for anterior dislocations, the femur is pushed posteriorly while the tibia is pulled anteriorly.
• Immobilization: Following a dislocation reduction, the limb must be immobilized in a long leg posterior splint with the knee in 15 to 20 degrees of flexion, ensuring access for serial neurovascular assessments.
• Patellar Dislocation Reduction: Reduce a laterally dislocated patella by applying gentle knee extension combined with medially directed (valgus) pressure on the displaced patella.
• The Locked Knee: Most commonly caused by a torn meniscus, a locked knee cannot actively or passively extend. Under procedural sedation, position the leg hanging over the table edge with the knee at 90 degrees of flexion, then apply longitudinal traction combined with internal and external rotation (or repeated McMurray maneuvers) to unlock the joint.
• Analgesia: An ultrasound-guided femoral nerve block is a highly valuable, underutilized nonopiate adjunct that provides excellent analgesia while avoiding systemic hypotension in trauma patients.

3. The Diagnostic Grid

• Targeted Physical Exam: An inability to perform a straight leg raise is a hallmark physical exam finding of a displaced transverse patella fracture or a rupture of the quadriceps/patellar tendon.
• Mandatory Vascular Checks: Popliteal artery injury is the major cause of morbidity and limb loss. Hard signs (absent pedal pulses, cool mottled foot, expanding popliteal hematoma, popliteal hemorrhage) mandate emergent surgical exploration or angiography in the OR. Soft signs (asymmetric pulses, dorsolateral paresthesias) mandate a CT angiogram (CTA) or duplex ultrasound.
• The ABI Pathway: If pulses are palpable and symmetric, measure the Ankle-Brachial Index (ABI). An ABI > 0.9 over a 24-hour observation period with serial checks every 3 to 4 hours effectively excludes significant popliteal injury. An ABI < 0.9 requires a CTA. Palpable pulses alone do not rule out arterial injury.

4. The Visual Board

• Explicit X-ray Interpretation: Standard trauma knee imaging requires Anteroposterior (AP) and lateral views, plus a "sunrise" view (essential for evaluating patellar fractures). Tunnel views (intercondylar notch) detect tibial spine fractures and loose bodies, while oblique views help identify subtle tibial plateau fractures.
• Occult Fractures: Tibial plateau fractures may be radiographically occult on plain films. If a patient is unable to bear weight after a knee injury and no fracture is seen on plain radiographs, advanced imaging (CT scan) must be obtained.

5. The Classification Matrix

• Ottawa Knee Rules: Radiography is indicated if the patient meets any of the following: Age >55, isolated patellar tenderness, isolated fibular head tenderness, flexion <90 degrees, or inability to bear weight for 4 steps (98.5% sensitivity).
• Pittsburgh Knee Rules: Radiography is indicated for a fall or blunt trauma mechanism PLUS either Age <12 or >50, OR inability to walk 4 weight-bearing steps in the ED (100% sensitivity).
• Gustilo-Anderson Classification: Used for open fractures. Type I (<1 cm), Type II (1-10 cm), Type IIIA (extensive soft tissue stripping), Type IIIB (periosteal stripping), Type IIIC (major vascular injury present). Guides antibiotic prophylaxis (e.g., adding an aminoglycoside for Type II/III).

6. The Danger Zone

• The Spontaneous Reduction Trap: Up to 50% of knee dislocations spontaneously reduce before ED arrival. A severely injured knee that is unstable in multiple directions must be assumed to be a spontaneously reduced dislocation, carrying the exact same catastrophic risk of popliteal artery injury.
• Irreducible Dislocations: Posterolateral knee dislocations may be completely irreducible via closed methods because the medial femoral condyle and medial collateral ligament (MCL) trap the joint, mandating emergent open reduction in the operating room.
• Referred Pediatric Pain: Missing a slipped capital femoral epiphysis (SCFE) because an adolescent patient strictly complains of knee pain. Always obtain frog-leg lateral hip radiographs in children with atraumatic knee pain.
• The Ischemic Clock: Popliteal artery injuries associated with knee dislocations must be identified and repaired within 8 hours, otherwise amputation may become necessary.

7. Mcq Masterclass

• "Inability to straight leg raise" = Displaced transverse patella fracture or rupture of the extensor mechanism (quad/patellar tendon).
• "Impaired dorsiflexion and decreased sensation over the first dorsal web space" = Common peroneal nerve injury, highly associated with knee dislocations.
• "Bumper injury" = Tibial plateau fracture resulting from valgus/varus force combined with an axial load.
• "Locked knee" = Meniscal tear preventing full extension.
• "Obese 11-year-old boy with atraumatic knee pain" = SCFE (referred pain from the hip).

8. The Boardroom Script

"I am consulting you for a [Patient Age]-year-old [Sex] who sustained a [Mechanism, e.g., high-speed MVC dashboard injury], resulting in a [Direction, e.g., anterior] knee dislocation. We performed a closed reduction under procedural sedation and the knee is currently immobilized in a posterior splint at 15 degrees of flexion. The patient currently lacks hard signs of vascular injury, but the Ankle-Brachial Index is [Value, e.g., 0.8], constituting a soft sign. A CT Angiogram has been ordered. The patient is prepped for admission for serial neurovascular checks and your definitive evaluation."