HPI:

• 23 y.o. male with acute left knee pain.
• Patient reports  a sharp pain and popping sensation in his left knee when he planted the lateral aspect of his dependent left foot and used momentum to return to a standing position.
• He was unable to fully extend his left knee immediately following the injury.
• He reports ability to bear weight, but with pain and difficulty. 

PE:

Knee exam: Some edema, no deformity. DP 2+. Decreased range of motion, lateral joint line tender to palpation. No instability or ligament laxity with anterior/ posterior drawer tests. Some pain with valgus maneuvers. Pt unable to extend past 60 degrees from 90 degree flexion, actively or passively.　

DDx:

• ACL tear causing severe edema
• Meniscal tear
• Inability to extend leg suggests partial patellar tendon rupture
• Fracture (e.g., tibial plateau, less likely femoral condylar or patellar)
• Patellar dislocation (ruled out with physical exam)

Workup:

• XR negative
• CT: 1. No acute osseous abnormality of the left knee. 2. Soft tissue infiltration surrounding the anterior cruciate ligament, concerning for tear. Consider follow-up evaluation with MRI knee.
• Pt was observed overnight for ortho consult and urgent MRI
• MRI showed: Anterior posterior cruciate ligaments are intact. The lateral collateral ligament is attenuated, suspicious for partial tear. The medial collateral ligament is intact. The quadriceps and patellar tendons are intact. There is displaced tear of the posterior horn and body of the lateral meniscus. Displaced meniscal fragment is present in the anterior intercondylar notch. The medial meniscus is intact. The articular cartilage is preserved. The bone marrow signal is unremarkable. There is no joint effusion. There is no Baker's cyst.

Pearls:

• Locked knee refers to inability to extend knee
• Causes of locked knee include: meniscus tear (#1), ACL tear, fracture, foreign body, or patellar dislocation
• Can attempt a closed reduction under procedural sedation: With knee flexed to 90 degrees, apply downward traction on the lower leg, then attempt internal and external rotation. 

Bengtzen R. Bengtzen R Bengtzen, Rachel. Knee Injuries. In: Tintinalli JE, Ma O, Yealy DM, Meckler GD, Stapczynski J, Cline DM, Thomas SH. Tintinalli J.E., & Ma O, & Yealy D.M., & Meckler G.D., & Stapczynski J, & Cline D.M., & Thomas S.H.(Eds.),Eds. Judith E. Tintinalli, et al.eds. Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 9e. McGraw-Hill; Accessed July 04, 2020. https://accessemergencymedicine.mhmedical.com/content.aspx?bookid=2353&s...