Urgent Care RAP
Urgent Care RAP
Hippo Education LLC.,
Knee X-rays
16 minutes Posted Oct 16, 2020 at 3:31 pm.
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Arun Sayal, MD runs through a thoughtful approach to the knee exam with Neda Frayha, MD, Matthieu DeClerck, MD. One that includes the Mnemonic “SLR-CDEF” as a reminder of what diagnosis we should think about with every knee injury. The mnemonic stands for Septic knee, Locked knee, Referred pain, Compartment syndrome, Dislocation (spontaneously reduced), Extensor mechanism disruption (Over 40, look over the patella – i.e., a quadriceps tear. Under 40 look under the patella  – i.e., a patellar tendon rupture), Fracture (radiographically occult …


  • History of injury and patient age often will narrow the differential for post-traumatic knee pain to a short list of possible diagnoses. 
  • Knee dislocations can occur in obese patients with very minor trauma and often spontaneously reduce resulting in ‘normal’ x-rays.
  • Adding oblique x-ray views improves the sensitivity in the diagnosis of tibial plateau fracture.
  • Radiologists ability to read films in a helpful way is highly influenced by the completeness of the history and the differential diagnoses of concern provided to them.  


  • History will often reveal the source of knee pain after trauma, as a full and careful exam is not generally possible due to pain, swelling, and spasm. 
    • For example, patients with an ACL tear will describe four classic historical features: 
      • Deceleration mechanism
      • Swelling within 1 hour
      • Sensation of a “pop” or shift at the knee joint
      • Inability to return to play
    • Patients with a meniscal tear will describe a twisting mechanism.
      • The force of twisting required to tear the meniscus decreases with age.  
        • Elderly patients can simply tear their meniscus by standing up.
    • Valgus (ie: knee bending inward) stress tends to cause different injuries depending on the age of the patient.
      • Salter-Harris Femur and/or Proximal fibula in adolescents
      • MCL injury in younger patients (ie: 20-30 years old)
      • Lateral tibial plateau fractures in older patients (ie: >50 years)
  • Examining patients before reviewing their x-rays will help to determine what to suspect clinically and look for radiographically. 
    • Other advantages of performing a history and physical prior to ordering x-rays include recognizing that additional views may be helpful and providing a more complete history for the radiologist interpreting the films. 
  • A mnemonic for x-ray ‘negative’ injuries of the knee that can prove useful is SLR-CDEF.
    • S - Septic joint
    • L - Locked knee (ie: when the knee cannot be fully extended) from meniscal injury
    • R - Referred pain (e.g. hip pathology) due to Obturator nerve irritation
      • Knee pain which is not reproduced when ranging the knee is suggestive of a referred source of pain.  
    • C - Compartment syndrome 
      • This can easily excluded by palpating the compartments and, when in doubt, comparing the firmness to the contralateral, uninjured side.
    • D - Dislocation (ie: at least 3 of 4 collateral ligaments have been disrupted)
      • Instability is the key finding indicating that a knee dislocation has likely occurred.
      • Knee dislocations commonly will spontaneously reduce, however, even if reduced, patients are at high risk of popliteal artery injury and subsequent ischemia/amputation. 
      • In very obese patients, knee dislocation can occur with minimal force (e.g. stepping off a curb).
    • E - Extensor Disruption (e.g. Patellar fracture, Patellar tendon rupture, and/or Quadriceps tendon rupture)
      • Patient will be unable to extend their knee/lower leg fully against gravity.
      • Patellar tendon rupture generally occurs in younger patients with high mechanism injury.
      • Quadriceps tendon rupture is more often spontaneous or low mechanism in older patients. 
    • F - Fracture (occult) 
      • The most common occult fracture of the knee is a lateral tibial plateau fracture.
        • Oblique knee films can allow for better examination for tibial plateau fractures.
      • Segond fractures involve the tibial spine. 
      • Osteochondral fragments from the patella can be seen radiographically after certain injuries in adolescent athletes, commonly gymnasts, and should be suspected if when there’s significant knee swelling.
      • Osteochondritis desicans is caused by a twisting mechanism (similar to medial meniscal injury) in adolescents.