Differential Diagnosis for Inability to Straighten Knee with Normal X-Ray

• Single most likely diagnosis
  • Quadriceps or patellar tendon rupture: This condition would prevent the patient from straightening their knee due to the disruption of the extensor mechanism, despite a normal X-ray since it involves soft tissue injury.
• Other Likely diagnoses
  • Meniscal tear: A meniscal tear can cause mechanical symptoms such as locking or catching, which might prevent the knee from fully straightening. An X-ray would not show this injury.
  • Ligamentous sprain: Significant ligamentous injury, especially to the posterior cruciate ligament (PCL), can lead to instability and difficulty in fully extending the knee.
  • Patellofemoral pain syndrome: While typically associated with pain, severe cases could potentially cause mechanical issues due to pain inhibition of the quadriceps muscle.
• Do Not Miss diagnoses
  • Septic arthritis: Although less common, this condition is a medical emergency. The inability to move a joint due to pain and swelling, along with systemic symptoms like fever, could indicate septic arthritis, which would not be visible on an X-ray.
  • Osteonecrosis: Early stages might not be visible on X-ray, and if the patient is unable to straighten their knee due to pain, this diagnosis should be considered, especially in high-risk populations.
• Rare diagnoses
  • Tumor or cyst: A soft tissue tumor or a cyst (like a synovial cyst) could mechanically obstruct knee movement, although this would be less common.
  • Neurological conditions: Certain neurological conditions affecting the lower limbs could result in weakness or paralysis of the muscles responsible for knee extension, such as a peripheral nerve injury or a spinal cord lesion.