Hello and welcome to HCM,
Difficulty in straightening of the leg has many reasons. Infectious arthritis
causes leads to restricted movement of the limb. However, infectious arthritis or septic arthritis
is associated with pain and fever
In your daughters case, there is no pain and no bony abnormality on X-ray.
This restriction in movement can be due to a soft tissue
This could be due to tight tendons or ligaments.
These details can be determined on MRI only.
You can go for MRI to determine the cause of resctricted movement.
Thanks and take care.
Dr Shailja P Wahal