This is a typical case of osteochondritis dissecans. For our case, the diagnosis could have been made by a simple X-ray of the knee, however, the role of MRI is more important in other less severe / obvious cases.

An MRI image is required to grade the stability and severity of an osteochondral injury and is used to plan management.

Since the bony fragment is displaced and the donor defect is well seen and filled with fluid, but no advanced osteoarthritic changes has took place, this case is stage IV.