The first X-ray was taken while the patient was laying down on the X-ray table with no weight through the knee. The joint space on the inside of the knee appears pretty thick (dark space between the bones highlighted in green).
However, to assess the true joint space and cartilage thickness, you must get an X-ray while standing, putting full body weight through the joint. Only then is the true joint space revealed.
The 2nd X-ray was taken of the same patient on the same day while standing. The true joint space is severely compromised with bone-on-bone appearance outlined in red.