Aortic valve replacement is necessary when the aortic valve has become diseased. The aortic valve can suffer from insufficiency (inability to perform adequately) or stenosis.

the aortic valve
The Aortic valve
The aortic semilunar valve lies between the left ventricle and the aorta.

The heart pumps blood from the left ventricle through the aortic semilunar valve into the Aorta.

The blood flows through the aorta after exiting the left ventricle through the aortic valve.

because the aortic valve is not functioning properly, i.e. prolapse