Slanted stirrups have been around for a long time.
The English saddle company Stubben has had them for over a hundred
The premise is very simple. In a traditional
stirrup, when you turn the fender 90° to the horse, the outside of
the stirrup is raised. This places your foot at about a 30° angle to
the horse, misaligning the bones in your knee and ankle. The outside
of my slanted stirrup is longer than the inside, so when you turn
that fender, the stirrup is parallel to the ground, relieving the
pressure and straightening your knees and ankles. So simple, but so
effective. I make them in a lot of types.