HOOF-pastern axis (HPA) refers to the arrangement of the equine digits, which are located below the fetlock. It is divided into pastern axis and hoof axis.

A line is taken from the dorsal aspect of the pastern and continues down the dorsal surface of the hoof, there is another line taken from the centre of the fetlock joint that dissects the pastern and coffin joint. If these lines are parallel, the correct HPA angulation occurs (Fig 1).