Knock-kneed, or close-kneed, horses have the entire knee set to the inside of the straight line from the chest to the toe.
This condition is usually accompanied by toed-out feet and some degree of outward rotation of the cannon bone.
As the horse moves and its legs strike the ground, there is unequal distribution of concussion on the column of bone, predisposing the horse to unsoundness.
For more information, see Relating Form to Function: Horse's Frontlegs, Front View

