Bench knee is a structural fault of the horse’s front legs. Bench, or offset, knees are characteristics of a horse with cannon bones set too far to the outside of the knee. In other words, the cannon bone is not centered symmetrically to the knee. This conformational fault increases the possibility for horses to develop splints. Here is an eXtension article that talks about conformation: Relating Form to Function: Horse’s Frontlegs, Front View.
The medial splint bone is under greater stress than normal, and medial splints are common. In bench knees, there is even more direct weight bearing on the medial splint bone, which in turn carries more weight to the interosseous ligament, increasing the possibility of splints.
Unfortunately, there is no trimming or shoeing that you can do to correct a horse with benched knees.