Promotion of thrush in contracted heels
When a hoof becomes too narrow in the heels, thrush can develop more easily in the frog. The frog and the lamina underneath become constricted and the sulci can become deep and narrow which creates a breeding ground for thrush-causing bacteria. Aside from proper stall/paddock hygiene the solution lies in restoring a physiologically shaped hoof.
The pictures, taken 5 months apart, illustrate this.