The heel-to-toe drop is the difference in height between the heel and the toe. There are no exact statistics or evidence that supports the theory that heel to drop has a direct impact on injuries.
It is not an exact science. One may even say that it is down to personal preference.
Some people find that a shoe with a higher drop will have less impact on the foot and ankle. It tends to direct the strain to the knees. A shoe with a lower drop has more impact on the knees.