There are many causes of ingrown toenails, such as the way the nails are cut, trauma, diseases of the nails (for example, fungus), and heredity. If the ingrown toenail is chronic, then it might need a procedure by a podiatrist to remove the nail border and destroy the root (matrix) in the corner to prevent the recurrence. Once this is healed, then cutting the nail straight across is correct. – Dr. Dennis Arnold, head of the International Pedicure Association
Click here to read A Doctor's Notes On Ingrown Toenail Surgery.