Nail growth direction revisited. Why do nails grow out instead of up?

In some investigations Kligman has been thought to have shown that the cul-de-sac in which the nail is formed determines that it should grow outward instead of upward. However, results obtained during the course of nail surgery for chronic paronychia and electron microscopic studies, along with genetic evidence, fail to confirm his observation on the effect of the proximal nail fold on nail growth direction. The whole subject of direction of nail growth remains questionable and worthy of further consideration.