While most nails generally stay smooth and intact, there are a number of conditions that can cause the nail plate to break, chip, crack, or otherwise split. This causes a poor cosmetic appearance, but it can also lead to pain and discomfort in some cases.
The most common and most well known cause of nail cracking and splitting is infection of a fungus. Nail fungus infection creates crumbling and splitting of nail material as the fungus breaks down the keratin material that the nail plate is made of. As the nail breaks down, it becomes discolored, chipped, and breaks down into crumbly, spongy pieces. The nail can even loosen off the skin base, and may partially or completely pull off with minimal effort or injury.
Another common cause of nail breakage is injury. The injury can come from something heaving dropping on or hitting the foot, or from kicking the toe into something solid. Runner’s nails can also develop these changes over time due to repeated bruising and damage from the force on the toes during running. As the force of all these injuries transfers to the fragile nail root cells at the base of the nail that grow the nail out, permanent damage can occur. The result is a nail that has a disruption in its growth pattern, which takes the form of a lengthwise split in the nail plate as it grows out, or a weakening of the nail plate as it grows longer, resulting in chipping or cracking near the end of the nail. The nail can also lift loose from the underlying skin as it grows out, or it can grow crooked, thickened, or inward.
Tight shoes can similarly damage nail root cells as a crush or kicking injury, only the damage takes place slowly over time. The persistent pressure on the nail root cells from the tight shoe gradually damages the cells to the point that they do not function properly, and the nail that grows from this is abnormal.
Finally, certain medical conditions can cause nail plate cracking, splitting, and chipping. These include such diverse skin conditions as psoriasis, autoimmune diseases like lupus, and some medical treatments like chemotherapy.