An undermined hoof wall is a separation between layers of the hoof wall, most frequently of the inner-most layer (between the stratum internum and the stratum medium) of the hoof wall. The sensitive lamina remain covered by a thin layer of horn. The ensuing cavity is usually filled with a crumbling type of horn caused by bacterial and/or fungal digestion of the tubular and lamellar horn. The lesions may extend from the bearing surface up to the coronary band.

A disease affecting the horn in the region of the white line. The horn in the white line is generally softer than the horn in the adjacent hoof wall and it may be additionally weakened by diseases or disorders of the hoof such as laminitis. These factors contribute to the increased potential for the horn of the white line to become damaged due to mechanical factors (e.g., small stones that become lodged in the horn) or invading micro-organisms (e.g., bacteria or fungi) that lead to disintegration of the horn. White line disease is a progressive disorder that can lead to damage in the stratum internum or the innermost layers of the stratum medium of the hoof wall proximally to the white line. Damage can become extensive and cavities can be formed within the hoof wall. This results in an undermined hoof wall, a condition very similar to and often confused with seedy toe.