The situation that a crown can be re-cemented is when the cement bond fails either to the crown or to the tooth and there has not been any decay or damage to the dental crown. Shaping the tooth for the crown in a way to increase retention can prevent so much reliance on the bond of the cement. Six degrees of taper and a minimum of 3mm wall height is recommended. Some back teeth are very short and that can be difficult to achieve. Often times old crowns over time the cement can wash out leaving space for bacteria to get under it and cause decay.