One of the common problems we see as dentists is cracked teeth. Any tooth can fracture, just like driving a truck over pavement enough times can crack the pavement, but the most commo reason is from the weaknesses caused by drilling and filling teeth. Large amalgam fillings are like a lump of soft lead in a crystal glass, slowly creeping and expanding over the years until the tooth cracks apart.
While most cracks are horizontal and a cusp cracks of the tooth, a large number are vertical cracks extending into the nerve of the tooth and splitting the root in half.
It is best to replace any metal fillings if any sign of cracking is evident, but replacement with tooth coloured fillings may not be strong enough.
The tooth should be supported with a full coverage of porcelain, or the best alternative which is gold. The porcelain has a higher strength than plastic fillings but can still crack. The gold will wear at the same rate as your own teeth and will not break but there is a cosmetic compromise.
Talk to your dentist about which option is most suitable for you.