A crown or an onlay? The difference?

All dental students are trained to do crowns which cover all 5 surfaces of the tooth. A good deal of skill is involved in designing and preparing crowns. Onlays are different in that they only cover part of the tooth.  To design and prepare an onlay requires a good deal more designing skill.  When done properly, either one will bring the tooth back to 100% of its original strength.  The reason an onlay is better is that good tooth structure is saved.  The original tooth structure is superior to any man made material and so if we can save any tooth structure, the patient is far better off.  Crowns and onlays can be made out of gold, all porcelain,porcelain fused to gold, or plastics.  We use Cerec Technology to fabricate porcelain bonded restorations.  Bonding is superior to cementing and metal does not bond well to tooth structure.  Crowns or onlays are done to replace large mercury fillings, resin fillings, or to repair fractured teeth.