A dental crown functions as a cap placed over a damaged tooth to restore its shape, size, strength, and appearance. Molars, the large teeth located in the back of the mouth, are the primary workhorses of the chewing system, withstanding immense forces during biting and grinding. These posterior teeth require crowns that prioritize extreme durability and fracture resistance over purely aesthetic concerns. The ideal molar crown must handle the high mechanical stresses of chewing without breaking or wearing down prematurely.
Primary Materials Used for Molar Crowns
Full metal crowns, often referred to as gold crowns, are made from alloys that combine gold with other metals like palladium, silver, or base metals such as nickel and chromium. This material has historically been the standard for posterior restorations because of its unmatched strength and ability to resist fracture. The metal is highly biocompatible and requires minimal reduction of the underlying tooth structure for placement.
Porcelain-fused-to-metal (PFM) crowns represent a traditional compromise, offering a balance between strength and appearance. They consist of a metal substructure, which provides mechanical strength, covered by a layer of tooth-colored porcelain. The metal core prevents catastrophic failure, making it a reliable option for molars, but the porcelain veneer can occasionally chip or fracture under heavy load.
Solid Zirconia crowns have emerged as a modern standard, made entirely from zirconium dioxide, a polycrystalline ceramic material. Monolithic zirconia is milled from a solid block and is known for its remarkable strength. This high-strength ceramic offers an aesthetic benefit compared to metal while maintaining durability suitable for the back of the mouth.
High-strength all-ceramics, such as lithium disilicate (often branded as E-Max), are glass-ceramic materials that balance moderate strength with superior light-handling properties. While they are stronger than traditional porcelain, they are reserved for premolars or situations where aesthetics are a significant concern on a molar. They are not recommended for the most posterior molars or patients with severe grinding habits.
Performance Factors Critical to Posterior Durability
The ability to withstand heavy chewing forces without breaking is measured by compressive strength and fracture resistance. Full metal crowns, especially gold alloys, have exceptional fracture toughness, flexing slightly under pressure rather than fracturing. Solid zirconia also exhibits high resistance to fracture, making it a reliable choice for the high-stress environment of the molar.
The ceramic layer on PFM crowns is the weakest point. Metal-ceramic crowns showed a higher mean compressive strength before chipping than veneered zirconia crowns. Monolithic zirconia avoids this weakness by being a single, uniform material, which makes it less likely to crack or split under high bite forces.
The crown material’s wear effect on the opposing natural tooth structure is also important. Gold alloys and other metals are gentle on the opposing enamel because of their smooth surface and lower hardness compared to ceramics. Conversely, the high hardness of ceramic materials, including zirconia and the porcelain used in PFM crowns, can cause increased wear on the enamel of the opposing teeth.
Achieving a precise marginal fit and seal is important for the long-term functional durability of any crown, preventing micro-leakage and recurrent decay. Gold crowns have a long clinical track record and can last for decades, with a reported survival rate of over 95% at 10 years. Zirconia crowns also show comparable longevity, often lasting 10 to 15 years or more, with their fracture resistance approaching that of metal alloys.
Matching Crown Type to Patient Needs
For maximum strength and longevity, particularly in the rearmost molars or for patients who clench or grind their teeth, full metal or solid zirconia crowns are the strongest recommendations. Full gold remains the traditional standard for its predictable performance and gentleness on opposing teeth, while monolithic zirconia provides comparable strength with a tooth-colored appearance.
When budget is a major consideration, or when a balance of function and moderate aesthetics is desired, the Porcelain-Fused-to-Metal (PFM) crown is often selected. PFM crowns offer a metal core for stability and a tooth-colored exterior, though the porcelain is susceptible to chipping, and a metal margin may become visible over time as gums recede.
When aesthetics are a higher priority, solid zirconia or high-strength all-ceramics like lithium disilicate are preferred. Solid zirconia is the go-to option for strength and aesthetics in the posterior mouth, though a high-translucency version may sacrifice a small amount of strength for a more natural look.