Introduction
A crown is judged three times: on the die, on the master cast in the articulator, and finally in the mouth. The same questions apply each time, grouped under the indices used to classify restorations: morphology, function, prevention and esthetics.
Morphologic requirements
- Restores the natural anatomy of the tooth: its outline, cusps, ridges and grooves, scaled to the neighbours and the patient's age.
- Contour follows the natural crest of curvature. An over-contoured (bulky) crown traps plaque and pushes on the gingiva; an under-contoured crown lets food strike the gingival margin.
- Proximal contacts sit at the right height and are as tight as the neighbours' contacts.
Functional requirements
- Occlusion in harmony with the opposing teeth and jaw movements: stable contacts in the intercuspal position, no premature contacts or interferences.
- Forces directed along the long axis of the tooth.
- Strength: thick enough not to fracture or wear through; for metal at least 0.5 mm, for ceramic 1–2 mm.
- Good retention: it stays on under chewing forces.