A patient presents with a small amalgam restoration in the occlusal sulcus. Clinical examination reveals extensive secondary caries undermining the cusps, with insufficient remaining tooth structure for a simple restoration. What is the most appropriate treatment?

When a tooth has extensive caries that undermines the cusps and leaves insufficient sound tooth structure to support a direct restoration, a full-coverage crown (whether all-ceramic, PFM, or gold) is indicated to protect the weakened cusps from fracture and restore proper occlusal function. Simply replacing the filling with a larger composite restoration would leave structurally compromised cusps at high risk of fracture. Extraction is premature if the tooth is restorable with a crown. Monitoring is inappropriate when active caries is present. The 'crown threshold' is generally reached when caries or existing restorations cover more than half the intercuspal distance.