TIME’S INEXORABLE passage is the subject of my main column. On a related note, of importance to dentists everywhere: How long do we expect our restorations to last? As a student, I was told that every restoration should last 10, 15 or in some cases 20 years. That was long ago — are we still setting ourselves up for failure with outsize expectations?

The closest medical analogy I can think of is hip and knee replacements. The latest data indicate that orthopedists have a 90 to 95 percent chance that the new joint will last 10 years, and 80 to 85 percent that it’ll be good two decades. Insurance companies tell dentists a crown should last five years; some allow for seven.

How easy is it to put a lifespan on restorations? We wouldn’t expect posterior composites to last as long as anterior composites. Studies show that the most common reason patients need their composites replaced is recurrent caries, followed by (for posterior composites) resin fracture. There are two main factors: the dentist him- or herself, and the patient’s risk profile. As dentists, we need to focus on our adhesive-placement techniques, light curing, ending mar- gins on enamel and — that ticking clock again — minimizing placement time if possible.

Patient risk factors fall into two primary categories: caries risk and re- current caries at gingival margins. This isn’t surprising, given that the gingival margin risk for caries spurred the restoration in the first place. For a look at how restorations age and why they fail, click the QR code at right to view my latest presentation.