In a year when the Standard & Poor's 500 Index reached new records on a regular basis, the best-performing stock is a relatively small provider of dental products.

Align Technology Inc., the maker of the clear teeth-straightener Invisalign, is on pace to become the index's winner this year with a 136 percent increase as of Dec. 28. With estimated revenue of $1.49 billion in 2017, the San Jose, California-based company's sales have almost doubled in just three years as popularity of its smile-correcting products grew. It remains tiny within the S&P 500, where average annual sales are $22 billion.

The drivers of growth: American teens, and China. Wall Street is betting Align has more room to grow in 2018, especially after the shares declined this month: 11 of the 12 analysts covering the stock recommend buying it, with one advising to hold it. Their average 12-month price target is $273.73, based on data compiled by Bloomberg, which would be a 21 percent increase from current levels.

Good health

In a testament to how good a year 2017 was for some health-care stocks, four health companies are among the S&P 500's 20 biggest gains, three of them businesses with less than $3 billion in annual revenue.