The markets have been on a roll this year, as the S&P 500's (SNPINDEX:^GSPC) picked up more than 18% year to date. The health-care sector's moved right along with the index in 2013, as many leading medical stocks have seen shares roar higher and reward investors with stellar gains.

Not every stock's joined the party, however. A small but select group of laggards hasn't been able to break into the green in 2013 -- and three members of the S&P have left investors shaking their heads. In the following video, Motley Fool contributor Dan Carroll takes you through the index's three worst medical stocks of the year and examines whether this trio of laggards can turn things around in the year's second half.