Mako had its biggest one-day decline since the shares began trading in February 2008 after the company cut its sales forecast for the RIO robotic surgical arm system for the second time in two months. Mako estimated 42 to 48 systems will be sold this year. On May 7, Mako had cut the forecast to 52 to 58 from as much as 62. The stock is down 57 percent this year.