A tried and true comment on insider transactions is that insiders sell for a wide variety of reasons (among them, diversification, desire for cash to fund major purchases, etc.) but buy for primarily one reason (to profit from the buy). Greg Lucier is buying NUVA extensively.
Notice what happened last time he bought extensively, around 5/7/14 - 5/14/14: SInce then, according to Yahoo Finance thru 6/8/15, NUVA is up 45.17%, SPY is up 10.86% and the health care sector as per XLV is up 28.71%. Best wishes with NUVA investing.