By Jon Kamp Of DOW JONES NEWSWIRES NuVasive Inc.'s (NUVA) second-quarter earnings more than doubled behind surging sales as the maker of devices for spinal-fusion surgery continued to shake off challenging market conditions seen by competitors. The San Diego-based company boosted its full-year earnings guidance Tuesday, raised the lower end of its sales forecast and maintained its outlook for sales growth next year. Shares rose 2.8%, to $35.00 in after-hours trading. Reports last week from Johnson & Johnson (JNJ) and Stryker Corp. (SYK) added to signs the $8 billion to $9 billion spinal-device market is facing pressure on product prices and procedure volumes that is restraining growth. But NuVasive, which markets tools for a unique procedure the company says cuts blood loss and hospital stays, has managed to maintain its high-growth pace by taking market share. NuVasive posted second-quarter earnings of $6.7 million, or 17 cents per share, up from $2.8 million, or 7 cents, a year ago. Excluding stock-based compensation and other costs, it said earnings were 42 cents in the recent quarter. Sales rose 35%, to $119.6 million. Analysts surveyed by Thomson Reuters had forecast, on average, earnings of 29 cents a share on sales of $117.7 million. Looking ahead, the company said it now expects full-year sales of $485 million to $500 million, with the bottom end of that range up $5 million from the prior forecast. It forecast earnings excluding items of $1.50 to $1.64 per share, compared with the prior $1.13 to $1.25 per share forecast. Analysts had forecast full-year earnings of $1.21 per share on sales of $490.3 million. Alex Lukianov, NuVasive's chairman and chief executive, said on a conference call that the company continues to expect sales growth of 25% to 30% next year. He also said, as he has before, that product innovations could fuel faster growth. -By Jon Kamp, Dow Jones Newswires; 617-654-6728; jon.kamp@dowjones.com