CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (37,145)	$ (41,660)	$ (39,216)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	26,785	29,212	24,940
Provision for inventory reserve	4,371	5,572	1,680
Provision for allowance for doubtful accounts	347	68	319
Stock-based compensation	5,932	6,956	11,188
Accretion of discounts and amortization of issuance costs of convertible senior notes	2,358	1,604	0
Deferred income taxes	(1,657)	(33)	0
Other	(14)	0	0
Changes in operating assets and liabilities:
Accounts receivable	(2,834)	(9,381)	(5,082)
Inventory	(6,965)	(3,439)	(8,766)
Prepaid expenses and other assets	(9,741)	(10,256)	(9,738)
Accounts payable, accrued expenses, and accrued payroll liabilities	5,750	8,059	6,365
Net cash used in operating activities	(12,813)	(13,298)	(18,310)
Investing activities
Purchase of surgical instruments	(11,066)	(12,275)	(10,905)
Purchase of property, plant and equipment	(3,920)	(17,439)	(2,787)
Changes in cash restricted for leasehold improvements	61	6,608	0
Purchase of intangible assets	(2,958)	(1,307)	(588)
Net cash used in investing activities	(17,883)	(24,413)	(14,280)
Financing activities
Borrowings on bank line of credit	0	19,500	25,000
Payments on bank line of credit	0	(19,500)	(25,000)
Proceeds from issuance of convertible senior notes, net of issuance costs	0	47,108	0
Proceeds from issuances of common stock, net of issuance costs	0	0	54,209
Principal payments under capital lease	(973)	(219)	0
Issuances and exercise of stock-based compensation benefit plans, net of income tax	9,397	2,244	2,017
Net cash provided by financing activities	8,424	49,133	56,226
Effect of exchange rate changes on cash and cash equivalents	725	(557)	(401)
Net change in cash and cash equivalents	(21,547)	10,865	23,235
Cash and cash equivalents at beginning of period	45,511	34,646	11,411
Cash and cash equivalents at end of period	23,964	45,511	34,646
Significant non-cash investing and financing activities
Buildings under capital lease	0	0	26,469
Leasehold improvements, including property under capital lease	0	171	6,884
Additions to property, plant and equipment	250	0	0
Capital lease obligation	0	1,708	33,938
Accretion of discount on convertible senior notes	2,281	807	0
Common stock offering costs	0	0	52
Cash paid for:
Income taxes	168	159	126
Interest	$ 2,231	$ 382	$ 428