Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 34,756	$ 45,986	$ 121,780	$ 137,154
United States
Disaggregation of Revenue [Line Items]
Revenue	25,699	30,877	85,060	93,672
International
Disaggregation of Revenue [Line Items]
Revenue	9,057	15,109	36,720	43,482
Implant-based
Disaggregation of Revenue [Line Items]
Revenue	30,070	36,041	98,748	108,738
Implant-based | United States
Disaggregation of Revenue [Line Items]
Revenue	25,194	29,949	82,480	91,934
Implant-based | International
Disaggregation of Revenue [Line Items]
Revenue	4,876	6,092	16,268	16,804
Shipment-based
Disaggregation of Revenue [Line Items]
Revenue	4,686	9,945	23,032	28,416
Shipment-based | United States
Disaggregation of Revenue [Line Items]
Revenue	505	928	2,580	1,738
Shipment-based | International
Disaggregation of Revenue [Line Items]
Revenue	$ 4,181	$ 9,017	$ 20,452	$ 26,678