Subsequent events - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions						1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 09, 2018	Mar. 08, 2018	Mar. 30, 2017	Mar. 22, 2017	Feb. 28, 2018	Mar. 31, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Exercise price of warrants per share					$ 2.00
Warrant expiration period					5 years			5 years
Series A Convertible Preferred Stock								5,000
Forecast | Amended Credit Facility With Mid Cap
Subsequent Event [Line Items]
Minimum liquidity covenant amount	$ 5.0						$ 5.0
Maximum | Amended Credit Facility With Mid Cap
Subsequent Event [Line Items]
Effective date of financial covenants of Amended Credit Facility				2018-04
Maximum | Forecast | Amended Credit Facility With Mid Cap
Subsequent Event [Line Items]
Effective date of financial covenants of Amended Credit Facility	2019-04
Minimum | Amended Credit Facility With Mid Cap
Subsequent Event [Line Items]
Effective date of financial covenants of Amended Credit Facility				2017-04
Minimum | Forecast | Amended Credit Facility With Mid Cap
Subsequent Event [Line Items]
Effective date of financial covenants of Amended Credit Facility	2018-04
Collaborative Arrangement | Elite Medical Holdings and Pac 3 Surgical | Forecast
Subsequent Event [Line Items]
Gain on contract termination and settlement							$ 6.3
Common Stock
Subsequent Event [Line Items]
Shares issued for acquisition of intangible assets, shares								285,000
Number of warrants issued								7,763,582
Exercise price of warrants per share								$ 2.00
Newly created stock					1,809,628
Common Stock | Maximum
Subsequent Event [Line Items]
Number of warrants issued					9,432,000
Subsequent Event
Subsequent Event [Line Items]
Proceeds from private placement			$ 39.7
Subsequent Event | Amended Credit Facility With Mid Cap | Globus facility agreement
Subsequent Event [Line Items]
Minimum liquidity covenant amount			$ 5.0
Subsequent Event | Maximum | Amended Credit Facility With Mid Cap | Globus facility agreement
Subsequent Event [Line Items]
Effective date of financial covenants of Amended Credit Facility			2019-04
Subsequent Event | Minimum | Amended Credit Facility With Mid Cap | Globus facility agreement
Subsequent Event [Line Items]
Effective date of financial covenants of Amended Credit Facility			2018-04
Subsequent Event | Collaborative Arrangement | Elite Medical Holdings and Pac 3 Surgical
Subsequent Event [Line Items]
Payments of settlement						$ 0.4
Subsequent Event | First Close of Private Placement
Subsequent Event [Line Items]
Proceeds from private placement			$ 39.7
Subsequent Event | Private Placement
Subsequent Event [Line Items]
Exercise price of warrants per share			$ 3.50
Proceeds from private placement			$ 45.2
Subsequent Event | Private Placement | Series B Convertible Preferred Stock
Subsequent Event [Line Items]
Newly created stock			14.3
Subsequent Event | Private Placement | Director and Executive Officers
Subsequent Event [Line Items]
Convertible note convertible to common stock			2,100,000
Number of warrants issued			1,700,000
Exercise price of warrants per share			$ 3.50
Shares issued price per share			$ 3.15
Preferred stock convertible to common stock			2,100,000
Subsequent Event | Private Placement | Director and Executive Officers | Series B Convertible Preferred Stock
Subsequent Event [Line Items]
Series A Convertible Preferred Stock			6,400,000
Subsequent Event | Private Placement | Armistice Capital Agreement
Subsequent Event [Line Items]
Number of warrants issued			1,800,000
Exercise price of warrants per share			$ 3.50
Warrants exercised			2,400,000
Proceeds from exercise of warrant			$ 4.8
Warrant expiration period			5 years
Subsequent Event | Second Close of Private Placement
Subsequent Event [Line Items]
Expected maximum period to close private placement			5 days
Subsequent Event | Common Stock | Private Placement | Maximum
Subsequent Event [Line Items]
Number of warrants issued			12,200,000
Subsequent Event | Common Stock | Private Placement | Series B Convertible Preferred Stock
Subsequent Event [Line Items]
Number of Common Shares Issued Pursuant to Conversion of Series A Convertible Preferred Stock			14,300,000
Shares issued price per share			$ 3.15
Subsequent Event | SafeOp Surgical, Inc.
Subsequent Event [Line Items]
Business acquisition date		Mar. 09, 2018
Cash payment for acquisition from net proceeds of private placement		$ 15.0
Convertible notes issued		$ 3.0
Convertible note convertible to common stock		931,667
Number of warrants issued		2,200,000
Exercise price of warrants per share		$ 3.50
Additional shares of common stock issuable subject to performance milestones		1,330,263
Preferred stock convertible to common stock		931,667
Subsequent Event | SafeOp Surgical, Inc. | Common Stock
Subsequent Event [Line Items]
Shares issued for acquisition of intangible assets, shares		3,265,132