Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 4,770	$ 3,465
Less: accumulated depreciation and amortization	(2,545)	(2,636)
Property and equipment, net	2,225	829
Furniture and Fixtures and Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	129	131
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	163
Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	266	226
Capitalized Surgical Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	3,733	2,652
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	150	150
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	191	168
Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 138	$ 138