Patents and other intangible assets are recorded at cost, or when acquired as a part of a business combination, at estimated fair value. These assets are amortized on a straight-line basis over the useful lives of the assets.

		December 31,
(U.S. Dollars, in thousands)	Weighted Average Amortization Period	2018			2017
Cost
Patents	10 years	$	39,085		$	38,621
Developed technology	10 years		12,400			—
IPR&D	Indefinite		26,800			—
License and other	7 years		14,654			10,276
Trademarks—finite lived	9 years		840			533
	9 years		93,779			49,430
Accumulated amortization
Patents		$	(35,016	)	$	(34,151	)
Developed technology			(827	)		—
IPR&D			—			—
License and other			(5,744	)		(4,625	)
Trademarks—finite lived			(295	)		(193	)
			(41,882	)		(38,969	)
Patents and other intangible assets, net		$	51,897		$	10,461

Intangible assets related to IPR&D projects are considered to be indefinite-lived until the completion or abandonment of the associated research and development efforts. During the period the assets are considered indefinite-lived, they will not be amortized but will be tested for impairment. Impairment testing is performed at least annually or when a triggering event occurs that could indicate a potential impairment. If and when development is complete, which generally occurs when regulatory approval to market a product is obtained, the associated assets are deemed finite-lived and are amortized over a period that best reflects the economic benefits provided by these assets.

In February 2019, the Company obtained FDA approval of the M6-C artificial cervical disc. As such, amortization of the IPR&D intangible asset attributable to these research and development activities will commence starting in February 2019, for which the Company expects to use a 10 year amortization period.

Amortization expense for intangible assets was $2.7 million, $1.8 million and $1.8 million for the years ended December 31, 2018, 2017 and 2016, respectively. Future amortization expense for intangible assets, including IPR&D, is estimated as follows:

(U.S. Dollars, in thousands)	Amortization
2019	$	6,604
2020		6,205
2021		6,146
2022		6,138
2023		5,495
Thereafter		21,309
Total	$	51,897