INTANGIBLE ASSETS

The gross carrying amounts and net book values of the Company’s intangible assets were as follows (in thousands):



	December 31, 2019											December 31, 2018
Product rights	Remaining Useful Life (In years)	Gross Carrying Amount		Accumulated Amortization			Impairment			Net Book Value		Gross Carrying Amount		Accumulated Amortization			Net Book Value
NUCYNTA	6.0	$	1,019,978	$	(455,192	)	$	(189,790	)	$	374,996	$	1,019,978	$	(360,891	)	$	659,087
CAMBIA	4.0	51,360		(31,027		)	—			20,333		51,360		(25,891		)	25,469
Zipsor	2.2	27,250		(22,044		)	—			5,206		27,250		(19,707		)	7,543
		$	1,098,588	$	(508,263	)	(189,790		)	$	400,535	$	1,098,588	$	(406,489	)	$	692,099

Amortization expense was $101.8 million, $101.8 million and $102.7 million for the years ended December 31, 2019, 2018 and 2017, respectively.

The Company evaluates the intangible assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. At December 31, 2019 the Company determined there were indicators of impairment present related to the NUCYNTA intangible asset based on current unfavorable commercial outlook resulting in a downward revision to the expected future cash flows from the NUCYNTA franchise. As a result, we recognized an impairment loss of $189.8 million on the NUCYNTA intangible asset to reduce the carrying value of $564.8 million to its estimated fair value of $375.0 million at December 31, 2019. The evaluation of fair value was determined under ASC 820, Fair Value Measurement (ASC 820) as the price that would be received to sell the asset in an orderly transaction between market participants at the measurement date of December 31, 2019. The fair value was based on a combination of an income approach and the observable transaction price from Collegium’s purchase of the NUCYTNA franchise in February 2020. The income approach consisted of the present value of future cash flows that a market participant would expect to receive from holding the asset in its current use. This included assumptions of a market participant’s view such as, but not limited to, future product net sales, related operating expenses, competitive landscape, and a discount rate to reflect the risk inherent in the future cash flows.

The Company expects future amortization expense, inclusive of the effect of the sale of the NUCYNTA asset on February 13, 2020, to be as follows (in thousands):



Year Ending December 31,	Estimated Amortization Expense
2020	$	13,400
2021	7,473
2022	5,668
2023	4,925
Thereafter	—
Total	$	31,466