4.    GOODWILL AND OTHER INTANGIBLE ASSETS

Goodwill

The following table summarizes the changes in goodwill for the nine months ended September 30, 2021:

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Gross goodwill and accumulated impairment losses was $73,812 and $7,585, respectively, at September 30, 2021 and $73,899 and $7,585, respectively, at December 31, 2020.

Intangible Assets

Intangible assets such as certain customer relationships and patents on core technologies and product technologies are amortizable over their estimated useful lives. Certain trade names and trademarks which provide exclusive and perpetual rights to manufacture and sell their respective products are deemed indefinite-lived and are therefore not subject to amortization.

Intangible assets consisted of the following as of September 30, 2021 and December 31, 2020:

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The Company recorded amortization expense of $2,174 and $2,203 for the three months ended September 30, 2021 and 2020, respectively, of which $198 and $290 was included in cost of goods sold in the consolidated statements of operations and comprehensive (loss) income for the respective periods. Amortization expense for the nine months ended September 30, 2021 and 2020 was $6,538 and $7,047, respectively, of which $596 and $1,090 was included in cost of goods sold in the consolidated statements of operations and comprehensive (loss) income for the respective periods.

The estimated amortization expense for finite-lived intangible assets for the remaining three months of 2021, the next four years and thereafter is as follows:

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