Operating Expenses
Selling, General and Administrative Expense. Selling, general and administrative ("SG&A") expenses increased $12.8 million, or 14.7%, for the three-month period ended September 30, 2024 compared to the corresponding period of 2023. As a percentage of sales, SG&A expenses were 29.3% for the three-month period ended September 30, 2024, compared to 27.6% for the corresponding period of 2023. For the three-month period ended September 30, 2024, SG&A expenses increased compared to the corresponding period of 2023, primarily due to an increase in labor related costs associated with headcount additions and employee termination benefits in connection with the integration activities for the EGS Acquisition, increased consulting and legal costs associated with acquisition due diligence, increased advertising and promotional expenses.
SG&A expenses increased $10.7 million, or 3.9%, for the nine-month period ended September 30, 2024 compared to the corresponding period of 2023. As a percentage of sales, SG&A expenses were 28.8% for the nine-month period ended September 30, 2024, compared to 29.8% for the corresponding period of 2023. For the nine-month period ended September 30, 2024, SG&A expenses increased compared to the corresponding period of 2023 primarily due to an increase in labor-related costs in our sales and marketing operations due to increased headcount to support growth and acquisitions, an increase of variable compensation linked to company performance, an increase of stock-based compensation expense associated with new equity grants, and an increased investment in advertising and promotional expenses.
Research and Development Expenses. Research and development (”R&D”) expenses for the three-month period ended September 30, 2024 were $20.5 million, up 4.5%, when compared to R&D expenses in the corresponding period of 2023 of $19.6 million. For the three-month period ended September 30, 2024, R&D expenses increased compared to the corresponding period of 2023 primarily due to increased labor costs due to increased headcount and increased materials for projects, offset partially by decreased regulatory costs related to clinical studies.
R&D expenses for the nine-month period ended September 30, 2024 were $62.3 million, up 1.9%, when compared to R&D expenses in the corresponding period of 2023 of $61.1 million. For the nine-month period ended September 30, 2024, R&D expenses increased compared to the corresponding period of 2023 primarily due to increased labor costs due to increased headcount, increased materials for projects, and increased costs related to clinical studies, offset partially by lower regulatory costs related to implementation of the Medical Device Regulation in the E.U.
Impairment Charges. For the three and nine-month periods ended September 30, 2024, we recognized no impairment charges. For the three-month period ended September 30, 2023, we recognized no impairment charges. For the nine-month period ended September 30, 2023, we recorded impairment charges of $270 thousand due to the acquisition and subsequent write-off of our equity investment in Bluegrass.
Contingent Consideration Expense. For the three and nine-month periods ended September 30, 2024, we recognized contingent consideration expense from changes in the estimated fair value of our contingent consideration obligations stemming from our previously disclosed business acquisitions of $0.1 million and $0.3 million, respectively, compared to contingent consideration expense of $0.6 million and $2.2 million for the three and nine-month periods ended September 30, 2023, respectively. Expense in each period related to changes in the probability and timing of achieving certain revenue and operational milestones, as well as expense for the passage of time.
Acquired In-process Research and Development. For the three and nine-month periods ended September 30, 2024, we recognized no acquired in-process research and development costs. For the three-month period ended September 30, 2023, we recognized no acquired in-process research and development costs. For the nine-month period ended September 30, 2023, we recognized $1.6 million in acquired in-process research and development costs primarily associated with the assets we acquired from Advanced Radiation Therapy, LLC (“ART”) on May 1, 2023.