Our gross profit as a percentage of sales increased to 44.9% for the six-month period ended June 30, 2022, compared to 44.6% for the six-month period ended June 30, 2021. The increase in gross profit percentage was primarily due to changes in product mix, lower standard costs from efficiencies gained in our foundations for growth program and lower intangible amortization expense as a percentage of sales, offset partially by unfavorable variances primarily from the impact of inflationary pressures on material costs and higher freight costs.
Operating Expenses
Selling, General and Administrative Expense. Selling, general and administrative ("SG&A") expenses decreased ($6.1) million, or (6.6)%, for the three-month period ended June 30, 2022 compared to the corresponding period of 2021. As a percentage of sales, SG&A expenses were 29.0% for the three-month period ended June 30, 2022, compared to 32.7% for the corresponding period of 2021. For the three-month period ended June 30, 2022, SG&A expenses decreased compared to the corresponding period of 2021 primarily due to $6.1 million of contract termination costs recorded in SG&A during the three-month period ended June 30, 2021 to renegotiate certain terms of an acquisition agreement.
SG&A expenses decreased ($3.1) million, or (1.8)%, for the six-month period ended June 30, 2022 compared to the corresponding period of 2021. As a percentage of sales, SG&A expenses were 29.7% for the six-month period ended June 30, 2022, compared to 32.6% for the corresponding period of 2021. For the six-month period ended June 30, 2022, SG&A expenses decreased compared to the corresponding period of 2021 primarily due to $6.1 million of contract termination costs recorded in SG&A during the three-month period ended June 30, 2021 to renegotiate certain terms of an acquisition agreement and $4.4 million decrease in acquisition related costs, partially offset by increased labor related costs associated with headcount.
Research and Development Expenses. Research and development ("R&D") expenses for the three-month period ended June 30, 2022 were $18.5 million, up 5.0%, when compared to R&D expenses in the corresponding period of 2021 of $17.6 million. R&D expenses for the six-month period ended June 30, 2022 were $35.9 million, up 5.9%, when compared to R&D expenses in the corresponding period of 2021 of $33.9 million. The increases in R&D expenses for the three and six-month periods ended June 30, 2022 compared to the corresponding periods in 2021 were largely due to higher labor-related costs, increased clinical expenses for certain R&D projects (including clinical trials for our Embosphere® Microspheres and WRAPSODYTM Endoprosthesis) and higher expenses related to implementation of the Medical Device Regulation in the European Union.
Impairment Charges. For the three-month period ended June 30, 2022, we recorded no impairment charges. For the three-month period ended June 30, 2021, we recorded impairment charges of $4.3 million. These impairments included $1.6 million of intangible assets and $1.3 million of property and equipment due to the planned discontinuance of the Advocate™ Peripheral Angioplasty Balloon product line, sold under our license agreements with ArraVasc, and $1.4 million of impairments of certain right-of-use “ROU” operating lease assets due to site consolidation decisions and changes in our projected cash flows for the underlying assets.
For the six-month period ended June 30, 2022, we recorded impairment charges of $1.7 million of intangible assets due to the divestiture of the STD Pharmaceutical business, which we completed on April 30, 2022. For the six-month period ended June 30, 2021 we recorded $4.3 million of impairment charges, as described above.
Contingent Consideration Expense. For the three and six-month periods ended June 30, 2022, 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 $1.2 million and $3.8 million, respectively, compared to contingent consideration expense of $1.8 million and $2.2 million for the three and six-month periods ended June 30, 2021. 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 six-month periods ended June 30, 2022, we recognized $6.7 million in acquired in-process research and development costs primarily associated with our acquisition of Restore Endosystems. We did not incur in-process research and development charges during the three and six-month periods ended June 30, 2021.