On March 5, 2018, we amended our senior credit facility to, among other things, extend the maturity date of the $25.0 million revolving credit facility, with a $5.0 million letter of credit sub-facility, to March 5, 2020, and provide for a $20.0 million term loan. We incurred $9.0 million in term loan borrowings to refinance $7.9 million of term loan borrowings and $1.1 million of equipment loans and have $11.0 million of term loan borrowings available for general corporate purposes. On August 17, 2018, we amended our senior credit facility, to provide for a $4.0 million equipment advance which we used solely to purchase equipment.

On April 26, 2019, we amended our senior credit facility to, among other things, extend the maturity date of the $25.0 million revolving credit facility, with a $5.0 million letter of credit sub-facility, to May 1, 2021 and refinance the existing term loan and equipment advance with a new $23.5 million term loan. The term loan will amortize over 36 months, beginning on May 1, 2020, following an initial 12-month interest-only period, and mature on April 1, 2023. We may prepay the term loan at any time, subject to a prepayment fee equal to 2.0% of the outstanding principal amount if prepaid on or before April 26, 2020, or 1.0% of the outstanding principal amount if prepaid after April 26, 2020, but on or before April 26, 2021. There were no changes to the term loan interest rates or financial covenants as a result of this amendment.

The loans accrue interest, at our option, at (1) a LIBOR rate determined in accordance with the senior credit facility, plus a margin ranging from 2.75% to 3.50%, or (2) a prime rate determined in accordance with the senior credit facility, plus a margin ranging from 0.00% to 0.75%. The margin percentage is determined based upon whether the outstanding borrowing is a term loan or revolving loan and our adjusted EBITDA for the preceding 12-month period. Interest is due and payable in arrears monthly for prime rate loans and at the end of an interest period for LIBOR rate loans.

The senior credit facility includes financial and operating covenants, including a minimum liquidity ratio and maximum adjusted EBITDA loss threshold that both apply in the event we do not maintain a minimum liquidity threshold, which are set forth in detail in the credit facility agreement. We were in compliance with the covenants under our senior credit facility as of September 30, 2019 and December 31, 2018. Our obligations under the senior credit facility are collateralized by substantially all our assets other than intellectual property.

At September 30, 2019, $23.5 million of term loan borrowings were outstanding, excluding unamortized debt issuance costs of $117,000, and no revolver borrowings were outstanding. Term loan interest rates were 5.75%. The following table presents the scheduled principal maturities as of September 30, 2019 (in thousands):

2019	$	—
2020		5,222
2021		7,833
2022		7,833
2023		2,612
Total	$	23,500