
	Number of Shares	Weighted- Average Exercise Price
Outstanding as of December 31, 2021	2,009,513	$8.73

Exercised	(39,267)	5.09
Canceled and forfeited	(14,229)	12.36
Outstanding as of June 30, 2022	1,956,017	10.22

As of June 30, 2022, the unrecognized compensation cost related to stock options was $0.7 million, which is expected to be recognized over a period of approximately 0.5 years.

Early exercises of stock options under the Company's 2008 Stock Option Plan are subject to a right of repurchase by the Company of any unvested shares. The repurchase rights lapse over the original vesting period of the options. The Company accounts for the cash received in consideration for the early exercised options as a liability included in accrued liabilities, which is then reclassified to stockholders’ equity as the options vest. As of June 30, 2022 and December 31, 2021, the Company had no shares subject to repurchase.

Restricted stock units (“RSUs”) are share awards that entitle the holder to receive freely tradable shares of the Company’s common stock upon vesting. RSUs generally vest over two to four years based upon continued services and are settled at vesting in shares of the Company's common stock. The grant date fair value of the RSUs is equal to the closing price of the Company’s common stock on the grant date.

In January 2022, the Company granted performance-based restricted stock unit awards subject to market and service vesting conditions to certain executive officers under SI-BONE's 2018 Equity Incentive Plan (“PSUs”). The shares subject to the PSUs vest over a three-year performance period beginning January 1, 2022 and ending December 31, 2024. The actual number of PSUs that will vest in each measurement period will be determined by the Compensation Committee based on the Company’s total shareholder return (“TSR”) relative to the TSR of the Median Peer Companies (as defined in the award agreement). The grant date fair value of each stock award with a market condition was determined using the Monte Carlo valuation model. The table below summarizes the assumptions used to estimate the grant date fair value of the PSUs granted:


	Six Months Ended June 30, 2022
Expected volatility of common stock				48.9%	to	58.7%
Expected volatility of peer companies						24.2%	to	152.5%
Correlation coefficient of peer companies				(0.13)	to	1.00
Risk-free interest rate						0.4%	to	1.2%
Dividend yield	—%	to	1.0%


	RSUs		PSUs
	Number of Shares	Weighted Average Grant Date Fair Value	Number of Shares	Weighted Average Grant Date Fair Value
Outstanding as of December 31, 2021	1,566,522	$25.17	—	$—
Granted	1,130,467	21.30	155,596	19.50
Vested	(395,489)	24.88	—	—
Canceled and forfeited	(129,123)	23.81	—	—
Outstanding as of June 30, 2022	2,172,377	23.29	155,596	19.50

As of June 30, 2022, the unrecognized compensation cost related to the RSUs was $42.2 million, which is expected to be recognized over a period of approximately 2.8 years. As of June 30, 2022, the unrecognized compensation cost related to the PSUs was $2.3 million, which is expected to be recognized over a period of approximately 2.5 years.

The Company’s 2018 Employee Stock Purchase Plan (the “ESPP”) allows eligible employees to purchase shares of the Company's common stock through payroll deductions at the price equal to 85% of the lesser of the fair market value of the stock as of the first date or the ending date of each six month offering period. The offering period generally commences in May and November. On March 26, 2020, the Company's Compensation Committee approved the amendment of the terms of future offerings under the ESPP which, among other things, increased the maximum number of shares that may be purchased on any single purchase date, provided for automatic enrollment in a new offering, and provided that the offering which commenced in May 2020 be twelve months in duration and consist of two purchase periods.

The fair value of the ESPP shares is estimated using the Black-Scholes option pricing model, which is being amortized over the requisite service period. The Company issued 112,773 and 104,861 shares under ESPP, representing approximately $1.6 million in employee contributions, for the six months ended June 30, 2022 and 2021. As of June 30, 2022 and December 31, 2021, total accumulated ESPP related employee payroll deductions amounted to $0.2 million and $0.3 million, respectively, which were included within accrued compensation and related expenses in the condensed consolidated balance sheets.


	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
	(in thousands)
Cost of goods sold	$	117	$	129	$	238	$	303
Sales and marketing	2,746		2,125		5,340		4,024
Research and development	664		404		1,297		823
General and administrative	2,224		1,599		4,383		3,137
	$	5,751	$	4,257	$	11,258	$	8,287