In May 2009, the Company’s shareholders approved the 2009 Employee Stock Purchase Plan, under which eligible employees may contribute up to 15

% of their earnings toward the quarterly purchase of the Company’s common stock. The plan makes available 0.9

million shares of the Company’s common stock, which includes the remaining shares available under the 1996 Employee Stock Purchase Plan. As of December 31, 2019, 1.5

million shares have been issued under both the 2009 and 1996 Employee Stock Purchase Plans. Each plan period lasts three months beginning on January 1, April 1, July 1 and October 1 of each year.

The purchase price for each share of stock is the lesser of 90% of the market price on the first day of the plan period or 100% of the market price on the last day of the plan period.

Stock-based compensation expense related to this plan was $1

million for each of the years ended December 31, 2019, 2018 and 2017, respectively.

The Company accounts for stock-based compensation costs in accordance with the accounting standards for stock-based compensation, which require that all share-based payments to employees be recognized in the statements of operations, based on their grant date fair values. The Company recognizes the expense using the straight-line attribution method. The stock-based compensation expense recognized in the consolidated statements of operations is based on awards that ultimately are expected to vest; therefore, the amount of expense has been reduced for estimated forfeitures. Forfeitures are estimated based on historical experience. If actual results differ significantly from these estimates, stock-based compensation expense and the Company’s results of operations could be materially impacted. In addition, if the Company employs different assumptions in the application of these standards, the compensation expense that the Company records in the future periods may differ significantly from what the Company has recorded in the current period.

The consolidated statements of operations for the years ended December 31, 2019, 2018 and 2017 include the following stock-based compensation expense related to stock option awards, restricted stock awards, restricted stock unit awards, performance stock unit awards and the employee stock purchase plan (in thousands):


	2019		2018		2017
Cost of sales	$	2,271	$	2,212	$	3,032
Selling and administrative expenses		30,907		30,443		33,335
Research and development expenses		5,399		4,886		3,069

Total stock-based compensation	$	38,577	$	37,541	$	39,436

During the years ended December 31, 2019, 2018 and 2017, the Company recognized less than $1 million, $1 million and $4 million of expense, respectively, of stock-based compensation related to the modification of certain stock awards upon the retirement of senior executives.

Stock Options

In determining the fair value of the stock options, the Company makes a variety of assumptions and estimates, including volatility measures, expected yields and expected stock option lives. The fair value of each option grant was estimated on the date of grant using the Black-Scholes option pricing model. The Company uses implied volatility on its publicly-traded options as the basis for its estimate of expected volatility. The Company believes that implied volatility is the most appropriate indicator of expected volatility because it is generally reflective of historical volatility and expectations of how future volatility will differ from historical volatility. The expected life assumption for grants is based on historical experience for the population of

non-qualified

stock option exercises. The risk-free interest rate is the yield currently available on U.S. Treasury

zero-coupon

issues with a remaining term approximating the expected term used as the input to the Black-Scholes model. The relevant data used to determine the value of the stock options granted during the year ended December 31, 2019, 2018 and 2017 are as follows:


Options Issued and Significant Assumptions Used to Estimate Option Fair Values	2019			2018			2017
Options issued in thousands		146			321			389
Risk-free interest rate		2.5	%		2.7	%		2.2	%
Expected life in years		5			6			6
Expected volatility		24.5	%		25.3	%		22.7	%
Expected dividends		—			—			—


Weighted-Average Exercise Price and Fair Value of Options on the Date of Grant	2019		2018		2017
Exercise price	$	230.37	$	196.78	$	170.24
Fair value	$	61.75	$	59.89	$	45.73

The following table summarizes stock option activity for the plans for the year ended December 31, 2019 (in thousands, except per share data):


	Number of Shares			Exercise Price per Share					Weighted- Average Exercise Price per Share
Outstanding at December 31, 2018		1,790		$	38.09	to	$	208.47	$	142.47
Granted		146		$	183.41	to	$	238.52	$	230.37
Exercised		(406	)	$	38.09	to	$	208.47	$	113.06
Canceled		(75	)	$	113.36	to	$	238.52	$	159.67

Outstanding at December 31, 2019		1,455		$	61.63	to	$	238.52	$	158.61

The following table details the options outstanding at December 31, 2019 by range of exercise prices (in thousands, except per share data):


Exercise Price Range	Number of Shares Outstanding		Weighted- Average Exercise Price		Remaining Contractual Life of Options Outstanding		Number of Shares Exercisable		Weighted- Average Exercise Price
$61.63 to $128.93		529	$	117.10		5.1		430	$	114.85
$128.94 to $192.62		520	$	160.06		7.7		208	$	149.37
$192.63 to $238.52		406	$	210.84		8.2		91	$	196.83

Total		1,455	$	158.61		6.9		729	$	134.94

During 2019, 2018 and 2017, the total intrinsic value of the stock options exercised (i.e., the difference between the market price at exercise and the price paid by the employee to exercise the options) was $45 million,

$44 million

and

$76 million, respectively. The total cash received from the exercise of these stock options

was $46 million,

$45 million

and

$91 million for the years ended December 31, 2019, 2018 and 2017, respectively.

The aggregate intrinsic value of the outstanding stock options at December 31, 2019 was $110 million. Options exercisable at December 31, 2019, 2018 and 2017

were 0.7 million

0.8 million

and

0.9 million, respectively. The weighted-average exercise prices of options exercisable at December 31, 2019, 2018 and 2017

were $134.94,

$117.08

and

$103.63, respectively. The weighted-average remaining contractual life of the exercisable outstanding stock options at December 31, 2019 was 6 years.

The aggregate intrinsic value of stock options exercisable as of December 31, 2019 was $72 million.

At December 31, 2019, the Company had 1.4 million stock options that are vested and expected to vest. The intrinsic value, weighted-average exercise price and remaining contractual life of the vested and expected to vest stock options were $109 million, $158.33 and 6.9 years, respectively, at December 31, 2019.

As of December 31, 2019, there were $32 million of total unrecognized compensation costs related to unvested stock option awards that are expected to vest. These costs are expected to be recognized over a weighted-average period of 3.2 years.

Restricted Stock

During the years ended December 31, 2019, 2018 and 2017, the Company granted

five

thousand

five thousand and

eight

thousand shares of restricted stock, respectively. The weighted-average fair value per share on the grant date of the restricted stock granted in 2019, 2018 and 2017 was $183.41,

$194.73 and

$130.35, respectively. The Company has recorded $1 million of compensation expense in each of the years ended December 31, 2019, 2018 and 2017 related to the restricted stock grants. As of December 31, 2019, the Company had

five

thousand unvested shares of restricted stock outstanding, which have been fully expensed.

Restricted Stock Units

The following table summarizes the unvested restricted stock unit award activity for the year ended December 31, 2019 (in thousands, except per share data):


	Shares			Weighted-Average Grant Dat e Fair Value per Share
Unvested at December 31, 2018		304		$	153.31
Granted		86		$	235.31
Vested		(104	)	$	139.07
Forfeited		(26	)	$	167.60

Unvested at December 31, 2019		260		$	184.70

Restricted stock units are generally granted annually in February and vest in equal annual installments over a five-year period. The amount of compensation costs recognized for the years ended December 31, 2019, 2018 and 2017 on the restricted stock units expected to vest were $14 million, $16 million and $17 million, respectively. As of December 31, 2019, there were $33 million of total unrecognized compensation costs related to the restricted stock unit awards that are expected to vest. These costs are expected to be recognized over a weighted-average period of 3.3 years.

Performance Stock Units

The Company’s performance stock units are equity compensation awards with a market vesting condition based on the Company’s Total Shareholder Return (“TSR”) relative to the TSR of the components of the S&P Health

Care Index. TSR is the change in value of a stock price over time, including the reinvestment of dividends.

The vesting schedule ranges from 0% to 200% of the target shares awarded.

In determining the fair value of the performance stock units, the Company makes a variety of assumptions and estimates, including volatility measures, expected yields and expected terms. The fair value of each performance stock unit grant was estimated on the date of grant using the Monte Carlo simulation model. The Company uses implied volatility on its publicly-traded options as the basis for its estimate of expected volatility. The Company believes that implied volatility is the most appropriate indicator of expected volatility because it is

generally reflective of historical volatility and expectations of how future volatility will differ from historical volatility. The expected life assumption for grants is based on the performance period of the underlying performance stock units. The risk-free interest rate is the yield currently available on U.S. Treasury zero-coupon issues with a remaining term approximating the expected term used as the input to the Monte Carlo simulation model. The correlation coefficient is used to model the way in which each company in the S&P Health Care Index tends to move in relation to each other during the performance period. The relevant data used to determine the value of the performance stock units granted during the year ended December 31, 2019, 2018 and 2017 are as follows:


Performance Stock Units Issued and Significant Assumptions Used to Estimate Fair Values	2019			2018			2017
Performance stock units issued in thousands		13			40			40
Risk-free interest rate		2.4	%		2.4	%		1.6	%
Expected life in years		2.8			3.0			3.0
Expected volatility		23.5	%		22.0	%		20.9	%
Average volatility of peer companies		26.2	%		25.9	%		25.6	%
Correlation Coefficient		34.2	%		35.9	%		37.8	%
Expected dividends		—			—			—

The following table summarizes the unvested performance stock unit award activity for the year ended December 31, 2019 (in thousands, except per share data):


	Shares			Weighted-Average Fair Value per Share
Unvested at December 31, 2018		100		$	212.34
Granted		13		$	372.68
Forfeited		(8	)	$	200.26

Unvested at December 31, 2019		105		$	233.11

The amount of compensation costs recognized for the years ended December 31, 2019

, 2018

and 2017 on the performance stock units expected to vest were $

million

, $5 million

and

less than

million, respectively

. As of Decem

er 31, 2019, there were $

million of total unrecognized compensation costs related to the restricted stock unit awards that are expected to vest. These costs are expected to be recognized over a weighted-average period of

1.9

years.