Dec 31 (Reuters) - Stryker Corp said it would buy Patient Safety Technologies Inc for $120 million, adding to its portfolio a device that reduces the risk of surgical sponges being left in patients after surgery.

The medical device maker said it would pay $2.22 per share for Patient Safety, a 50 percent premium to the stock's closing price on Monday.

Patient Safety shares rose to $2.20 on Tuesday morning.

The device, SurgiCount Safety-Sponge System, consists of a mobile computer that tracks bar-coded sponges used in surgeries.

The deal is expected to close in the first quarter of 2014.

Stryker said the device would be added to its instruments division, which includes surgical equipment and surgical navigation systems.

BofA Merrill Lynch was Patient Safety's financial adviser and Latham & Watkins LLP was its legal counsel.

Shares of Patient Safety, which has a market capitalization of $57.5 million, were at $2.19 in early trading.

Stryker shares were little changed at $75.18 on the New York Stock Exchange.