Northwell Health has acquired a 70-percent stake in Suffolk Surgery Center, an outpatient practice in Shirley.

Suffolk Surgery Center specializes in ophthalmologic, orthopedic, gastroenterology, podiatric, pain management and colorectal surgeries. Northwell didn’t disclose the terms of the deal in its announcement late Monday.

Suffolk Surgery Center is the seventh joint-venture surgery center that Northwell has partnered with as it expands its outpatient business. The New Hyde Park-based health system, which is the largest private employer in the state, said it plans to open nearly 20 additional ambulatory surgical centers throughout New York, including one in Syosset.

Northwell Health also operates ambulatory surgery centers in New Hyde Park, Manhasset and Staten Island.

Separately, it also has agreed to buy 51 percent of the Melville Surgery Center on Walt Whitman Road, Robert Scoskie, managing director at Northwell Ventures, the health system’s investment arm, said in an interview.

Scoskie said the Melville purchase is waiting for state approval.

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Ambulatory health care service is growing on Long Island. Employment in the field grew 5 percent in the last year to 90,000, according to the state Department of Labor. Ambulatory centers are growing in popularity as health systems look to move patients away from more expensive hospital stays.

Suffolk Surgery Center has 40 surgeons credentialed to perform surgery there and handles about 7,000 patients annually.

The 8,500-square-foot facility at 1500 William Floyd Pkwy. has four operating rooms.

Suffolk Surgery Center has 9 physician owners and a staff of about 40 including nurses, administrators, receptionists and surgical technicians, Scoskie said.