State health officials in Rhode Island were investigating how a surgeon operated on the wrong knee of a patient Friday at the Miriam Hospital, part of a medical network that was reprimanded and fined $50,000 last year for three mistaken surgeries at another of its facilities.

The latest mistake happened while a surgeon performed a knee arthroscopy, a procedure in which doctors insert a pencil-sized camera into the knee so they can diagnose a malady. In some cases, surgeons will repair or remove damaged tissue.

After the operation was complete, the surgeon realized the mistake and operated on the correct knee, hospital officials said.

Miriam spokeswoman Linda Shelton would not identify the doctor or patient, whom the hospital said was "doing well." Lifespan, the not-for-profit corporation that operates the hospital, has started an investigation into the accident.

"We deeply regret that such a mistake occurred," Lifespan said in a written statement. "We have apologized to, and are working closely with, the patient and the family."

The state Department of Health was alerted to the mistake Friday and sent a team to investigate, department spokeswoman Helen Drew said.

"We're obviously greatly concerned about the incident and the patient," she said.

Lifespan revised its policies following a string of wrong-side surgeries at Rhode Island Hospital, a teaching facility for Brown University's medical school.

In November, the chief resident started operating on the wrong side of an 82-year-old patient's brain, health officials said. A different doctor performed neurosurgery on the wrong side of another patient's head in February.

In August, a patient died several weeks after a third doctor operated on the wrong side of his brain. Afterward, state health authorities ordered the hospital to take steps to avoid similar accidents in the future, including an independent review of its neurosurgery practices and better verification from doctors of surgery plans.