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09/23/09 8:16 PM ET

Arias undergoes right knee surgery

Righty has small bone fragment removed from back of knee

HOUSTON -- Astros relief pitcher Alberto Arias underwent surgery on his right knee Wednesday afternoon at The Methodist Hospital.

During the surgery, which was performed by team medical director Dr. David Lintner, Arias had a small bone fragment removed from the back of his right knee. The bone fragment had been irritating a tendon. Arias, 25, has been on the disabled list since Aug. 24 with soreness in his right knee.

Arias, who went 2-1 with a 3.35 ERA in 42 games this year, was supposed to be able to walk on his own by Wednesday night.

Meanwhile, Roy Oswalt will fly to Dallas, Texas, on Thursday to get a second opinion on his ailing lower back. Oswalt has been diagnosed with a bulging disc and inflammation of a joint in his lower back and is seeking a second opinion from Dr. Andrew Dossett of Presbyterian Hospital in Dallas.

Brian McTaggart is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.