The Cardinals and co-ace Chris Carpenter have elected to end the former Cy Young Award winner's attempted comeback in order to undergo surgery due to continued nerve-related shoulder problems that flared anew following the pitcher's most recent side session.

Surgery to alleviate Carpenter's thoracic outlet syndrome is now considered a necessity, whether or not he pitched this season. Given the halting pace of his return from a condition that has plagued him since spring training, Carpenter has opted to seek the procedure sooner than later in order to optimize his chances of pitching next February. A date for the surgery has not been announced.

The club scrubbed Carpenter's scheduled bullpen session Monday and has no timetable for Carpenter's next session, according to a club source. Carpenter visited Dallas-based specialist Dr. Gregory Pearl last Thursday to discuss his alternatives as he confronts thoracic outlet syndrome, a rare condition that compresses nerves and blood vessels in a region between the collarbone and rib cage.