TEMPE, Ariz. -- The recovery and rehabilitation Arizona Cardinals quarterback Carson Palmer remembers after tearing his ACL in 2006 isn't the same as what he's about to embark on for the next eight or nine months.

Orthopedic philosophies and rehab techniques have changed in the past eight years, but the toughest part of Palmer's process, experts say, may be dealing with a second ACL injury at age 34.

“There's no question that our ability to recondition after an ACL reconstruction will change as we get older,” said Brian Cole, team physician for the Chicago Bulls and Chicago White Sox.

Recovery, Cole said, is inversely related to age. But since Palmer's first ACL injury, doctors have become more aggressive during rehab to get athletes back on the field or court, he added. That's been obvious with Cardinals safety Tyrann Mathieu's return and defensive end Darnell Dockett's ahead-of-schedule recovery.

ESPN injury analyst Stephania Bell said success and return-to-activity rates for a second ACL are still high but it may take longer than the typical eight months to get back. She said Palmer may need as much as nine to 12 months to be cleared for everything. Palmer hopes to be back by organized team activities but Cardinals coach Bruce Arians said July 1 may be more realistic -- putting his recovery at less than eight months.

Palmer's road back to the field began Tuesday, when he had surgery to repair the ACL using his patellar tendon. Cole said 75 to 85 percent of team physicians opt to use patellar tendons if they're sturdy enough. In 2006, Palmer's ACL was replaced with a cadaver ligament.

By using a patellar tendon, Palmer will likely face a more strenuous recovery early in the process, Bell said. One significant part of his rehab will be getting his kneecap moving as soon after surgery as possible so he doesn't develop scar tissue and “unusual stiffness” around the kneecap.

“It's a strong graft,” she said. “That will be something that will be different for him because he wasn't concerned with that when he did the other type of graft before. But other than that, it's probably more straightforward for him.”

While he's trying to get his range of motion back, Palmer will be looking at a series of mental hurdles, said former Carolina Panthers and New York Giants receiver Domenik Hixon, who tore his right ACL twice within 14 months.

Soon after the second injury, which he suffered in September 2011, Hixon said he began to doubt whether he was going to get another chance to play. He also was worried about defenders around his knees and going across the middle.

But recovering from the second surgery was easier than the first, he said.

“The unknown is gone, you've done it once before, you know what's expected,” Hixon said. “I knew the timetable where I'm supposed to be at and why they were measuring certain things and you're not going into it blind.

“I think that helped out a lot from the mental aspect.”

One significant change in rehab philosophy since 2006 is not using a clock to determine when an athlete will return to action, Cole said.

Since 1998, there have been 1,254 NFL players with knee-related injuries that were placed on injured reserve, according to ESPN Stats & Information. Of those, 249 were identified to have ACL injuries. And just nine injured -- before Palmer -- the same one at least twice.

Palmer's tearing the same ACL -- or a revision, as it's called in the medical community -- almost nine years after the first tear is rare, Cole said. Retear rates range between 2 to 7 percent depending on the sport and graft, Cole said.

“This is a little unusual in that it's a later failure,” Cole said. “In some respects you can argue this is an entirely new injury.”

His surgery Tuesday went “great” and “very, very small cartilage” was repaired, Arians said. Cole said retorn ACLs tend to have more cartilage damage than initial injuries.

Since the last time Palmer had his ACL replaced, doctors have an increased understanding of tunnel placement in the knee because, Cole said, they've become more educated in the “normal anatomy” of a knee with an original ACL.

With Day 1 of Palmer's recovery over, he already has benefited from the improvements and advancements of philosophy. He's on the verge of finding out how different his recovery and rehabilitation will be eight years after his first ACL.

“I'm mentally prepared,” Palmer said last week. “I'm mentally strong and I'm going to grind this thing out. I know the mindset you need to take, and that's the one-day-at-a-time thing, and it's baby steps and it's doing calf raises and small, little incremental movements and all these little tedious things that you don't feel like are doing anything but you have to do them.

“But you have to do what they tell you to do.”