Nearly 11 months after doctors reconstructed his blown-out right knee, Green Bay Packers linebacker Nick Barnett still can hear it.
The sound is a faint crackling, a sort of cccccht Barnett can't help but notice whenever the scar tissue grinds around his new anterior cruciate ligament.
He knows it's normal for someone who scars as much as he does. That just about anyone who undergoes ACL reconstruction feels different the first year, sometimes 18 months. That the noises and the soreness and the swelling all is part of the process of coming back from a significant injury that fells dozens of NFL players every year.
"Ten years ago, people didn't come back from a torn ACL," Barnett said this week. "I think you really have to approach it like that -- you have to work as if there's a chance you might not be coming back. I took it as that, but as I get stronger and stronger, I just got more excited about coming back."
Recent research shows NFL players are coming back from ACL reconstruction at a higher rate than ever. Surgical advances, more experienced physicians and improved rehabilitation techniques mean veterans such as Barnett -- and teammates Mark Tauscher and Will Blackmon, who also have torn ACLs in the past year -- more likely than not will play again, with no significant impact on their longevity.
But there also is evidence players may struggle to return to their previous performance levels, particularly if they've also sustained cartilage damage. With no reliable method for regenerating cartilage, and no way to truly re-create a "normal" knee, there always is a chance a player who undergoes reconstruction surgery never will be quite the same.
"Sometimes, they lose only a small step," said Dr. Brian J. Sennett, chief of sports medicine at the University of Pennsylvania Department of Orthropaedic Surgery.
"But in the NFL, the margin -- say you're a receiver -- between beating the defender regularly or being even with the defender is sometimes only that half-step."
The surgery itself has become relatively standardized.
In simple terms, surgeons drill two tunnels -- one in the tibia (lower leg bone) that enters the knee joint, and one in the femur (thigh bone) starting at the exact spot where the new ACL will be attached.
The torn ACL is useless; it can't be sewn back together, because it won't properly heal. Instead, patients and surgeons can select whether to replace it with an autograft (a graft from the patient's patella tendon or hamstring) or an allograft (a graft from a cadaver).
Surgeons snake the graft through one tunnel, through the middle of the knee joint and into the other tunnel, then attach both ends with a metal screw or something else, depending on the type of graft.
The biggest breakthrough was the entry into common practice of arthroscopy -- use of a tiny, fiberoptic camera often called a "scope" that can be inserted into an incision only 1/8 inch long -- which made the procedure less invasive and allowed doctors to ensure the graft landed in the right place.
"We used to just -- it was kind of a guesstimate and you'd drill and stick it in," said Dr. Patrick McKenzie, the Packers' team physician since the early 1990s.
"That was before I was even in practice. But the complication rate in the old days was very, very high -- 25 percent major complication rate -- and in this day and age, it's very, very low."
So few doctors could properly perform the surgery even 20 years ago that, when McKenzie would examine prospects at the NFL's scouting combine, he found that many ACL reconstructions had failed.
There remain concerns about players who had ACL injuries in college; a forthcoming study authored by Dr. Robert H. Brophy, assistant professor Washington University School of Medicine and assistant team physician for the St. Louis Rams, found that players who came to the NFL combine between 1987 and 2000 with previous ACL injuries were slightly less likely to play in the NFL.
However, the study also found players who did get into the league after ACL surgery stayed there almost exactly as long as their healthy-kneed counterparts.
"It's not going to predict any given athlete," Brophy said. "It's a combination of their injury, their athletic ability, their positions -- all of those factors come into play. So, you can't go to one guy and say, 'This is going to have a definitive effect.' This is something that we look at in a population."
Likewise, the impact on performance is difficult to quantify, depending on a player's position.
A 2006 study co-authored by Sennett examined the statistical performance of 31 running backs and receivers who sustained 33 ACL injuries between 1998 and 2002. Most of the players returned to action within nine to 12 months; only 21 percent never played in another regular-season game.
But when compared to a control group of 146 players set up to account for football-specific variables such as team success and scheme changes, the performance of the players who had ACL reconstructions went from above average in the three years before surgery to below average in the three years after. Researchers found the average drop in power rating -- a weighted sum of yards and touchdowns -- fell by roughly one-third post-injury.
"They don't always come back," Sennett said, "but when they do come back, they actually come back less than they were beforehand."
Sennett listed decreased speed, power and proprioception -- the knee's ability to know where it is in 3D space -- as possible factors in the decline. However, he acknowledged there also are human factors at play, such as teams moving on to younger players and leaving reduced opportunities when the injured player returns.
Brophy's study, which included 94 players, indicated the pro prospects of linemen and linebackers were affected most negatively by ACL injuries. But McKenzie believes the most skilled athletes, such as running backs and receivers Sennett studied, need the most time to regain top form.
"If you're kind of a slow, pudgy guy that can't jump -- how long does it take to be slow, pudgy and not jump?" McKenzie said. "If you're a gifted athlete that has a lot of neuromuscular skills, that's a high skill level (and) the neuromuscular recovery is a big part."
There also is a mental hurdle players must overcome. For Barnett, it was learning to ignore the crackling and believe his knee was fine. For skill-position players, such as quarterbacks, it's believing they can take a hit without getting reinjured.
Cincinnati quarterback Carson Palmer, who tore multiple knee ligaments on a low hit in a January 2006 playoff game, told the Los Angeles Times this summer that "still to this day, I'm trying to get over this mental block.
"It's just something where you've got to go out on a limb and just say, 'I'm going to step through every throw.' Because what happens is, the ball starts sailing on you, balls start dying, interceptions happen, tipped balls happen and your completion percentage drastically goes down."
ACL surgery generally is performed as soon as the swelling goes down, usually within weeks. The "prehab" time can last much longer if the player has additional ligament damage like Tauscher, who sustained his injury Dec. 7 and didn't have the reconstruction until Jan. 15.
During the waiting period, the most significant choice players must make is whether to have their new ACL created with an autograft or allograft.
Some surgeons speak strongly in favor of using one's own patella tendon or, in fewer cases, the hamstring because high-level athletes tend to stretch allografts at a higher rate and there's a lower chance the body will reject its own tissue. Barnett opted for the cadaver because he didn't want to weaken another part of his body, citing former teammate Mike McKenzie's broken patella after McKenzie's torn ACL was repaired with a patella graft.
"It's a big question," said Dr. McKenzie, who has averaged two to three ACL reconstructions a year since joining the Packers.
"I think allografts have a very prominent and helpful role, meaning the cadaver grafts. But I think it's all individualized. The patellar tendon grafts have been the gold standard forever."
As recently as the 1980s, post-operative patients often were placed in casts with their knees bent, leaving their legs thin and stiff when the casts came off six weeks later. But rehabilitation now begins almost immediately after surgery, progressing quickly from pain control to range of motion, strengthening, functional work such as running and jumping and then finally to football movements.
Although advanced methods have allowed some players to come back more quickly, McKenzie generally sets a timeline of nine to 11 months. Hence, the Packers held Barnett out of training camp practices until Aug. 17, almost exactly nine months following his surgery, and didn't re-sign Tauscher until Oct. 12. A similar timeline would mean Blackmon, injured Oct. 5, might be cleared just in time for training camp next summer.
The primary barricade to a speedy return is cartilage damage, either from old injuries or whatever caused the ACL to snap. While the majority of ACL injuries (around 70 percent) are noncontact, such as those Barnett and Blackmon sustained when their feet caught awkwardly on the FieldTurf in the Metrodome, the most damaging often are the direct hits that can harm the other three ligaments that stabilize the knee and the cartilage surrounding it.
Without sufficient cartilage, a reconstructed knee is more likely to remain swollen and painful. Brophy's study found that combine prospects with a history of meniscal cartilage damage had their NFL careers reduced by 1 1/2 years; those who had both meniscus and ACL surgery played nearly two years less.
"The real question, how can we regrow and restore the cartilage that's been injured?" said Dr. Eric L. Chehab, an orthopaedic surgeon in the Illinois Bone and Joint Institute's sports medicine division.
"That remains a challenge that we haven't met yet. At this point, they have to deal with it."
There is "a lot of excitement," Chehab said, about new reconstruction techniques being tested, including a "double-bundle" method that utilizes two smaller grafts in an attempt to better replicate the function of an ACL.
The ultimate goal is to create a normal knee -- not just one that roughly replicates its function, with some subtle biomechanical differences.
"We continue to get better at it as we get more data and more information, certainly for the long-term implications," Chehab said. "There's no question that repairing an ACL will protect the cartilage from being reinjured, but we still don't have the answer whether it prevents arthritis down the road. We can take an athlete whose knee is completely unstable and essentially would end their career once they've torn the ACL, and we can restore their chance of getting back in the league. It certainly is a successful surgery. It takes a knee that runs a 90 percent chance of being unstable to a 90 percent chance of being stable, and that's a big deal."
Although there seem to be a large number of high-profile players who have suffered ACL tears in recent years -- a list that includes Pro Bowl quarterbacks Palmer, Philip Rivers, Donovan McNabb and Tom Brady -- McKenzie said NFL physicians' annual survey indicates the frequency of the injury hasn't changed much over the past two decades.
McKenzie averages two or three ACL surgeries per year for the Packers; the injuries tend to be come disproportionately in training camp because of increased exposure from larger rosters and two-a-day practices. According to Press-Gazette research, a dozen NFL players have sustained season-ending ACL injuries since training camps opened in late July, most recently Blackmon and Miami running back Patrick Cobbs.
When Barnett sustained his injury, one of his first calls was to Packers linebacker Brady Poppinga, who tore the ACL in his left knee covering a kickoff in his first NFL start in 2005.
Poppinga referred to his recovery as a great experience -- "obviously, I would never wish it upon anybody and I don't want to do it again"-- because it helped him regain focus on what was truly important to him.
"You don't use it as an excuse," Poppinga said. "More than anything, you use it as something to springboard you -- you use it as something to get you motivated. If you look at it in a way to where it's a downer, I think it can really kind of suck your life out of who you are."
That was Barnett's other challenge -- working harder during the offseason than he ever had in his career, knowing his body fat was down to 10 percent and his body was otherwise healthy, yet being stuck on the sidelines well into August because of the knee.
He says he's back to the old Nick Barnett now, the same guy who never had knee trouble and had missed only two starts in six seasons before the ACL tore. And he says he no longer worries at all about the knee -- even when he hears that noise.
"The knee doesn't feel exactly the same as the left knee," Barnett said. "But I do feel strong and fast and everything I can do -- there's nothing I can't do, nothing that limits me."