The relief pitchers underwent elbow reconstruction on back-to-back days last April, Madson with Dr. Lewis Yocum, the Angels team physician and an expert on the procedure, and Kohn with Dr. James Andrews, the renowned Alabama surgeon whose patients include Michael Jordan, Jack Nicklaus and Robert Griffin III.
Most popular baseball stories
Yocum replaced Madson's torn ulnar collateral ligament with a tendon from the right-hander's right forearm, the first - and most common - place surgeons look for grafting material.
Madson, signed to be the Angels' closer, began throwing in November. He progressed to the mound in January but suffered a setback Feb. 1 and was shut down for 2 1/2 weeks because of elbow soreness.
He has resumed throwing but will open the season on the disabled list, with no clear return date.
Kohn, who was effective for the Angels in 2010, began throwing in November and was letting his fastball go by December. He threw off a mound about 20 times before spring training and has already made three appearances in exhibition games, his fastball hitting 93 mph this week.
But the experts say that doesn't necessarily mean Kohn's leg was a better choice than Madson's arm.
"The fact that one pitcher is struggling and the other is not has nothing to do with where we got the graft from," said Andrews, one of the nation's top sports orthopedic surgeons for knee, elbow and shoulder injuries.
"There are so many factors, such as age and mileage. Bone spurs or arthritis can cause complications. The biggest thing is the healing response, which is as varied as the colors of the rainbow - some heal quickly, some take forever."
Madson, 32, is a lanky 6-foot-6, 200-pounder who has thrown 630 big league innings and 743 minor league innings since 1998.
Kohn, 26, is a compact, 6-foot, 200-pounder who has thrown 332/3 major league innings and 1831/3 minor league innings since 2008.
"Everyone's body is different," Madson said. "I don't know that it's comparing apples to apples. Your throwing motion, how your body heals and age all have something to do with it. Healing is a human thing, and the older you get, the slower you heal."
Since Dr. Frank Jobe performed the first elbow reconstruction on then 32-year-old left-hander Tommy John in 1974, extending the pitcher's career by 14 years, there have been only "very minor changes" in the procedure, Yocum said.
The injured UCL, which is located on the inside of the elbow, is replaced with a tendon taken from somewhere else in the patient's body. For decades, the graft was passed through tunnels drilled in the ulna and humerus bones and woven into a figure-eight pattern to reconstruct the ligament.
A newer technique developed in 2006, known as the "docking procedure," is less invasive and entails splitting the muscles rather than detaching them. Only one hole instead of three is drilled, allowing the ulnar nerve to remain in place.
Rehabilitation still takes about a year to 18 months for players to return to their previous levels of ability, but the success rate has improved slightly using the docking procedure. Though numerous careers have been saved by Tommy John surgery, its overall success rate is actually about 85 percent, Yocum and Andrews said.
"People think it's 100 percent successful," Andrews said. "It's not."
Perhaps the biggest change in the procedure occurred a decade ago, when doctors began harvesting hamstring tendons, which are a little thicker and longer than those found in the wrist, forearm, calf or foot, for grafts.
"Some guys are using the hamstring tendon almost routinely because it's sturdier," Yocum said. "They definitely hold up and can give you more length and substance. If you have bone spurs to remove and you know you need more tissue, you take one of the tendons out of the leg."
So why wouldn't doctors use the hamstring tendon all the time?
"It's more complicated because you have to bandage that area up and rehab the leg," Andrews said. "We usually don't take from the leg unless the wrist tendon is absent or too short or too thin."
The palmaris tendon that runs up the arm from the wrist is a popular grafting target because it is conveniently located and expendable. But 20 percent-25 percent of the population is born without the tendon, Yocum and Andrews said, forcing doctors to look elsewhere for grafts.
"It's one of God's little tricks," Yocum said. "It makes us work a little harder."
Both Andrews, who performs 225-250 Tommy John surgeries a year, and Yocum, who performs 75-100 procedures, said they used tendons from the wrist about 60 percent of the time and from the hamstring 40 percent of the time.
"If hamstrings were bullet-proof, everyone would be doing them," Yocum said. "We've been using the wrist tendon since Frank started, and it's worked out well. But you're starting to see a trend toward more hamstring tendons as people get more comfortable with it and it has more of a track record."
Doctors will sometimes make a small incision in the wrist to check the palmaris tendon to see if it's adequate to replace the UCL.
In Madson's case, Yocum said, "He had two really good wrist tendons."
But clean procedures do not guarantee successful outcomes. The rehabilitation process is key, and Madson, the former Philadelphia Phillies closer, admits he may have pushed himself "too hard, too soon," as he got excited about the possibility of being ready by the April 1 season opener.
Dodgers outfielder Carl Crawford, who had Tommy John surgery last August, was shut down last week after experiencing nerve irritation around the elbow. Pitchers John Lackey of the Boston Red Sox, Mike Pelfrey of the Minnesota Twins and Daniel Hudson of the Arizona Diamondbacks are also trying to come back this season after undergoing the procedure.
"A lot of guys have setbacks at 10 or 11 months," Yocum said. "You get so close, and your body tells you it takes a year. That doesn't mean you can't come back sooner. But you can't push too hard. You have to respect the process."
Kohn, who relies on a fastball and slider, said he was aggressive in his rehabilitation, and in several followup visits with Andrews there was no bruising or buildup of scar tissue in the elbow.
"Knock on wood, but everything feels great right now," said Kohn, who had a 2-0 record and 2.11 earned-run average in 24 games for the Angels after a July call-up in 2010.
"I don't know why, but it just worked out that I was able to come back pretty quick. My mechanics are a little different, and my delivery is a little smoother, but the ball is coming out of my hand great. Overall, I feel better than I ever have."
Manager Mike Scioscia has noticed.
"The arm stroke, the velocity, the ball coming out of his hand . . . it all looks great," Scioscia said. "He has a power arm."
So does Madson, when healthy. His fastball averaged 94 mph in 2011, when he had a 4-2 record, 2.37 ERA and converted 32 of 34 saves for the Phillies. He also has an excellent changeup, a curve and a cut fastball.
But it will be several weeks before Madson pitches in an exhibition game, and it could be several months before the Angels know whether Madson, who signed a one-year, $3.5-million deal, will regain his 2011 form.
"I've had big league pitchers bounce back in nine months, and I've had some who take a year and a half," Andrews said. "The worst thing you can do is tell a professional athlete he should be well in 12 months, and they're not. You can't rush Mother Nature."