​A deep dive on returning to sport after ACL surgery.
You tear your ACL. You go through 8 months of grueling rehab. You miss out on 8 months of playing your favorite sports with your friends. You go through 8 months of isolation and FOMO. It sucks.
You know what sucks worse? Re-tearing your ACL 3 months later.
Around 25% of athletes who tear their ACL will re-tear that same ACL or tear the other ACL within 2 years of the initial surgery. That number is absurdly high. With 1 out of every 4 people re-tearing, clearly we’re not doing something right.
So, what’s the problem?
- Insufficient graft healing and/or surgical error? Personally, I think this is usually the least likely culprit. It does not explain contralateral (other leg) tears at all. Surgeons are pretty good at surgery.
- Graft type? Maybe! Some grafts are better than others. The most extreme example is that an allograft (a graft from a cadaver’s body) is 5.5x more likely to fail than an autograft (a graft from a different part of your own body). But, athletes aren’t getting allografts in 2024, so I don’t think this plays a big role today.
- Age? Younger athletes are more at risk for graft failure- the odds of re-injury decrease 0.09 for every year increase in age. But, that’s not helpful. You’re not going to change your age, so let’s focus on things we can control.
- Time of return? Now we’re on to something. This is heavily supported by literature. Stay tuned for more.
- Poor function and return to sport training? Undoubtedly one of the most likely culprits. Let’s dive in.
Here’s the TL;DR version:
- Different sports and activities have different risk odds; your athlete should be cleared for these gradually.
- Both time and function tell us when your athlete is ready to return. Neither alone is sufficient.
- Limb symmetry is important, but only if the non-surgical leg is getting stronger too.
- You have to test isolated quad strength- there’s no way around it.
- Different hops give us important information about whether your athlete’s body is prepared to handle high loads at fast speeds.
- ACLs normally tear during agility movements, so we need to test agility.
- Psychology matters. If your athlete doesn’t think they’re ready, even if all of their physical numbers are great, then they’re more likely to get injured.
- Not all situations are the same, some people should be willing to tolerate more risk than others.
What is “cleared?”
Our question at hand is: when is your athlete ready to return to your sport? When are they cleared?
Being “cleared” should be nuanced, not binary.
Being cleared isn’t a toggle switch, it’s a dimmer. We don’t (or at least we shouldn’t) toggle the switch up from being “not cleared” to “cleared for everything.”
Instead, we spin the dimmer to gradually be cleared for more activities.
Different sports, positions, and drills have different demands. Some of them are riskier than others. Your athlete is going to be cleared to swing a golf club well before they’re cleared to play in a live football game.
Father Time vs Mother Function
The graft takes somewhere around 2 years to complete ligamentization. That means whatever they put in your athlete’s knee (usually a piece of patellar tendon, quadriceps, or hamstring) takes about 2 years to transform into a ligament.
Does that mean they should wait 2 years to return to playing to ensure they have a more ligament-like ligament in there? Not really.
We have pretty solid evidence to support that for every month that an athlete returns to their sport before 9 months, they are 50% more likely to re-tear. So, if they returned at 8 months they’re 50% more likely to re-tear, at 7 months they’re 100% more likely to re-tear, at 6 months they’re 150% more likely to re-tear, and so-on.
After that, time alone seems to stop being predictive of re-injury.
That’s where function swoops in. We need to let biology run its course, but we also need to get as strong and powerful as possible to protect the graft.
The answer isn’t time or function, it’s time and function. If your athlete is crushing return to performance tests at 6 months, they’re still going to be at an increased risk of re-injury as opposed to if they waited until 9 months.
If they’re at 11 months, but they’re still weak, slow, and hesitant, they’re still going to be at an increased risk of re-injury.
Relatively absolute
Most return-to-sport tests compare your athlete’s surgical leg to their other leg. This is called the limb symmetry index (LSI).
If they kick 100 lbs on their healthy leg and 60 lbs on their surgical leg, their LSI for that test is 60%.
Research has suggested that the threshold of “good enough” if usually between 85-95%. If your athlete’s surgical leg does 90% as good as their other leg, then that’s good enough to return.
Does that make sense, though? Let’s consider Little Johnny who tore his left ACL 6 months ago.
Fortunately, we have a lot of information on Little Johnny. Before he got hurt, Johnny’s quad strength was 75 lbs on both legs and his single leg hop for distance was 50″ on both sides.
His physical therapist focused really hard on getting his left (surgical) leg as strong as possible. Johnny always did his home exercises and worked his tail off at therapy. At 6 months, his physical therapist tested him and… his left leg was 90% as strong and powerful as his right leg! Woohoo!
He kicked 60 lbs on his right leg and 54 lbs on his left leg. He jumped 48″ on his right leg and 43″ on his left leg.
Wait… that doesn’t sound so good?
His numbers before his injury were clearly not good enough, because that’s where he was when he got injured. Then, he took 6 months and did no targeted training on his right (non-surgical) leg.
Logically, that leg got weaker. So, now, we’re using the leg that is even weaker than the time of injury as the standard for return? Actually, we’re using 90% of that weaker leg as the standard?
90% of suck is 💩.
Context is important. Your athlete needs to be strong in an absolute sense and strong relative to the other leg.
We need to aim to be stronger than they were pre-injury before returning, not as strong as they were. They should be training the heck out of the non-surgical leg throughout the entire process.
Your athlete should be chasing a moving goal post. Aiming for limb symmetry is important, but only in the context of the non-surgical side continuing to get stronger and more powerful.
Strength
It’s the quads, until it’s not the quads. They’re the lynchpin item that has to be pulled first.
We have to measure quad strength. Furthermore, we have to measure it in isolation.
A lot of clinicians will use compound movements (like a leg press) to test strength. Compound movements use multiple joints and multiple muscles. Athletes are good compensators. If their quad isn’t strong enough, they’ll still test well and get the weight up by calling in reinforcements from the other joints/muscles (e.g. using hip extensors more).
Isolating leg extension gives them no opportunity to use anything besides the quads. They can’t cheat it.
We use a crane scale kick test. With the knee joint right at the edge of the table, the strap is lengthened to allow the athlete to kick out to 60 degrees. Over a period of 10 seconds, the athlete gradually works up to kicking 100% (we usually do two warm-up trials where we work up to 70%, then 90% over 10 second periods). The athlete is allowed to lean back as desired.
Our goal is to get to 90% LSI (with context of LSI given above in mind).
It’s important to note that the compound strength movements are also really important. They just can’t completely supplant isolated testing.
We want to get reverse lunges and trap bar deadlifts (or similar lifts) as strong as possible throughout the rehab process.
Power
In the 1980’s hops were introduced as a standard for ACL rehab and they’ve remained a mainstay since.
Hops involve jumping on one leg and landing on that same leg. They’re a good proxy for an athlete’s capability to safely change direction, because they require the athlete to produce a lot of force concentrically, then control the landing eccentrically.
We use a combination of horizontal and vertical hops, as well as a combination of single and repeat hops.
- Single hop for distance
- Triple hop for distance
- Single leg vertical jump
- Single leg 4-jump test
Again, our goal is to get to 90% LSI (with context of LSI given above in mind).
Agility
ACL injuries are nearly synonymous with cutting, pivoting, and twisting. So, we better make sure that your athlete is ready to cut, pivot, and twist without issue.
Furthermore, there’s growing evidence to support that these re-tears are exacerbated by cognitive load.
Huh?
That means the cutting motion alone isn’t normally the issue- it’s cutting while the brain is focused on something else. This could be tracking a ball, mirroring a person, or looking down field to see what’s going on.
To test agility, we use a Reactive 10-10 Shuttle.
The athlete has to sprint 10 yards toward the therapist/coach. Right before the athlete gets to the 10 yard mark the therapist points right or left, the athlete has to turn over that shoulder and then sprint back through the starting line.
This is timed on laser timers.
This is usually our final hurdle of objective testing. I’m not even going to test a timed Reactive 10-10 Shuttle until strength and power testing is cleared- the risk vs. reward ratio isn’t favorable.
Again, our goal is to get to 90% LSI (with context of LSI given above in mind).
Confidence
One of the most sure fire ways to get hurt in any sport is to go out there scared or confused.
We need to make sure that your athlete is both physically and psychologically ready to return. I like to use the ACL-RSI survey to gauge how they’re feeling internally about their own ability to return. The ACL-RSI is a 12 question survey.
- Are you confident that you can perform at your previous level of sport participation?
- Do you think you are likely to re-injury your knee by participating in your sport?
- Are you nervous about playing your sport?
- Are you confident that your knee will not give way by playing your sport?
- Are you confident that you could play your sport without concern for your knee?
- Do you find it frustrating to have to consider your knee with respect to your sport?
- Are you fearful of re-injuring your knee by playing your sport?
- Are you confident about your knee holding up under pressure?
- Are you afraid of accidentally injuring your knee by playing your sport?
- Do thoughts of having to go through surgery and rehabilitation prevent you from playing your sport?
- Are you confident about your ability to perform well at your sport?
- Do you feel relaxed about playing your sport?
The athlete answers on a scale of 0-10 for each question, where 0 indicated not ready and 10 indicates very ready. The total score goes up to 120, with 65 being a rough threshold suggesting readiness.
Risk tolerance
I hate to break it to you, but there will always be a chance that your athlete re-tears their ACL. The only way to guarantee that they don’t is to force them to sit on the couch and binge Netflix for every second of the rest of their life.
Participating in a sport puts your athlete at risk for having an ACL injury. The more cutting, pivoting, and twisting involved in that sport, the greater risk they have.
Our goal is not to clear you when your are not at any risk whatsoever, but to clear you when you’ve reached the risk tolerance threshold that you are comfortable with.
Context is going to play a big role here. (Doesn’t it always?)
A professional athlete who has to participate in training camp in August or they won’t have any chance of making a team should tolerate more risk than a 14 year old athlete who has a spring 7 on 7 league starting up.
It actually makes sense for the professional athlete to try to come back at 6 months if that’s when training camp starts. It would make no sense for the 14 year old athlete to think twice about playing 7 on 7 that spring.
Black and white answers rarely exist. Get comfortable in the gray.
A quick thought on the gray
Our brain likes clear-cut answers. It gets uncomfortable with too many choices and too many possibilities.
It distorts our reality and makes it seem like there are clear right and wrong solutions. Just look out how people behave in political endeavors.
The truth is, the world is gray. There’s nuance everywhere.
The more comfortable you get with that, the more likely you will be able to accomplish things that others can’t, because you’ll see the world in a way that others won’t.
Embrace uncertainty if you ever want to achieve greatness.
More from me…
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Thanks so much for your help in spreading the word about long-term athlete development!
Be >,
Zach
Dr. Zach Guiser, PT, DPT, CSCS