Is 9 Months the Safe Time for ACL Return to Sport?
Maybe not...
For years, the message in ACL rehab has been clear and consistent: wait at least nine months before returning to sport.
This guideline isn't arbitrary; it's backed by influential research (Grindem et al 2016, Kyritsis et al 2016 & Beischer et al 2020) showing that delaying your return to at least 9 months can significantly cut your risk of re-injury.
But the world of sports medicine is always evolving.
A compelling new study by Kotsifaki et al (2025) has just been published that adds a crucial layer of nuance to this debate.
It asks a powerful question:
“If an athlete is physically ready and has passed all their functional tests, does the 9-month deadline still hold the same power?”
The findings don't necessarily erase what we already know, but they challenge us to look beyond the calendar and focus more on how an athlete is prepared.
Let's dive into what this means for you.
What the Study Found
A team of researchers prospectively followed 530 male athletes in pivoting sports for two years after their ACL reconstruction (ACLR). The aim was to see what factors influenced their return to sport and whether the timing of that return actually increased the risk of another knee injury for those who were physically ready.
Here are the key takeaways that really stood out to me:
The single most important factor for a successful return to sport wasn't the time since surgery, but rather completing the rehabilitation program and meeting objective discharge criteria. Athletes who met these criteria were almost six times more likely to return to their pre-injury sport (OR 5.71). This shows a clear "dose-response" relationship: the more quality rehab you do, the better your chances of getting back out there.
For the athletes who successfully completed their rehab and passed all their discharge tests, returning to sport before or after the 9-month mark made no difference to their risk of a new knee or ACL injury.
Overall, 72% of the athletes returned to pivoting sports within two years. However, the success rates varied dramatically based on rehab completion. A whopping 90% of athletes who met the discharge criteria returned to their sport. In contrast, only 46.2% of those who dropped out of rehab early managed to return.
If there is one point I want you to take away from this paper, it's this:
Adherence to a structured, progressive rehabilitation program and meeting objective discharge criteria were the keys to success, regardless of the surgical details.
The study's conclusion states it perfectly: adherence to the rehab protocol, progressing based on objective criteria, and meeting those criteria at the end were associated with a high likelihood of returning to pivoting sports, "irrespective of surgery type or comorbidities".
The data showed that factors like the type of graft used, whether a meniscus repair was performed, or if a lateral augmentation was added to the surgery were not significantly associated with an athlete's return to sport. The factors that were significant were all related to rehabilitation: how often the athlete attended, whether they completed sport-specific sessions, and ultimately, whether they passed their final tests. This tells us that while surgery fixes the structure, it's the hard work in the gym and on the field that truly dictates the outcome.
What were the criteria?
So, what did it actually mean to "meet the discharge criteria" in this study?
It wasn't just a clinical test of the knee for stability, and a handshake from the surgeon. Clearance was a comprehensive process based on passing a battery of tests.
Athletes who were cleared to return to sport had to achieve:
Pain-free testing
Over 90% quads and hamstring strength symmetry
Over 90% jump symmetry (horizontal and vertical tests)
Successful completion of a sport-specific on-field protocol
Education on injury prevention and a maintenance program
This highlights that being "ready" is a multi-faceted status. It's about being strong, being powerful, moving well without pain, and being educated on how to stay healthy long-term.
So, how does this compare to past research?
Many of you who follow ACL research will know about two landmark studies that have heavily influenced my practice: Grindem et al. (2016) and Kyritsis et al. (2016).
So how does this new paper fit in?
The study from Grindem and colleagues was pivotal, famously showing that for each month an athlete delayed their return to sport (up to 9 months), their reinjury risk was reduced by about 50%. This is the study that really solidified the 9-month guideline in many clinicians' minds.
The work by Kyritsis and colleagues found that professional football players who failed to meet a battery of six clinical discharge criteria before returning to sport had a four times greater risk of rupturing their ACL graft.
This new study doesn't necessarily contradict this past research; it refines it.
It supports the finding from Kyritsis et al. on the critical importance of meeting discharge criteria. And it adds a new layer to the Grindem et al. findings by suggesting that if an athlete has met those strict functional and objective criteria, the 9-month timeline may be less important for injury risk. This really strengthens the argument that our focus should be more on 'how' an athlete returns, not just 'when'.
A word of caution…
Now, before we adjust our thinking, it's crucial to understand the context of this study—something I always stress at The ACL Hub. The participants in this research were all male, with an average age of 26-27, and had a high pre-injury activity level (Tegner score of 8 or above).
This means we need to be cautious about applying these findings to everyone. Female athletes, who were not included in the study, may have different risk profiles. The same goes for older or less active individuals.
Actionable tips for patients
Focus on the 'How', not just the 'When': Your energy is best spent on the quality of your rehabilitation, not just counting down the days on a calendar.
Stick with your rehab program. The study shows a clear link between attending sessions, especially sport-specific ones, and a successful return to sport.
Ensure your progression through rehab and your ultimate clearance to return to sport are based on objective tests of your strength and function, not just time. You need to earn your return to sport.
Actionable tips for health professionals
Emphasise that time is a necessary component for biological healing, but it's not sufficient on its own to ensure a safe return to sport. The focus should be on meeting performance criteria.
Implement regular, objective testing throughout the rehabilitation process to guide progression. At least monthly. This not only informs your clinical decision-making but also helps in goal setting and motivating the athlete.
While guidelines are helpful, each athlete's journey is unique. Factors like age, sport, complications, and psychological readiness all play a role. Use objective data to tailor the rehab and make a shared decision with the athlete about their return to sport.
This study provides powerful evidence that a criteria-based approach to ACL rehabilitation is what truly matters.
It shifts the conversation from a rigid timeline to a more personalised, evidence-informed process. It’s not about a fast or slow return; it's about a successful return.
Thanks as always for reading!
Your in health,
Mick Hughes (ACL Hub Founder)
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