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ACL Injury Rehabilitation - What The Research Says.

ACL Re-Injury Statistics

Rehabilitation following an ACL injury can be a long and strenuous process, with a typical return to sport timeline ranging from 9-12 months. Whilst it has been reported that 80% of ACL patients return to sport, only 55% return to their pre-injury level of competitive sport. Furthermore, the research shows that even after ACL reconstruction surgery, the risk of a second ACL injury - either a re-injury to the same knee (ipsilateral) or an ACL injury to the opposite knee (contralateral) - is alarmingly high. Many factors influence re-injury risk, including age, sex, activity level, graft placement, graft type, time from surgery, and neuromuscular and biomechanical control of the lower limbs. Multiple reviews have concluded that the decision on when the athlete should return to sport is typically based on both subjective parameters (satisfactory clinical examination, normal joint function, self-reported knee function) and objective criteria (muscular strength, knee laxity, limb symmetry index, functional testing). Additionally, the athlete's performance in on-field tasks and drills which mimic the sport the athlete is returning to, can be used to subjectively assess their readiness to return to sport. However, with so many factors at play, no definitive criteria for returning to sport whilst minimising the risk of re-injury, has yet been established.


Age as a Risk Factor

A young athlete who returns to sport following ACL reconstruction is at a much greater risk of a second ACL injury compared to their elder counterparts. Re-injury rates in athletes aged younger than 20 years have been reported as high as 30% in the literature. The probability of a second ACL injury for these younger athletes compared to athletes over the age of 20, is increased three to sixfold. In addition, the increased risk in this group is apparent immediately upon returning to sport. The evidence strongly indicates that second ACL injury risk is greatest within the first 2 years after ACL reconstruction for young athletes returning back to high-level sport. The majority of studies indicate that a young athlete who returns to sport within 1 year is 15 times more likely to sustain a second ACL injury (be it to the same leg or opposite leg) than a healthy athlete with no previous knee injuries. However, a systematic review by (Wiggins et al. 2016) assessed 19 ACL studies and concluded that the risk of a young athlete with a prior ACL injury sustaining a second ACL injury was as high as 30 to 40 times greater than that of a healthy, un-injured athlete of the same age. Their review also reported that this elevated risk remains evident within 2 years of returning to sport, when an athlete is approximately 6 times more likely to sustain a second ACL injury than their un-injured counterparts.

In regards to slightly older athletes, research from (Salmon et al. 2005) investigated the rate of a second ACL injury within 5 years of ACL reconstruction, in a population with a median age of 28 years. The authors found that 12% of subjects (1 in 8.3) sustained a second ACL injury within the 5 year period post ACL reconstruction. In a 10-year follow-up study on this same cohort, it was found that 27% of subjects (1 in 3.7) had sustained a subsequent ACL injury, suggesting that a greater length of time post-surgery does not exactly equate to a decreased risk of another ACL injury, in elder athletes.


Males vs Females

The literature indicates that the risk of a second ACL injury is higher in female athletes than males. A study by (Paterno et al. 2012) on athletes who returned to high-risk sports (sports involving cutting, turning and pivoting) after ACL reconstruction, found that 25.4% of athletes suffered a second ACL injury within 1 year of their return to play. This staggering 25.4% consisted of 6.3% re-injuring their ipsilateral leg and 19.1% sustaining a contralateral ACL injury. Compared to their control group (healthy athletes with no history of an ACL injury), those athletes who sustained an initial ACL injury and went on to have ACL reconstruction surgery, were 15 times more likely to sustain a second ACL injury. Interestingly, the study found that female athletes who sustained a prior ACL injury, were 16 times more likely to sustain a second ACL injury relative to the control group, and 4 times more likely to sustain a second ACL injury relative to males with a previous ACL reconstruction. Similar research by (Ahlden et al. 2012) on the rate of re-injury in athletes (median age of 25 years) who had previous ACL reconstruction surgery, found that 9.1% of subjects sustained a second ACL injury within 5 years of returning to sport. However, of the female soccer players aged 15-18 years within the cohort, as high as 22% of these sustained a second ACL injury. Of this 22%, 11.8% accounted for an ACL re-tear on the previously injured leg, with 10.2% accounting for an ACL tear to the opposite (previously un-injured) leg. These statistics demonstrate that the risk of a second ACL injury is not exclusive to the previously injured leg, as a large body of evidence exists which shows the rate of injury to the contralateral leg is nearly on par with that of the ipsilateral leg.


Minimizing the risk of Re-Injury

On a positive note, there are a number of measures that, if complied with, can significantly reduce the risk of re-injury upon return to sport. Firstly, research shows that for every month the return to sport is delayed (until 9 months after ACL reconstruction), the rate of knee re-injury is reduced by 51%. Taking this into account, athletes should be encouraged to delay their return to sport until at least 9 months post-operation, to increase their chances of a successful return. After the 9 month marker however, the research shows no significant reason to delay the return to sport any longer.

Secondly, a quadriceps strength limb symmetry of above 90% has been shown by multiple studies to correlate with a significantly reduced risk of re-injury upon return to sport. One such study by (Grindem, Snyder-Mackler, Moksnes et al. 2016) reported that 33.3% of athletes returning from ACL injury with a quadriceps limb symmetry index (LSI) less than 90%, suffered a subsequent re-injury. In comparison, the same study found that only 12.5% of athletes with a quadriceps LSI higher than 90% suffered a subsequent re-injury.

Thirdly, passing a return to play test battery consisting of various hopping tests (performed both fresh and under fatigue) have been associated with a reduced risk of re-injury upon return to sport. Examples of these return to play hopping tests can be found here. Additionally, numerous studies have reported associations between various psychological variables and a successful return to sport. These psychological variables consist of questionnaires such as the Tampa Scale of Kinesiophobia, the ACL Return to Sport after Injury Scale and a Knee Self-Efficacy Scale.

Finally the importance of remaining compliant and consistent with the rehabilitation programme for the duration of the rehabilitation process has been shown to correlate with higher rates of successful returns to sport. Adequate rehabilitation should consist of pre-operative and post-operative rehab and should continue long after the athlete has returned to sport. An appropriate rehabilitation programme will have positive effects on quadriceps LSI and functional testing. These markers of muscular strength and functional performance, along with strict time-based criteria, athlete self-reported knee function, and self-reported confidence levels, should all contribute to the final decision on when to return to sport.



Factors that contribute to a lower risk of re-injury upon return to sport:

  • Older than 20 years of age at the time of injury.

  • Male.

  • Wait at least 9 months before returning to sport.

  • Follow a rehabilitation programme from a Physiotherapist or Strength and Conditioning Coach both pre and post-operation.

  • Quadriceps strength within 90% of un-injured limb.

  • Pass a return to play testing battery consisting of functional tests assessing jumping ability across three planes of movement.

  • Complete a progression of drills mimicing the demands of the sport you are returning to, before participating in regular training session.

  • Continue your rehabilitation programme long after you have returned to sport.


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