The main finding in this study was that using an RTS test battery comprising 2 hop tests and 2 PROs reduced the passing rate, and, thus, was more demanding, compared with using a battery of 5 MF tests (19% versus 29%). Interestingly, there was a very strong correlation[24] between the two different test batteries. Therefore, a clinic without advanced testing equipment to measure strength can use 2 hop tests and 2 psychological PROs as criteria for RTS. The results are comparable or even better than a comprehensive battery of 5 MF tests (strength and hop). There was no significant correlation between passing MF test batteries and reaching cut-offs for either KOOS QoL or ACL-RSI, which indicates that the use of only MF tests or only psychological outcomes is likely insufficient as RTS criteria.
Our results suggest that 1 in every 2 patients passed the RTS criteria and achieved symmetrical knee function, when the decision was based on reaching leg symmetry in 2 unilateral hop tests. With test batteries that comprise more tests, the passing rates decreased, in agreement with the literature.[8–10] More tests, thus, increase the demands on the patients recovery after ACL reconstruction. When 5 MF tests with or without 2 PROs were used, the passing rate, compared with only 2 hop tests, decreased from 47% to approximately 29% and 13%, respectively. The use of only 2 hop tests to determine symmetrical muscle function can, therefore, not be recommended, as approximately 30% of patients run the risk of being classified as false positives.
Current recommendations for RTS evaluation,25 are strongly supported by results from the present study, suggesting that batteries of tests should comprise strength and hop tests, as well as PROs.[25, 26] In our cohort, a very small proportion of patients met our recommended RTS criteria at 1 year after ACL reconstruction. This result indicates that clinical settings, included in Project ACL, and responsible medical professionals for the treatment of the patients in this study, need to better prepare patients in order to make a safe RTS.
The results of low psychological readiness to RTS and unacceptable low knee-related QoL suggest that some patients have recovered MF without recovering the psychological outcome. Psychological factors are important during rehabilitation,[27, 28] where for example, high fear of re-injury can prevent patients from returning to their preinjury level of sport.[29–31] Furthermore, a lower psychological readiness to RTS 1 year after ACL reconstruction is associated with a higher risk of a second ACL injury,[23] supporting that it is important to include psychological PROs in RTS decision-making, alongside tests of MF.
In this study, a smaller proportion of patients met the criteria for an acceptable ACL-RSI compared with KOOS QoL. The ACL-RSI was developed to assess psychological readiness to RTS.[17] However, the impact RTS has on ACL-RSI, i.e. whether RTS leads to high psychological readiness or whether high psychological readiness leads to RTS, is yet to be studied. Patients who do not RTS after ACL reconstruction can report poor knee-related QoL up to 20 years after surgery, compared with patients who RTS.[32] However, the use of both the KOOS QoL and the ACL-RSI led to more patients being identified as not “recovered” compared with using only MF tests. Future studies are needed to better understand how individual psychological profiles are related to a safe RTS.
Given the high rate of new knee-related injuries in patients after ACL reconstruction [33, 34] and the assumption that patients who RTS might not have been ready for it, more emphasis should be placed on preparing patients for RTS test battery criteria during rehabilitation, especially as passing RTS test batteries can reduce the risk of re-injury.[4]
Limitations and strengths
A limitation of this study is that we did not determine the different test batteries effectiveness to reduce the risk of a second ACL injury. Even though there is evidence[4, 35] suggesting that patients who meet certain cut-offs in RTS test batteries have lower risk for a second ACL injury, there is an ongoing debate [36],[37] about the evidence and the validity of RTS testing.
The use of the LSI is a limitation since the patients’ healthy limb can have reduced strength after ACL reconstruction,[38] meaning that tests of muscle function may overestimate the function of the operated limb.[39] Results in the present study might therefore be falsely high, which strengthens the recommendation that RTS criteria are important to meet before RTS.
Patients who suffered a second ACL rupture were excluded from the present study in order to create a group of patients that was as homogeneous as possible. Future studies will show how the different batteries of tests assessed in this study affect the risk of a new ACL injury.
The primary strength of this study is the relatively large number of patients included. Another strength is the choice of PROs, since the KOOS and the ACL-RSI was used, and these PROs have high methodological quality.[40]