Passing a limb symmetry test does not mean a patient is cleared from a second ACL injury. Simonsson (2025) tested five limb symmetry thresholds against real reinjury outcomes in 233 athletes after ACL reconstruction. None of them separated the athletes who tore the graft again from the ones who did not. Nearly 1 in 4 young athletes who return to sport after ACL reconstruction suffer a second ACL injury (Wiggins 2016). The symmetry number most clinics use to clear that return did not predict which athletes those would be.
What did Simonsson 2025 actually test?
The Project ACL cohort followed 233 athletes after ACL reconstruction (Simonsson 2025). Researchers checked five limb symmetry thresholds against who actually suffered a second ACL injury. None of the five told apart the reinjured athletes from the ones who stayed healthy. That is a direct test of the number most return-to-sport batteries lean on. It was run against real outcomes, not against a passing score.
This matters because the symmetry number is usually the last gate before clearance. A patient hits 90% on a hop test or a strength dynamometer. The chart notes "symmetry criteria met," and the return-to-sport conversation moves forward. Simonsson's result challenges that workflow directly. Most clinics treat that gate as the finish line. It did not separate the athletes who got hurt again from the ones who stayed healthy.
Why can 97% symmetry still hide a real deficit?
A symmetry percentage can be reached by compensating around the knee, not recovering it. Kotsifaki (2022) found that athletes hit 97% symmetry on a triple hop distance test. Yet the involved knee produced only 51% to 66% of the work the other leg did. The hip, pelvis, and trunk picked up the difference and produced a passing score. That is one plausible reason a single symmetry number missed the at-risk athletes in the Simonsson cohort.
Does this mean limb symmetry testing has no value?
No. Grindem (2016) tested a decision rule combining delayed return to sport with quadriceps strength symmetry. It cut reinjury risk by 84% in the Delaware-Oslo ACL cohort. Symmetry is a real input. The problem is treating one symmetry number, checked once at discharge, as the whole decision. Grindem's result came from stacking strength symmetry with time and other criteria. It did not come from a single threshold standing alone.
How does this change Monday morning?
A discharge visit built around one hop-distance or strength symmetry score is not a complete return-to-sport decision, even at 90% or higher. Pair strength and hop symmetry testing with a reactive measure. A reactive measure adds a decision under time pressure, not a planned single-leg output. QuickBoard's Agility Rating captures bilateral reaction time and choice accuracy under full body weight, across five dimensions. That is a different question than whether the limb can match force output on a planned hop. A patient can hit every strength and hop symmetry target. That patient can still respond slower and less accurately on the involved side when the step is not planned in advance. That is exactly the reactive gap a static symmetry number cannot see. Strength symmetry testing and functional progressions bill under CPT 97530 (Therapeutic Activities) or CPT 97112 (Neuromuscular Re-education). The Agility Rating assessment bills separately under CPT 97750 (Physical Performance Test), in 15-minute increments, on a non-eval day. Run both before the clearance conversation. Do not rely on the symmetry number alone.
Takeaways
- Simonsson (2025) tested five limb symmetry thresholds against 233 ACL athletes. None predicted who suffered a second ACL injury.
- Nearly 1 in 4 young athletes who return to sport after ACL reconstruction suffer a second ACL injury (Wiggins 2016).
- A 97% symmetry score can still hide a real deficit. The involved knee did only 51% to 66% of the work in Kotsifaki's cohort (Kotsifaki 2022).
- Delaying return to sport and meeting strength symmetry together cut reinjury risk 84% in the Delaware-Oslo cohort (Grindem 2016). Symmetry belongs in a decision rule, not as a standalone pass score.
- Add a reactive, cognitive-motor measure alongside strength symmetry before clearing return to sport.
Symmetry testing still matters. It is not sufficient by itself. See Ortho Rehab and ACL Return to Sport for the full clinical use case. The published Research library covers the QuickBoard studies behind the Agility Rating, and How QuickBoard Works covers the reactive assessment model.
Citations
- Simonsson 2025
- Grindem 2016
- Kotsifaki 2022
- Wiggins 2016
See QuickBoard research for full citation details.