Symmetry Indexes Can Overestimate Knee Function After ACL Injury
E, Failla MJ, & Snyder-Mackler L. J Ortho
Sport Phys Ther
. 2017; Published online ahead of print
Take Home Message: If we compare a post-surgical knee to the
uninvolved limb it may be ideal to test the uninvolved limb before surgery
rather than later.
A patient trying to return to their
previous level of activity after an anterior cruciate ligament (ACL)
reconstruction must work hard to improve strength and function.  Upon return, they are still at a high risk
for another ACL tear and long-term complications from their injury.  Clinicians commonly use limb symmetry indexes
to assess what is “normal” for a patient during hop and strength tests.  However, the uninvolved limb might be a poor standard
to hold a patient to.  The authors of
this study compared limb symmetry indexes and estimated pre-injury capacity among
70 ACL reconstruction patients.  Each
participant completed an evaluation prior to an ACL reconstruction and again at
6-months post ACL reconstruction. The evaluation included quadriceps strength
and 4 single-legged hop tests. At 2 years after the ACL reconstruction,
participants reported if they experienced a new ACL injury in the prior 2
years. The authors estimated pre-injury capacity by using a participant’s involved
limb at 6 months post surgery in comparison to their uninvolved limb at the
initial evaluation. This differs from the limb symmetry index, which relies on
the uninvolved limb’s performance at the 6 month follow-up.  The table below demonstrates that less people
passed a 90% criteria on all tests with the estimated preinjury capacity index
(20 people) compared with the limb symmetry index (40 people). During the two
year follow up, 11 participants reported a second ACL injury and 8 of these
participants passed the recommended 90% limb symmetry index cut off.  However, only 2 of the 11 participants passed
the 90% estimated preinjury capacity cut off. 
Table. Disagreement between limb
symmetry index and estimated preinjury capacity.
Failed 90% estimated preinjury
capacity index on all tests (50 people)
Passed 90% estimated preinjury
capacity index on all tests (20 people)
Failed 90% limb symmetry index on
all tests (30 people)
suffered another injury)
4 people
Passed 90% limb symmetry index on
all tests (40 people)
suffered another injury)
suffered another injury)
These authors found that using the
strength and functional performance of the uninvolved limb at the initial evaluation
for comparative norms was better than assessing and comparing both limbs at 6
months after surgery.  This is very
interesting because there may be adaptations that occur due to decreased
physical activity levels during the post-surgical period.  So comparing the limbs after this relative
deconditioning may underestimate where the healthy limb usually is.  It would also be interesting to see how the
injured limb performed prior to injury (for example, during preseason
screening) and how those measurements compare to the non-injured limb at the
time of injury. This presurgery assessment strategy, however, is still
vulnerable to arguments that the healthy limb may not be a good reference point
at all because there may have been predisposing reason as to why the person got
injured in the first place that affected both limbs.  Regardless, these findings support the use of
the estimated preinjury capacity index instead of the limb symmetry index. This
would be appealing because it is using a snapshot of where they were closest to
their competitive physically active state. 
It would not take much longer to do these assessments on the healthy
limb at your initial evaluation during pre-surgical rehabilitation.  It would be interesting to continue to follow
these patients out and not only compare their injury rates, but also monitor
their patient-reported outcomes and risk for osteoarthritis.
for Discussion:  What criteria do you use
for return to play?  Do you establish any
pre-injury or pre-surgery norms for the lower extremity in your initial
by: Nicole Cattano
by: Jeffrey Driban