The purpose of this investigation was to determine if preoperative and postoperative single-legged hop tests predict self-reported knee function 1 year after ACL reconstruction. Preoperative hop tests did not significantly predict self-reported knee function 1 year after ACL reconstruction. However, hop tests conducted 6 months postoperatively statistically significantly predicted self-reported knee function (P < .02). Of the 4 hop tests, the 6-m timed hop and the crossover hop were the best predictors and the only 2 tests in which the confidence intervals of the discriminatory accuracy (AUC) were above 0.5. With a specificity of 90% (), the 6-m timed hop test was the most useful test for identifying patients with self-reported knee function below normal ranges at 1 year. The crossover hop test had a sensitivity of 88% () and was thus the test that most accurately identified patients with knee function within normal ranges. These results have significant implications for the clinical management of patients after ACL reconstruction as the ease of application of these hop tests allows for a practical method to determine the need for directed interventions to restore medium-term knee function.
In the current study, the 6-m timed hop was the strongest independent predictor and had the highest discriminatory accuracy. Patients with knee function below normal ranges were more than 5 times more likely to have 6-m timed hop LSI lower than 88% at 6 months than those with knee function within normal ranges. Previous work by Fitzgerald et al11
has shown that the 6-m timed hop along with other variables can identify those with poor dynamic knee stability from those with good knee stability early after ACL injury. In those individuals with good dynamic knee stability, this test can also discriminate between those who did not successfully return to high-level sports from those who did.11
The 6-m timed hop, the least demanding hop,23,46
can effectively challenge neuromuscular control in patients who have deficits in limb-to-limb differences. These asymmetries strongly predict those who will have knee function below normal ranges at 1 year. Limb asymmetries may be the result of underlying impairments, such as quadriceps weakness,26,53
that may contribute to low hop performance and should be identified and corrected. In addition, limb asymmetries in athletes may result in suboptimal performance on the playing field and are linked with an increased risk of a second ACL injury.22,38,43
Sports-specific activities are more challenging than landing from a planned hop in a controlled environment, and thus the deficits seen in single-legged hop performance may be magnified in sports-specific activities, potentially predisposing the ipsilateral or contralateral knee to injury. Our data suggest that patients demonstrating an LSI less than 88% on a 6-m timed hop test may benefit from continued rehabilitation to address impairments and functional performance in an attempt to normalize function.35,42
Restoration of symmetrical function between limbs remains an important goal of postoperative rehabilitation, and limb asymmetry for all patients remains a risk for poor knee function.
The crossover hop predicted knee function within normal ranges, with similar predictive ability and discriminatory accuracy as the 6-m timed hop. Patients with knee function within normal ranges were 4 times more likely to have crossover hop LSI greater than 95% than those with knee function below normal ranges. The crossover hop is the most demanding hop test as it imposes forces in frontal and transverse planes, combined with multiple hops in the sagittal plane. Side-to-side differences are minimized in these patients, likely indicating superior neuromuscular control, therefore increasing the probability of knee function within normal ranges at 1 year. These patients appear to have improved their ability to generate and attenuate forces on the involved limb.35,37
Furthermore, patients with high limb symmetry after ACL reconstruction favor the involved limb less during functional activities, are more confident in its function, move more symmetrically, and are more likely to attempt full competition sports.3,31
Patients with minimal side-to-side differences on the crossover hop test at 6 months possibly will have good knee function at 1 year if they continue with their current training regimen.
The timing of hop testing affects prediction models. Conducting hop tests 6 months after ACL reconstruction is appropriate for predictive purposes. The ability to predict knee function is better after a period of rehabilitation than acutely after injury or surgery.9,19
Six months after surgery is a typical time point in which patients begin to transition to sports or preinjury activities,30
so administering hop testing at this time point provides clinicians valuable information relevant to patients’ current function, their progress during rehabilitation and the necessity for additional rehabilitation, and their readiness to return to sporting activities.
Self-reported measures are frequently used as an outcome variable for knee function.8,29,51
Patient demographics, health behaviors, concomitant injuries, meniscal surgery, and knee extension range-of-motion loss have been identified as predictors of self-reported knee function.21,29,44
Lower knee function based on self-report outcomes after ACL reconstruction has been associated with lower patient satisfaction and fear of reinjury.28,31
Impaired medium-term self-reported knee function was predictive of symptomatic radiographic knee osteoarthritis 10 to 15 years after reconstruction.41
Being able to ascertain after ACL surgery those patients who will more likely have knee function below normal ranges highlights the importance of identifying modifiable predictive factors, such as the 6-m timed hop LSI, that can be readily administered by clinicians to develop targeted interventions to potentially improve knee function, increase patient satisfaction, and reduce the risk of symptomatic arthritis.
Preoperative hop testing was not predictive of postoperative knee function. Eitzen et al8
found that neither the preoperative triple nor the stair hop test was predictive of knee function 2 years after reconstruction. The LSI after preoperative rehabilitation does not delineate between those who will have IKDC 2000 scores within normal ranges compared with those with scores below normal ranges 1 year after surgery. The median LSI for all hop tests preoperatively was greater than 93% after preoperative neuromuscular rehabilitation, indicating as a group, patients were able to achieve adequate limb symmetry in all 4 hops. Although prediction is better after rehabilitation than before rehabilitation, the ACL reconstruction is a “game-changer,” and the passive stability it provides likely makes the preoperative functional hop performance irrelevant to outcome after ACL reconstruction.34,48
Although preoperative impairments, such as quadriceps weakness and knee extension range of motion, can influence knee function after reconstruction,8,50
preoperative hop performance does not predict postoperative knee function.
Furthermore, in patients who are treated nonoperatively, hop tests provide a different assessment. Fitzgerald et al11
found that the 6-m timed hop test along with other measures can predict return to previous activity level in the short term. In a prospective cohort study in nonoperatively treated individuals with ACL injury, Grindem et al,16
using a similar design and methodology of the current study, reported that only the single-hop test significantly predicted self-reported knee after 1 year. Therefore, the effect of a stabilization procedure such as ACL reconstruction may affect which hop tests predict knee function.
Many factors may contribute to the failure to achieve self-reported knee function within normal ranges. Our group was young and active and may only be generalizable to this physically active age population. This study had some incomplete data. Some patients did not meet the minimal criteria for allowing hop testing (weakness, effusion, recurrent instability) at each time period. The results can only be generalized to those with isolated ACL injuries or asymptomatic concomitant injuries and cannot be generalized to individuals with symptomatic concomitant injuries. The hop test battery was administered in the same fashion each session11,40
and may not be generalized to different hop test paradigms. A portion of patients did not complete either 6-month or 1-year testing, reducing the participants available for analysis.