The main findings of the present study were that the LSI for the single hop test, IKDC evaluation score, ACL-RSI score, and LSI for the quadricep at 6 months after ACLR predict the RTS of non-elite athletes. Furthermore, athletes who resume to sport 1 year after ACLR may have less abnormal and asymmetric gait behavior than those who do not resume to sport.
Knee function and RTS
Clinical efforts should be made to maximize quadriceps strength to achieve symmetry between limbs after ACLR. A postoperative LSI of 90% LSI for the quadricep is considered to be important for successful RTS [32]. Although the optimal strength threshold has not been determined [9], mean LSI deficits in quadricep strength > 10% are related to an increased incidence of osteoarthritis [33] and worse self-reported function following ACL reconstruction [34], as well narrower tibiofemoral joint spaces [35]. The present finding that quadricep strength is a useful predictor of RTS is not consistent with some previous reports of weak evidence for such an association in competitive athletes [16, 36]. However, Grindem et al. [37] found that quadricep strength effectively predicted reinjury after return to level I sports. Thus, predictors of RTS may differ according to the sport level and suitable predictors for different populations should be identified.
The hop test reflects knee recovery and is simple and feasible to administer in clinical practice. In this study, the LSIs for both hop tests were significantly lower in the nRTS group than in the RTS group, and the cut-off LSI of 84.4% for the single hop test predicted RTS. These findings are in line with the results of a systematic review, which yielded a predictive cut-off point of 85% for the single hop test [38]. We emphasize that these values are predictors of, rather than objective criteria for, RTS. LSI > 90% for the single hop test is generally considered to fulfill the criteria for RTS [39].
The findings of this study suggest that in addition to physical treatment, psychological counseling is needed to improve the chances of successful RTS and reduce the risk of ACL reinjury. The ACL-RSI score was significantly lower in the nRTS group than in the RTS group in this study, consistent with previous findings. Kitaguchiet al. [16] reported that ACL-RSI scores < 55 at 6 months after reconstruction were related to a greater risk of unsuccessful RTS at 1 year after reconstruction in athletes. Müller et al. [15] also found that the ACL–RSI score was among the strongest predictors of return to pre-injury sports. Psychological factors such as fear of pain, fear of reinjury, lack of confidence, and depression most commonly influence the RTS [8]. In this study, we could not determine whether patients’ RTS was based on psychological readiness or whether the patients became psychologically well prepared due to the RTS. Factors reported to be related to psychological preparation for the RTS include self-reported outcomes and knee function [17, 40]. In addition, male sex, younger age, and more frequent participation in activities were found to positively influence psychological readiness at 12 months after ACL reconstruction [40].
Three-dimensional knee kinematics and RTS
Evidence from this study supports the addition of kinematics testing to RTS test batteries. Our observation of significant differences in kinematic variables between involved and uninvolved knees in the nRTS group at 6 months after ACLR is consistent with the findings of Di Stasi et al. [18], who demonstrated greater kinematic asymmetry between limbs in patients who did fulfill the RTS criteria. The involved knees had larger external rotation angles in this study; increased external rotation of the tibia has been related directly to a change in peak contact pressure in the patellofemoral joint [41]. Moreover, kinematic changes are related to the initial stage of knee osteoarthritis [22]; the lifetime incidence of imaging-defined posttraumatic osteoarthritis in the patellofemoral and tibiofemoral joints after ACLR is 30–90% [42, 43]. Knee kinematics are multifactorial and affected by static structural integrity [44], muscle strength [45], joint proprioception, neuromuscular adaptation and control [46], psychological factors (e.g., fear of re-injury) [47] and the use of compensatory strategies [48]. Knee laxity, muscle strength, and psychological factors were assessed in this study, the knee kinematics were not restored to normal despite good performance in strength and psychological readiness. Further studies are needed to explore the causes of poor knee kinematics recovery.
The flexion-extension ROM differed significantly between the involved and uninvolved knees in the RTS group in this study. Palmieri-Smith et al. [49] observed greater knee kinematics asymmetry in the sagittal plane when the quadriceps strength was low in patients who were cleared for RTS after ACLR. Thus, sagittal knee kinematics do not depend only on the quadricep strength and further targeted interventions are needed.
The current study has several limitations. The sample was relatively small, especially with regard to the nRTS group. Although we were able to identify trends and obtain significant results with the current sample, research conducted with larger samples may enable further refinement of the cut-off points for the predictive variables. Furthermore, this study focused on non-elite athletes with isolated ACL injuries; thus, the findings may not be generalizable to patients with other injuries or sport levels. Finally, based on previous research, the follow-up period for this study was 1 year; long-term knee function was not assessed. Future studies should be conducted with longer follow-up periods to enable the exploration of long-term changes in knee function after ACLR.