The purpose of this study was to compare the gait characteristics of athletes who passed RTS criteria and athletes who did not pass RTS criteria 6 months after ACL reconstruction. Our data supported our initial hypotheses that patients who did not pass RTS criteria would demonstrate smaller peak knee angles and lower extensor moments on the involved knee. Interestingly, the expected compensatory strategy of increased ipsilateral hip extensor moments and power absorption was not found; rather, the poorly functioning patients appeared to rely on an altered contralateral hip strategy while simultaneously limiting motion and attenuating forces about the injured knee. The functional examination used to determine RTS readiness in this study appears most useful in identifying athletes with significant gait abnormalities after ACL reconstruction.
Patients who did not pass RTS criteria demonstrated statistically significant differences between limbs on all kinematic and kinetic variables at the knee. Specifically, lower peak knee flexion, knee extensor moment, and peak power absorption were characteristic of their reconstructed knee. Kinetic asymmetries at the knee were found in both groups of patients; however, the magnitude of knee extensor moment asymmetry in the patients who did not pass RTS criteria was more than twice that of the group who passed RTS criteria. Furthermore, knee joint motion asymmetries appeared to be characteristic of the group who did not pass RTS criteria. Early after injury, noncopers adopt a stiffened knee strategy by reducing the motion at the knee and co-contracting the supporting musculature in a likely attempt to avoid repeated episodes of joint instability.39
It is apparent from our data and from previous findings17,38
that this stiffened knee strategy can persist in athletes in spite of successful ligament reconstruction and may more accurately reflect the functional deficits of those who do not meet RTS criteria.
The compensatory movement behaviors of ACL-injured athletes are not confined to the reconstructed knee. The contrasting hip strategy found between the limbs of the patients who did not pass RTS criteria reinforces the concept that a unilateral ACL injury can elicit a “bilateral kinetic response”16
by which the injured patient also adopts an altered neuromuscular strategy for the uninvolved limb. This contralateral hip compensation appears to persist even after surgery and supervised rehabilitation for some athletes. In addition to absorbing power in the uninvolved hip early during the stance phase, the patients who did not pass our RTS criteria appeared to “overflex” their uninvolved knee during weight acceptance. This specific adaptation has not previously been described in this group of ACL-reconstructed athletes but may be another manifestation of the aberrant contralateral limb–loading patterns adopted by some patients after surgery.36,37
The hip strategy of the patients who failed RTS criteria partially supported our second hypothesis concerning asymmetries at the hip. While clinically meaningful limb differences in peak hip flexion angle and hip power were found, these asymmetries appeared to stem more from a compensatory strategy of the uninvolved hip rather than from an adaptation of the involved hip. Although not originally hypothesized, this adaptation was not totally unexpected. Roewer and colleagues38
also identified this strategy in noncopers 6 months after ACL reconstruction in which the uninvolved hip absorbed power and the involved hip generated power early in weight acceptance. Our findings indicate that this asymmetry may be specific to noncopers with poorer functional performance after ACL reconstruction and does not wholly define this cohort of athletes.
The first several months after reconstruction represent a time of marked movement abnormalities5,9,17,37,38
often in spite of high-level sport performance.36,37
Ideally, a cluster of clinical tests and measures assessing the functional abilities of athletes would directly and robustly relate to the comparable measures of biomechanical impairments. Our data indicate that while performance on clinical tests and measures demonstrates some relationship with meaningful movement deficits and asymmetry, they may be only part of the ideal formula for informing RTS decision making. Neuromuscular asymmetries during dynamic movement may not only affect sport performance but also be highly predictive of the risk of a second knee injury. Paterno and colleagues37
prospectively followed 56 athletes for 1 year after their medical discharge to determine the biomechanical risk factors related to reinjury. Nearly one quarter of their patients experienced a second ACL injury. Increased knee abduction motion, asymmetry in internal hip rotation moment, asymmetry in knee extensor moments during a drop vertical jump, and deficits on single-legged postural assessment predicted the second injury with excellent sensitivity (92%) and specificity (88%). In a cohort of nearly 300 ACL-reconstructed athletes, Laboute and colleagues25
reported a nearly 3-fold increase in an athlete’s risk of reinjury if the patient returned to activity within the first 7 months after surgery. Regardless of functional ability, returning to sport within 6 months of surgery may place athletes at an increased risk for reinjury, particularly if movement asymmetries are also present. Concurrent assessments of functional ability and movement symmetry may be the ideal method of assessment for RTS readiness in a group of athletes at great risk for second knee injuries. Three-dimensional biomechanical analysis of human movement, however, is time intensive, highly technical, and expensive. Ultimately, identification of highly sensitive and specific clinical correlates to the biomechanical measures predictive of poor function and risk of a second injury will enhance RTS decision making for these athletes.
Perhaps of equal importance is the purported relationship between abnormal movement patterns and the development of osteoarthritis after ACL injury. Anterior cruciate ligament reconstruction alone cannot prevent degenerative joint changes,23,27,44,45,48
as it is possible that the intra-articular cellular response that occurs early after injury already initiates this process.26,28,34
The initiation and progression of osteoarthritis may be a result of a cyclic pattern of pathomechanics in the wake of ACL injury.3,47
By this proposed framework, the continued disturbance in the biomechanics of the joint by way of altered limb motion provides an intracapsular environment conducive to the degenerative process.3
The abnormal movement strategies adopted in response to ACL injury are also not ubiquitously resolved with surgery.15–17,36–38
The persistence of these maladaptations during tasks as simple as walking may have serious consequences to the long-term health of the injured joint.
Our study population was limited to noncopers, who represent the poorest performing group of ACL-deficient athletes. Although the majority of ACL-injured athletes are noncopers,21
these findings may not translate to all ACL-deficient patients who undergo reconstruction. Similarly, these data do not indicate that all noncopers who go on to fail RTS testing after ACL reconstruction demonstrate gross movement asymmetries, nor are all noncopers who go on to pass our RTS criteria after ACL reconstruction completely free of movement dysfunction. While concomitant assessment of functional performance and gait symmetry may enhance RTS decision making, widespread application of motion capture techniques is not feasible. The development and validation of RTS criteria that accurately identify athletes with movement asymmetries are warranted.
Noncopers are a notoriously variable group of ACL-injured athletes.18,30
Previous work has highlighted the difficulty with which these athletes resume normal function and adopt symmetrical movement strategies in the months after ACL reconstruction.10,17,19,38
In an attempt to more accurately characterize the movement patterns of these athletes after surgery, we stratified our cohort by functional performance on a validated clinical testing battery.13
By this method, the lower functioning patients (ie, those who did not pass RTS criteria) demonstrated more meaningful limb asymmetries during gait than did their higher functioning counterparts. Furthermore, patients who failed RTS criteria were older and had a longer time from injury to surgery. While these factors may also influence RTS success in this cohort, examination of this relationship was not the purpose of this study.
To our knowledge, this work is the first to distinguish the movement patterns between higher and lower functioning athletes after ACL reconstruction. Importantly, athletes who have been classified as noncopers before surgery and then demonstrate poorer function after ACL reconstruction may be more likely to have clinically significant neuromuscular asymmetries compared with those who pass RTS testing. Movement asymmetries, specifically, are identifiable and modifiable risk factors for a second ACL rupture in athletes who have already returned to activity.37
Additional targeted therapy may be warranted to address flawed movement mechanics and maximize function in some noncopers after ACL reconstruction. Neuromuscular interventions targeting abnormal and asymmetrical movement behaviors in athletes preparing to return to sport may not only enhance functional performance but also mitigate the risk for secondary injury and reduce the likelihood of developing debilitating osteoar-thritis of the knee joint.