Background Information as to how anterior cruciate ligament (ACL) injury and reconstructive surgery (ACLR) alter lower extremity biomechanics may improve rehabilitation and return to play guidelines, reducing the risk for repeat ACL injury.
Aim To compare lower extremity biomechanics before ACL injury and after subsequent ACLR for the injured and uninjured leg.
Methods Baseline unilateral lower extremity biomechanics were collected on the dominant leg of participants without ACL injury when they entered the Joint Undertaking to Monitor and Prevent ACL (JUMP-ACL) study. Thirty-one participants with subsequent ACL injury, reconstructive surgery and full return to physical activity completed repeat, follow-up biomechanical testing, as did 39 uninjured, matched controls. Not all injured participants suffered injury to the dominant leg, requiring separation of those with ACL injury into two groups: ACLR-injured leg group (n=12) and ACLR-uninjured leg group (n=19). We compared the landing biomechanics of these three groups (ACLR-injured leg, ACLR-uninjured leg, control) before ACL injury (baseline) with biomechanics after ACL injury, surgery and return to physical activity (follow-up).
Results ACL injury and ACLR altered lower extremity biomechanics, as both ACLR groups demonstrated increases in frontal plane movement (increased hip adduction and knee valgus). The ACLR-injured leg group also exhibited decreased sagittal plane loading (decreased anterior tibial shear force, knee extension moment and hip flexion moment). No high-risk biomechanical changes were observed in control group participants.
Conclusions ACL injury and ACLR caused movement pattern alterations of the injured and uninjured leg that have previously shown to increase the risk for future non-contact ACL injury.
- Knee injuries
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Athletes with prior anterior cruciate ligament (ACL) injury and reconstructive surgery (ACLR) have altered lower extremity biomechanics compared to those who have never suffered an ACL injury1–4 and demonstrate significant differences between the injured and uninjured legs.3–7 These alterations are potentially important since individuals with previous ACL injury have a 5–15-fold increased risk for subsequent ACL injury,8–10 increased development of osteoarthritis,11 ,12 and decreased physical activity.13 ,14 Preliminary evidence that lower extremity biomechanics are predictive of subsequent non-contact ACL injury in those with ACLR15 suggests that ACL injury and ACLR might in themselves induce biomechanical movement pattern alterations that result in greater risk for subsequent injury and poor outcomes.
There is, however, no substantial evidence to demonstrate that ACL injury and ACLR induce these deleterious changes, as previous reports of biomechanical measures collected prior to and following ACL injury have been limited to case reports.16 ,17 Therefore, it is presently unknown if altered biomechanics following ACL injury and ACLR are the result of the injury and surgery, or if these alterations were present prior to injury and may have contributed to the initial injury. The ability to distinguish pre-existing biomechanical alterations from those induced by ACL injury and surgery would provide valuable information to optimise rehabilitation regimens and provide a safer return to physical activity following ACLR.
The purpose of this study was to compare lower extremity biomechanics before ACL injury and after subsequent ACLR for the injured and uninjured leg. Previous studies indicate asymmetrical lower extremity biomechanics following ACL injury and that the risk for repeat injury commonly affects the injured and uninjured leg,10 ,18–20 so we chose to study the effect of ACL injury on the biomechanics of the injured and uninjured leg in two separate groups. These analyses allow us to determine if differences in lower extremity biomechanics were present prior to ACL injury or only after ACLR.
This study employed repeated measures, case-cohort research design. All participants were recruited from the Joint Undertaking to Monitor and Prevent ACL Injury (JUMP-ACL) study, a multiyear, multisite prospective study to identify risk factors for ACL injury. Participants were enrolled at service academies in the USA; United States Air Force Academy, United States Military Academy and United States Naval Academy, and completed initial biomechanical testing (baseline) for the JUMP-ACL study during the summer of their enrolment year, and prospectively monitored during his/her career at the service academy for ACL injuries.
Participants identified for enrolment in this study (follow-up), were limited to those with complete baseline (prior to initial ACL injury event) biomechanical data and who had no history of ACL injury prior to enrolment. ACL-injured participants (cases) were identified as having suffered an ACL injury during their enrolment in the study, and were still enrolled in the JUMP-ACL study and respective service academy. For each case, three controls were identified for follow-up, matched based on gender, cohort year and service academy. Testing at baseline for the JUMP-ACL study captured unilateral lower extremity biomechanics, and not all ACL injuries for the cases occurred on the tested leg: 12 injured the tested leg and 19 injured the non-tested leg. Hence, the cases (n=31) were further subdivided into two separate groups; ACLR-injured leg (ACLR-INJ; n=12) for those who injured the tested leg and ACLR-uninjured leg (ACLR-UNINJ; n=19) for those who injured the non-tested leg. Twenty-nine of the cases had no history of ACL injury at baseline. Two cases were retained in the ACLR-INJ group with previous ACL injury because their uninjured leg was tested at baseline and the same leg was subsequently injured, representing data that qualified them for the ACLR-INJ group. This resulted in three groups (ACLR-INJ, ACLR-UNINJ and control) for analysis, and allowed us to independently assess effects of ACL injury and ACLR in the injured and uninjured leg.
All procedures were conducted after institutional review board (IRB) approval for each institution. Informed consent was obtained prior to data collection for all participants with prescribed procedures to avoid coercion. Each participant performed a double leg jump-landing manoeuvre at baseline and follow-up (figure 1)21 ,22 and similar manoeuvres have been previously used to identify prospective risk factors for initial23 and subsequent15 ACL injury and represents a common movement pattern in sports and physical activity. Participants were required to stand atop a 30 cm box located at a distance from the front edge of the force plate equal to half of their body height. They then jumped forward towards the force plate, landed with only the leg of interest making contact with the force plate, and immediately performed a maximal effort vertical jump. Detailed data collection methods were established and followed for each data collection session to ensure consistency.
Biomechanical data were collected using an electromagnetic tracking system (Ascension Technologies Inc, Burlington, Vermont, USA) integrated with a non-conductive force plate (Bertec Co, Columbus, Ohio, USA). Prior to data collection, sensors were placed on the shank and thigh of the leg, and pelvis. The position of the medial and lateral malleoli, medial and lateral femoral epicondyles, and the anterior superior iliac spines relative to the segment sensors was recorded using a movable sensor. The ankle joint centre and knee joint centres were estimated as the midpoint between the malleoli and femoral epicondyles, respectively, and the hip joint centre was estimated based on the location of the anterior superior iliac spines according to the Bell method.24 A segment-link model of the shank, thigh and pelvis was developed based on these points, with the shank segment defined by the ankle and knee joint centres and the shank sensor, the thigh segment defined by the knee and hip joint centres and the thigh sensor, and the pelvis as the anterior superior iliac spines and the pelvis sensor. Local right-handed axis systems were embedded in each segment to describe position and orientation. Knee and hip joint angles were defined as the shank position relative to the thigh and thigh position relative to the pelvis, respectively. Joint angles at each were calculated using an Euler sequence with first rotation defining flexion/extension, second rotation defining valgus/varus or adduction/abduction, and third rotation defining internal/external rotation. All biomechanical data collection was consistent with previously described procedures.21 ,22
All kinematic data were sampled at 144 Hz and kinetic data were sampled at 1444 Hz. Kinematic data were lowpass filtered using a fourth order Butterworth filter (14.5 Hz) and all biomechanical data were collected and exported using the Motion Monitor Software (Innovative Sports Training, Inc, Chicago, Illinois, USA) consistent with previously described methods.21 ,22 Moments were normalised to the product of body height (m) and weight (N), and are reported here as normalised internal joint moments. The vertical ground reaction force (VGRF) and anterior tibial shear force (ATSF) data were normalised to body weight (N). All dependent variables during the time points of interest were calculated using a customised MATLAB (Mathworks, Inc, Natick, Massachusetts, USA) programme. Values for each variable were recorded at initial ground contact (IGC), defined as the first time point at which the VGRF exceeded 10 N, and peak values in each direction during the landing phase (LP) of the double leg jump-landing, defined as the time from IGC to peak knee flexion.
Changes in the dependent variables were assessed via 3 (groups: ACLR-INJ, ACLR-UNINJ and control)×2 (time: baseline, follow-up) mixed model analysis of covariance (gender), for each dependent variable. Post hoc analyses consisted of Tukey's HSD (honest significant difference), and were performed for any significant interaction effect. An α level of 0.05 was set a priori to determine statistical significance for all analyses (IBM SPSS V.19, SPSS, Inc, an IBM company, Chicago, Illinois, USA).
Participant demographics and anthropometrics for baseline and follow-up testing sessions are summarised in table 1. We were unable to obtain graft type for five members of the ACLR-INJ group, and six of the ACLR-UNINJ group. The ACLR-INJ group reported three bone-patella tendon-bone autografts and four hamstrings autografts. The ACLR-UNINJ group reported five bone-patella tendon-bone autografts, seven hamstrings autografts and one Achilles tendon allograft. Participants completed a similar rehabilitation programme, and were healthy at the time of testing.
Group chronological data for testing sessions, ACL injury and surgery are presented in table 2.
Initial ground contact
We observed significant interactions for frontal plane knee (F(2,66)=3.957, p=0.024) and hip (F(2,66)=3.773, p=0.028) angles at IGC. Post hoc analysis indicated no difference among groups for either variable at baseline. From baseline to follow-up, the control group demonstrated no change, while both ACLR groups demonstrated a significant increase in knee valgus and hip adduction angle at IGC. At follow-up there was no difference between ACLR groups for either variable, suggesting a similar pattern of change in both. Both ACLR groups landed with greater hip adduction at follow-up as compared to the control group.
We observed significant interactions for peak knee varus angle (F(2,66)=5.198, p=0.008), peak knee valgus angle (F(2,66)= 3.768, p=0.028) and peak knee internal rotation angle (F(2,66)=4.204, p=0.019). Again, there was no difference among groups at baseline. The control group drove the significant interaction for peak knee internal rotation angle, as they demonstrated a significant increase from baseline to follow-up testing. No significant change in knee rotation angle was observed for the ACLR groups and the peak knee internal rotation angle for the control group was greater than the ACLR-UNINJ group at follow-up. The control group demonstrated no other significant change in peak kinematics during the LP. Both ACLR groups demonstrated a significant decrease in peak knee varus angle, and the ACLR-UNINJ group demonstrated a significant increase in peak knee valgus angle from baseline to follow-up testing. There was a similar increase in peak knee valgus angle for the ACLR-INJ group but it did not achieve statistical significance. There was no difference among groups at follow-up for peak knee varus and valgus angle.
We observed significant interactions for peak knee extension moment (F(2,66)=4.509, p=0.015), peak hip flexion moment (F(2,66)=3.847, p=0.026) and peak ATSF (F(2,66)=4.530, p=0.014). These were the result of changes from baseline to follow-up for the ACLR-INJ group only. There was no significant difference among groups at baseline, but the ACLR-INJ group demonstrated a significant decrease in peak internal knee extension moment, peak internal hip flexion moment and peak ATSF from baseline to follow-up. This change resulted in the ACLR-INJ group demonstrating lower values for each variable as compared to the ACLR-UNINJ group at follow-up.
We believe the most important finding of this study is that movement patterns, which remain remarkably consistent in uninjured controls, are altered by ACL injury and reconstructive surgery. These alterations that occur in the injured and uninjured limb, are not ameliorated through traditional physical therapy rehabilitation regimens, and result in postinjury movement patterns identical to those shown to be predictive of future ACL injury.15 ,23 The consistency of the control group's biomechanics from baseline testing to follow-up testing—only peak knee internal rotation angle during the LP changed—highlights the native resiliency of these biomechanical patterns. Control group movement patterns remained essentially unchanged despite the participants enrolling into military service requiring high levels of physical activity and training over approximately 2 years of follow-up, but as we have demonstrated here for the first time, these normally stable movement patterns are susceptible to change following ACL injury.
We postulate that the altered movement patterns we observed following ACL injury may stem from decreased internal knee extension moment of the injured knee following injury. As seen in our data and elsewhere, decreasing knee extension moment correlates with a decrease in ATSF, and decreased shear force on the reconstructed ACL graft.25 ,26 Therefore, the decreased ATSF in the ACLR-INJ group represents a decrease in loading of the injured limb and reconstructed ACL. To explain what influences ATSF and subsequent loading of the ACL, Sell et al27 demonstrated that peak posterior ground reaction force, normalised external knee flexion moment, knee flexion angle, quadriceps activation and gender were predictive of ATSF during a similar double-leg jump-landing in healthy participants. Of these variables, decreased internal knee extension moment (ie, the internal response to an external knee flexion moment) and quadriceps activation are the likely cause of our observation as there was no difference among groups for peak knee flexion angle, gender was controlled for in our statistical model, and a post hoc analysis of peak posterior ground reaction force indicated no difference among groups (F(2,66)=0.210, p=0.811).
Decreased internal knee extension moment has been previously observed in those with ACLR.1 ,7 ,28 ,29 Internal knee extension moment is an estimation of the moment generated by the quadriceps as they act eccentrically to brake against the external knee flexion moment of landing. ACLR patients in our study may have decreased their internal knee extension moment as an active strategy to reduce loading of the reconstructed ACL.25 The decreased internal knee extension moment may also be due to an involuntary reduction in quadriceps muscle activation as a result of injury. Palmieri-Smith et al30 observed an interaction between reduced quadriceps activation imposed by a knee effusion model produced by saline injection and subsequent reduction in knee extension moment during a single-legged jump landing. Therefore, we believe our finding of reduced sagittal plane loading is likely the result of quadriceps dysfunction: a dyad of reduced quadriceps activation (voluntary or involuntary) and reduced quadriceps strength, which has been previously observed in those with ACLR.1 ,7 ,28 ,29
Quadriceps dysfunction post-ACLR may decrease the body's ability to resist landing forces in the frontal plane, as previous observations indicate the quadriceps have the ability to resist frontal plane loading at the knee.31–34 We hypothesise that our observation of increased hip adduction and knee valgus is a compensatory force absorption strategy as quadriceps dysfunction results in decreased frontal plane absorption while VGRF remains unchanged. Therefore, following injury, individuals land with the same amount of lower extremity loading and VGRF, but have a decreased ability to resist this loading in the frontal plane because of injury induced quadriceps dysfunction. They therefore compensate by absorbing the remaining force through increased medial displacement of the knee and hip. This is one possible mechanism whereby quadriceps dysfunction in the injured limb may directly induce increased knee valgus and hip adduction during landing. This postulation of altered quadriceps use on frontal plane motion has been previously reported by Palmieri-Smith et al35 as they observed an association between decreased preparatory activation of the quadriceps and increased peak knee valgus angle in healthy women during landing.
The ACLR-UNINJ group demonstrated similar patterns of increased hip adduction and knee valgus but without changes in internal knee extension moment or ATSF. Similar changes to the uninjured leg may be indicative of a global deficit in quadriceps activation following ACLR. Bilateral reduction in quadriceps activity has been previously observed for those with unilateral ACL injury.36–39 Urbach et al37 demonstrated that voluntary activation of the quadriceps decreases for both legs following ACLR. They also observed that before ACLR, quadriceps force in the injured leg was less than the uninjured leg, and differences remained relatively stable 2 years after ACLR. It is possible that a lower level of quadriceps dysfunction may have been present during rehabilitation and produced kinematic alterations in our ACLR-UNINJ group. Alterations in knee moments or loading may not have been observed because quadriceps dysfunction recovered disproportionally between legs, with the uninjured leg returning to normal while still displaying altered kinematics. Whatever the cause, this observation helps explain why ACL reinjury commonly occurs in the uninjured leg,10 ,18–20 as increased medial displacement of the knee is predictive of initial ACL injury and reinjury for those with ACLR.15 ,23 This is tempered by the fact that data collected for the ACLR groups consisted of two separate groups of individuals; therefore, the observed movement patterns may be unique to each sample. The possible mechanism responsible for these movement alterations—residual quadriceps dysfunction or the promotion of symmetrical movement during rehabilitation—needs to be further investigated.
These results have important clinical implications, as this investigation provides the first evidence—other than case reports16 ,17—to describe how ACL injury and ACLR may increase the risk for recurrent ACL injury via increased medial displacement of the hip and knee.15 ,23 The altered movement patterns were consistent across our injured population despite not controlling for injury mechanism, graft type or rehabilitation protocol. This suggests these alterations are characteristic of ACL injury itself and correcting these faulty patterns may be important for a variety of patients with differing mechanisms of initial ACL injury. We believe these data indicate that acquiring adequate quadriceps eccentric strength and symmetric loading of the injured and uninjured knees following ACLR may help decrease the development of faulty movement patterns. Also, symmetrical motion between limbs is often encouraged during rehabilitation for a number of reasons, and because we observed alterations in the uninjured leg, we encourage clinicians to not use this as a strict rule, as others have previously suggested.36 ,38 Rather, our findings suggest that rehabilitation should focus on the quality of movement in the injured and uninjured leg alike which will likely achieve symmetrical movement and movement that is less likely to lead to further injury.
Finally, we would recommend careful assessment of individuals with ACLR who wish to continue participating in sport and exercise. Our participants were required by military order to complete similar standard-of-care post-ACLR rehabilitation programmes available without cost to them. Yet approximately 21 months postsurgery, our young, healthy participants still displayed dramatic deficits in movement and joint loading. This suggests that current rehabilitation practices may not adequately address deficits induced by ACL injury. Prolonged reinjury prevention in the form of movement retraining and strengthening programmes may be needed, especially given recent evidence that the duration of a movement retraining programme profoundly influences retention.40
Our study is not without limitations. There were variable periods of time between baseline and ACL injury, and return to physical activity and follow-up during which changes may have occurred that could have influenced our results. The lack of change in biomechanics of the control group, however, strongly supports that the changes we observed with this study are the result of ACL injury and ACLR only. The consistency of our findings among a relatively heterogeneous group in terms of injury mechanism and graft type also strengthens the evidence that these changes are due to injury. These factors were not controlled for intentionally in hopes of being able to include as many participants as possible without negatively affecting the internal validity of this study, as the opportunity to conduct this study was rare and valuable.
In conclusion, our observations indicate that following ACL injury, physically active individuals with ACLR demonstrate an increase in knee valgus and hip adduction during a double leg jump-landing. This is present in separate groups examining the injured and uninjured leg. These exact alterations in movement have been previously identified as prospective risk factors for non-contact ACL injury and likely explain why those with previous ACL injury are at greatly increased risk for secondary ACL injury.
What are the new findings?
This study demonstrates for the first time how preinjury lower extremity biomechanics change following anterior cruciate ligament (ACL) injury and reconstructive surgery (ACLR).
Hip adduction and knee valgus increase following ACL injury and ACLR in the injured and uninjured leg.
Movement pattern changes are accompanied by a decrease in internal knee extension moment, internal hip flexion moment and anterior tibial shear force in the injured leg.
How might it impact on clinical practice in the near future?
Since these movement patterns have been previously identified as prospective risk factors for non-contact ACL injury, this study provides and explains why and how athletes with initial ACL injury are at an increased risk for secondary ACL injury, and suggests how rehabilitation may be improved to reduce these movement patterns and reduce the risk for injury.
Contributors BMG, SWM, AIB and DAP contributed to conception and design of the study, data acquisition, analysis of data and interpretation of data. JTB and JHW contributed to conception and design of the study, and interpretation of data. All authors contributed to drafting, revising and approving the manuscript.
Funding The Joint Undertaking to Monitor and Prevent Anterior Cruciate Ligament (JUMP-ACL) study was funded by a grant provided by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (#R01-AR050461001) and pilot work was funded by the American Orthopedic Society For Sports Medicine.
Competing interests None.
Patient consent Obtained.
Ethics approval Office of Human Research Ethics, The University of North Carolina at Chapel Hill.
Provenance and peer review Not commissioned; externally peer reviewed.
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