ACL-reconstructed and ACL-deficient individuals show differentiated trunk, hip, and knee kinematics during vertical hops more than 20 years post-injury

The main finding of the present study is that the combined movement strategies of the trunk, hip, and knee differed for the two treatment groups and compared to healthy controls during the VH. More specifically, ACL_PT demonstrated kinematics characterized by reduced hip flexion, knee flexion, and knee rotation, while ACL_R presented the greatest trunk flexion, hip flexion, and hip rotation. The greater difference between legs in anterior tibial laxity for ACL_PT may explain these movement strategies. ACL_PT also presented lower hop height than ACL_R and CTRL. Despite this, there were similar scores for the two ACL groups on Tegner activity scale and IPAQ, and higher scores on the KOOS subscales ADL, SPORT, and QOL for ACL_PT compared to ACL_R, indicating that ACL_PT had not adapted a more restricted movement strategy compared to ACL_R. This may indicate that knee scores alone do not capture the entire picture with regard to knee function evaluation, and thus, motion analysis provides additional value.

In the absence of an ACL, knee stability relies on the remaining static and dynamic stabilizers to restrict the tibia from excessive anterior displacement and rotation. Since the ACL has an oblique medial orientation from the femur to the tibia [2], a ruptured ACL may result in a more internally rotated tibial position during passive motion. Indeed, in knees where the ACL has been removed, an increased internal rotation has been shown during application of an axial tibial force [16]. In the present study, the observed less pronounced knee internal rotation, in fact an externally rotated knee, in combination with less knee flexion in Take-off for ACL_PT compared to CTRL, may therefore be a knee-protective strategy to avoid positions of internal rotation where give way might occur. The same strategy was found during Landing, although with the addition of less hip flexion and greater hip adduction when compared to both ACL_R and CTRL. Since an internal tibial torque also induces a coupled anterior tibial translation relative to the femur [13], this further supports a protective avoidance strategy of potentially risky knee positions for ACL_PT. Similar results of reduced knee sagittal plane movement for ACL-deficient persons compared to ACL_R and CTRL groups have been shown in landings of a one-leg hop for distance [22]. This movement strategy seems not to be explained by quadriceps strength since no differences were shown between any of these groups for knee extension strength, although both treatment groups had lower knee flexor strength than controls. In contrast, in our study population only the concentric knee extension strength, but not concentric flexion strength, was lower for both ACL_PT and ACL_R compared to CTRL, as presented previously [26].

ACL_R adapted a strategy where they increased trunk and hip flexion compared to both ACL_PT and CTRL. Increased trunk flexion was indeed observed in both phases for the injured compared to the non-injured leg for ACL_R. Similar results with greater trunk flexion between legs for ACL-reconstructed persons in another study further revealed a forward shift of the centre of pressure, which resulted in a more anterior position of the ground reaction force vector in relation to the hip, knee, and ankle joint axes [18]. This strategy shifted the joint moment from the knee to the adjacent joints. Instructions to land with increased trunk flexion also result in increased hip and knee flexion, but do not influence movement in the frontal or transversal planes, when compared to a preferred landing strategy during the vertical drop landing [4]. On the other hand, a landing strategy with a more upright trunk flexion results in increases in peak vertical ground reaction force, peak knee extensor moment, hip moment, and quadriceps amplitude [24]. A small knee flexion angle also increases the patellar tendon insertion angle and decreases the hamstrings insertion angle, which results in an increased anterior tibial shear force for a given anterior tibial shear load [4]. The ACL_PT group would therefore seem to have adopted a movement strategy that restricts movement, although provokes an anterior tibial translation when compared to the other groups. This movement strategy was particularly evident when comparing their injured leg to the non-injured leg in Landing, demonstrating less knee flexion and knee internal rotation for the injured leg. In addition, the altered kinematics also for the non-injured leg in ACL_PT compared to both ACL_R and CTRL further indicate a crossover effect displayed to a greater extent than for ACL_R. The restrained movement strategy for ACL_PT for both legs may indicate a greater vulnerability to challenging loadings of the knee in one-leg hops [21], possibly also increasing the risk of injury for the intact ACL of the other leg, and contributing to development of OA [2]. Of particular relevance here may be the average differences in knee rotation of 5.9° in Take-off and 6.7° in Landing which were shown for ACL_PT compared to CTRL for I/ND legs. These are above the value of 5° shown to accelerate cartilage thinning [3], which may be detrimental to future knee health. Particular consideration should be given to the coupled average differences in knee flexion of 8.3° in Take-off and 9.9° in Landing between the same groups. For comparison, there were non-significant average values of knee flexion and knee rotation for ACL_R and CTRL in Take-off of 2.3° and 4.5°, respectively, and in Landing of 2.9° and 4.7°, respectively. Hop tests with a greater emphasis on rotational demands may enhance this difference for ACL_R compared to CTRL due to rotational instability that seems to remain after ACL reconstruction [10, 29]. Previously reported results from the same population show that both treatment groups have different kinematics compared to the same controls also in the two-leg squat and side hop tests with knee rotational instability [10] and in the one-leg hop for distance where less knee flexion and knee internal rotation were shown [27]. The results of the present study further corroborate these earlier findings but now extend the analysis of movement strategies to also include trunk and hip movements using multivariate methods.

There are very few long-term kinematic studies, and the existing ones do not indicate any differences between ACL-injured persons and controls. von Porat et al. [30] reported similar knee kinematic and kinetic results for ACL-injured persons and matched healthy controls during gait, step up, and crossover hop tests at 16 years post-ACL injury. Ortiz et al. [20] also reported similar hip and knee kinematic outcomes between physically active women with ACL reconstruction compared to healthy, non-injured women in drop jump and up-down tasks. However, only 12 persons (six treated with surgery, six without) participated in the 16-year follow-up and just 13 ACL-injured persons, with a mean time of 7.2 years (range of 1–16 years) after reconstruction, participated in the latter study. These smaller studies with mixed groups might be under-powered, and thus, the inconsistent results may depend on study populations, group sizes, and variations in assessment and follow-up times.

A general insight into clinical importance is that knee scores alone most likely do not capture the entire picture with regard to evaluation of knee function. Analysis of movement strategies certainly provides additional value, especially when evaluating treatment. A VH test is easy to administer in the clinic, is not time-consuming, enables comparisons of biomechanical asymmetries between injured and non-uninjured sides, and can be used to determine progress in rehabilitation. Different kinematics for both ACL groups are of clinical relevance since instability may exist irrespective of treatment. Both ACL_PT and ACL_R had >90% LSI for hop height, despite demonstrating different movement patterns and low self-estimated knee function. Interpretation of results based on LSI scores or absolute measures from hop performances should therefore be analysed with caution when evaluating knee function if used without consideration of other knee function measures. Similar results have been found for ACL-injured persons achieving >90% in dynamic tests despite presenting altered kinematics [19] or having self-estimated unstable knees [6, 30]. Hopefully, better and simpler motion analysis of movement strategies in the clinics may be developed based on more detailed analyses such as in the present study. Our results highlight the importance of incorporating the trunk and hip when evaluating movement strategies for ACL-injured persons in the clinics rather than only focusing on the knee joint.

Some limitations of the present study should be acknowledged. The use of skin markers implies risks of soft tissue artefacts influencing validity and reliability. To reduce this, clusters of markers were used on areas with large muscle groups for better approximations. Soft tissue artefacts seem to be similar between subjects and hence should not mask group differences [8]. Comparisons between the different treatment groups could also be questioned as this was not an RCT. Long-term kinematic studies are, however, very rare, and the biomechanical conditions from these cohorts were described without necessarily drawing direct conclusions about treatment recommendations. It should be mentioned that major improvements in AC reconstruction techniques has occurred during the past 20 years and need to be considered in relation to our results. Multiple health-related aspects over the years may also play a role, such as the precise impact and course of OA that may vary [17]. However, no effects of the degree of radiological knee OA on knee kinematics was found in an earlier report on the same study population [25]. The higher Tegner activity score for CTRL compared to both treatment groups could be questioned, but was partly explained by the high score assigned to occasional participation in downhill skiing during leisure time for many participants as they live in a northern climate. No group differences for IPAQ scores may indicate that the amount of activity does not explain the different movement patterns. Also, since BMI did not present any correlations with or influenced any of the kinematic results, the groups included in this study could be considered comparable for this biomechanical investigation.