Anterior cruciate ligament (ACL) injury is one of the most common knee injuries in sport (30). Most patients undergo surgical reconstruction as a means to reestablish static knee stability (29). Unfortunately, ACL reconstruction and traditional rehabilitation do not restore lower extremity joint mechanics (3,14), creating a scenario in which there is greater risk for ACL reinjury (37) and early-onset osteoarthritis (3,28). Specifically, during level ground walking and jump-landing tasks, patients with reconstructed ACL who have been cleared for full physical activity demonstrate reduced peak knee flexion angles (27) and smaller peak extension moments (16,27) in concert with excessive knee abduction moments (8). Biomechanical alterations have also been observed proximal to the knee, with patients with ACL injury experiencing decreased hip flexion (14,19) and increased hip adduction (19) angles. Sagittal and frontal plane alterations in hip biomechanics during gait may lead to aberrant loads at the knee joint, creating a mechanism thought to lead to the onset and progression of posttraumatic osteoarthritis (3).
Given that posttraumatic knee joint degeneration is hypothesized to develop from chronic and consistent abnormal joint loading (3), it seems plausible that abnormal biomechanics during daily tasks have prominent and detrimental long-term consequences on knee joint health. Few articles have assessed biomechanical adaptations of patients with ACL injury during more challenging activities of daily living, such as stair ambulation. Stair walking is known to produce larger joint angles and moments compared with level ground walking, indicating that ascending and descending stairs are more biomechanically demanding tasks (2,39). Limited data suggest that individuals with reconstructed ACL, specifically those greater than 1 yr after surgery and cleared for full physical participation, ambulate stairs with less flexed (20) and more abducted knee joints (18). This shift in knee joint position likely affects the location of tibiofemoral joint contact area (3), which could be detrimental to long-term joint health by creating an abnormal load distribution on the articular cartilage (3). Although data that separately examine biomechanical alterations in patients with ACL injury or reconstructed ACL exist (18,20,46), there remains a paucity of evidence that longitudinally evaluates changes in biomechanics throughout the injury process, specifically during stair walking. Data generated during level ground walking indicate that biomechanical adaptations occur early in the injury process and can progress in the years after surgery (15). Understanding how these biomechanical alterations progress throughout the course of injury and reconstruction can help identify biomechanical risk factors that could lead to the progression of joint degeneration. Furthermore, identifying deleterious biomechanics early in the injury process could help guide presurgery therapeutic programs, such as early gait retraining, to counter these abnormalities.
As such, the primary purpose of this investigation was to longitudinally evaluate peak sagittal and frontal plane knee and hip joint angles and moments, joint angles at initial contact, and joint excursions during stair ascent and descent in patients with acute ACL injury, and again at a time when these individuals returned to activity after reconstruction. It was our hypothesis that patients with ACL injury would ambulate stairs with more extended knee and hip joints before surgery, demonstrate smaller knee extension moments, and use less sagittal plane knee joint motion in the injured limb compared with that in the uninjured limb and healthy controls. Furthermore, as patients with ACL injury have demonstrated alterations in level ground walking gait and jump-landing biomechanics upon return to sport participation (8,16,27), we anticipated that these alterations during stair walking would persist and also be present when they were cleared for unrestricted activity.
Twenty patients who had sustained a unilateral, primary ACL rupture, confirmed by physician examination and magnetic resonance imaging, were recruited from the Department of Orthopedic Surgery at the University Medical Center (Table 1). Patients with ACL injury reported no history of orthopedic surgery or major musculoskeletal injury in either lower extremity limbs. After reconstruction, all patients were entered into a standardized rehabilitation protocol by the orthopedic surgeon and physical therapy clinic of the University Medical Center (see Document, Supplemental Digital Content 1, Standardized ACL rehabilitation protocol, http://links.lww.com/MSS/A558).
Twenty healthy control participants free of any history of major lower extremity musculoskeletal joint injury or surgery and lower extremity musculoskeletal injury of any kind within the past 6 months were also recruited and demographically matched to patients with ACL injury using age, height, mass, and activity level. Patient activity level was evaluated using the Tegner score, and self-reported function was assessed via the international knee documentation committee (IKDC) form (Table 1). The biomedical institutional review board approved this study, and all participants provided written informed consent before enrolment.
This longitudinal, case–control investigation involved two testing sessions: presurgery (after injury but before reconstruction) and 6 months postsurgery (cleared by physician for unrestricted activity). The two testing sessions for healthy participants were separated by an average of 27.7 ± 1.4 wk (Table 1). All outcomes were collected bilaterally in both groups, with limbs of the healthy group being matched to those of the ACL group on the basis of limb dominance. Limb dominance was determined by asking the patient which limb they would prefer to use to kick a ball (23).
Lower extremity joint mechanics were evaluated during stair ambulation using a 12-camera Eagle motion capture system (Motion Analysis Corp., Santa Rosa, CA) and an AMTI OR5-6 force plate (dimensions, 50.8 × 50.8 × 8.25 cm; Advanced Motion Technology, Inc., Watertown, MA). Before collection, participants were equipped with 35 retroreflective markers (bilateral acromioclavicular joints, anterior superior iliac spines, posterior superior iliac spines, iliac crests, greater trochanters, anterior thigh, medial and lateral femoral condyles, proximal, lateral and anterior shanks, medial and lateral malleoli, dorsal navicular, second metatarsal head, base of the fifth metatarsal, posterior calcanei, and on cervical vertebra seven). After an initial static trial, participants performed five ascending trials and five descending trials for each limb on a custom-built four-stair staircase (rise, 17 cm; run, 25 cm). An 80-lb box (50 × 50 × 35 cm), separate from the rest of the staircase, was custom-fit to be positioned directly atop the force platform. To help avoid unwanted movement of the box, double-sided carpet tape was affixed to the force plate surface. The staircase had an opening to receive the box and allow it to serve as the second step but without contacting the rest of the staircase. The known weight of the box was accounted for in the calculation of ground reaction forces (1). To familiarize themselves with the task, participants were allowed two practice trials of both stair ascent and descent. Participants then performed each trial at a self-selected pace and were allowed adequate rest periods between each trial (Fig. 1). Gait speed was calculated by evaluating the velocity of the marker placed on cervical vertebra seven. Importantly, every trial was initiated from a standing position, in which the participant’s first step was a level ground step toward the staircase with either their left or right limb and their second step was a step up or down the staircase with the opposite limb.
Data sampling and reduction
Cortex motion capture/processing software (version 188.8.131.522; Motion Analysis Corporation) was used for collecting and tracking all motion trials, and Visual 3D (version 4.0; C-Motion, Inc., Germantown, MD) software was used for postprocessing. A kinematic model consisting of seven skeletal segments (bilateral foot, shank and thigh segments, and pelvis) was created using the static trial (33). Lower limb joint rotations were defined on the basis of the initial static trial of each participant that was aligned with the three-dimensional laboratory coordinate system. Joint rotations during the stair task were calculated using the Cardan rotation sequence (12) and expressed relative to the static trial for each participant (32). Kinematic data were sampled at 200 Hz (22), and synchronized ground reaction force data were sampled at 2000 Hz and filtered using a fourth-order, zero-lag, low-pass Butterworth filter with a frequency cutoff of 6 Hz (22,35). Filtered kinematic and ground reaction force data were processed in the Visual 3D software using a standard inverse dynamics approach (45). Kinetic outputs were normalized to participant’s body height and mass (N·m·kg−1·m−1) and represented as internal moments (11).
Biomechanical data were time-normalized to 100% of the stance phase, with initial contact occurring when the limb of interest was in contact with the second step of the staircase and the vertical ground reaction force exceeded 10 N (32). Toe-off equated to the time when vertical ground reaction force fell below 10 N (32). Peak sagittal and frontal plane knee and hip joint angles and moments, joint angles at initial contact, and joint excursions (joint excursion = maximum joint angle – minimum joint angle) were obtained from the participant’s five individual trials of each task and averaged for analysis. Ensemble averages from the five ascending and five descending trials were calculated across stance for all rotations and moments and used to create biomechanical graphs for visual representation (Figs. 2 and 3) (33). Each graph was further partitioned into specific stages of stance phase: weight acceptance (WA), pull-up (PU), and forward continuance (FCN) for stair ascent and WA, FCN, and controlled lowering (CL) for stair descent. (31)
Independent t-tests were conducted to evaluate differences in demographic characteristics and gait speed during each task between groups. Separate 2 × 2 × 2 (group × time × limb) ANOVA tests were used to assess differences in peak sagittal and frontal plane knee and hip joint angles and moments, joint angles at initial contact, and joint excursions between the two groups (ACL and healthy) and two limbs (injured and uninjured) across the two time points of presurgery (average, 37.1 ± 15.3 d after injury) and 6 months after surgery (28.3 ± 2.9 wk after surgery). Where appropriate, Bonferroni post hoc multiple comparisons were performed using estimated marginal means. All statistical analyses were performed using SPSS statistical software, version 21 (IBM, Armonk, NY). Alpha level was set a priori at P ≤ 0.05.
All 40 participants completed both sessions; therefore, all patients were available for the 6-month follow-up analysis. Patients with ACL injury reported lower IKDC scores compared with those reported by healthy controls. No other demographic differences were observed (Table 1). Gait speed did not differ between the ACL (ascent pre, 0.65 ± 0.10 m·s−1; ascent post, 0.68 ± 0.11 m·s−1; descent pre, 0.78 ± 0.11; descent post, 0.79 ± 0.09) and healthy groups (ascent pre, 0.67 ± 0.08 m·s−1; ascent post, 0.68 ± 0.06 m·s−1; descent pre, 0.79 ± 0.07; descent post, 0.80 ± 0.06; P > 0.05). Means and SD for all outcome variables can be found in Tables 2 and 3. Knee and hip biomechanical curves (angles and moments) across 100% stance can be viewed in Figures 2 and 3.
During stair descent, there were no differences for any frontal or sagittal plane joint angles at peak or at initial contact (P > 0.05) (Table 2).
There was a significant limb–group effect for sagittal plane knee joint excursions (F1,38 = 4.95; P = 0.03) during stair descent, with the ACL-injured limb using less sagittal plane knee motion than the ACL-uninjured limb regardless of time point (P = 0.04) (Table 2). There was a significant limb–group effect for peak knee extension moment (F1,38 = 3.45; P = 0.03). Specifically, the ACL-injured limb demonstrated smaller knee extension moments than the ACL-uninjured (P = 0.01), healthy matched injured, and healthy matched uninjured limbs (P = 0.04) regardless of time point (Table 2). Lastly, a significant group main effect for peak knee abduction moment (F1,38 = 6.75; P = 0.01) was observed during stair descent, with the ACL group demonstrating smaller peak knee abduction moments regardless of limb or time (P = 0.01).
During stair ascent, there was a significant limb–group–time effect for peak knee extension angle (F1,38 = 3.97; P = 0.05) and a significant limb–group effect for knee flexion angle at initial contact (F1,38 = 4.07; P = 0.05) (Table 3) Regardless of time, both the ACL-injured (P = 0.001) and uninjured limbs (P = 0.006) displayed less knee flexion angle at initial contact and also experienced greater peak extension angles than the matched injured (P = 0.005) and uninjured limbs (P = 0.03) of the healthy group. It should be noted that participants never achieved a position of knee extension during stair ascent; therefore, we operationally defined peak knee extension as the amount of knee flexion at the knee’s most extended position during that task and is represented as a negative value in Table 3 (18). A significant limb–group effect was also observed for sagittal plane knee joint excursion during ascent (F1,38 = 10.57; P = 0.002) (Table 3), with the ACL injured limb utilizing less total sagittal plane knee motion than the ACL-uninjured limb (P < 0.001) and both healthy matched limbs (Table 3, P = 0.002).
A significant limb–group effect was observed for peak knee extension moment (Table 3) (F1,38 = 4.87; P = 0.03), and it was discovered that the ACL-injured limb demonstrated smaller peak knee extension moments than the ACL-uninjured limb (P = 0.002) and both healthy matched limbs (P = 0.01) regardless of time (Table 3).
Lastly, significant group main effects were discovered for hip abduction angle at initial contact (F1,38 = 10.39; P = 0.003) and frontal plane hip joint excursions during stair ascent (F1,38 = 21.79; P < 0.001). The ACL group initiated stair ascent with more hip abduction (P = 0.003) and used more total frontal plane hip motion (P < 0.001) than the healthy group regardless of time and limb (Table 3).
We hypothesized that patients with ACL injury would ambulate stairs with more extended knee and hip joints before surgery, experience smaller knee extension moments, and use less sagittal plane knee joint motion in the injured limb compared with both the uninjured and healthy matched control limbs. We expected these alterations to persist when patients were cleared for unrestricted activity. Our hypothesis was partially confirmed, as individuals with ACL injury used less knee flexion bilaterally, at initial contact (WA), and at peak extension (which occurred during the FCN phase of stance) compared with healthy controls during stair ascent. These alterations were observed before surgery (37.1 ± 15.3 d after injury) and when these patients returned to unrestricted activity (28.3 ± 2.9 wk after surgery). We also observed smaller knee extension moments (occurring in the PU phase of stance during ascent and in the CL phase of descent) in the ACL-injured limb compared with the contralateral uninjured and healthy matched control limbs. Although we did not detect any differences in hip joint angles at peak or initial contact during stair descent, patients with ACL reconstruction contacted the stair using greater hip abduction bilaterally and used greater frontal plane hip motion than healthy controls during stair ascent.
Previous investigations have separately examined biomechanical alterations during stair walking in ACL-deficient (18) and ACL-reconstructed limbs (18,20,46) and have concluded that individuals with ACL injury ambulate stairs with less flexed (20) and more abducted knee joint angles (18). In the current study, we detected less flexed knee joints in ACL-injured limbs in the absence of kinematic alterations in the frontal plane during stair ascent. It is hypothesized that a shift in knee flexion angle likely affects the location of tibiofemoral joint contact (20). A shift in tibiofemoral contact pattern could be detrimental to long-term joint health, as this would lead to abnormal load distribution on the articular cartilage (3). As such, it seems plausible that cumulative effects of small alterations in knee joint flexion angles over time, whether it be at initial contact, peak, or through the entire range of motion, may have detrimental effects on knee joint degeneration. Clinically, however, it remains unclear how these relatively small alterations in knee flexion angle during WA (approximately 8° difference at initial contact) or throughout the entire stance phase (5° difference in sagittal plane knee excursion) contribute to posttraumatic knee joint degeneration. Notably, it should be highlighted that our data suggest these alterations in knee flexion angle occur mainly during stair ascent, which has been shown to be a more demanding biomechanical task than stair descent (39). Although this is the first investigation to longitudinally track patients across the recovery process after ACL injury, it is clear that more research is needed to fully understand the extent of these kinematic alterations and ultimately what effect they have on posttraumatic joint degeneration.
An interesting finding of this study was that patients with ACL injury used less sagittal plane knee joint range of motion throughout stance phase of stair ascent while experiencing greater frontal plane hip joint range of motion. Abnormal movement patterns at a joint have the potential to influence joint biomechanics both proximal and distal to the injury (19). It is likely that a decrease in sagittal plane knee joint motion initiates abnormal movement patterns proximal in the kinetic chain, ultimately affecting hip joint motion. Less sagittal plane knee excursion has also been observed during stair walking in patients with knee osteoarthritis (10), and others have demonstrated increased frontal plane hip motion in patients with reconstructed ACL during dynamic tasks (19). It is also plausible that increased strength at the hip played a role in allowing these patients to utilize more motion at the hip joint, creating a new strategy in which they were able to complete the task. Hip strengthening was a specific focus of the standard rehabilitation protocol that these patients completed, and although hip muscle strength measures were not recorded, increasing hip strength has been a strategy previously used by clinicians to help correct abnormal biomechanics following knee joint injury (17,38). In contrast, these alterations were also observed before surgery, and some data suggest that hip strength is not associated with hip and knee joint biomechanics during stair walking (7) (specifically in patellofemoral pain syndrome patients), making it unclear if the post-surgical rehabilitation positively affected hip biomechanics in the current set of patients. Therefore, it seems more likely that this reduction in sagittal plane knee motion and increase in frontal plane hip motion is a compensatory strategy in response to the ACL injury. Further, it is possible these alterations may contribute to the development of post-traumatic osteoarthritis overtime; however, more research is needed to determine the influence these biomechanical alterations have on the progression of joint degeneration.
Similar to previous investigations, we observed smaller knee joint extension moments in the ACL-injured limb as compared with those in the uninjured limb (20,46) and healthy individuals (18,46) during stair ambulation. This reduction in knee extension moment is thought to result from persistent deficits in quadriceps strength that are commonly observed in patients with injured and reconstructed ACL (25). Weakness of the muscle causes a decrease in the patient’s ability to produce enough quadriceps force to eccentrically control the limb through the entire range of motion (3). Furthermore, smaller moments in the involved limb may develop because of planned biomechanical adaptations, where the individual with ACL injury attempts to reduce stress on the injured joint and avoid painful movements related to compressive forces (41). Previous investigations have observed reduced knee joint moments at times of greater than 1 yr after reconstruction (20,24,26,46), and this is the first investigation to identify these alterations both before and after reconstruction in the same cohort of patients. Therefore, it is likely that changes in knee joint function developed in response to the acute ACL rupture and that reconstructive surgery and traditional rehabilitation do not appear to restore normal biomechanics during daily tasks in these patents. However, this investigation was not prospective in nature, and therefore, we cannot conclude whether these biomechanical alterations were present before, and potentially contributed to, the ACL injury in the current patients. Although recent evidence suggests that baseline movement patterns are altered after injury and reconstruction (19), further work in the area of ACL injury risk and prevention should aim to prospectively evaluate lower extremity biomechanics during a variety of tasks.
Investigators continue to discover (20,46) higher joint moments in the uninjured ACL limb, which seems to warrant the investigation into rates of contralateral ACL injury and posttraumatic osteoarthritis of the contralateral limb. The unloading of the injured joint, whether due to quadriceps weakness or pain avoidance, may place undue stress on the contralateral limb, creating a scenario for injury on the uninvolved limb. Rates of osteoarthritis in the contralateral limb are not investigated as regularly as posttraumatic osteoarthritis in the injured limb; however, rates of contralateral limb osteoarthritis are reported to be approximately 20%–30% (5,34), which is significantly less frequent than reports of ipsilateral posttraumatic osteoarthritis (50%–60%) (5). Although instinctively counterintuitive, it has been suggested that the unloading observed in the injured limb may actually contribute to these high rates of osteoarthritis, as unloading previously loaded articular cartilage may negatively affect joint health by changing the biochemical composition of the cartilage and compromise its structure (4,9). Similarly, less is known regarding the rates of contralateral ACL rupture, however reported rates of contralateral ACL injury range from 5% to 30% (21,37,40). It is also important to note that knee joint asymmetry is suggested to contribute to secondary ACL rupture (37) and posttraumatic osteoarthritis (36). The asymmetrical shift in biomechanics after injury and reconstruction may ultimately negatively affect joint health and stability in these patients. More research is needed to understand the clinical meaning of patients with ACL injury shifting joint loads to the uninvolved limb and how this adaptation ultimately affects bilateral long-term joint health.
Although the return to high-level dynamic activities on a reconstructed, but not normal, knee may increase loads on the knee joint that over time can lead to joint degeneration and osteoarthritis (3,42), little is known regarding the cumulative effects of small biomechanical alterations that are potentially adopted during activities of daily living, such as level ground walking and stair ambulation. Our patients with ACL injury experienced biomechanical differences from healthy controls before surgery, potentially indicating that these alterations develop early in the injury process. Furthermore, traditional reconstructive surgery and therapeutic rehabilitation did not restore these biomechanical alterations to normal levels. This is in agreement with previous investigations evaluating level ground walking, which found early biomechanical responses to injury (15). The lack of any gait retraining or biofeedback interventions during the current rehabilitation protocol stands out as areas in which further consideration is warranted to help correct these abnormal gait patterns. Therefore, it is possible that patients may benefit from early, presurgery gait retraining interventions to correct abnormal gait adaptations before they become permanent movement patterns. Gait retraining programs have shown varying levels of success in patients with reconstructed ACL (13); however, it is possible that after reconstruction, poor biomechanical strategies have already been adopted. Therefore, clinicians may have better success instituting gait retraining interventions before surgery and training with a variety of low- and high-demanding activities, to help limit the development of deleterious biomechanical strategies. However, gait retraining interventions alone may not restore normal biomechanics and may need to be supplemented with neuromuscular control and strengthening programs to help combat the deficits in muscle function common in this population. Furthermore, outside of advancements in therapeutic rehabilitation, improvements in surgical procedures must also be evaluated in an attempt to minimize joint and limb asymmetry in both structure and function.
One limitation of this investigation is that biomechanics were only captured during the second step of stair ascent or descent. It is possible that movement patterns further up or down that staircase (i.e., the 10th step) may be different from those during the participant’s second step. Similarly, previous research has demonstrated that a participant’s starting position during stair walking tasks can influence joint biomechanics in healthy populations and that initiating stair ascent or descent from a walking position may create greater joint moments and powers (43,44). The current study only used one starting position (standing with an initial step); however, it remains possible that the biomechanics of patients with ACL injury may be more or less affected when initiating stair ascent/descent from a walk. Future research is needed to understand the influence that starting position has on joint biomechanics during stair walking in populations with ACL injury. Another limitation is that our biomechanical model did not have a trunk segment. As such, we did not account for movement of the trunk during these tasks. It is possible that patients with ACL injury adopted a strategy in which they altered trunk movement patterns to limit changes in their lower extremity. Recent evidence has determined that excessive lateral trunk flexion is a possible biomechanical strategy adopted by patients with reconstructed ACL (6). Accordingly, future research would benefit from evaluating trunk movement during stair ascent and descent.
Patients with ACL injury used less knee flexion bilaterally at initial contact and at peak compared with healthy controls during stair ascent, with less sagittal plane knee joint excursions. We also observed that regardless of time point, the ACL-injured limb experienced smaller knee extension moments compared with the contralateral uninjured and healthy matched control limbs. Importantly, biomechanical alterations were observed before surgery and persisted when patients were cleared for unrestricted activity. Therefore, patients may benefit from early gait retraining interventions. More research is needed to understand the effect of biomechanical alterations during activities of daily living on the development of posttraumatic osteoarthritis after ACL injury.
No conflicts of interest or sources of funding were declared by the authors of this article. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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