Can we modify gait asymmetry after ACL reconstruction?

Background:

The majority of individuals undergo anterior cruciate ligament (ACL) reconstruction after sustaining an ACL tear. After surgery, individuals take on progressive rehabilitation aimed to reduce pain and inflammation and restore physical function, and oftentimes the focus quickly shifts to return to sport. Insult to the neurologic system after an ACL tear is less commonly the focus during rehabilitation. ACL tears are an orthopedic injury with neuromuscular impairments. Undergoing ACL reconstructive surgery restores passive joint stability, but does not normalize aberrant knee joint biomechanics associated with the development of post-traumatic osteoarthritis. Since ACL reconstruction uses an avascular, aneural tendon graft to replace the ruptured ACL, one result is compromised somatosensory feedback, reduced afferent input to the central nervous system, and impaired neuromuscular control. These deficits contribute to asymmetric gait mechanics that can persist for years after ACL reconstruction despite full recovery of strength and clinically assessed function. Aberrant knee joint biomechanics during walking are associated with the development of radiographic osteoarthritis as early as 5 years after ACL reconstruction, implicating knee mechanics as an important target of interest in rehabilitation.

Central Nervous System Changes

Previous blogs done by Dave Sherman, Meredith Chaput, Harjiv Singh, Dustin Grooms, and Lindsey Lepley have spoken to the intricacies of how ACL rupture affects both muscle function and the nervous system. To piggy back and expand on these consequences, central nervous system (CNS) adaptations after ACL rupture may be a key reason for why there are changes in gait mechanics after knee joint injury. The uninjured ACL, together with the quadriceps and knee joint capsule, contains mechanoreceptors, which contribute to the functional stability of the knee joint by providing proprioception, or information about the position of the knee in space. Rupturing the ACL results in disruption of ascending sensory information into the CNS, ultimately causing alterations in neuromuscular control surrounding the knee joint. Although ACL reconstruction can restore some passive stability and proprioception, the number and functionality of mechanoreceptors, which provide critical sensory information to the nervous system) is significantly diminished. Consequently, after ACL reconstruction, individuals demonstrate protracted deficits in the neuromuscular control of gait, one of the first movements reintegrated after surgery and arguably most important functions necessary for high quality of life outcomes after surgery.

Figure 1: Cascade of events related to the development of post-traumatic osteoarthritis after ACL injury.

Gait mechanics overview

Changes in knee joint mechanics, typically defined as smaller knee flexion and extension angles, and reduced knee extensor moments in the involved limb during the stance phase of gait, are highly characteristic mechanics of asymmetric gait. Sagittal plane interlimb gait asymmetries (i.e., peak knee flexion and extension angles, peak internal knee extension moment) are observed early after injury and reconstruction and many remain unresolved for years, despite extensive rehabilitation. These interlimb gait asymmetries after ACL reconstruction are associated with the development of post-traumatic knee osteoarthritis. Early development of knee osteoarthritis after ACL reconstruction is linked to reduced levels of sports and recreational participation, increased pain, lower self-reported quality of life, and premature total knee arthroplasty. Current evidence-based physical therapy fails to adequately restore symmetric knee mechanics, underscoring a need to develop rehabilitation that effectively targets these gait asymmetries. Prior literature does suggest a relationship between quadriceps strength and asymmetric gait mechanics early after ACLR, with quadriceps weakness being related to smaller angles and moments in the involved knee in the stance phase during both walking and jogging. This weakness is substantial, with the involved side being <80% of the uninvolved side. Other data suggests that gait asymmetries linger beyond strength resolution, suggesting resolving quadriceps strength does not restore aberrant gait mechanics after ACL reconstruction. Another phenomenon discussed is a loading strategy similar to ‘learned nonuse’ that occurs in survivors of stroke, which may also underlie gait asymmetry after surgery, where habits are formed early on and linger throughout rehab as they have now become ‘learned’ gait strategies.

What have we tried and where should we go?

Traditional physical therapy after ACL reconstruction aims to rehabilitate the orthopedic clinical impairments (e.g., muscle weakness, joint range of motion) after ACL rupture, but fails to adequately address aberrant knee mechanics, which may be a manifestation of this neuromuscular problem. There is a critical need to develop clinical interventions that directly address the aberrant knee mechanics after ACL reconstruction, potentially through neuromuscular adaptation. Understanding the adaptability of knee mechanics after ACL reconstruction is a necessary first step towards guiding the development of interventions directly targeting the aberrant mechanics implicated in PTOA. Perturbation training, an intervention developed in an attempt to enhance the sensorimotor characteristics of muscle, improves pre-operative gait asymmetries, suggesting flexibility of the neuromuscular system after ACL injury. Perturbation training is a neuromuscular intervention where you impose perturbations (i.e., purposeful manipulation to the support surface) to the patient, which is aimed to decrease abnormal movement patterns. Improved gait mechanics from perturbation training, however, are not maintained when administered post-operatively, suggesting surgery may provide a further deafferentation. Perturbation training can be time intensive, and is non-specific to the gait impairments. Gait-specific interventions targeting the involved limb may assist in normalizing interlimb mechanics contributing to post-traumatic osteoarthritis development, and rely on the neuroplastic, adaptive properties of the CNS to learn a new motor pattern. In order to develop appropriate clinical interventions that directly target these interlimb gait asymmetries relating to PTOA, we must determine if individuals after ACL reconstruction are able to learn new gait patterns via adapting knee joint mechanics during walking. Ultimately, interventions that improve gait asymmetries related to the development of osteoarthritis are critical to improving long-term outcomes after ACL rupture and reconstruction.

Understanding the ability to learn

Split-belt treadmill adaptation, a modality shown to improve intra and inter-limb gait patterns in neurologic patient populations, may offer a task-specific way to elicit key neuromuscular adaptations in individuals after ACL reconstruction. A split-belt treadmill is a tool used to study the acquisition of a new gait pattern through motor adaptation. The process of motor adaptation is an error-driven form of motor learning, which is typically shown through a change in movement strategy in response to a perturbation. Motor adaptation works by causing an individual to recalibrate their movement in response to a perturbation. In a more real-world setting, we can consider walking on a boardwalk and then transitioning to a beach, where the first few steps may feel kind of funny, but the brain automatically adjusts to the new environment and you start walking normally again after a few minutes getting used to it. In this case, the perturbation is differing belt speeds. Split-belt treadmills, as the name implies, have a separate belt under each leg that each has a motor and force place embedded. Individuals can walk either with both legs at the same speed (i.e., tied) or with the legs going at different speeds (i.e., split).

Leveraging well-studied principles from split-belt treadmill adaptation to study gait deficits in individuals after ACL reconstruction may be an opportune place to start understanding CNS adaptability in these individuals. The split-belt treadmill adaptation paradigm allows researchers the opportunity to study changes in knee joint mechanics and learning in real-time. The use of a split-belt treadmill to approach changes in walking mechanics offers a task-specific approach to targeting the mechanical deficits that underlie aberrant gait mechanics after ACL reconstruction. Limited research overall has focused on the ability of individuals after ACL reconstruction to learn new gait patterns. Typical intervention after ACL reconstruction is not always gait-specific, and focuses primarily on strength and functional deficits. While feedback aimed at teaching symmetric gait strategies largely uses explicit cuing (e.g., "strike with your heel,” “bend your knee more while walking,” “roll off your toe”), split-belt adaptation is a more implicit form of learning, which naturally may adapt the nervous system through perturbation rather than explicit verbal instructions.

My dissertation work is built on the premise that effective clinical interventions for improving asymmetric movement after ACL reconstruction should capitalize on the neuroplastic, adaptive properties of the CNS. The adaptability of the CNS has been demonstrated via split-belt treadmill adaptation in other populations with gait impairments including neurologic (e.g., stroke) and post-amputation. We currently do not know if individuals after ACL reconstruction are able to learn new, more symmetric knee joint mechanics that could minimize the risk of developing post-traumatic osteoarthritis. If individuals demonstrate reduced capabilities to learn and retain new, symmetric knee joint mechanics, there may be a fundamental impairment in the neural control of gait after joint injury, which would directly impact the way clinical interventions are designed and executed. A deeper understanding of motor learning abilities in individuals after ACL reconstruction is necessary to develop clinical interventions to correct detrimental gait mechanics.

Figure 2: Split-belt treadmill with separate motors and embedded force plate.

Our understanding of the ability of the neuromuscular system to adapt after ACL reconstruction is limited to one published study, which shows adaptable spatiotemporal parameters (i.e., step length, stance time) following a single session of a split-belt adaptation paradigm. This study did not focus on knee mechanics relevant to the development of post-traumatic osteoarthritis (i.e., knee joint excursions and loading). However, the ability for individuals after ACL reconstruction to adapt spatiotemporal parameters does suggest a capacity to adapt knee joint mechanics associated with the development of post-traumatic osteoarthritis. Through studying the various learning strategies in individuals after ACL reconstruction, we can create targeted clinical interventions to address asymmetric movement patterns. Transfer of a new motor skill to a person’s free-living environment is a critical marker of the effectiveness of rehabilitation. To ultimately develop rehabilitation interventions targeting knee mechanics after ACL injury, it is important to preliminarily understand the ability for individuals to adapt the aberrant knee mechanics, learn new gait patterns, and retain new gait patterns. To assess this using a split-belt treadmill, we can look at re-learning and retention, as well as over-ground carry over. Gait adaptation studies in individuals post-stroke have shown some promising steps towards the transfer of newly learned gait patterns to new environmental contexts. Specifically after ACL reconstruction, motor learning principles such as manipulation of feedback and task-specificity assist in the transfer of dynamic knee control from the clinic to athletic environments. These data suggest manipulating various training parameters may be a way to encourage clinicians to facilitate transfer of learning. Understanding the ability to adapt knee mechanics, and eventually to transfer learned mechanics into every day overground walking, is a critical first step. While we are currently in the exploratory phase of this research, we hope that the results will inform the eventual development of clinical interventions. These data will provide a direction on learning capacity in ACL-injured individuals, which will further inform clinical learning strategies. We have preliminary evidence of learning in 7 participants after ACLR during split-belt treadmill adaptation. Our pilot data show adaptation of peak knee flexion angle during stance, and we are continuing to collect both individuals after ACL reconstruction and healthy, matched-control participants for my ongoing dissertation work. We aim to collect 15 individuals in each group in order to publish our data on this topic. In addition to gait mechanics, we will be collecting strength, patient-reported outcome measures, demographics, and active joint position sense (proprioception measurement).

Conclusion

I wrote this blog post in hopes of expanding on the role of CNS adaptations in understanding changes in gait mechanics after ACL reconstruction. Resolving gait asymmetry via real-time feedback, perturbation training, and other interventions is a hot topic in ACL literature, but learning abilities of individuals after ACL rupture should be considered when designing studies for this impairment. Once we understand if knee joint biomechanics can be altered to improve gait symmetry, and if patients after ACL reconstruction are able to adapt their mechanics, we can determine the capacity for patients after ACL reconstruction to resolve gait asymmetries and further inform clinical interventions targeting gait mechanics.

Elanna Arhos

Dr. Arhos is a physical therapist and postdoctoral researcher at The Ohio State University Wexner Medical Center. She has been involved in researching clinical and biomechanical outcomes after ACL rupture and reconstruction. Her dissertation work from her PhD in Biomechanics and Movement Science at the University of Delaware focused on clinical factors associated with the development of posttraumatic osteoarthritis after ACL rupture, and gait adaptability after ACL reconstruction. She has published her research in peer-reviewed journals and presented her research at the national and international level. Dr. Arhos’ research has been supported by the Foundation for Physical Therapy Research and the National Institutes of Health.

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