gms | German Medical Science

Introduction: Stride-to-stride variability (variations among subsequent strides) has been attributed not only to noise (i.e due to measurement error) but also to the underlying mechanisms of the neuromusculoskeletal system that produce human gait. This inherent variability provides the healthy human body with the flexibility to respond to unpredictable stimuli and stresses and becomes impaired with aging and disease (i.e. Parkinson’s). These findings are the result of using nonlinear tools to explore variability changes which are usually masked when traditional analysis (i.e. standard deviations) is adopted. Approximate Entropy (ApEn) (a non linear tool that evaluates variability and complexity) has already been used in medical domains such as cardiology and neurology. It has been found that ApEn correlates well with pathology while, in some cases, it is predictive of subsequent clinical changes. In addition, a hypothesis has been developed, the “loss of complexity hypothesis”, where decreased complexity is associated with loss of healthy flexibility rendering the system more rigid and unable to adapt to stresses. Our aim was to assess the variability of the ACL deficient knee, since ACL rupture is a common musculoskeletal injury and is accompanied by altered gait patterns and future pathology in the joint.

Methods: In this retrospective study, ten subjects with unilateral deficiency walked on a treadmill at their self-selected speed, 20% faster, and 20% slower, while kinematics were collected (50 Hz) from 80 consecutive strides for each condition. The ApEn (low values signify increased regularity and decreased complexity) of the resulted knee joint flexion-extension time series was calculated. A two by three (side by speed) analysis of variance, repeated on both factors, was performed on the ApEn group means. Post-hoc analysis using paired t-tests was employed when appropriate. Level of evidence: II

Results: Significantly smaller ApEn values were found in the ACL deficient knee when compared with the contralateral intact (F = 5.57, p = 0.022), for all speeds. ApEn values significantly increased (F = 5.79, p = 0.005) with increases in walking speed.

Conclusion: The altered properties of the ACL deficient knee, which exhibits more regular and less complex patterns than the contralateral intact knee, may decrease the adaptability of the system rendering it less able to adjust to perturbations. This could provide an explanation for the increased future pathology found in the deficient knee. ApEn can be an important tool in assessing pathology and therapeutic interventions in orthopaedics.