title: A novel approach to measure variability in the anterior cruciate ligament deficient knee during walking: the use of the approximate entropy in orthopaedics.
abstract: OBJECTIVE
  The evaluation of variability of biological rhythmic activities through measures such as Approximate Entropy (ApEn) has provided important information regarding pathology in disciplines such as cardiology and neurology. This research lead to the "loss of complexity hypothesis" where decreased variability is associated with loss of healthy flexibility rendering the system more rigid and unable to adapt to stresses. ApEn as a measure of variability and complexity, correlates well with pathology while, in some cases, it is predictive of subsequent clinical changes. The study of human gait could benefit from the application of ApEn since it is also a rhythmical oscillation. 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. We hypothesized that the ACL deficient knee will exhibit more regular and less variable walking patterns than the contralateral intact knee.
  
  
  METHODS
  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 of the resulted knee joint flexion-extension time series was calculated.
  
  
  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.
  
  
  CONCLUSIONS
  The altered properties of the ACL deficient knee, which exhibits more regular and less variable patterns than the contralateral intact knee, may decrease the adaptability of the system rendering it less able to adjust to perturbations. This could explain the increased future pathology found in the deficient knee. ApEn can be an important tool in assessing pathology and therapeutic interventions in orthopaedics.
authors.affiliation: Orthopaedic Sports Medicine Center of Ioannina, Department of Orthopeadic Surgery, University of Ioannina, Ioannina, Greece. oaki@cc.uoi.gr
authors.forename: Anastasios D
authors.initials: AD
authors.lastname: Georgoulis
authors.forename: Constantina
authors.initials: C
authors.lastname: Moraiti
authors.forename: Stavros
authors.initials: S
authors.lastname: Ristanis
authors.forename: Nicholas
authors.initials: N
authors.lastname: Stergiou
mesh_terms: D000328:Adult; D016118:Anterior Cruciate Ligament / Q000503:physiopathology*; D000070598:Anterior Cruciate Ligament Injuries; D019277:Entropy; D005080:Exercise Test; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology; D007719:Knee Joint / Q000503:physiopathology*; D008297:Male; D008433:Mathematics; D008875:Middle Aged; D009985:Orthopedics / Q000379:methods*; D010507:Periodicity; D016059:Range of Motion, Articular / Q000502:physiology; D016138:Walking / Q000502:physiology*