title: Subfailure injury affects the relaxation behavior of rabbit ACL.
abstract: OBJECTIVE
  To study changes in viscoelasticity of a ligament due to an incomplete or a subfailure injury.
  
  
  METHODS
  An in vitro study of anterior cruciate ligament preparations.
  
  
  BACKGROUND
  The viscoelastic properties are an inherent part of the physical characteristics of a ligament. An injury to a ligament alters both its elastic and viscous properties. Although the effects of several parameters on the mechanical properties of a ligament have been studied, there is no information in the literature concerning the effect of an incomplete or subfailure injury on its viscoelastic behavior.
  
  
  METHODS
  Ten pairs of rabbit femur-anterior cruciate ligament-tibia specimens were used. A standardized relaxation (Relax) test was adopted to quantify the viscoelastic behavior, before and after a subfailure injury. One member of the pair was subjected to three sequential tests: Relax 1; Relax 2; and stretch to failure. The other member of the pair was subjected to other three tests: Relax 3; 80% subfailure injury, i.e. stretch of 80% of failure deformation; and Relax 4.
  
  
  RESULTS
  We found that the relaxation test by itself (Relax 1 vs Relax 2), did not affect the viscoelasticity of the ligament. On the other hand, the 80% subfailure injury (Relax 3 vs Relax 4) affected the ligament viscoelastic behavior. The force was decreased by about 50% at time zero (10.46 vs 4.79 N, p = 0.014), and at 180 s (8.14 vs 4.11 N, p = 0.018). Fitting a three-element linear viscoelastic solid model to our data, we found the serial spring stiffness to decrease by about 50% (p = 0.01), the parallel spring remained unchanged, and there was a tendency for the dashpot coefficient to decrease (by 57%, p = 0.09).
  
  
  CONCLUSIONS
  The 80% subfailure injury decreased the initial stiffness of the ligament, and tended to decrease its viscoelastic property.
  
  
  CONCLUSIONS
  Subfailure or incomplete injuries of ligaments are more common than the complete injuries. The present study describes the effects, on both the elastic and viscous properties, of a ligament subjected to a subfailure injury.
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, CT 06520-8071, USA. manohar.panjabi@yale.edu
authors.forename: M M
authors.initials: MM
authors.lastname: Panjabi
authors.forename: P
authors.initials: P
authors.lastname: Moy
authors.forename: T R
authors.initials: TR
authors.lastname: Oxland
authors.forename: J
authors.initials: J
authors.lastname: Cholewicki
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D016118:Anterior Cruciate Ligament / Q000503:physiopathology*; D000070598:Anterior Cruciate Ligament Injuries; D004548:Elasticity; D066298:In Vitro Techniques; D011817:Rabbits; D013314:Stress, Mechanical