Subfailure injury affects the relaxation behavior of rabbit ACL

Objective. To study changes in viscoelasticity of a ligament due to an inco mplete or a subfailure injury. Design. An in vitro study of anterior cruciate ligament preparations. Background. The viscoelastic properties are an inherent part of the physica l characteristics of a ligament. An injury to a ligament alters both its el astic and viscous properties. Although the effects of several parameters on the mechanical properties of a ligament have been studied, there is no inf ormation in the literature concerning the effect of an incomplete or subfai lure injury on its viscoelastic behavior. Methods. Ten pairs of rabbit femur-anterior cruciate ligament-tibia specime ns were used. A standardized relaxation (Relax) test was adopted to quantif y the viscoelastic behavior, before and after a subfailure injury. One memb er of the pair was subjected to three sequential tests: Relax 1; Relax 2; a nd 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 failur e 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 viscoelast ic 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 3.11 N, p = 0.018). Fitting a three -element linear viscoelastic solid model to our data, we found the serial s pring stiffness to decrease by about 50% (p = 0.01), the parallel spring re mained unchanged, and there was a tendency for the dashpot coefficient to d ecrease (by 57%, p = 0.09). Conclusions. The 80% subfailure injury decreased the initial stiffness of t he ligament, and tended to decrease its viscoelastic property.