Equivalence of single and incremental subfailure stretches of rabbit anterior cruciate ligament

Experimental models are often used in the laboratory to produce incomplete soft-tissue injuries simulating those observed clinically. Single and incre mental stretch protocols have been utilized. The latter has many advantages over the former. This study was designed to determine if incremental and s ingle ligamentous stretches are biomechanically equivalent. Eleven paired f resh rabbit bone-anterior cruciate ligament-bone preparations were used. On e of each pair (single-stretch protocol) was stretched to 88% of the averag e failure deformation and then stretched to failure. The other ligament (in cremental-stretch protocol) was stretched to 55, 66, 77, and 88% of the ave rage failure deformation and then stretched to failure. All stretches were performed at 1.2 m/sec. Stress-relaxation tests were performed before and a fter the 88% stretch fur both stretch protocols. Relaxation curves were par ameterized as forces at six time points and were also fitted to a three-ele ment model. Load-deformation curves recorded during stretch to failure were characterized by eight parameters. Each incremental stretch step produced a significant increase in deformation, indicating alteration in the mechani cal properties of the ligament. Both groups of ligaments, when intact, exhi bited no differences in relaxation curves (p > 0.2). The 88% stretches, pro duced by each of the two stretch protocols, significantly altered the visco elastic behavior of the ligaments (p < 0.002). However, after the 88% stret ch, there were no differences in either viscoelastic (p > 0.1) or load-defo rmation (p > 0.1) parameters of the two stretch protocols. In conclusion, t he 88% subfailure stretch significantly altered the mechanical properties o f ligament, and the incremental and single stretches were biomechanically e quivalent.