Summary

Cyclic Loading and Torsion Decrease Load to Failure of the ACL: A Biomechanical Study

Abstract

It has been demonstrated that repetitive load may contribute to tendon and ligament injury. However, it is unclear if repetitive loading and torsion is a factor in ACL tears. We investigated of ACL strain behavior, energy dissipation, and load-to-failure under cyclic loading.

30 fresh knees were obtained from Yorkshire breed pigs (2-4 mo, ~ 90lb), an established model for ACL evaluation. ACL and menisci were preserved and the femur and tibia were potted with the tibia aligned 20o with load sensor on a servo-hydraulic MTS (Fig 1). Three groups of 10 knees underwent testing: CONTROL; 100 cycles axial load (AXIAL); 100 cycles with 30o internal rotation of tibia (TORSION). Following initial preload of 1-5N, cyclic loading to 520 N was performed at 0.5 Hz. Load to failure testing was performed with unidirectional tensile loading at 1 mm/sec until rupture. Load and displacement data captured at frequency of 100 Hz. Ultimate force was measured directly from the load-displacement curves. Post-ultimate failure yielding was not considered.
Descriptive statistics were utilized to outline the load-to-failure samples. Continuous variables were expressed as mean ± standard deviation (SD), while categorical variables were described using frequencies and percentages. Before conducting inferential analyses, a Shapiro-Wilk test was utilized to evaluate the normal distribution of the data. After confirming normality, a one-way analysis of variance (ANOVA) was chosen to explore the potential relationship between the applied load and the number of cycles to ACL failure. Subsequent post hoc analyses were performed using the Tukey honestly significant difference (HSD) test to scrutinize specific group variations.
Cyclic loading resulted in signficant differences in load to failure between CONTROL, AXIAL, and TORSION, with decreasing load to failure in that order. (Fig 2) There was a 20% decrease after 100 cycles in AXIAL (829.8+133.2 N) vs CONTROL (1034.7+308.0 N). Load to failure for TORSION (486.0+100.0 N) was 53% less than CONTROL The one-way ANOVA yielded statistically significant differences in the maximum failure force among the various loading conditions (p = 0.0039).
Relatively slow cyclic loading to submaximal failure resulted in significant decreases in the load to failure of the ACL. Cyclic loading under torsion of the knee joint reduced load to failure of the ACL by over 50%. This is consistent with other authors using human cadaveric specimens and in vivo. Wotjys loaded knees to 3x and 4x BW and created failures in 5/10, 8/10 specimens (AJSM 2013). Nuccio found that in humans 10 min passive cyclic loading (.1 H, 200 N) resulted in 20.7% ↑ATT (Phy Ther Sp 2018).

Cyclic loading commonly occurs during normal sporting events, and poor motion patterns may result in increased rotation of the knee joint during repetitive activity. This could possibly increase the risk of ACL rupture in athletes.