Mechanical response of ankle ligaments at low loads

Background: The aim of this study was to examine the mechanical behavior of human ankle ligaments at low forces. Predominantly, ankle ligaments have been studied under the auspices of ligament injury. While the mechanical properties of a ligament when tested to failure provide a basis for comparisons, the loads and displacement do not reflect normal physiologic loading. Methods: Eight fresh-frozen ankles (mean age 65) were dissected to expose the ligaments surrounding the talocrural joint. Eight ankle ligaments were studied and included: medially - anterior tibiotalar (ATTL), posterior tibiotalar (PTTL), tibiocalcaneal (TCL); laterally-anterior tibiofibular (ATiFL), posterior tibiofibular (PTiFL), anterior talofibular (ATFL), posterior talofibular (PTFL), and calcaneofibular (CFL). Stress relaxation tests were carried out at 30% and 10% strain. The peak load and area under the curve were assessed for all experiments. Results: Significant differences were found for the average peak loads of the elastic response between 30% and 10% strain for each ligament (p<.05). At 10% strain the relationship between the ligaments on the medial and lateral side revealed a Pearson R value of .991 (p =.087). No significant difference was found between the strain energies of the various ligaments (p>.05). The anterior talofibular ligament was found to possess similar relaxation results to the medial ligaments. The calcaneofibular ligament relaxed up to 10% more compared to the anterior talofibular for the same relaxation period. The mechanical testing was performed in uniaxial tension and did not consider off-axis loading that may occur in vivo during ankle motion. Conclusions: The stress relaxation experiments revealed all ligaments to relax even when loaded to less than 5 N, reflecting the viscoelastic nature of ligaments. The stress relaxation results show that the anterior talofibular ligament does not relax to the same extent as the other lateral ligaments. Examining the properties of human ankle ligaments at low loads has revealed some new findings. Clinical Relevance: This study highlights the need to understand the synergistic effects of the ligaments. This is important for reconstruction and arthroplasty procedures.