Abstract
Characterising material properties of ligaments is essential in the analysis of human morbidity and mortality of low-speed sporting accidents, high-speed road traffic accidents, and very high-speed battlefield injuries. At lower strain rates the elastic modulus and ultimate stress increase relative to strain rate, although very high strain rate testing has not been performed to date.
A porcine stifle joint lateral collateral ligament experiment was conducted that simulated the strain rates that occur during across a full range of different human knee ligament injuries. Tensile testing was performed at five strain rates, each an order of magnitude apart, in the range 100–104%/s. Seven specimens were tested at each rate. Three loading techniques were used: 1) screw-driven, 2) servo-hydraulic, 3) drop weight rig with tensile impact adaptor. Cross sectional area was measured by counting pixels on a standardized digital photograph of an alginate-paste cast of the mid-substance of each sample. Strain was measured directly from the mid-substance of each ligament by high-speed video extensometry. Stress-strain curves were produced and used to quantify the elastic modulus, failure strain and ultimate stress at each strain rate.
Across the range of strain rates, elastic modulus increased from 288 to 905 MPa (p< 0.05), and ultimate stress increased from 39.9 to 77.3 MPa (p< 0.05). A relationship between strain rate and both, elastic modulus and ultimate stress was identified. Strain rate sensitivity was not observed at very fast strain rates.
Ligament strength increases when strain rates are high. These data provide an explanation for very high strain rate failure of ligaments under extreme loading conditions, that can be considered protective of bone fracture, such as can be seen in traumatic through knee amputations in blast injuries.
