Effect of a single impact loading on the structure and mechanical properties of articular cartilage

Aim To study the effect of a single impact on the structure and mechanical properties of cartilage. Materials and method Osteochondral plugs harvested from bovine femora were subjected each to a single impact using an in-house designed drop-tower. Impact masses of different values were released from different drop heights in selected combinations to apply stresses at strain rates and impact energies within specific ranges. Changes in the storage and loss moduli were estimated from cyclic compressive loading test undertaken before and after impact. The conditions of these tests were set to those occurring during walking and running. The extent of the damage on cartilage surface and depth was assessed using optical and confocal microscopy. Results The storage modulus varied slightly at level walking and running after performing impact tests up to the impact energy of 0.736 J. However, the decrease in the storage modulus was considerable at the impact energy of 1.962 J for test conditions representing both walking and running. This impact energy resulted in strain rate of 1500 s−1, stress of 25 MPa and energy absorbed per unit volume of 12.79 mJ/mm3. After impact the loss modulus increased over the loading cycles at all energies. Severe fissures were observed across the cartilage surface and through its thickness at levels of impact energy equal or grater than 1.472 J. Conclusions This study identified thresholds for the strain rate, impact stress and impact energy, which caused permanent changes in the mechanical properties and damage to structure of cartilage.