Swelling significantly affects the material properties of the menisci in compression

This study has evaluated the swelling of meniscal test samples associated with altered osmotic environments. Meniscal samples were cut and weighed, then placed in one of 3 solutions: deionized water, phosphate buffered saline (PBS) or 2× concentration PBS. The amount of swelling in meniscal samples was solution independent with average swelling greater than 20%. The effect of this swelling on the mechanical properties of the samples was evaluated under confined compression testing. Samples were measured using a photogrammetric technique at the time of sample preparation and again after 1 h in PBS. Meniscal samples used for mechanical testing swelled on average 18% in thickness after 1 h in isotonic PBS. Free-swollen tissue was 1/3 as stiff at equilibrium as those that were recompressed to their original thickness prior to testing. Secant moduli at peak stress were nine times greater in the recompressed samples than the free-swollen samples. Relaxation times were faster in swollen samples, indicating increased permeability compared to recompressed specimen. Swelling pressure in the tissue averaged 14.4 kPa in isotonic PBS, identifying that the menisci are pre-stressed structures within the knee joint. Histological analysis identified that the quantity of swelling is related to both the osmotic pressure generated by proteoglycans and the local collagen architecture in the sample. This is the first study to quantify the amount and swelling in meniscal test samples. This swelling behavior significantly influences the properties of the tissue in compression and should be addressed in future mechanical testing and protocol development for the menisci.