Articular cartilage. Strong adsorption and cohesion of phospholipids with the quaternary ammonium cations providing satisfactory lubrication of natural joints.

Much evidence supports the hypothesis that surface-active phospholipid (SAPL) molecules on articular cartilage (AC) adsorbed to negatively-charged proteoglycan matrix form phospholipid (membrane), are negatively charged surface (-PO4 -) and hydrophilic. In Hills cartilage model (1984) phospholipids adsorbed to cartilage surface act as boundary lubricants making the surface extremely hydrophobic. Hydrophobic surface of AC has gained no support in all experimental facts presented in this paper and the current literature showing that AC is amphoteric and hydrophilic with the negatively charged surface (-PO4 -). The interfacial energy of the model membrane of spherical lipid bilayers evident from phosphatidylcholine "bell-shaped curve" has amphoteric character and lowest energy in lubrication at a pH 7.4 ± 1 of the natural joint.