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Scientists may have found a culprit behind osteoarthritis. Emotion Matters/Stocksy
  • Synovial joints, one of the most common types of joints between bones, are characterized by the presence of a cavity filled with viscous synovial fluid between the two joint bones.
  • The molecules in the synovial fluid form a lubricating film on the layer of cartilage that covers the bone surface, thus helping to reduce friction between the joint or articular cartilage tissue.
  • Osteoarthritis involves irreparable degeneration of cartilage tissue at the synovial joint that is partly caused by changes in the composition of synovial fluid, but how these changes influence its lubricating properties is not understood.
  • Findings from a new study show how these changes in synovial fluid composition could influence its ability to form a thick, lubricating film on the articular cartilage tissue, potentially influencing the resistance to wear and tear of the joint.

Osteoarthritis is the most common joint disease in the United States, affecting an estimated 32.5 million adults. Osteoarthritis is caused by the wear and tear of the synovial joint, leading to the degeneration of the articular cartilage that forms the synovial joint.

Studies suggest that changes in the composition of the synovial fluid reduce its ability to lubricate the articulating cartilage tissue, thus contributing to cartilage damage.

A new study published inBiointerphasessuggests that changes in the composition of the synovial fluid in osteoarthritis could hinder the ability of synovial fluid molecules to form a protective, lubricating film on the surface of articular cartilage tissue, contributing to the wear and tear of the cartilage.

Synovial joints are one of the three major types of joints. Unlike other joint types, synovial joints are characterized by the presence of a cavity filled with synovial fluid at the interface between the two articulating bones.

The surface of the bones that form a synovial joint is covered with a layer of cartilage. The presence of the synovial fluid at the joint reduces friction between the cartilage during movements and allows free movement between the bones.

Synovial fluid is a viscous fluid composed of hyaluronic acid, phospholipids, and lubricin. Hyaluronic acid is a polysaccharide (complex sugar), whereas phospholipids are composed of fatty acids and are generally found in the membrane of cells.

These phospholipids can self-assemble to either form bilayers, i.e., double-layered membranes, or vesicles, which are sacs containing fluid enclosed by a lipid membrane. Phospholipids can form a film around the cartilage layer and help reduce friction at the joint.

Moreover, hyaluronic acid can bind to these phospholipids and potentially influence the lubricating properties of phospholipids. However, evidence supporting the role of hyaluronic acid in synovial fluid in lubricating the boundary between articulating bones has been inconsistent.

Osteoarthritis is a degenerative condition involving damage to the cartilage layer covering the articulating bones. In addition, osteoarthritis is associated with changes in the composition of the synovial fluid that alter its lubricating properties and promote the erosion of the cartilage.

Specifically, osteoarthritis involves the breakdown of longer chains of high molecular weight hyaluronic acid polymer into shorter chains that have a lower molecular weight. The concentration of hyaluronic acid is also reduced tenfold in osteoarthritis.

Scientists do not completely understand how these changes in synovial fluid composition in a diseased state impact its lubricating properties.

In the present study, the researchers conducted experiments in the laboratory to examine the impact of the concentration and molecular weight of hyaluronic acid on its ability to form a complex with phospholipids.

They found that the concentration and molecular weight of hyaluronic acid indeed impacted the structure of bilayer membranes of vesicles composed of phospholipid-hyaluronic acid in a buffer solution.

The researchers then examined the characteristics of films formed by hyaluronic acid-phospholipid assemblies on a gold surface that acted as a sensor. The mixing of high molecular weight hyaluronic acid with phospholipids resulted in the formation of a thick, continuous, and disordered or amorphous film on a gold surface. Increasing the concentration of high molecular weight hyaluronic acid led to a decrease in the thickness of the film.

Such an amorphous thick film is considered to effectively lubricate the movement at a synovial joint. Moreover, such a film is also more likely to retain its integrity and be more resistant to wear and tear, thus more effectively protecting the articulate cartilage tissue. In contrast, the interaction of low molecular weight hyaluronic acid with phospholipids limited the formation of a thick film on the gold surface.

These results suggest that the breakdown of hyaluronic acid resulting in a decline in molecular weight, as observed in osteoarthritis, could lead to an increase in friction and wear and tear of articular cartilage.

The study’s co-author Rosa Maria Espinosa-Marzal, a professor at the University of Illinois at Urbana-Champaign, said:

“Our results show low molecular weight hyaluronic acid, which mimics osteoarthritis-diseased joints, hinders the adsorption of the hyaluronic-acid-lipid complex. The lack of the formation of an amorphous film on the surface may reflect a consequence of osteoarthritis since this film should help reduce friction and wear.”

However, Dr. Jacob Klein, a professor at the Weizmann Institute of Science, Israel, cautioned that the study does not directly show a link between potential changes in synovial fluid composition and its impact on lubricating properties.

“I would have liked to see friction experiments to correlate with their structural measurements, which they didn’t do; so it is not completely clear to me what are the direct implications for lubrication,” Dr. Klein noted.