Molecular Weight

Hyaluronan (HA) is present within articular cartilage where it is synthesised by chondrocytes and in synovial fluid where it is synthesised by type B synoviocytes.  Hyaluronan can exist as Hyaluronic acid, sodium hyaluronate or as hyaluronate depending on the environment in which it is found, and all terms are used interchangeably.  It has been recognised for many years and in several species that in osteoarthritis (OA) the molecular weight and concentration of HA were diminished by one-half to one-third of their normal values, giving rise to the concept of visco-supplementation.

Hyaluronan imparts the viscoelastic nature to synovial fluid, which means it behaves as a viscous solution at low sheer rates and is elastic in nature at high sheer rates.  In synovial fluid HA also lubricates the synovial membrane/cartilage interface (boundary lubrication) and physically excludes active inflammatory components and leucocytes from the joint cavity, a mechanism known as steric exclusion.  Hyaluronan has additional direct anti-inflammatory effects and has been shown to decrease fibroblastic pannus formation in osteoarthritic joints.

The functional mechanisms of HA are directly dependent on the molecular weight and concentration of HA.  This concept should be kept in mind when choosing from the assorted preparations of HA available for use.  The molecular weight of equine synovial fluid HA has been reported to range between 2-3 million Daltons, while the reported concentration of HA ranges between 0.33 and 1.5mg/ml.  For the available HA products, molecular weight and price are typically directly and positively correlated.  Therefore, the high molecular weight preparations are recommended due to their increased efficacy and longer duration of action.

The dosing routing for hyaluronan in horses has been arrived at based on clinical impressions, and there is wide variation in how horses respond clinically to HA administration.  When administered for idiopathic synovitis, HA would typically be injected intra-articularly (i.a.) every week  for 4 injections.  There is no rest period required after HA administration.  The manufacturer recommended doses are based on use in a fetlock or carpus, so when using HA in a large joint such as a stifle, one should probably administer a double dose.