The effects of hyaluronic acid on articular chondrocytes 

This preclinical study from 2005 forms the theoretical basis for which future investigations of HA use in degenerative joint conditions was based. The authors were able to demonstrate the beneficial impact of hyaluronic acid in joint spaces. While studies previously suggested that HA only offered temporary relief as a viscosupplementation therapy (restoring the elasticity and viscosity of synovial fluid to normal or higher levels.) 

References:

Akmal M, Singh A, Anand A, Kesani A, Aslam N, Goodship A, Bentley G. The effects of hyaluronic acid on articular chondrocytes. J Bone Joint Surg Br. 2005 Aug;87(8):1143-9.