SynopsisThe dynamic viscoelastic properties of hyaluronic acid solutions have been measured over the frequency range 0.02-1.67 cps. The effects of varying temperature, hyaluronic acid concentration, pH, and ionic strength on the dynamic shear moduli were studied. The solutions exhibited a sharp transition from viscous to elastic behavior as the strain frequency increased. No entanglement coupling of the hyaluronic acid molecules was evident over the concentration range 2 . M . 0 mg./ml. Solutions at pH 2.5 showed a pronounced elastic behavior relative to both higher and lower pH's. This effect was attributed to a stiffening of the hyaluronic acid molecule at this pH.

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Hylan, a hyaluronan derivative, was chemically cross-linked with divinyl sulfone to produce a water-insoluble gel. This gel was fragmented into a gel slurry and evaluated for particle size, biocompatibility, and residence times in selected tissues. Hylan gels used in this study are made up of pseudoplastic, deformable gel particles with greater elasticity (at all frequencies) and greater viscosity (shear rates, 0.01 sec-1) than the water soluble hylan polymer. Hylan gel was injected intradermally and subdermally in mice and was found to produce a minimal reaction at 24 h; thereafter (up to 7 weeks) there was no significant tissue reaction. Intradermal injection of [3H]-hylan gel in guinea pigs revealed a minimal tissue reaction after 1 week, and measurement of radioactivity in the tissue at 1, 2, and 4 weeks revealed only a slight decrease in the total amount of injected radioactivity. The immunogenic activity of hylan gel was evaluated in rabbits; unmodified hylan gel, degraded hylan gel, and hylan gel ovalbumin conjugate were used to immunize rabbits. No antibody production to any hylan gel sample was detected, although control rabbits immunized with ovalbumin developed titers > 400 of antiovalbumin antibodies by day 21, as measured by the passive cutaneous anaphylaxis assay (PCA). Last, serum from owl monkeys (Aotus trivirgatus) in which hylan gel had been placed intravitreally for up to 3 years contained no detectable anti-hylan gel antibodies (PCA assay). Skin tests on these monkeys were also negative.

Nomenclature of hyaluronic acid 'Hyaluronic acid' is the name given by Karl Meyer to a polysaccharide composed of N-acetylglucosamine and glucuronic acid. The first part of the name, 'hyal-', originates from hyaloid (vitreous), because Meyer & Palmer (1934) first found this polysaccharide in the ' vitreous humor' of the bovine eye. Subsequently, it was found in numerous prokaryote and eukaryote systems. To designate the polyanion, the term 'hyaluronate', without a corresponding cation, is used extensively in the biochemical literature. In agreement with the recommendations on 'Polysaccharide Nomenclature' (IUB-IUPAC Joint Commission on Biochemical Nomenclature, 1982/3), we suggest that the term 'hyaluronan' be used for this polysaccharide generally, irrespective of its degree of dissociation.

The availability of elastoviscous solutions of highly purified hyaluronan has created two new therapeutic methods in human and veterinary medicine: viscosurgery and viscosupplementation. Viscosurgical tools and implants are widely used in ophthalmology and have been suggested for use in otology. Viscosupplementation of joint fluid using elastoviscous hyaluronan solutions is widely used in the treatment of equine traumatic arthritis. It was also suggested for use in idiopathic osteoarthritis in humans, but this application has not received wide acceptance. Cross-linked forms of hyaluronan have been developed and given the generic name of hylans. Water-insoluble soft gels of hylans are ideally suitable as viscosurgical implants to prevent postoperative adhesions and to control scar formation. Hylan solutions are being used in arthroscopic viscosurgery. Hylan devices in various forms (gels, tubes, membranes) have been used in animal studies for matrix engineering, the purpose of which is to control and direct tissue regeneration and augmentation. The biology of hyaluronan. Wiley, Chichester (Ciba Foundation Symposium 143) p 265-280In this paper we shall review past and present efforts to use hyaluronan as a therapeutic agent in human and veterinary medicine and explain the rationale behind its use. Our presentation will be limited to two areas of medical application -viscosurgery and viscosupplementation. We shall also discuss the reasons why cross-linked derivatives of hyaluronan were recently developed and will review their intended clinical uses.

Objective. To compare 3 different hyaluronan (HA) preparations used as therapeutic agents for osteoarthritis pain in humans in order to establish the degree to which a single application affects the sensitivity of nociceptors in both the normal and the acutely inflamed rat joint.Methods. In anesthetized rats, single-unit recordings were performed from the medial articular nerve of the right knee joint under normal conditions and during an acute experimental arthritis. Fifty fine afferent units (conduction velocities 0.8-15.3 meters/second) responded to passive movements of the knee joint. They were exposed to a torque meter-controlled, standardized stimulus protocol consisting of innocuous and noxious inward and outward rotations of the joint. This stimulus protocol of 50 seconds' duration was repeated every 5 minutes for 2-3 hours. Three commercially available HA preparations and a buffer solution, the solvent of these preparations, were injected intraarticularly after discharges resulting from 6 stimulus protocols were averaged and used as controls.Results. Both in normal and in inflamed joints, the injection of Hyalgan did not reduce nerve impulse frequency of the evoked discharges. The injections of Orthovisc had no effect in normal joints, but produced a transient frequency reduction of the evoked discharge in inflamed joints. Synvisc significantly reduced (by an average of 50%) the impulse discharge in both normal and inflamed joints 50 minutes after injection, and this level of impulse discharge continued until the end of the recording period (120-130 minutes after injection). The buffer, which had elastoviscous properties substantially different from those of Hyalgan, Orthovisc, and Synvisc, had no such effect.Conclusion. We conclude that the elastoviscous properties of HA solutions are determining factors in reducing pain-eliciting nerve activity both in normal and in inflamed rat joints.Elastoviscous hyaluronan (HA) solutions and HA derivatives (hylans) have been used in veterinary medicine since the early 1970s for treatment of arthritis pain (for review, see ref. 1). This therapeutic approach was based on the finding that long-lasting analgesia could be achieved in the joints of arthritic horses by replacing the pathologic synovial fluid, which has a low elastoviscosity, with a highly purified HA solution of normal elastoviscosity but significantly greater concentration than that of the healthy joint fluid. As a result of this discovery, highly purified HA and hylan solutions became available worldwide for the treatment of arthritis pain in humans and animals (for review, see refs. 2 and 3). This new therapeutic paradigm was called viscosupplementation.HA products of variable elastoviscosity have been used in viscosupplementation. The differences in rheologic properties (elastoviscosity) among these therapeutic products are primarily due to variations in their average molecular weight. Clinical studies in human osteoarthritis showed that in order to achieve clinically significant short-term and long-...

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The conformation of the polysaccharide hyaluronan (HA) has been investigated by tapping mode atomic force microscopy in air. HA deposited on a prehydrated mica surface favored an extended conformation, attributed to molecular combing and inhibition of subsequent chain recoil by adhesion to the structured water layer covering the surface. HA deposited on freshly cleaved mica served as a defect in a partially structured water layer, and favored relaxed, weakly helical, coiled conformations. Intramolecularly condensed forms of HA were also observed, ranging from pearl necklace forms to thick rods. The condensation is attributed to weak adhesion to the mica surface, counterion-mediated attractive electrostatic interactions between polyelectrolytes, and hydration effects. Intermolecular association of both extended and condensed forms of HA was observed to result in the formation of networks and twisted fibers, in which the chain direction is not necessarily parallel to the fiber direction. Whereas the relaxed coil and partially condensed conformations of HA are relevant to the native structure of liquid connective tissues, fully condensed rods may be more relevant for HA tethered to a cell surface or intracellular HA, and fibrous forms may be relevant for HA subjected to shear flow in tight intercellular spaces or in protein-HA complexes.