Hyaluronic Acid (HA) is a naturally occurring substance found throughout the human body, utilized widely in modern medicine and aesthetics. Classified as a glycosaminoglycan, this long, unbranched carbohydrate is known for its capacity to bind and retain water. Its unique physical properties make it a foundational component in connective tissues, supporting hydration, lubrication, and structural integrity. Due to its biocompatibility, HA is used in medical treatments ranging from joint injections to ophthalmic surgery and cosmetic fillers.
Hyaluronic Acid: Structure and Natural Biological Function
Hyaluronic acid is a linear polysaccharide composed of repeating disaccharide units: D-glucuronic acid and N-acetyl-D-glucosamine. This non-sulfated chemical structure allows the molecule to form numerous hydrogen bonds, enabling its exceptional ability to attract and hold moisture. A single HA molecule can bind up to 1,000 times its own weight in water, a property highly exploited in medical applications.
This water-binding capacity creates the molecule’s viscoelastic and gel-like consistency, allowing it to act as an effective lubricant and shock absorber. The average adult body contains approximately 15 grams of HA, with about a third turning over daily. Highest concentrations are found in the skin, where HA contributes to hydration and elasticity, and in the connective tissues.
HA is a major component of the extracellular matrix (ECM), the scaffolding that supports cells in various tissues. It is particularly abundant in the synovial fluid of joints, the vitreous humor of the eye, and the dermis of the skin. In joints, HA provides the viscosity and lubrication necessary for smooth movement. In the eye, it helps maintain the gel-like structure of the vitreous humor and protects delicate tissues.
The function of HA depends highly on its molecular weight, which ranges from very low to several million Daltons. High-molecular-weight HA forms a protective, structural matrix responsible for lubrication and viscosity. Conversely, lower-molecular-weight fragments penetrate tissues more easily and are associated with signaling cellular processes like tissue repair and inflammation.
Therapeutic Use in Orthopedics and Ophthalmology
The viscoelastic properties of hyaluronic acid make it a valuable treatment in orthopedics, particularly for managing osteoarthritis (OA). In OA, naturally occurring HA in the synovial fluid breaks down, reducing the fluid’s viscosity and shock-absorbing capability. This loss contributes to joint pain and stiffness, most commonly in the knee.
Physicians address this loss using viscosupplementation, which involves injecting an HA solution directly into the affected joint space. This injectable HA temporarily replaces the depleted synovial fluid, restoring the joint’s lubrication and cushioning. The goal is to reduce friction between joint surfaces, alleviate pain, and improve mobility for patients who have not responded adequately to other non-surgical therapies.
In ophthalmology, HA has been a long-standing component of surgical procedures, dating back to the 1950s. It is used as a surgical aid during operations such as cataract removal and corneal transplants. A highly purified, concentrated HA solution is introduced into the eye during these procedures to maintain the stability of the anterior chamber and protect surrounding delicate tissues.
HA is also a common ingredient in eye drops used to treat dry eye syndrome. The hydrating and protective qualities of the molecule help stabilize the tear film and moisturize the eye surface. These ophthalmic uses leverage HA’s biocompatibility and its ability to function as a clear, protective, lubricating gel.
Dermatological and Aesthetic Applications
The skin holds over half of the body’s total hyaluronic acid and relies on the molecule for hydration, firmness, and appearance. As natural HA production decreases with age and sun exposure, the skin loses moisture retention, leading to dryness and the formation of lines and wrinkles. External HA applications address this through topical products and injectable treatments.
Topical products like serums and moisturizers utilize HA to draw moisture into the outer layer of the skin. Product effectiveness relates to the molecular weight of the HA used. High-molecular-weight HA remains on the skin’s surface, forming a moisture-retaining film that reduces water loss.
Conversely, lower-molecular-weight HA is smaller and may penetrate deeper into the epidermis, offering hydration and potentially stimulating tissue repair processes. Topical application of HA improves skin smoothness, plumping, and overall hydration, minimizing the appearance of fine lines.
In aesthetic medicine, HA is the primary component of dermal fillers, used to restore volume and contour facial features. For injectable applications, HA molecules are chemically modified through cross-linking, creating a thicker, more stable gel. This cross-linked HA resists rapid breakdown by the body, providing long-lasting structural support to areas that have lost volume, such as the cheeks, lips, and nasolabial folds.
Sourcing and Safety Considerations
HA used in pharmaceutical and aesthetic products is sourced primarily through modern biotechnological methods. The most common method is microbial fermentation, using specific non-animal bacteria, such as Streptococcus zooepidemicus, to produce high-purity HA. This process is favored for its scalability, high yield, and reduced risk of transferring animal-borne pathogens or causing allergic reactions.
Historically, HA was extracted from animal tissues, such as rooster combs, but this method is less common due to concerns about purity, contamination, and ethical sourcing. Medical-grade HA must meet stringent quality standards, including ultra-high purity and precise rheological properties, verified through compliance with international pharmacopeia standards.
Hyaluronic acid is generally well-tolerated because it is naturally present in the body. Side effects, when they occur, are typically associated with the administration method rather than the molecule itself. After an intra-articular injection, patients may experience temporary pain, swelling, or warmth at the injection site.
Common adverse effects from dermal fillers include temporary bruising, swelling, and redness at the injection site. These reactions usually resolve within a few days to a week. Before undergoing any HA-based therapeutic or aesthetic treatment, patients should consult a qualified medical professional to discuss the procedure and ensure it is appropriate for individual health needs.