Biostimulation and Bioregeneration in Injectable Aesthetics

The terminology surrounding regenerative aesthetics has become increasingly imprecise. A recent review on regeneration in aesthetic medicine highlights the need for clearer biologic definitions to prevent semantic drift in the clinical literature.¹

Within a mechanistic framework, biostimulation refers to a predictable tissue response triggered by implanted, foreign biomaterials that stimulate endogenous fibroblast activity and extracellular matrix (ECM) deposition. This response typically includes increased production of collagen types I and III, dermal thickening, and measurable improvements in tissue biomechanical properties.^2–4

Bioregeneration, by contrast, represents a more comprehensive biological process involving coordinated ECM remodeling, regulated inflammatory signaling, angiogenesis, and restoration of tissue function.1 In adult human skin, most aesthetic interventions achieve durable reparative remodeling rather than complete regeneration of native tissue architecture.

In practical terms, the distinction can be summarized as follows: biostimulation involves fibroblast activation, collagen deposition, and dermal thickening; bioregeneration refers to restoration of tissue quality, ECM remodeling and organization, and vascular and immune balance. 

EVIDENCE-BASED MECHANISMS OF PLLA AND CAHA

Interest in regenerative aesthetics has grown rapidly as clinicians increasingly seek treatments capable of producing durable structural and qualitative improvements in aging skin. Poly-L-lactic acid (PLLA) and calcium hydroxylapatite (CaHA) operate primarily through biostimulatory pathways that can lead to broader regenerative-like improvements in tissue quality over time. These injectable biostimulators have emerged as central tools for stimulating endogenous ECM remodeling rather than simply providing transient volume replacement.

These materials are often described clinically as “regenerative,” but recent literature suggests that the biological processes induced by injectable biostimulators are more accurately described as controlled reparative remodeling rather than true morphogenetic regeneration of native tissue architecture.¹ Understanding the mechanisms by which these materials interact with host tissue is essential for accurate terminology, patient counseling, and optimal clinical outcomes.

This article reviews the biological mechanisms and clinical evidence supporting PLLA and CaHA as biostimulatory injectables while distinguishing between the concepts of biostimulation and bioregeneration in aesthetic medicine.

PLLA: MECHANISMS OF COLLAGEN BIOSTIMULATION

Evolution
Injectable PLLA was first approved in Europe in 1999 for the cosmetic correction of wrinkles and scars. Early use was associated with papules and nodules due to limited understanding of optimal dilution, hydration, and injection technique; however, refinements in reconstitution and placement methods substantially improved both safety and aesthetic outcomes. PLLA later became widely used for HIV-associated facial lipoatrophy and broader facial volumization.⁵ In 2023, PLLA received US Food and Drug Administration approval for periosteal implantation for the correction of deep dermal and subcutaneous facial volume loss, further expanding its role in regenerative aesthetic dermatology.⁶

*Image generated by ChatGPT based on information provided within the article

Composition and Particle Characteristics
Injectable PLLA is supplied as a lyophilized powder composed of PLLA microparticles with carboxymethylcellulose and mannitol excipients. The microparticles typically measure ~40 μm to 63 μm in diameter, a size range that reduces rapid phagocytosis while allowing injection through fine needles or cannulas.^4,7 After reconstitution and injection, the carrier solution is absorbed while PLLA particles remain within the dermis or subdermis, where they initiate a localized foreign body response that drives tissue remodeling.

Host Response and Collagen Formation
Histologic studies have demonstrated that PLLA particles become surrounded by macrophages and fibroblasts shortly after implantation.⁴ Over time, these cells stimulate progressive deposition of new collagen fibers around the particles.

Early collagen deposition typically consists of collagen type III, which gradually transitions toward collagen type I during later phases of remodeling.⁴ This collagen maturation process parallels the time course of clinical improvement observed with PLLA treatment, which typically evolves over several months. 

Gene expression analyses in treated tissues demonstrate upregulation of multiple remodeling mediators, including transforming growth factor-β1 (TGF-β1) and tissue inhibitor of metalloproteinases-1 (TIMP1), supporting the concept that PLLA induces active ECM remodeling rather than passive volumization.⁴

Macrophage Polarization and Controlled Inflammation
Recent mechanistic studies have clarified the role of immune signaling in PLLA-induced tissue remodeling. Experimental models demonstrate that PLLA exposure promotes polarization of macrophages toward an M2 phenotype, characterized by anti-inflammatory and reparative functions.²

M2 macrophages produce cytokines including interleukin-4 (IL-4), interleukin-13 (IL-13), and transforming growth factor-β (TGF-β), which stimulate fibroblast activity and promote collagen synthesis.² This regulated inflammatory cascade represents a key biological bridge between the initial foreign body response and subsequent tissue remodeling. This response illustrates that inflammation in aesthetic medicine is not inherently detrimental; when properly regulated, it can serve as a driver of constructive tissue remodeling.

Polymer Degradation
PLLA degradation occurs through hydrolysis of ester bonds within the polymer backbone, producing lactic acid and soluble oligomers that are eventually metabolized through the tricarboxylic acid cycle or gluconeogenesis.⁷

Degradation kinetics are influenced by several factors, including polymer molecular weight, crystallinity, stereochemistry, particle morphology, and local tissue conditions, such as pH and enzymatic activity.⁷ Degradation products may influence the surrounding microenvironment, but immune cell recruitment appears to be primarily driven by protein adsorption and innate immune recognition of the implanted material rather than by pH changes alone.

PLLA and Osteogenesis
PLLA enhances osteoblast differentiation and promotes bone regeneration.⁸ The current literature supports PLLA’s osteogenic function when used specifically as scaffold material.^8–10 Mechanistically, PLLA matrices upregulate bone sialoprotein (BSP), alkaline phosphatase activity, and calcium content.⁸ PLLA also creates an immune microenvironment with M2 polarization conductive to osteogenesis. However, PLLA has low osteoconductivity and poor cellular adhesion, and generates inflammatory degradation byproducts, all limiting its ability to directly induce new bone formation.¹¹

Combining PLLA with other bioactive molecules, such as CaHA, can enhance its osteogenic potential. In vitro studies with nanohydroxyapatite-coated PLLA fibers have shown increased alkaline phosphatase activity, biomineralization, and bone-related gene expression, leading to ectopic bone formation.¹² PLLA/hydroxyapatite composite scaffolds have improved cell adhesion, enhanced osteointegration, and accelerated bone healing.^13–16 Combination therapy with CaHA requires careful consideration of the patient’s aesthetic goals. Synergism can lead to excess stimulation of osteoblasts, resulting in soft tissue overgrowth and excessive bony prominence. 

When injected supraperiosteally, PLLA promotes facial architectural integrity. Through low-level osteoconductivity, PLLA functions to offset age-related and hormone-related bone loss, particularly in perimenopausal and postmenopausal women. Representative before-and-after images reveal improved retained bone structure and improved skin quality after PLLA treatment (Figure 1). 

Figure 1. Representative before-and-after images demonstrate improvements in retained bone structure and skin quality after PLLA treatment.

CAHA: STRUCTURAL SUPPORT AND BIOSTIMULATORY DISPERSION

Characteristics
Calcium hydroxylapatite fillers consist of synthetic CaHA microspheres suspended within a carboxymethylcellulose gel carrier. Particle sizes typically range from ~25 μm to 45 μm.¹⁷

When injected without dilution, CaHA functions primarily as a structural filler providing immediate volumization and mechanical support. However, dilution protocols have enabled broader use of CaHA as a biostimulatory agent for improving skin quality.

Many clinical applications involving diluted or hyperdiluted CaHA are considered off-label and rely on expert consensus and practitioner experience rather than formal regulatory indications.¹⁸

Fibroblast Activation and ECM Remodeling
Experimental studies demonstrate that CaHA microspheres can directly activate fibroblasts through physical contact, triggering increased collagen synthesis and ECM production.¹⁹

Systematic reviews of CaHA mechanisms suggest that treatment may also influence additional regenerative pathways, including elastin fiber formation and angiogenesis, although the evidence base remains heterogeneous and further controlled studies are needed.²⁰

Histologic Evidence
Human biopsy studies have demonstrated persistent CaHA microspheres within treated tissue several months after injection, surrounded by newly formed collagen fibers and fibroblast activity.²¹ Additional investigations report increases in dermal proteoglycans and elastin components, suggesting that CaHA may influence multiple structural elements of the ECM.²²

Rheology and Dilution-Dependent Behavior
The rheologic properties of CaHA change significantly with dilution. Studies evaluating CaHA rheology demonstrate that dilutions >~1:1 reduce elastic modulus and increase fluid-like behavior.²³

These rheologic changes help explain the clinical distinction between undiluted CaHA, which provides structural volumization, and hyperdiluted CaHA, which disperses broadly through tissue and promotes collagen biostimulation. By increasing dispersion of microspheres across a larger tissue area, hyperdilution may activate a greater number of fibroblasts and promote widespread dermal remodeling.

CLINICAL EVIDENCE FOR SKIN QUALITY IMPROVEMENT

PLLA and Lateral Cheek Treatment
Several randomized clinical trials provide evidence that PLLA can improve skin quality in addition to correcting facial volume loss. A randomized controlled study evaluating PLLA for moderate to severe cheek wrinkles demonstrated significantly higher responder rates compared with untreated controls over a 12-month period. Investigators and patients reported improvements not only in wrinkle severity but also in radiance, firmness, and overall skin appearance.²⁴

A multicenter double-blind randomized trial comparing repeated PLLA injections with saline control found statistically significant improvements in skin elasticity and hydration, as well as investigator-assessed improvements in smoothness and radiance at 12 months.³ These findings support the concept that PLLA functions as both a volumizing and skin-quality treatment.

Hyperdiluted CaHA for Dermal Remodeling
Consensus recommendations for hyperdiluted CaHA describe dilution ratios typically ranging from 1:2 to 1:4 depending on treatment area and tissue thickness.¹⁸ These protocols generally involve superficial injection planes designed to stimulate dermal remodeling and improve skin laxity.

Mechanistic studies indicate that increased dispersion of CaHA microspheres with hyperdilution enables interaction with a larger population of fibroblasts, providing a plausible explanation for observed improvements in dermal quality.¹⁹ Systematic reviews further suggest that CaHA may influence multiple ECM components, including collagen, elastin, and vascular remodeling signals.²⁰

STRUCTURAL ARCHITECTURE AND FACIAL AGING

Facial aging is influenced not only by soft tissue changes but also by remodeling of the facial skeleton. Predictable age-related changes occur in the maxilla, orbit, and midface, altering the structural support for overlying tissues.²⁵ Studies in aged human skin demonstrate that restoring mechanical support within the dermal microenvironment can reactivate fibroblast function through mechanotransduction pathways, leading to improved collagen synthesis and dermal structure.²⁶

Whereas biostimulatory fillers may improve soft tissue support and external facial architecture, current evidence does not demonstrate direct prevention of skeletal bone resorption. Such claims should therefore be considered theoretical rather than established.

SAFETY CONSIDERATIONS

Both PLLA and CaHA product labeling emphasize core safety principles, including slow injection, minimal injection pressure, and avoidance of intravascular injection.17 Reviews of dermal filler complications identify 2 primary mechanisms of vascular injury: intravascular embolization and extravascular vascular compression. These events may lead to tissue ischemia, necrosis, or, rarely, visual complications, highlighting the importance of careful injection technique and informed patient consent.²⁷ PLLA treatments may also rarely produce delayed nodules or foreign body granulomas, reflecting the same inflammatory pathways responsible for its biostimulatory effect.

CONCLUSION

PLLA and CaHA are among the most extensively studied particulate collagen biostimulators in aesthetic medicine. Both materials induce controlled inflammatory signaling that activates fibroblasts, stimulates ECM production, and promotes gradual tissue remodeling.

Although these processes are often described clinically as regeneration, current evidence suggests that they are more accurately characterized as durable reparative remodeling, resulting in measurable improvements in dermal structure, biomechanical properties, and overall skin quality. Understanding the mechanisms and limitations of these materials allows clinicians to use them more effectively while maintaining scientific precision in the rapidly evolving field of regenerative aesthetics.