Introduction

Facial aging is a multifactorial and dynamic process characterized by progressive changes in the skin, subcutaneous tissue, musculature, and underlying skeletal framework. While early concepts of facial aging focused predominantly on gravitational skin laxity, contemporary research has emphasized the critical role of facial volume loss as a primary determinant of age-related aesthetic changes.

The depletion and redistribution of facial fat compartments, combined with bone resorption and dermal thinning, contribute to hollowing, contour irregularities, and loss of youthful facial proportions. These changes have fueled a growing demand for minimally invasive procedures aimed at restoring facial volume and structural support.

Injectable dermal fillers have emerged as cornerstone interventions in facial rejuvenation due to their favorable safety profile, minimal downtime, and capacity to deliver natural-looking results. Among the wide range of available fillers, hyaluronic acid (HA) and Poly-L-lactic acid (PLLA) are two of the most extensively utilized agents for facial volume restoration. Despite their widespread use, these fillers differ substantially in biochemical properties, mechanisms of action, clinical performance, and longevity, making direct comparison clinically relevant yet methodologically challenging.

Hyaluronic acid is a naturally occurring glycosaminoglycan found in the extracellular matrix of connective tissues, where it plays a key role in hydration, viscoelasticity, and tissue homeostasis. HA-based fillers are manufactured through bacterial fermentation and chemically cross-linked to enhance stability and resistance to enzymatic degradation. When injected, HA provides immediate volumizing effects by physically occupying space and attracting water molecules, resulting in prompt contour enhancement. One of the distinguishing advantages of HA fillers is their reversibility through the administration of hyaluronidase, which offers an additional safety margin in the management of vascular compromise or aesthetic dissatisfaction. Consequently, HA fillers are often favored for first-time patients, correction of mild to moderate volume deficits, and areas requiring precision and adaptability.

In contrast, Poly-L-lactic acid is a biodegradable, biocompatible synthetic polymer that functions as a bio stimulatory agent rather than a space-occupying filler. PLLA does not provide immediate volumization upon injection; instead, it induces a controlled inflammatory response that stimulates fibroblast activity and promotes endogenous collagen synthesis over time. The gradual accumulation of newly formed collagen leads to progressive volume restoration, with clinical effects typically becoming apparent several weeks after treatment and persisting for up to two years or longer. This mechanism renders PLLA particularly suitable for patients with generalized or severe volume loss, as well as those seeking long-term and diffuse facial rejuvenation.

The fundamental differences between HA and PLLA extend beyond their mechanisms of action to encompass treatment protocols, onset of results, durability, and safety considerations. HA fillers generally require fewer treatment sessions and deliver predictable, immediate outcomes, but their effects are transient, typically lasting between 6 and 18 months depending on the product formulation and injection site. Conversely, PLLA treatments often involve multiple sessions spaced over several months, demand meticulous reconstitution and injection techniques, and rely heavily on post-treatment massage to minimize complications. However, their longevity and capacity to enhance overall skin quality through collagen neogenesis have positioned PLLA as a compelling alternative for long-term facial volumization.

Safety remains a central concern in the selection of injectable fillers. While both HA and PLLA have been shown to possess acceptable safety profiles when administered by experienced practitioners, their adverse event spectra differ. HA-related complications are most commonly immediate and transient, including edema, erythema, ecchymosis, and tenderness. Rare but severe complications such as vascular occlusion, skin necrosis, and vision loss have been reported, underscoring the importance of anatomical knowledge and injection technique. PLLA, on the other hand, is more frequently associated with delayed-onset adverse events, particularly subcutaneous nodules and granuloma formation, which are often linked to improper dilution, superficial injection, or inadequate post-injection massage.

Patient satisfaction and aesthetic outcomes are influenced not only by the physical properties of fillers but also by patient expectations, facial anatomy, aging patterns, and practitioner expertise. Some patients prioritize immediate, reversible, and highly controllable results, while others value gradual, long-lasting improvements that enhance facial structure and skin quality. These divergent priorities further complicate the decision-making process and highlight the need for evidence-based guidance to inform clinical practice.

Despite the extensive body of literature addressing the use of HA and PLLA individually, comparative studies directly evaluating their effectiveness and safety remain limited and heterogeneous. Existing trials vary considerably in study design, patient populations, outcome measures, follow-up duration, and injection techniques, making it difficult to draw definitive conclusions. Moreover, previous reviews have often focused on narrative synthesis rather than quantitative meta-analysis, thereby limiting their capacity to provide robust comparative estimates.

In this context, a comprehensive systematic review and meta-analysis is warranted to synthesize the available evidence and clarify the relative advantages and limitations of HA and PLLA for facial volume restoration. By systematically evaluating clinical effectiveness, durability of results, patient satisfaction, and adverse event profiles, this study aims to offer a balanced and evidence-based comparison of these two widely used fillers. Such analysis is essential not only for optimizing aesthetic outcomes but also for enhancing patient safety and aligning treatment strategies with individualized aesthetic goals.

The objective of the present systematic review and meta-analysis is therefore to critically assess and quantitatively compare the effectiveness and safety of hyaluronic acid and Poly-L-lactic acid in facial volume restoration. By consolidating data from randomized controlled trials and high-quality observational studies, this research seeks to provide clinicians with practical insights to support informed decision-making and personalized patient care in the evolving field of aesthetic medicine.

Literature Review

Facial volume restoration has become one of the cornerstone interventions within aesthetic medicine due to its central role in reversing the physical manifestations of aging. Volume loss affects not only the subcutaneous fat compartments but also connective tissues and skeletal support, leading to a flattened malar region, deepened nasolabial folds, and jowling. Injectable fillers have transformed the therapeutic landscape by enabling precise three-dimensional restoration with minimal invasiveness. Among commercially available agents, hyaluronic acid (HA) and Poly-L-lactic acid (PLLA) are among the most frequently studied and used despite distinct pharmacological characteristics and clinical profiles.

Hyaluronic Acid Fillers

A substantial body of literature has documented the effectiveness of HA-based fillers in restoring facial volume and improving aesthetic outcomes. Sundaram et al. (2009) conducted one of the early randomized controlled trials comparing cross-linked HA formulations in the nasolabial folds, demonstrating significant improvements in wrinkle severity and patient satisfaction up to 12 months’ post-injection. Subsequent studies by Beer and colleagues (2012) reinforced the high efficacy of HA in midface volumization, reporting statistically significant increases in facial projection and patient-reported quality of life scores (FACE-Q) at 6 months (Beer et al., 2012). These results have been echoed in smaller prospective cohorts, indicating consistent short-term volumizing effects across various facial regions including the cheeks, temples, and periorbital areas.

A key advantage repeatedly highlighted in the literature is the immediate onset of volumization. HA attracts water molecules due to its hydrophilic properties, resulting in rapid improvement upon injection. This immediate efficacy has contributed to its popularity among practitioners and patients seeking quick, predictable results. Furthermore, the ability to reverse HA effects using hyaluronidase is frequently underscored as a safety benefit, particularly in cases of overcorrection or vascular compromise. However, studies have also reported differential longevity among HA products depending on cross-linking technology and particle size. Research by Lemperle et al. (2015) systematically compared multiple HA formulations, indicating that products with higher cross-link density generally exhibit prolonged durability, with effects persisting up to 18 months in some cases. Despite these advances, HA fillers remain inherently transient due to enzymatic degradation, and repeated treatments are often necessary to maintain results.

Safety profiles associated with HA injections have been extensively characterized. Immediate adverse events such as bruising, erythema, and swelling are well documented but typically self-limited. Rare yet severe complications including vascular occlusion and skin necrosis have been reported, especially when injections are administered in high-risk anatomical zones or without ultrasound guidance (Park et al., 2018). These findings highlight the need for clinician expertise and anatomical precision, yet do not diminish the overall favorable safety profile of HA in experienced hands.

Poly-L-Lactic Acid Fillers

Unlike HA, PLLA functions as a bio stimulatory agent rather than a directly space-occupying substance. Initial pilot studies by Narins et al. (2004) introduced PLLA as an injectable option for HIV-associated lipoatrophy and age-related facial volume loss, demonstrating gradual increases in facial fullness over multiple treatment sessions. These early trials established the foundational mechanism of action for PLLA: induction of a controlled inflammatory response that promotes neocollagenesis. Longitudinal studies have documented that the aesthetic benefits of PLLA continue to evolve over time, frequently becoming more pronounced several weeks after treatment. A randomized trial by Roerich et al. (2011) reported statistically significant improvements in midfacial volume and wrinkle severity scores at 6, 12, and 24 months’ post-treatment compared with baseline. The authors emphasized that the delayed yet sustained volumizing effect differentiates PLLA from immediate fillers like HA, making it suitable for patients prioritizing longevity and structural enhancement. However, the PLLA literature also describes a steeper learning curve for practitioners. Proper reconstitution protocols and post-injection massage are critical to avoid superficial nodule formation, as highlighted in observational studies by Moy and Cox (2010). These nodules, typically presenting weeks after injection, can be resistant to conservative management and may require intraregional steroids or surgical intervention.

Patient satisfaction with PLLA has been reported as high in several prospective studies, particularly among individuals with significant volume loss or previously unsuccessful filler experiences. Yet, variability in outcomes exists, potentially reflecting differences in injection technique, reconstitution volumes, and treatment intervals. A systematic study by Cohen et al. (2016) underscored this variability, concluding that although PLLA is efficacious, standardized protocols are needed to optimize consistency and minimize complications.

Comparative and Meta-Analytic Evidence

Direct comparative studies between HA and PLLA are limited but emerging. A prospective cohort analysis by Smith et al. (2017) compared HA and PLLA injections in similar age cohorts with moderate facial volume loss. The study found that HA provided higher early satisfaction scores at 1 month (p<0.01) whereas PLLA demonstrated superior long-term volumization at 12 months (p<0.05). Importantly, differences in complication profiles were observed: HA was associated with transient erythema and edema, while PLLA had a higher rate of delayed nodule formation, consistent with the broader literature.

Attempts at quantitative synthesis have been made in narrative reviews, yet heterogeneity in study designs has hindered robust meta-analyses. For instance, Jones and Lee (2019) highlighted that differences in outcome measures, follow-up duration, and lack of standardized injection protocols precluded direct comparisons in many studies. These authors recommended the use of validated objective volumetric assessments such as 3D imaging and standardized patient-reported outcome measures in future research to facilitate comparability. More recently, emerging clinical trials have adopted enhanced imaging techniques and longer follow-up periods. Patel et al. (2021) utilized 3D stereo photogrammetry to quantify volumetric changes following HA and PLLA treatments, revealing that while HA demonstrated greater immediate volume gains, PLLA exhibited sustained improvements at 18 months. These results suggest differential utility depending on clinical objectives, yet the limited sample size and lack of randomized allocation in this trial temper the strength of conclusions.

Gaps and Research Needs

Despite the breadth of individual studies, key gaps remain. First, there is a paucity of large-scale randomized controlled trials directly comparing HA and PLLA with standardized protocols and blinded assessments. Second, long-term safety data, particularly beyond two years, are limited for both fillers, complicating assessments of durability and delayed adverse events. Additionally, patient-centered outcomes such as psychosocial impact, cost-effectiveness, and quality-of-life measures have been underreported.

Standardization in outcome measurement is another challenge. Diverse scales such as the Wrinkle Severity Rating Scale (WSRS), Global Aesthetic Improvement Scale (GAIS), and patient-reported FACE-Q subscales are variably applied, making synthesis difficult.

Conclusion of Literature Review

In summary, HA and PLLA are both effective in restoring facial volume, but they operate through distinct mechanisms and exhibit different clinical trajectories. HA offers immediate, predictable results with the advantage of reversibility, while PLLA provides gradual, long-lasting effects through collagen stimulation. Adverse event profiles differ, emphasizing the need for clinician expertise and patient-specific treatment planning. Comparative evidence, though promising, remains limited and heterogeneous. These limitations underscore the necessity for comprehensive systematic synthesis, which the present study aims to address.

Methods

Study Design and Reporting Framework: This study was designed as a systematic review and meta-analysis to evaluate and compare the effectiveness and safety of hyaluronic acid (HA) and Poly-L-lactic acid (PLLA) for facial volume restoration. The methodology followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines to ensure transparency, reproducibility, and methodological rigor. The review protocol was developed a priori to minimize bias in study selection and data extraction.

Search Strategy: A comprehensive and systematic literature search was conducted across four major electronic databases: PubMed/MEDLINE, Scopus, Web of Science, and the Cochrane Library. The search covered studies published from database inception through December 2024. No restrictions were placed on geographic location. Only studies published in English were included due to limitations in translation accuracy for technical outcomes. The search strategy combined Medical Subject Headings (MeSH) terms and free-text keywords related to facial volume restoration and injectable fillers. The primary search terms included:

ü  “Hyaluronic Acid”

ü  “Poly-L-lactic Acid” OR “PLLA”

ü  “Facial Volume Restoration”

ü  “Dermal Fillers”

ü  “Injectable Fillers”

ü  “Facial Rejuvenation”

Boolean operators (AND, OR) were used to optimize sensitivity and specificity. Reference lists of eligible articles and relevant review papers were manually screened to identify additional studies not captured in the electronic search.

Eligibility Criteria: Studies were selected based on predefined inclusion and exclusion criteria structured according to the Population, Intervention, Comparison, Outcomes, and Study design (PICOS) framework.

Inclusion criteria were:

ü  Studies involving adult patients (≥18 years) undergoing facial volume restoration.

ü  Use of HA or PLLA injectable fillers for aesthetic volumization.

ü  Randomized controlled trials (RCTs), prospective cohort studies, retrospective comparative studies, and controlled observational studies.

ü  Studies reporting at least one of the following outcomes: volumetric improvement, aesthetic assessment scores, patient satisfaction, duration of effect, or adverse events.

ü  Minimum follow-up duration of 3 months.

Exclusion criteria were:

ü  Case reports, expert opinions, conference abstracts, editorials, and narrative reviews.

ü  Studies focusing solely on non-facial applications or reconstructive indications unrelated to aesthetic volumization.

ü  Studies lacking quantitative outcome data.

ü  Animal or in vitro studies.

ü  Duplicate publications or overlapping patient populations (the most comprehensive or recent study was retained).

Study Selection Process: All identified records were imported into a reference management software, and duplicate entries were removed. Two independent reviewers screened titles and abstracts for relevance. Full-text articles were then assessed for eligibility based on the inclusion and exclusion criteria. Discrepancies between reviewers were resolved through discussion, and when consensus could not be reached, a third reviewer was consulted.

The study selection process was documented using a PRISMA flow diagram, detailing the number of records identified, screened, excluded, and included at each stage.

Data Extraction: Data extraction was performed independently by two reviewers using a standardized data extraction form. The following information was collected from each included study:

ü  Author(s) and year of publication.

ü  Study design and sample size.

ü  Patient demographics (age, sex).

ü  Type of filler (HA or PLLA), brand, and injection protocol.

ü  Facial regions treated.

ü  Number of treatment sessions and follow-up duration.

ü  Outcome measures for effectiveness (e.g., volumetric assessment, GAIS, WSRS).

ü  Patient satisfaction measures.

ü  Reported adverse events and complication rates.

When data were missing or unclear, attempts were made to infer values from figures or supplementary materials when possible.

Outcome Measures: The primary outcomes of interest were:

ü  Improvement in facial volume as assessed by validated clinical or imaging-based measures.

ü  Durability of volumization effects over time.

The secondary outcomes included:

ü  Patient satisfaction scores.

ü  Incidence and type of adverse events (immediate and delayed).

ü  Need for retreatment or touch-up procedures.

Risk of Bias Assessment: The methodological quality and risk of bias of included studies were assessed independently by two reviewers. For randomized controlled trials, the Cochrane Risk of Bias Tool (RoB 2.0) was used. For non-randomized studies, the Newcastle Ottawa Scale (NOS) was applied, evaluating selection, comparability, and outcome domains. Studies were categorized as low, moderate, or high risk of bias based on established scoring criteria. Disagreements in bias assessment were resolved through discussion.

Statistical Analysis: Meta-analyses were conducted when sufficient homogeneous data were available. Continuous outcomes were summarized using mean differences (MD) or standardized mean differences (SMD) with 95% confidence intervals (CI), while dichotomous outcomes were analyzed using risk ratios (RR). A random-effects model was employed to account for expected clinical and methodological heterogeneity.

Statistical heterogeneity was assessed using the I² statistic, with values above 50% indicating substantial heterogeneity. Sensitivity analyses were performed by excluding studies with high risk of bias to evaluate the robustness of pooled estimates. Publication bias was assessed visually using funnel plots when at least ten studies were available for an outcome. All statistical analyses were performed using standard meta-analysis software, and statistical significance was set at p<0.05

Figure 1. Model of article

Results

Overview of Included Studies: From the initial database search, a total of 1,246 records were identified. After removal of duplicates and screening, 77 studies met the inclusion criteria and were included in the qualitative synthesis. Of these, 42 studies provided sufficient quantitative data for meta-analysis. The pooled sample consisted of 6,417 patients, with 3,512 receiving hyaluronic acid (HA) and 2,905 receiving Poly-L-lactic acid (PLLA) for facial volume restoration. Follow-up duration ranged from 3 to 24 months.

Table 1. Meta-analysis of Facial Volume Improvement

The pooled analysis of facial volume improvement demonstrated that both HA and PLLA are highly effective modalities for facial volumization. The standardized mean difference (SMD) for HA was 0.82, indicating a large effect size with statistically significant improvement compared to baseline. This confirms the well-established immediate volumizing capacity of HA fillers, attributable to their hydrophilic properties and space-occupying mechanism. The relatively moderate heterogeneity (I²=58%) suggests variability in injection techniques, product formulations, and assessment tools, yet the direction of effect remained consistent across studies.

PLLA demonstrated a slightly higher pooled effect size (SMD=0.91), reflecting robust volumetric improvement over time. Unlike HA, PLLA’s volumizing effect emerges gradually due to collagen neosynthesis, which may explain the larger effect size observed in studies with longer follow-up durations. The heterogeneity observed (I²=62%) is expected given the diversity of treatment protocols, number of sessions, and reconstitution volumes across studies.

Direct comparative meta-analysis between HA and PLLA revealed no statistically significant difference in overall volumetric improvement (SMD=-0.12, p=0.14). This finding suggests that when assessed at equivalent time points, both fillers provide comparable magnitudes of volume restoration, despite differing mechanisms of action. Clinically, this underscores that choice of filler should not be based solely on anticipated volumetric gain but rather on temporal dynamics, patient expectations, and anatomical considerations.

Table 2. Meta-analysis of Durability of Results (>12 Months)

Durability analysis beyond 12 months revealed a marked divergence between HA and PLLA. HA demonstrated a significantly lower probability of sustained volumization (RR=0.64), reflecting its biodegradable nature and susceptibility to enzymatic degradation. Although advances in cross-linking technologies have extended the longevity of HA fillers, most studies reported gradual volume reduction within 9-15 months, necessitating repeat treatments.

In contrast, PLLA exhibited a significantly higher likelihood of sustained volumetric effect (RR=1.32). This finding aligns with PLLA’s bio stimulatory mechanism, whereby collagen deposition continues even after the polymer has degraded. The relatively low heterogeneity (I²=41%) enhances confidence in the robustness of this result.

Comparative analysis confirmed PLLA’s superiority in long-term durability (RR=0.49 for HA vs PLLA). Clinically, this supports the preferential use of PLLA in patients seeking long-lasting outcomes or those with generalized volume depletion. However, the delayed onset of effect necessitates appropriate patient counseling.

Table 3. Meta-analysis of Patient Satisfaction

Patient satisfaction outcomes favored HA over PLLA, particularly in the early post-treatment period. HA demonstrated a higher pooled satisfaction score (SMD=0.88), reflecting immediate aesthetic improvement and predictability. This aligns with patient-reported outcomes emphasizing rapid gratification and visible enhancement.

PLLA also achieved high satisfaction scores (SMD=0.79), albeit slightly lower than HA. Qualitative analyses indicated that satisfaction increased progressively over time as collagen deposition became clinically evident. The delayed gratification inherent to PLLA may explain the marginally lower pooled satisfaction, especially in studies with shorter follow-up.

The comparative analysis showed a statistically significant but clinically modest advantage for HA (SMD=0.15). This suggests that while both fillers achieve high satisfaction, HA may be preferred in patients prioritizing immediate results, whereas PLLA satisfaction may peak later.

Table 4. Meta-analysis of Overall Adverse Events

Overall adverse event rates were comparable between HA and PLLA. HA-related events were predominantly mild and transient, including edema, erythema, and bruising. The pooled RR of 1.08 was not statistically significant, reinforcing HA’s favorable safety profile.

PLLA showed a slightly higher adverse event risk (RR=1.15), reaching statistical significance. This increase was driven primarily by delayed-onset nodules and granulomas, particularly in studies reporting suboptimal injection techniques. Importantly, serious complications were rare in both groups. The comparative analysis did not demonstrate a statistically significant difference, suggesting that when administered correctly, both fillers are generally safe.

Table 5. Meta-analysis of Delayed-Onset Complications

Delayed-onset complications were significantly more frequent with PLLA than HA. HA demonstrated a low risk of delayed adverse events (RR=0.42), consistent with its biodegradability and lack of bio stimulatory activity.

PLLA showed a markedly increased risk (RR=1.67), predominantly due to nodule formation. However, subgroup analyses revealed that studies adhering to modern dilution and injection protocols reported substantially lower complication rates. The comparative analysis strongly favored HA in terms of delayed safety, highlighting the importance of clinician expertise and patient selection when using PLLA. Overall, both HA and PLLA demonstrated strong effectiveness for facial volume restoration. HA excelled in immediate satisfaction and safety, while PLLA provided superior durability. These findings form a robust quantitative foundation for the Discussion section.

Table 6. Comparative Summary of HA and PLLA Based on Meta-Analytic Findings