Hyaluronic Acid Injection Techniques for Lip Augmentation-Comparison of Linear, Retrograde, and Microdeposit Approaches: A Systematic Review.

Hyaluronic acid (HA) is widely used for lip augmentation; however, the safety and performance of injection techniques and instruments (needle vs cannula) remain unclear. The aim of this study was to evaluate the safety of HA injection techniques for lip augmentation and to assess patient satisfaction, injected volume, and associations with technique, instrument, and HA product. A systematic review was conducted accordance with PRISMA 2020 and registered in PROSPERO (CRD420251123328). PubMed/MEDLINE, Embase, Cochrane Library, LILACS, SciELO, and Scopus were searched from January 1, 2010, to August 16, 2025, for studies involving adults undergoing HA lip augmentation. Eligible designs included randomized and nonrandomized clinical trials, cohort studies, and systematic reviews. The primary outcome was safety, defined by type and timing of complications. Secondary outcomes included patient satisfaction, injected volume, and associations with technique, instrument, and HA product. Risk of bias was assessed using RoB 2 and ROBINS-I, and findings were synthesized narratively. Sixteen studies including 3692 patients were analyzed. Most protocols used 1 to 2 mL per session, commonly employing linear retrograde techniques with serial puncture, fan patterns, or microdeposits. Adverse events were mild and transient, including edema, ecchymosis, tenderness, and nodularity, with no reported cases of vascular occlusion or vision-threatening events. Needle-only and combined needle plus cannula approaches showed comparable safety profiles. Patient satisfaction was high in most patients. HA lip augmentation appears effective and generally safe when performed by experienced injectors. Current evidence does not support a single superior technique, instrument, or HA formulation. Standardized studies are needed to define best-practice protocols.