## 4 Discussion  
The demand for non-surgical procedures to correct blemishes has grown considerably in recent years, and the injection of hyaluronic acid (HA) represents the second most common procedure after the injection of botulinum toxin, with an increase in demand of 60% from 2014 to 2018 [29]. Hyaluronic acid-based treatments offer a valid alternative to some surgical interventions, with immediate results, little or no recovery times, and the possibility to repeat the procedure if needed [30; 31; 32]. Treatment options for the tear-trough deformity can be surgical [33], alloplastic implant [34], or autologous fat grafting [35]. An alternative treatment is represented by calcium hydroxyl-apatite (CaHA), which acts as a bio stimulating material, inducing the formation of new collagen with consequent replenishing anointing effect. Unlike hyaluronic acid-based fillers for which we can reverse possible complications through hyaluronase use, CaHa has no antidote, and its use is therefore recommended to more experienced injectors [36]. Soft tissue augmentation with HA fillers is a common and minimally invasive procedure, although not devoid of possible complications [37; 38]. Although the injection of HA products is generally well tolerated, rare serious complications can occur when used to treat tear trough deformities. Accidental retinal artery occlusion by either direct injection or compression is a very rare complication that can occur due to the anatomical complexity of this area [17]. The onset of this complication is related to the presence of numerous branches of the ophthalmic artery in the periocular region, whose direct or indirect involvement during the use of HA fillers can cause blindness [39; 40]. However, in 2012, [PERSON] et al. suggested that local adverse events were related to injection techniques (needle or cannula) and not to the different properties of the fillers; various patient factors, the selection of the product to be used, the choice of the injection procedure, and the devices used, are essential to obtain more satisfactory results and reduce the occurrence of adverse effects [41; 42]. It is very important to select the appropriate filler, in relation to the anatomical characteristics of the region to be treated, in order to avoid complications, and it is useful to know the rheological properties of the fillers, their physiology, the dimensions and concentrations of the particles and the properties derived from the level of cross-linking of HA. In the treatment of the peri-ocular region, the use of a filler with high G\({}^{\prime}\) and low affinity for water such as Restylane (Galderma) reduces the incidence of side effects [43]. In the present study, 830 patients who underwent injections of hyaluronic acid-based fillers for tear trough deformity corrections were analyzed. Of these, 404 patients (48.7%) received hyaluronic acid (HA) gel filler Restylane, 175 patients (21.1%) received Teosyal Puresense Redensity 2, 150 patients (18.1%) received Juvederm Ultra plus XC, and Juvederm Voluma was used in 101 patients (12.1%). In the majority of cases, doses from 0.1 to 1.2 mL per side were administered, precisely in 683 patients (82.2%), while in 147 patients, the dose is higher with an average of 3.1 mL (17.8%). About 69.2% of patients underwent injection through a needle, while cannulas were used in about 30.8%. In 454 patients (54.69%), the filler was placed at the supra-periosteal level, while in 376 patients (45.4%), it was placed in the suborbicular and subcutaneous plane. The concentrations of hyaluronic acid present in the different products available vary depending on the market; however, high concentrations and the use of large HA particles are related to a higher incidence of soft tissue edema [24] as well as other complications [44].