### Results of Individual Studies
No severe adverse effects were revealed; malar edema, blue-gray dyschromia, and contour irregularity were present in both short-term and long-term follow-up, but malar edema occurred early, often after the first injection. [PERSON] et al., in a retrospective observational study in 2018, evaluated safety for the treatment of infraorbital hollowing with Juvederm Voluma XC (Allergan Inc., Dublin, Ireland) in 101 patients from February 2016 to March 2017 [25]. They used 27-gauge microcannulas to fill the infraorbital region, and the filler was injected in a supraperiosteal or submuscular plane. The total mean injection volume was 1.0 mL of HA gel. A total of 12 patients (12%) had adverse events related to the injection of Juvederm Voluma XC. Of those 12 patients, 3 had more than 1 adverse event (25%). Despite the thin skin of the periorbital region, only one patient had the Tyndall effect. Three patients developed diffuse doughy edema of the infraorbital area. Ten patients had bruising and only two had contour irregularities. Most of these adverse events were temporary, with only three patients requiring hyaluronidase to reverse the injection. [PERSON] et al., in 2019, in an interventional non-randomized observational study, treated 150 patients with Hyaluronic acid filler gel Juvederm Ultra Plus XC (Allergan Inc., Dublin, Ireland) between January 2017 and February 2018 [26]. They introduced a new procedure for the tear trough treatment based on just three bolus injections called the Tick technique. The volume injected was different according to their grade of depression: 0.3 mL for Hirmand grade 1, 0.4 mL for grade 2, and 0.5 mL for grade 3 using a 31G 6-mm-long needle with perpendicular bolus release at the above periosteal level. Fifteen percent of patients underwent touch-ups for optimal correction with boost injections from 0.1 to 0.2 mL. No serious complications were revealed. Immediately after the injections, 12/150 patients (8.0%) had swelling, and redness was seen in 6/150 patients (4.0%), pain in 3/150 patients (2.0%), and bruising in 3/150 patients (2.0%). After 1 week post-treatment, 7/147 patients had swelling (4.7%), and bruising was recorded in 2/147 patients (1.4%). [PERSON] et al., in 2020, injected Teosyl Puresense Redensity 2 in 24 patients for the tear trough treatment. The procedure was performed in the supra-periosteal layer, with injections using exclusively cannulas [27]. In the post-injection period, they observed: moderate swelling in 1 patient, mild swelling in 22 patients, bruising in 1 patient, and pre-syncopal symptoms in 1 patient. At 2 weeks, they observed: mild swelling in six patients, moderate swelling in one patient, bruising in two patients, puffiness in four patients, mild asymmetry in one patient, watery eye in one patient, and overall minimal difference in one patient. At 4 weeks, they observed: swelling in two patients and puffiness in one patient. [PERSON] et al., in 2021, in a retrospective case note review, treated 165 patients with a hyaluronic acid product, Restylane Vital light (Galderma, Watford, UK), that was injected in the pre-septal region of the tear trough [28]. The manufacturer's supplied syringe was used, which was secured to a 31-gauge 4 mm needle (TSK Mesotherapy Needle). The treatment was performed to a visual endpoint of small subdermal \"bubbles\" placed at regular intervals of 3-5 mm within the upper 1/3 rd of the superomedial tear trough and the pre-septal hollow. A hundred patients noted variable bruising that lasted a median of 3 days (range 2-7 days). All patients recorded having some visible localized eyelid bumps in the treated area, which subsided in all eyelids by 4 days (median), range 2-7 days. Two patients had persistent eyelid swelling. In one of these patients, swelling resolved after 6 weeks to achieve the desired result, whilst the other patient needed the filler dissolved. Three patients developed Tyndall (blue-grey discoloration) on follow-up (one patient at 3 months, and two patients at 6 months follow-up).