# Non-Surgical Touch-Up with Hyaluronic Acid Fillers Following Facial Reconstructive Surgery
## 4 Discussion
The process of crosslinking accomplishes this purpose. This can be achieved with 1,4-butanediol diglycidyl ether (BDDE), divinyl sulfone, bis(carbodimide), polyethylene glycol diglycidyl ether (PEG) or 1,2,7,8-diepoxyoctane. The degree of crosslinking will enhance the persistence of the HA filler in the tissues, but excessive crosslinking may reduce biocompatibility, causing a foreign-body reaction and encapsulation [99]. Crosslinking will also change the rheological properties of the gel of the HA filler. Harder fillers with a greater degree of crosslinking are more difficult to deliver, causing more discomfort during the injection. Nevertheless, they are able to better resist the dynamic forces of muscle movement and could have longer longevity. Moreover, they may feel firmer under the cutaneous surface and are better suited for volumizing deeper volumetric deficiencies in thicker-skinned individuals. Conversely, softer gels with a lesser degree of crosslinking, are more comfortable for the patients and easier to be injected. Being softer under the skin, they are better suited for the thinner tissue in case of periorbital reconstruction or perioral regions rehabilitation [95; 99]. Nonetheless, appropriate filler longevity can provide structural stability and resiliency, which is a requirement for a successful facial reconstruction. Thus, it is important for facial plastic surgeons to consider the delicate balance between stability and degradability when choosing which fillers are best suited for facial restoration. HA fillers produce lasting results, up to 18 months, depending on individual variability and crosslinking rate, HA concentration, and injective technique [100]. As described in the literature, deep injection could stimulate periosteal stem cells, allowing a semi-permanent effect [101]. In a prospective study by [PERSON] [102], 86% of participants treated with HA fillers reported improvement in their lips at 9 months after treatment. Cohesivity is an essential feature of filler implants, defined as the capacity of a gel not to dissociate. This property is certainly important during HA filler distribution into the tissues of the treated area and it is known to affect the lifting capacity of the gels [99]. In addition, one of the most significant--but often overlooked--factor of a successful HAfiller injection is pain control, not only in minimizing discomfort, but also in improving patient satisfaction and reducing procedural downtime. For this reason, local anesthetic, such as lidocaine, has been incorporated in HA fillers as an ancillary active substance with the aim to enhance patient comfort during the injection. Furthermore, lidocaine reduces erythema, bruising, and swelling as a result of its antihistamine property [99]. Together with proper selection of the appropriate reconstructive surgery-based technique, knowledge of rheological properties is mandatory for the selection of the HA filler, with the aim to achieve the planned rehabilitative purposes [95]. According to HA filler rheology, four main parameters should be considered to describe viscoelastic properties of a filler: G\(*\) refers to viscoelastic properties or hardness, G\({}^{\prime}\) measures elastic properties, G\({}^{\prime\prime}\) measures viscous properties, and tan delta measures the ratio between viscous and elastic properties [103]. It is paramount to use the appropriate filler and the correct technique to achieve an outstanding restorative outcome. Nowadays, HA fillers are no longer only used to address wrinkles, but also to restore volume and facial enrhythmy, according to reconstructive surgery principles. Our strategy of injection is based on surgical concepts, where long-lasting results is the \"rule\". In a case of trauma sequelae, the level of injection is not only superficial, but also involves a deep plane as a periosteal layer [101]. Physicians must be guided by the safety of the patient and by an aesthetic approach, respectful of proportions that could be missing after previous surgical procedures or traumatic events.