Introduction

Aesthetic medicine is a rapidly evolving field. Over the last decade, there has been a conceptual shift infacial rejuvenation, from removal and reconstructive surgeries to volume restoration and facial lifting using multiple modalities. While we strive to move the profession forward with new technologies and products, it is very important for us to revisit the basics, and to understand the principal physiological changes in facial tissue that come into play as a result of aging. Furthermore, understanding the rheology of dermal fillers, and making full use of its potential, is important to ensure predictable and reproducible results.

Discussion
Understanding the pathophysiological changes of facial aging is critical in the course of restoring volume, and could recalibrate the manner in which the maturing face is treated. The five layers of aging, namely skin, fat, superficial muscular aponeurotic system (SMAS) and retaining ligaments, muscles, and bones, undergo aging interdependently. The skin layer becomes atrophic with the loss of hyaluronic acid, collagen and elastin. The fat under the skin experiences downward shift and displacement. Both SMAS (which serves as the base for soft tissues)and the retaining ligaments (which act as anchors)stabilize the skin to the underlying skeletal structures. The weakening of these deep connective tissues leads to stretching and loosening of basal and anchoring support, contributing to facial sagging. Muscle atrophy, muscle hypertrophy, muscle hyperactivity as well as skeletal changes due to bone resorption complete this interdependent system of facial aging. Aesthetic physicians can do more harm than good if they overlook the right layers that require attention.

An awareness of the characters of fillers — their “rheology” – is very important in producing predictable great results in enhancing and rejuvenating the face. Placement of fillers of the wrong rheology in the wrong plane at the wrong place with the wrong amount can cause great distortion to the natural look of a face. One such example is overfilled syndrome, which is very often overlooked by even the most experienced aesthetic physicians. Therefore, it is proposed to look beyond filling the face with volume, but using the right filler rheology at the right place, with the optimal amount to give support to the underlying structures and to restore to what it used to be. The idea of structural support takes a multitude of factors into consideration, including how treatments should be individualized based on gender, ethnicity, age and the cultural perception of youth and beauty. Together, these factors create a treatment procedure that suits the individual patient, thus providing better outcomes while maintaining a natural appearance.
With the numerous filler types and brands currently available in the market, deciding which facial filler to use, when, where, why to use it, its longevity and safety, is not a straightforward process. The various types of fillers in the market can be broadly divided into three simple groups: the voluminizing, enhancing and refining fillers.

The ‘voluminizing’ fillers are those with great lifting capacity, with certain features of plasticity, and generally do well in deeper layers such as supraosteal and subcutaneous layers. Generally, a small amount is enough to create the needed lifting and voluminizing results. Ideal areas for placement include, but are not limited to, infraorbital areas along the orbicular is retaining ligaments, temporal areas (i.e., between the superficial and deep temporal fascia), and prejowl sulcus, just medial to the mandibular cutaneous ligaments.
The ‘enhancing’ fillers require high elasticity and cohesivity with minimum characteristic of plasticity, in order for them to stay in shape, without fearing its migration and losing its shape. Such fillers are best in regions such as the nasal bridge, nasal spine, canine fossa, chin and mandibular augmentation.
The ‘refining’ fillers require high cohesivity with low viscosity to enablethem to “blend in” and integrate with the soft tissues, without breaking down into pieces. Such fillers spread well while not breaking down, making them ideal for highly dynamic and mobile areas of the face, including the periorbital and perioral regions.

Understanding the rheological properties of fillers allows the physician to make informed decisions on the best layer and area to apply the product. This is executed by targeting the retaining ligaments, restoring structure, particularly in areas such as the orbicular is retaining ligament and the zygomatic cutaneous ligament, among others. This underscores the importance of understanding volume loss of the specific layers of facial tissue, and the need for structural support.

Conclusion
The challenge for an aesthetic physician is where to draw the limit in our quest to develop a rejuvenated face. Injectology is not only confined to techniques and safety of injection. Understanding the functional anatomy, deep structures and layers, danger zones, and rheology of the fillers are equally important to excel in the art of filler-based face sculpting these days. While preventing distorted faces and overfilled syndrome in the long run is vital, patient satisfaction and well-being should always be a priority.