CPD: Explaining Dermal Filler Characteristics

Explaining Dermal Filler Characteristics

Dr Ahmed El Houssieny takes a look at key physical properties of HA dermal fillers and explores the difference they make in practice

Injectable soft tissue fillers or dermal fillers have become an integral part of aesthetic medicine with around 60 products available in the UK.1 When, as practitioners, we choose a dermal filler, what guides our choice? We are likely to be making a judgement about the suitability of that filler for the procedure we have in mind. Yet, what are the fundamental properties of fillers that inform this judgement? Hyaluronic acid (HA) fillers are the most common filler type, being used in almost 80% of filler procedures in the US.2 However, they vary greatly from one another in terms of their physical properties due to the different technological processes used in their development.3 This article aims to explore some of the key properties of HA dermal fillers and asks how we take them into account to optimise our choice of filler for any given procedure or area of the face. The focus will be on the rheological properties of G-prime (elasticity) and G double prime (viscosity), as well as cohesivity and concentration of HA.

Fillers and forces Dermal fillers injected into the face at any level or location are subject to different stresses throughout their lifespan. This starts with the extrusion force exerted to eject the filler through the needle or cannula during the injection process and extends through the moulding and massage, which places and integrates the filler.3,4 When in place, the filler is subject to stresses from adjacent tissues, as well as from facial expression and external forces, such as rubbing a cheek or physical exercise.4,5 The stresses in question are compression and stretch forces (applied perpendicular to the filler) and shear forces (applied along the surface of the filler or torsion) as shown in Figure 1. 4 The array of fillers available to practitioners of aesthetic medicine all possess properties that respond to these forces in a way that enables them to be effective. However, levels of elasticity, viscosity and cohesivity, as well as other properties, vary greatly between different products. It is important that the practitioner chooses a filler with an appropriate profile to respond to the forces exerted upon it during injection and for the duration of its placement in any given facial area. Elasticity and G-prime The G-prime (or G’) is a rheological property of a dermal filler and, in my experience, is one that often forms a central part of any discussion around filler choice. The G-prime or G’, also called the ‘storage modulus’ or ‘elastic modulus’, measures elasticity or the extent to which a filler can recover its shape after deformation due to shear stress.4 Manufacturers measure the G-prime of dermal fillers under stress using a rheometer.6 Values in commercially available HA dermal fillers are dependent on each filler’s chemical structure and range from 10 to 1,000 pascals.7-9 This gives a wide choice for the practitioner looking for an appropriate product. Generally speaking, G-prime is seen as a measure that gives an indication of where in the face the filler may be used.10 Higher G-prime fillers are better able to resist force and, as a result, are recommended in the literature and by manufacturers where deeper injections are needed or where volume needs to be added.8,10-13 Conversely, a lower G-prime means a more fluid filler that may be suitable for larger areas or where softness is required, such as the tear trough.10,11 However, the effects of G-prime are not independent from other filler properties. Cohesivity and viscosity each work in tandem with G-prime, as well as with one another, and this interaction must also be taken into consideration when choosing an appropriate HA filler.8,9,14 G-prime, G double prime (discussed next), and cohesivity work together in dermal fillers to produce the desired results but, due to differences in manufacturing, there are many different combinations to choose from, each with its own particular effects. Borrell and colleagues, for example, found that lift was achieved in two smooth HA fillers with high cohesivity but lower G-prime.14 This was in contrast to a more granular filler, which achieved lift with a higher viscoelasticity and lower cohesivity.14 The practitioner must then consider what consistency of filler is most suited to the procedure. If a greater level of lift is required, a filler that combines high cohesivity with high G-prime has been shown to be effective.14 Thus, while G-prime is a central property to think about in the selection of an HA filler, it cannot be considered in isolation.

Viscosity and G double prime Another key measure to consider is G double prime (or G”). Also referred to as the viscous modulus, G double prime measures viscosity. Viscosity is the quality of thickness of a filler or its ability to resist flow and G double prime measures the inability of a filler gel to regain its original shape after the removal of shear stress.4 Like G-prime, G double prime is measured using a rheometer to assess gel that is placed under stress by being moved at a range of different rates or oscillation frequencies.6,9 Viscosity is crucial at the point of filler injection. It is a fundamental

Torsion

Compression

Lateral shear Stretching

Gel viscosity Cohesive gel Non-cohesive gel

Gel with low viscosity

Gel with high viscosity

Figure 2: Relation between cohesivity and viscosity in an HA dermal filler. Image recreated from Molliard et al. (2018).3

requirement that dermal filler can be extruded through a narrow needle or cannula. The more viscous a filler, the more difficult it is likely to be to inject.3,4 HA is a naturally viscous substance, but manufacturers use a range of technologies and formulations to ensure that it is of an appropriate viscosity for use.3 It has been suggested that viscosity is unimportant once a filler has been injected.4 However, others argue that viscosity plays a key role in integration of a filler gel into tissue.3,13 A low viscosity filler can spread, leading to a smoother integration with the skin tissues in treatment of superficial areas.15 Good integration is essential to achieving a natural effect.16 However, integration with tissues is not only the product of viscosity but also of a filler’s cohesivity. While, a low-viscosity filler may be desirable in terms of spread, a filler which combines low viscosity with a higher cohesivity allows spread without the gel losing its integrity. This also allows a filler to be moulded or massaged after injection to ensure correct placement and distribution within the tissues.3,17

Viscoelasticity Elasticity and viscosity are interdependent properties of dermal fillers and can be combined into one measurement. The measure for viscoelasticity is G* or the complex modulus which combines G-prime and G double prime. Viscoelasticity is also referred to as ‘hardness’, although it should be noted that this term refers to the cross-linked structure rather than a quality of the gel itself.4 All fillers require both elasticity and viscosity so that they can respond to the different stresses to which they are subject.4,5 Consider how a filler is required to flow out of the needle during injection in response to high shear stress (viscosity) and then to regain its shape once it has been injected and is exposed to low levels of shear stress (elasticity).4,5 Generally speaking, HA fillers are more elastic than viscous.18

Cohesivity It is important to consider the cohesivity of a filler when choosing an appropriate product; both its function and how it interacts with other filler properties. Cohesivity is the capacity of a dermal filler not to dissociate because of the mutual affinity of its molecules. In other words, it enables the filler to remain intact. In the case of HA fillers, internal cohesive forces hold together the individual crosslinked HA units that comprise the gel.14 In the absence of a standardised measure, a range of testing modalities has been used to show the clinical value of this property in HA dermal fillers.14,19 The Gavard-Sundaram observation-based scale has demonstrated variation in cohesivity levels, ranging from those fillers that fragment in water upon contact to those that retain their integral shape after an extended period of time.17,19 It has been suggested that cohesivity may not be required in addition to G-prime for the performance of a filler.15 Drop-weight assessment was used to show an inverse correlation between G-prime and cohesivity in fillers with an HA concentration of 20mg/mL or higher.15 Fillers with higher G-prime showed lower cohesivity, while lower G-prime gels showed higher cohesivity.15 However,compression force testing (quantitative) and dye diffusion (qualitative) have shown that lift capacity can be achieved using HA fillers with high

Property/modulus Suggested application Example treatment area

G’ (elasticity modulus)

G” (viscosity modulus) Low – fine lines and wrinkles (a)5,11,

Mid – moderate lines and wrinkles4

Higher – tissue projection/lift11,17

Lips5,9,11

Mild-to-moderate nasolabial folds4 Mild-to-moderate marionette lines4

Moderate-to-severe nasolabial folds11,23 Deep periosteal correction to restore contour to oval of the face5 Mid-face9

Low – superficial indications requiring spread (b)3 Fine lines, such as lips and around the neck3,9

Mid to high – lift and volumisation, not requiring spread8 Chin8 Mid-face8,9

Low – even spread and malleability4,5 Low-to-moderate cohesivity for the frontal region5

Mid – balance between vertical projection and malleability4,5 Low-to-moderate cohesivity for the superficial fatty compartment in the mid-face5 Mild-to-moderate marionette lines4

High – tissue projection/expansion and volumisation4,5,19 Deep fatty compartment in the mid-face5 Moderate-to-severe marionette lines4

Cohesivity

cohesivity and low G-prime, as discussed above.14 Cohesivity may contribute to lift capacity through tissue expansion, rather than through projection as G-prime does.19 Further, histology and immunohistochemistry have demonstrated the homogeneous integration and optimal spreading of cohesive fillers after intradermal injection compared with non-cohesive fillers.18 Cohesivity and viscosity work closely together (Figure 2), allowing spread into the tissues as required while ensuring that the gel does not fragment.3

Concentration of HA HA occurs naturally in vertebrates where it plays a part in maintaining extracellular space and lubrication, particularly of joints.21 In its natural state, however, HA has poor viscoelasticity and is quickly degraded by hyaluronidase in the body.22 As such, it is chemically altered to achieve the characteristics required for it to function well as a dermal filler. Changing HA concentration can alter the viscosity, elasticity and cohesivity of HA fillers.4,15,22 A high concentration of HA also increases the resistance to a filler gel being degraded by naturally occurring hyaluronidase, thus increasing the durability of the filler.22 Concentrations of HA in commercially available fillers are within the range 5-24mg/ml. Higher levels can make the gel too viscous to administer.9,18,23 Dilution of the filler with saline or lidocaine is one response to this difficulty and may lead to good results, however practitioners should be aware that reducing the concentration of HA by diluting it decreases the G*, G’ and G”.9 This can not only make the gel softer, less elastic and less viscous, but may also reduce its duration.4,9

What does this mean in practice? How do the properties outlined above guide the practitioner’s choice of dermal filler for their patients? One may start by considering the area to be injected. The properties of the chosen filler need to be appropriate for injection depth, skin thickness, the strength of the facial muscles in the area being injected and whether the skin is loose or tightly stretched.5 The filler needs to be able to resist deformation exerted upon it in any given facial area, whether through shear or compression forces from injection and throughout its lifespan.4,5 Crucially, being familiar with the properties of a dermal filler alone is not enough. Filler choice also needs to be based on the patient’s treatment goals, the quality of their patient’s skin and their facial musculature. These may vary greatly based on gender, age, ethnicity and personal preference.11 Broadly speaking, facial filler injections can be divided into two types: those into deep and bony areas of the face requiring higher levels of viscoelasticity or cohesivity, and those into softer more superficial tissue, requiring lower levels.18 However, there are nuances to take into account for both. A summary of key properties with example applications is given in Table 1. When treating dark circles around the eyes, for example, there is little compression from surrounding tissue and little shear stress in this part of the face.5 A low G-prime filler is appropriate: little lift or volume is required.5 Low cohesivity and low viscosity are also appropriate.5,24 Low-cohesivity filler is believed to spread evenly in the tissue and to reduce the risk of lump formation and of the bluish colouration (Tyndall effect) caused by too superficial a placement of HA filler.16 A smooth gel should ensure the filler is invisible once implanted.5 Similarly, for fine lines, a filler with lower cohesivity and low viscosity is suitable.3,4,5 However, the forces exerted in the individual patient’s face should be taken into account. In the perioral area, both compression and shear stresses are present and can vary in strength.5 Here, low-cohesivity may need to be combined with low-to-medium viscoelasticity or G-prime. 4 For deeper lines in the perioral area, a higher cohesivity may be needed.5 Filler used in this area needs to spread adequately and be malleable so that it can be fully integrated.5 Other areas require greater lift and volumisation: the temporal region, for example, with its thick skin and strong, large temporalis muscle. Shear stress is low in this area, but in order for deeply-injected filler to achieve lift and volume, HA fillers with high G-prime and a high cohesivity or high viscoelasticity are recommended.5,24 Similarly, where lift is required along with resistance to compression stress, such as in the chin, a high G-prime, high-cohesivity filler is appropriate.4,5 Injecting a low G-prime filler, or one with low cohesivity, in areas that require lift, volume and resistance to strong forces may not cause harm, but it will not achieve the desired projection.5 When considering levels of viscosity and cohesivity in a filler, it is worth remembering that lower viscosity is linked to ease of injection.3 However, the relationship between these properties also affects distribution and may affect your choice (see Figure 2). A filler with high viscosity and low cohesivity may disperse as microboluses in the dermis, in contrast with a low-viscosity, high-cohesivity filler which may distribute homogeneously.3,19 Intermediate viscosity and cohesivity have been shown to display an intermediate distribution pattern.19 On the other hand, medium to low cohesivity may be easier to mould on initial placement of the filler.4 Note that there may be more than one suitable filler option. A 2010 study, for example, found that lower volumes of a high G-prime dermal filler was found to be as effective in the treatment of nasolabial folds as a filler with a lower G-prime (firmness; 1,800 vs 660Pa) with fewer touchup treatments. 25 Equally, if the same degree of lift may be achieved using a filler with high cohesivity and low G-prime or vice versa, for example, there may be other qualities, such as duration of effect, that are central to filler choice. While following the broad principles on suitable filler properties for a facial filler procedure, take detail into account and tailor the choice of filler to the individual case. It may be useful to record results in your patients that note filler properties so that you can build on your own experience.

Conclusion The wide variation in property combinations in HA fillers has an impact on their suitability for a procedure and the results.18.19 Familiarity, then, with the key properties of available HA fillers supports the practitioner in making an appropriate choice. The selection of dermal filler with properties best tailored to the treatment in hand is central to achieving a natural-looking aesthetic outcome that has good duration and is inline with an individual patient’s requirements and expectations.

Dr Ahmed El Houssieny is a trained anaesthetist with a passion for aesthetics. He is an honorary lecturer at the University of Chester and an education provider on cosmetic procedures. Dr El Houssieny is registered with the General Medical Council, as well as being an associate member of the British College of Aesthetic Medicine. Qual: MBBCH

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