Compression ForceThemeasurements of the force during compression weremade using an ARES G2 (TA-instruments, Newcastle,DE) equipped with a 25 mm plate–plate measuringgeometry at a temperature of 25.0C. A 1 mL samplewas centered on the lower plate. After loading the sample,the upper platewas lowered to a position of 2.5mmabove the lower plate and the sample was allowed torelax for 120 seconds. The force was then measuredevery second while lowering the upper plate at a rate of0.0133 mm/s to reach a 0.9mmgap after 2minutes.Dispersion TimeThe dispersion time methodology is based on thatdescribed by Sundaram.15 The gels were first stained byadding 30 mL 1%solution of toluidine blue to 1 mL ofgel and mixing between 2 syringes for 3minutes.The gelwas then filled in a 1 mL BD glass syringe (Becton,Dickinson and Company, Franklin Lakes, NJ) and airwas removed from the gel by centrifugation. A 1,000mL beaker measuring 14.2 cm high and 11.7 cmdiameter was filled with 700 mL deionized water andplaced on amagnetic stirrer. Amagnetic stirring bar, 25mm long and 0.8 mm broad, was used at a rate of 170rpm. The syringe containing the stained gel was placedwith the orifice 2 cm above the surface of the water. AZwick material tester, Zwick BTC-FR 2.5 TH.D09(Zwick Roell, Ulm, Germany), was used to push the gelout of the syringe at a speed of 400 mm/min. Photographswere taken 15, 70 and 95 seconds after the gel hitthe surface. The beaker was also filmed for 10 minutes.Each photograph was graded by 5 evaluators, whowere blinded to the individual time points and samplesshown in each photograph, on a scale of: 1 = fullydispersed; 2 = partly dispersed; 3 = partly dispersed/partly cohesive; 4 = partly cohesive; 5 = fully cohesive.Drop WeightThe weight of an average fragment/drop of a HA gelthat is pushed through a defined vertical orifice ata constant speed was determined. The gel was filled ina 1 mL BD glass syringe (Becton, Dickinson andCompany) and the air was removed by centrifugation.An 18Gcannula with a plane orifice (Intramedic Luer-Stub Adapter 18 G; Becton, Dickinson and Company)was mounted on the syringe and with the use ofa Zwick material tester, Zwick BTC-FR 2.5 TH.D09(Zwick Roell), the gel was extruded at a constant speedof 7.5 mm per minute. When a constant force wasachieved, 10 fragments/drops were collected andweighed and the average weight was calculated.ResultsRheological PropertiesThe rheological properties of a gel are oftendescribed using the elastic modulus, G9, and theviscous modulus G$. Sometimes the complex modulus,G*, is also used, G* is a measure of the totalresistance to deformation. A firmer gel has a higherelastic modulus meaning that the response to deformationis mainly elastic. Softer gels have lower elasticmodulus and the ratio of elastic to viscous behavior isusually lower than for a firmer gel.The gels used for evaluating the different cohesionmethods are the Restylane and the Emervel families.To characterize the gels used in the study, the rheologicalproperties were first measured. The results fromthe frequency sweeps are shown in Figure 1. RES wasthe firstHAgel on the market. RES and PER are firmergels with a high elastic modulus. The Emervel family isa more recent family of HA fillers ranging from semifirmto soft with lower elastic modulus compared withRES and PER. The samples used for this investigationcover a large range in rheological properties with anelastic modulus from 25 to 800 Pa at 1 Hz.Perceived CohesionThe results from the sensory evaluation are shown inFigure 2. The Emervel family of products was perceivedto be more cohesive than RES and PER. Theperceived cohesion judged by the 79 HCPs was reasonablyconsistent with the judgements made by the 6scientists.The results were unexpectedly consistent between theparticipants, given that no detailed instructions hadbeen given on how to assess the level of cohesivity,apart from the definitions given in the methods section.Apparently sensory analysis, i.e., manual handlingand observation of the gel, yields a reasonable estimateof the cohesivity of anHAfiller. This may explain whymany investigators state that products they havestudied are cohesive without explaining how this wasdetermined. The drawback of sensory analysis is theneed for simultaneous comparison with other products,and the low resolution of scoring procedurescompared with instrumental analysis.Compression ForceIt has been suggested that compression force isdependent on the cohesive properties of a material.6,9,16 To validate this, the tests were first replicatedusing the previously described methodology andproducts.6,9,16