Comparative evaluation of the compressive strength surface hardness and porosity of a selection of capsule-mixed versus hand-mixed Glass lonomer cements

Abstract

Introduction: Glass ionomers are available in sets of powder and liquid constituents, which are dispensed using a scoop and dropper bottle system prior to hand-mixing by an operator. Glass ionomers are also available in capsulated form, which is mixed in a suitable mechanical mixing machine prior to clinical use. Capsulation enables uniform proportioning of the powder and liquid. In this context, mixing time will be correct as an automated process is utilised, resulting in a cement mixture that is optimal and reproducible, with minimal air entrapment. Manufacturers promote the capsulated form as being time saving, and easy to dispense, with more accurate adaptation because of the use of an applicator to place the material. Aim: The aim of this in vitro study was to compare the performance of hand-mixed glass ionomer materials with their capsule-mixed equivalents in terms of compressive strength, surface hardness and porosity. Materials and Methods: Four groups of 10 cylindrical specimens were manufactured for each of the four specified hand-mixed posterior glass ionomers for each test that was performed: Riva Self Cure (RSCH) (SDI Limited); GC Fuji IX GP (FIXH) (GC Corp); Ketac Universal (KUH) (3M ESPE) and Ketac Molar Easymix (KMH) (3M ESPE). Similarly, four groups of 10 cylindrical specimens were manufactured for each of the four equivalent capsule-mixed posterior glass ionomers for each test that was performed: Riva Self Cure (RSCC) (SDI Limited); GC Fuji IX GP (FIXC) (GC Corp); Ketac Universal Aplicap (KUC) (3M ESPE) and Ketac Molar Aplicap (KMC) (3M ESPE). The compressive fracture strength of each specimen was determined after 24 hours using a universal testing apparatus. A compressive load of 1 mm/min was applied to the 6 mm long axis of each specimen. The load to fracture was recorded and the compressive fracture strength was calculated. Within one hour after compressive strength testing, a selection of fragments from each specimen was examined by Scanning Electron Microscope (SEM). Fragments were vacuum gold-sputter-coated prior to SEM examination. The fragments were observed at an operating voltage of 10kV, and over a range of magnifications to investigate crack propagation. The surface hardness of each specimen was measured with a digital micro-hardness tester with Vickers diamond indenter. The indenter was set at a load of 500mN at five predetermined regions of each specimen, with a dwell-time of five seconds. The five readings for each specimen were computed and the mean VHN in N/mm2 for each specimen was determined. Each specimen was observed and analysed for porosity using Micro-CT. Three-dimensional reconstructions were made of each specimen and the number of voids per volume (mm3) of specimen, the total volume of voids (mm3) per volume of specimen and the volume percentage of voids per volume of specimen were calculated. Results: RSCH and RSCC showed statistically significant differences when compressive strength (p=0.027), volume of voids (p=0.005) and volume percentage of voids (p=0.005) were compared. No statistically significant differences were found between RSCH and RSCC when surface hardness (p=0.124) and number of voids (p=0.221) were compared. When compressive strength (p=0.254) and number of voids (p=0.210) of FIXH and FIXC were compared, no statistically significant differences were found. Statistically significant differences were found when surface hardness (p=0.031), volume of voids (p<0.001) and volume percentage of voids (p<0.001) of FIXH and FIXH were compared. No statistically significant difference was found when compressive strength (p=0.090) of KUH and KUC were compared. Statistically significant differences were found when surface hardness (p<0.001), number of voids (p<0.001), volume of voids (p=0.004) and volume percentage of voids (p=0.004) of KUH and KUC were compared. Statistically significant differences were found between KMH and KMC when compressive strength (p<0.001), surface hardness (p=0.006), number of voids (p=0.001), volume of voids (p=0.010) and volume percentage of voids (p=0.010) were compared. Conclusion: The current study suggests that RSCC is more advantageous for clinical use compared to RSCH. The results as to whether the capsule-mix or the hand-mix product are superior for the examined properties for GC Fuji IX GP are inconclusive. KUC surpassed KUH in tests performed and is therefore recommended for clinical use. KMC out-performed KMH in all tests conducted, and is therefore advocated for use in clinical practice.