title: Conventional glass-ionomer materials: A review of the developments in glass powder, polyacid liquid and the strategies of reinforcement.
abstract: OBJECTIVE
  The development of glass-ionomers (GIs) from the earliest experimental GI formulations to the modern day commercially available GIs was reviewed. The aim of the review was to identify the developments in the glass powder and polyacid liquid constituents of GIs since their inception in the late 1960s.
  
  
  METHODS
  The glass powder has undergone major changes from the earliest GI powder formulation (G200) in an effort to enhance the reactivity with the polyacid liquid. The GI liquids have also been optimised by the manufacturers in terms of polyacid composition, molecular weight and concentration to improve the handling characteristics. Despite these developments in the glass powder and polyacid liquid constituents, GIs cannot 'truly' be advocated for the restoration of posterior dentition due to the poor mechanical properties when compared with dental amalgam and resin-based composites (RBCs).
  
  
  METHODS
  Various attempts to improve the mechanical properties of GIs through substitution of reinforcing fillers to the GI powder or modification of the GI liquid were identified in the dental literature. Despite the claimed improvements in mechanical properties of the modified GIs, a wide variation in mixing and testing conditions was identified which prevented a valid assessment of the reported reinforcement strategies. When investigating a GI reinforcement strategy it is crucial that the mixing and testing conditions are standardised to allow a valid comparison between studies.
  
  
  METHODS
  The dental literature reporting the earliest experimental GIs to modern day commercially available GIs (1969-2015) was reviewed. In addition, full-text publications and abstracts published in English reporting various GI reinforcement strategies were included.
  
  
  CONCLUSIONS
  Nevertheless, major improvements in GI formulations through a reinforcement strategy have yet to be made to enable clinical usage of GIs for the restoration of posterior dentition.
  
  
  CONCLUSIONS
  GIs chemically are inherently weak but bond to sound tooth structure without the need for preconditioning or removal of sound tooth structure such that improvements in the mechanical properties of GIs would be desirable. Although advances have been made through different GI glass powder and polyacid liquid formulations over the past 40 years, further improvements in the mechanical properties of the current GIs are required to be indicated for the restoration of posterior dentition. The literature is replete with reports on GI reinforcement, however, improved reporting and control of mixing and testing conditions are required for a valid assessment of the reinforcement strategies.
authors.affiliation: Materials Science Unit, Dublin Dental University Hospital, Trinity College Dublin, Dublin 2, Ireland.
authors.forename: Mirza Shahzad
authors.initials: MS
authors.lastname: Baig
authors.affiliation: Materials Science Unit, Dublin Dental University Hospital, Trinity College Dublin, Dublin 2, Ireland. Electronic address: garry.fleming@dental.tcd.ie.
authors.forename: Garry J P
authors.initials: GJ
authors.lastname: Fleming
mesh_terms: D000180:Acrylic Resins; D003764:Dental Materials; D006801:Humans; D012822:Silicon Dioxide; D013314:Stress, Mechanical
chemical_list: D000180:Acrylic Resins; D003764:Dental Materials; C015897:glass ionomer; D012822:Silicon Dioxide
keywords: Glass powder; Glass-ionomer restorative; ISO 9917:2007; Mechanical testing protocol; Polyacid liquid; Reinforcement