The in¯uence of resin coating on the shear punch strength of a high strength auto-cure glass ionomer Jakob Leirskar a, * , Ha Êkon Nordbù a , Graham J. Mount b , Hien Ngo c a Department of Cariology, Dental Faculty, University of Oslo, Box 1109 Blindern, N-0317 Oslo, Norway b Department of Dentistry, Adelaide University, Adelaide, SA, Australia c Colgate Australian Clinical Dental Research Center, Adelaide University, Adelaide, SA, Australia Received 30 January 2001; revised 14 November 2001; accepted 18 December 2001 Abstract Objectives. The aim of this study is to examine the in¯uence of early water contact on the shear punch strength of a modern, high strength, auto-cure glass ionomer (Fuji IX GP). Methods. Specimens, 8.0 mm in diameter and approximately 1.25 mm thick, were prepared in metal washers, and allowed to remain covered by polyester strips for 7 min, to ensure completion of the initial set. A minimum of 32 specimens was prepared for each set of experiments and they were divided into two equal groups. Following removal of the strips, the surfaces of the specimens in Group 1 were immediately coated on both sides with Fuji Coat LC and light cured for 10 s. In Group 2 the specimens were left uncoated. All specimens were then placed in distilled water at 37 8C, and stored for periods of 24 h, 1, 2, 5, or 8 weeks. The strength of the specimens was measured at each time interval in the shear mode by a punch test in a universal testing machine. Results. The mean strength of the coated specimens in Group 1, which had been protected against exposure to water, increased signi®- cantly on ageing from 67.1 (24 h) to 78.0 MPa (1 week), with no further signi®cant change in strength. The mean strength of the specimens in Group 2 increased signi®cantly from 75.7 (24 h) to 83.3 (1 week) and 91.9 MPa (2 weeks), with no further signi®cant change in strength. Uncoated specimens were stronger at all points of time. The difference was statistically signi®cant at 24 h, 2, 5, and 8 weeks (P , 0.05), but not at 1 week P  0:061: Signi®cance. The results suggest that, contrary to the usual instructions for placement of glass ionomer, early access to water will positively in¯uence the shear punch strength of Fuji IX GP. q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Restorative material; Glass ionomer; Resin coating; Shear punch test; Mechanical strength 1. Introduction The use of glass ionomer materials in operative dentistry has gradually increased during the last decade as a result of improvements in the materials themselves, as well as changes in restorative techniques. Other factors include a greater demand for `esthetic' restorations, and the current debate on `the amalgam issue'. According to the manu- facturers, glass ionomer restorations are indicated for decid- uous teeth for the restoration of stress-bearing occlusal and proximal restorations. In permanent teeth, restorations using this material are especially indicated for use in erosion/abra- sion lesions without cavity preparation, in anterior proximal and cervical carious lesions, ®ssure seals, in transitional restorations and in minimal intervention cavity designs where occlusal load is limited. In stress-bearing areas, however, relatively high failure rates have been recorded, mainly as a result of fracture [1]. The adverse effect on the physical and mechanical proper- ties of the earlier types of auto-cure glass ionomers arising from premature exposure to water, or following prolonged dehydration, has been well documented. Immersion in an aqueous environment leads to water absorption and erosion, and dehydration causes crack formation [2±4]. Both problems adversely affect the strength, as well as the translucency, lead- ing to loss of esthetics in the restoration [5]. The strength and resistance to erosion rely on the formation of a relatively insoluble polyacid/cation matrix. This matrix takes time to form and the character of the ionic cross links change over the early life (,24 h) of the restoration from being pre- dominantly based on hydrolytically unstable calcium poly- acrylate to the more stable aluminium polyacrylate [6]. Exposing the newly placed glass ionomer to an aqueous Dental Materials 19 (2003) 87±91 dental materials 0109-5641/03/$ - see front matter q 2003 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S0109-5641(02)00016-7 www.elsevier.com/locate/dental * Corresponding author. Tel.: 147-22852138; fax: 147-22852344. E-mail address: jakoble@odont.uio.no (J. Leirskar).