THE EFFECT OF REPEATED HEAT-PRESSING ON THE BIAXIAL FLEXURAL STRENGTH AND SURFACE ROUGHNESS OF LITHIUM DISLILICATE GLASS-CERAMICS

Statement of problem: During heat-pressing of lithium dislilicate glass-ceramics, It is more economical to press several restorations from one ingot at the same time. However, this is often not possible and may result in a considerable amount of leftover material. It has been reported that the leftover materials after heat-pressing are reused in some dental laboratories. Sufficient knowledge about the consequences of such procedure is not available. The issue is thereby raised whether the leftover material should be discarded or reused.
Purpose: The aim of the present study was to evaluate the effect of repeated heat-pressing on biaxial flexural strength and surface roughness of lithium dislilicate pressable glass-ceramics (IPS e.max Press). As well as to describe the microstructural features present in fresh-pressed, and repressed material using Xray diffraction, EDAX and SEM
Materials and methods: Twenty pressed and repressed samples were prepared following the manufacturers’ recommendations measuring 15 mm × 1 mm per material. Biaxial flexure (piston on 3-ball method) was used to assess strength. X-ray diffraction was performed to identify the crystalline phases, and a scanning electron microscope was used to disclose microstructural features. Also surface roughness was evaluated
Results: BFS for pressed and repressed respectively; (375.8± 4.55) and (389.4±12.12) MPa; no significant difference was found between the Pressed and Repressed groups. Surface roughness for pressed glazed and unglazed respectively (0.27±0.05, and 1.14±0.16) – for repressed glazed and unglazed respectively (0.21±0.06 and 1.33±0.20) - for both groups, significant difference was found between Glazed and Un-Glazed ceramics. No statistical significant difference was found between Pressed and Repressed groups. Xray diffraction revealed the material is predominantly crystalline structure; lithium disilicate was identified to be the main crystalline phase, peaks after pressing and repeating pressing are similar, the crystalline phase assemblage did not change; however their radiation intensities (height) has, the dominant peak (highest peak) for the repressed sample is smaller compared to the pressed. EDAX results showed no change in composition between pressed and repressed samples. Microstuructural features SEM displayed a noted increase in crystal dimension after repressing
Conclusions. IPS e.max Press left over pressed buttons can be safely reused since repressing does not display consequent negative effects on Biaxial flexural strength and surface roughness, as well as microstructure.