Effects of multiple firing on the physical properties of CAD/CAM produced lithium disilicate for dental restorations

Objectives. To evaluate the effect of multiple thermal cycles on the physical and aesthetic properties of lithium disilicate CAD ceramic (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein). Methods. Experimental groups consisted of four test groups of crystallization firing plus one, three, five or seven “corrective” firings at the manufacturers recommended protocol of 820°C and a control “crystallization” group with one firing. Mechanical properties were analysed using the biaxial flexure strength method (n = 15) as per ISO6872 and scanning electron microscopy (SEM) was used to verify the origin of fracture (n = 1); fracture toughness change and hardness was determined by Vickers indentation method (n = 15). Physical crystalline phase change analysis was analysed with XRD (n = 1) (XPert PRO, Analytic, Almelo, Netherlands) and colour shift was quantified using a spectrophotometer (n = 5) (VITA Easyshade Compact, Vita GmbH, Bad Sackingen, Germany). Results. Mean biaxial flexure strengths and fracture toughness with standard deviations were: control 451(±57) (MPa) 1.60(±0.08) (MPa m1/2); +1 firing 390(±82) (MPa) 1.62(±0.09) (MPa m1/2); +3 firing 422(±125) (MPa) 1.63(±0.08) (MPa m1/2); +5 firing 417(±108) (MPa) 1.51(±0.24) (MPa m1/2); +7 firing 459(±106) (MPa) 1.53(±0.16) (MPa m1/2). XRD analysis showed lithium disilicate was the main crystal phase. Under all thermal conditions no further crystalline phases were identified. The colour of the specimens changed clinically significantly with increasing thermal cycles. Conclusion. There appears to be undesirable effects on strength, toughness and hardness due to thermal cycling IPS e.max CAD. There also appears to be a colour change effect as a result of increased numbers of firing cycles