For a thin veneer in lithium disilicate, GC Initial LiSi Press requires a cervical and labial reduction of at least 0.3 mm, while Ivoclar IPS e.max CAD requires at least 0.4 mm. For a crown, the two instructions for use converge almost exactly: a shoulder of at least 1.0 mm, and an incisal or occlusal reduction of about 1.5 mm. This reference brings together, indication by indication, the minimum preparation thicknesses published by the two most widely distributed lithium disilicate manufacturers in CAD/CAM laboratories.
What is the minimum thickness for a veneer in lithium disilicate?
The thin veneer calls for the smallest reduction of the two systems, but not the same one depending on the manufacturer. According to the GC Initial LiSi Press instructions for use, revised in November 2024, a thin veneer requires a reduction of at least 0.3 mm in the cervical and labial area, and 0.4 mm at the incisal edge, preferably located in the enamel. According to the Ivoclar Vivadent IPS e.max CAD Monolithic Solutions instructions for use, the minimum thickness of a thin veneer is 0.4 mm in the cervical and labial area, with a restoration thickness of 0.5 mm to be planned at the incisal edge.
For a full veneer, the two instructions for use give exactly the same value: a reduction of at least 0.6 mm in the cervical and labial area, and at least 0.7 mm at the incisal edge. For an occlusal veneer (table top), GC calls for an occlusal reduction of about 1.0 mm; Ivoclar calls for a circular shoulder of at least 1.0 mm and an occlusal reduction of at least 1.0 mm as well.
What is the minimum thickness for an anterior or posterior crown in lithium disilicate?
For an anterior crown, GC Initial LiSi Press calls for a shoulder or chamfer of at least 1.0 mm, an incisal reduction of about 1.5 mm, and a vestibular or oral reduction of about 1.2 mm. The IPS e.max CAD instructions for use give the same three figures for an anterior crown or bridge abutment: a shoulder of at least 1.0 mm, an incisal reduction of at least 1.5 mm, and a vestibular or oral reduction of at least 1.2 mm.
For a posterior crown, GC calls for a shoulder of at least 1.0 mm and an occlusal reduction of about 1.5 mm. Ivoclar adds a vestibular or oral reduction of about 1.5 mm in the premolar region, on top of the shoulder of at least 1.0 mm and the occlusal reduction of at least 1.5 mm. For a minimally invasive crown in IPS e.max CAD, which requires mandatory adhesive cementation, Ivoclar brings these three dimensions down to 1.0 mm each, but prohibits any wall thickness below 1 mm.
What minimum depth and width are needed for an inlay or onlay in lithium disilicate?
Here, both manufacturers give the same rule. For both an inlay and an onlay, the preparation depth in the fissure area must be at least 1.0 mm, and the isthmus width at least 1.0 mm as well, with cavity walls at 6° to the tooth's long axis and rounded internal edges to avoid stress concentration. For an onlay, a reduction of at least 1.0 mm in the cusp areas is added to these two values.
What is the maximum width for a 3-unit bridge in lithium disilicate?
On this point, GC Initial LiSi Press and Ivoclar IPS e.max CAD publish exactly the same limit: the pontic width must not exceed 11 mm in the anterior region (up to the canine), nor 9 mm in the premolar region (up to the second premolar as the terminal abutment). Beyond those figures, both instructions for use place the bridge outside the indication. In the layering technique (cut-back or layering), where a lithium disilicate framework is veneered with a cosmetic ceramic, Ivoclar specifies that the framework must make up at least 50% of the restoration's total thickness: for a total thickness of 1.5 mm, the IPS e.max CAD framework must measure at least 0.8 mm and the veneering ceramic at most 0.8 mm; for 1.8 mm, the framework rises to at least 1.0 mm.
Do the two manufacturers claim the same mechanical strength?
Strength is not directly comparable between the two systems, since GC does not publish a flexural strength value in the Initial LiSi Press preparation instructions. Ivoclar states, for IPS e.max CAD, a specified biaxial flexural strength of at least 360 MPa, with a typical average value of 530 MPa according to ISO 6872:2015. This figure is the one announced by the manufacturer for its own material, and does not replace a laboratory's own quality control. The lithium disilicate glass-ceramic block in the Dentev catalogue covers the same family of indications, crowns, veneers and inlays, but its own instructions for use should be checked for its exact minimum thicknesses, which may differ from those of GC and Ivoclar.
- Thin veneer, cervical and labial: 0.3 mm (GC Initial LiSi Press) or 0.4 mm (Ivoclar IPS e.max CAD)
- Full veneer, cervical and labial: 0.6 mm, incisal edge 0.7 mm (GC and Ivoclar, identical value)
- Inlay and onlay, depth and isthmus: at least 1.0 mm for both manufacturers
- Anterior crown: shoulder 1.0 mm, incisal reduction 1.5 mm, vestibular or oral reduction 1.2 mm (GC and Ivoclar, identical values)
- Posterior crown: shoulder 1.0 mm, occlusal reduction 1.5 mm (GC and Ivoclar)
- Maximum pontic width: 11 mm in the anterior region, 9 mm in the premolar region (GC and Ivoclar, identical value)
- IPS e.max CAD biaxial flexural strength: at least 360 MPa, typical average value 530 MPa (Ivoclar Vivadent instructions for use)
Sources
"IFU GC Initial LiSi Press EEP," GC Corporation and GC Europe, document 30000703, last revised 11/2024, "Preparation Design" chapter.
"IPS e.max CAD, Monolithic Solutions, Instructions for Use," Ivoclar Vivadent AG, 9494 Schaan, Liechtenstein, reference document 689914/en, "Preparation guidelines," "Minimum layer thicknesses" and "Material" chapters.
