Zirconia has emerged as a favored material in clinical dentistry owing to its biocompatibility and aesthetic appeal [1,2]. However, its inherent aesthetic limitations necessitate the use of porcelain veneers for color correction [3,4]. Although effective, these veneers are prone to fracture and chipping, thereby compromising the longevity of the material [5,6]. Advancements in materials science, including grain size refinement and compositional adjustment of zirconia, have played a crucial role in addressing these limitations, promoting the use of monolithic zirconia as a viable alternative to conventional veneer-containing materials [7], [8], [9]. This new material streamlines production, is compatible with CAD/CAM and additive manufacturing for enhanced cost efficiency, and allows tooth preparation in less time [10], [11], [12]. To enhance translucency and aesthetics, yttria (Y) oxide stabilizer content was increased from 3 mol% to 4–5 mol% (3Y, 4Y, and 5Y) [13,14]. An approach involving the introduction of multilayer monolithic zirconia (M-Zr) featuring a natural color gradient and refinement through precise CAD/CAM positioning diminishes the requirement of staining and guarantees a more natural tooth appearance [15,16].

The use of a stabilizer at high content enhances the superior translucency of M-Zr crowns, surpassing that of monolithic zirconia crowns and natural tooth enamel [17,18], in addition to reduction of alumina content and increase in grain size [17]. The key factors for the optimization of color matching and aesthetics of M-Zr crowns include the yttria content of zirconia (3Y, 4Y, and 5Y), thickness of restorations, and initial color of the zirconia block or disc [19]. Notably, thicknesses of >1.5 mm may lead to color deviations owing to the reduction of light transmission [20]. In clinical applications, the selection of appropriate cement shades and choice of suitable abutment materials are critical to achieve the desired aesthetic outcome [21,22]. A comparison of the fracture strength of zirconia and lithium disilicate materials showed that M-Zr crowns has a significantly higher fracture strength, substantially exceeding the average human biting force [23], [24], [25]. This suggests a higher safety margin, making M-Zr a preferred crown material for fixed dental prosthetics [26].

The utilization of M-Zr facilitates gradient transparency, coloration, and structural robustness, thereby enabling superior strength in the cervical region and heightened translucency at the incisal, thereby establishing a gradient effect within the dental prosthetics [21]. Currently, in dental clinical practice, the predominant compositions of yttrium oxide stabilizers are either 3Y+5Y or 4Y+5Y combinations. Literature reported that the selection of zirconia blocks, particularly variations in yttrium oxide stabilizer compositions [19], not only influences translucency gradients and color saturation but impacts mechanical strength [27,28]. In dental applications, the ability of M-Zr crowns to withstand prolonged biting forces without damage is underscored by the risk posed by inadequate mechanical strength, potentially leading to increased complications impacting patients, dentists, and dental technicians.[29,30] Existing research on the mechanical properties of M-Zr has focused on non-crown morphology specimens,[27] while studies on crown morphology have mainly concentrated on monolithic zirconia, thereby creating a notable gap in understanding the mechanical properties of M-Zr.[30], [31], [32] To bridge this gap, this study investigates the minimal thickness required for M-Zr crowns across different tooth positions and assesses the effect of thickness on translucency. This study posits the null hypothesis that the required minimal thickness of M-Zr crowns differs significantly across various tooth positions.