Micro-layering ceramic on zirconia: Two-body wear 

Micro-layering with ceramics is considered a promising compromise between monolithic zirconia and classic veneering. But how do these minimal ceramic layers behave under functional stress? The following article analyzes the two-body wear behavior of various micro-layering ceramics on zirconia crowns and classifies the results from a materials science perspective.

Zirconia has established itself as a material in restorative dentistry thanks to its excellent mechanical properties. Continuous development towards higher translucency has led to its increasing use in monolithic applications – even in aesthetically demanding cases. This significantly reduces the risk of chipping. However, one challenge remains: the abrasion behavior of zirconia does not correspond to the natural adaptation mechanism of human teeth.

Micro-layering with a focus on wear behavior

Against this background, the wear behavior of zirconia restorations comes into focus. Wear manifests itself as material loss and can be caused, among other things, by contact with antagonistic teeth, abrasive food components, and parafunctional activities.¹ In addition, the extent of wear is influenced by factors such as the material of the antagonist, the thickness and hardness of the enamel, neuromuscular forces, and material-related properties (such as hardness, fracture toughness, and frictional behavior).^1-3 Ideally, the wear and tear of dental restorations should be similar to that of natural teeth.

Since wear behavior is largely determined by the material properties of the contact surface, the design of the surface zone is becoming increasingly important. Against this backdrop, concepts that influence this zone are gaining significance. One such concept is micro-layering (minimal veneering) as an alternative to traditional veneering techniques. In this process, so-called micro-layering ceramics are applied in layer thicknesses of approximately 0,1 to 0,6 mm to anatomically shaped or minimally reduced zirconia frameworks.⁴ 

Micro-layering combines the advantages of monolithic and conventionally veneered zirconia restorations, as it is more conservative in terms of tooth structure, offers high aesthetic quality and reduces the risk of chipping.

Material and method

Zirconia crowns (Zolid HT+; Amann Girrbach) were coated with four different micro-layering ceramics (Ceramotion One Touch 3D; Dentaurum (CER), IQ SQIN; GC Europe NV (IQS); MiYO structure; Chemichl (MIY) and Structure; Estetic Ceram (STR)). Polished anatomical zirconia crowns (MZ), conventionally veneered zirconia crowns (zirconia; Estetic Ceram (VEN)), and human teeth served as a control group. 

The test specimens were subjected to a total of 600.000 cycles in the chewing simulator (Fig. 1). Impressions of the crowns and steatite antagonists were taken before aging, after 60.000 cycles, after 120.000 cycles, and after 600.000 cycles. These impressions were scanned to subsequently determine the volumetric wear of the crowns and antagonists using inspection software. A qualitative analysis of the wear areas was also performed (Fig. 2). Additionally, the particle size distribution of the ceramic materials was determined. 

Results

• Material and number of chewing cycles had a significant influence on volumetric material loss of crowns and antagonists.
• The crowns made with the tested ceramic materials showed greater material removal than HUM.
• As the number of chewing cycles increased, so did the material removal. 
• No material removal occurred with MZ crowns and HUM antagonists.
• After 600.000 cycles, the scaffold was exposed at MIY, STR and CER.
• The particle sizes increased in the following order: MIY < IQS < CER < STR < VEN.
• There was a negative correlation between particle size and volumetric material loss.

Conclusion

The results show that micro-layering with ceramics influences the wear behavior of zirconia restorations, but with significant material-dependent differences. Under long-term stress, the zirconia framework can become exposed in certain systems. For clinical application, this means that micro-layering should not be considered a purely aesthetic measure, but rather a material-scientific concept in which material selection, layer thickness, and indication must be carefully coordinated.