The aim of the present study is to evaluate the wear characteristics and clinical performance of lithium silicate crowns in comparison to monolithic zirconia crowns.

The quality of the overall restorations as well as gingival tissues will be also evaluated according to modified United States Public Health Service (USPHS) criteria: (Marginal adaptation, color match, anatomic form, integrity of restoration, secondary caries, retention, tooth sensitivity, gingival index and periodontal index) at baseline, 6 months and 1 year

Recently advances in ceramics have greatly improved the mechanical and optical properties of restorative materials to overcome the drawbacks of all ceramic restorations over decades such as fracture, chipping, crack, wear and delamination of veneer ceramics.

Up to date despite the popularity of all ceramic restoration, the clinicians have been worried about the wear of tooth enamel antagonist to ceramic materials.

The wear properties of the ceramic restoration can affect the rate of wear of the antagonist enamel. So the wear resistance of ceramic restoration must be the same as enamel.

Explanation for choice of the comparator:

Monolithic zirconia attracts many dentists worldwide due to its excellent mechanical properties, biocompatibility and appreciate aesthetics. Some in vivo studies demonstrated the clinical success of monolithic zirconia restorations as an antagonist to natural enamel with good marginal adaptation, accepted contour, occlusion and minimum gingival response.

Batson et al. studied the quality of CAD/CAM fabricated single tooth restorations (Ten zirconia restorations were compared to 12 metal ceramic and 10 lithium disilicate counterparts) .They found that were no significant differences between the studied crown systems. No difference of the gingival response among the different crown systems. Eighty percent of zirconia crowns needed no occlusal adjustment; also it showed the least amount of marginal discrepancy.

Lohbauer et al. evaluated the amount of wear on the antagonist occlusal surfaces of clinically placed monolithic zirconia premolar and molar crowns using optical profilometry after 2 years of cementation, they found that the mean volume loss for enamel antagonist contacts (n = 7) was measured to 361 μm and the mean of the maximum vertical loss to 204 μm. The mean volume loss for pure ceramic contacts (n = 10) was measured to 333 μm and the mean of the maximum vertical loss to 145 μm.

Mundhe et al.compared the wear of enamel opposing polished zirconia, glazed metal ceramic crowns and natural enamel as a control, one year after the cementation. They found that the occlusal wear of the antagonistic enamel one year after the cementation of metal ceramic crowns ranged from 69.20 ± 4.10 to 179.70 ± 8.09 μm, whereas, for zirconia crowns, it was from 42.10 ± 4.30 to 127.00 ± 5.03 μm.

Stober et al. reported that after 2 years of cementation of the monolithic zirconia crowns, Mean and maximum vertical loss of enamel in occlusal contact areas caused by monolithic zirconia (46 and 151 μm, respectively) was approximately double that caused by contralateral antagonistic enamel (19-26 and 75-115 μm, respectively).

A lithium silicate glass ceramic (obsidian ceramic) is newly introduced to the market. After crystallization, it exhibits an ideal combination of esthetics and strength with translucency that mirrors the vitality of natural teeth for fabrication of full anatomic anterior and posterior crowns. Obsidian ceramic restorations are highly resistant to chipping unlike other ceramics, due to their monolithic composition and average flexural strength of 385 MPa. Also Obsidian Milling Block owes its good wear resistance due to a very high content of ultrafine nanometer-size crystalline structure.

Inclusion Criteria:

• All subjects are required to be:

  1. Age range of the patients from 21-50 years old; able to read and sign the informed consent document, illiterate patient will be avoided.

  2. Patients able physically and psychologically to tolerate conventional restorative procedures.

  3. Patients with no active periodontal or pulpal diseases, having teeth with good restorations.

  4. Patients with teeth problems indicated for full coverage restoration (e.g. moderate to severe discoloration, coronal fracture where partial coverage would lack retention, malposed or malformed teeth).

  5. Patients with root canal treated teeth requiring full coverage restorations.

  6. Presence of an opposing natural tooth which is non-restored or minimally restored. (Minimally restored is defined as teeth which have no restoration greater than a Class II a restoration)

  7. The opposing arch doesn't have a full coverage restoration or a partial denture.

  8. Patients willing to return for follow-up examinations and evaluation.

     Exclusion Criteria:

  1. Patients with poor oral hygiene.
  2. Patients with psychiatric problems or unrealistic expectation (patient that has phobia from dental treatments or needle bunch).
  3. Patients have no opposite occluding dentition in the area intended for restoration
  4. Patients suffer from Para functional habits or temporomandibular disorders. (as those patient may have wear which will affect the results)