Removable implant overdentures have become a widely accepted and well-established treatment modality for the management of edentulous patients. To date, titanium is the biomaterial of choice for the fabrication of oral implants due to its superior mechanical properties and excellent biocompatibility. However, controversial reports about possible allergic reactions to titanium, together with the rise in the concept of “metal-free” dentistry, have propelled the search for alternative implant materials. Of particular interest is zirconia, specifically yttrium-stabilized tetragonal polycrystalline zirconia (Y-TZP). Preclinical studies, case reports and randomized clinical trials on the use of zirconia implants for rehabilitation of partially dentate patients have revealed favorable outcomes, albeit in the short-term. Despite these encouraging results, there is a lack of published literature on the use of zirconia implants for the support of overdentures.
This doctoral thesis aimed to evaluate and compare the clinical and prosthodontic outcomes of similar design zirconia and titanium implants used for the support of maxillary and mandibular overdentures in a randomized controlled trial. A novel protocol for implant distribution was adopted based on the need for improved biomechanics in using zirconia implants. The implant distribution was: in the maxilla, a mid-palatal implant and three anterior implants in the incisor and first premolar regions; in the mandible, a mid-symphyseal implant and bilateral distal implants in the first molar region. The implants were conventionally loaded four months after the surgical procedures.
The overall survival rate for all the implants in this trial was 75.9 %. After one year there was no significant difference in the survival rate between the two groups in either jaw. In the mandible, titanium implants showed a survival rate of 95.8% versus 90.9 % for the zirconia implants (P=0.47). The corresponding values in the maxilla were 71.9% and 55% for the titanium and zirconia groups respectively (P=0.14). Implants in the maxilla showed a significantly higher failure rate than those in the mandible for both groups (P< 0.05). Three implants fractured in the zirconia group.
The bone level changes around the implants were favorable after one year, with marginal bone loss of 0.18 mm for the titanium implants and 0.42 mm for the zirconia implants. A statistically significant difference was noted between the two groups (P = 0.009).
Mid-line fracture of the mandibular denture was the most commonly encountered prosthodontic complication. Reduced acrylic thickness around clips and increased masticatory forces were identified as the main reasons for fracture. No statistically significant difference was observed between the two groups (P > 0.05).
Fractrographic analysis of fractured zirconia implants was used to draw some recommendations regarding the designs of the existing implants and minimize such incidences in the future. Further research to optimize the design of zirconia implants for minimizing placement failures as well as for the support and retention of overdentures is still mandatory.
The qualitative study on the patients’ perception of the proposed treatment revealed that the novel implant protocol was acceptable to patients. The major perceived advantages of the treatment were functional improvement and increased social confidence. Cost was a significant barrier for edentulous patients seeking implant treatment. Participants’ knowledge of implant treatment is limited and should be enhanced.
The in-vivo and in-vitro findings of this thesis identify the potential use of zirconia implants for the support of overdentures. However, due to the increased marginal bone loss and higher fracture rate observed for zirconia implants compared to titanium ones, the use of zirconia implants should only be limited to patients with allergy to titanium or those requesting a metal-free restoration
