Based on the study’s findings, it cannot be recommended to replace individual molars by a single reduced-diameter implant.
Reduced-diameter (≤3.5 mm) implants can be useful in certain situations, and reviews have reported high survival rates. However, long-term data on reduced-diameter implants supporting fixed restorations are lacking, especially in the posterior area. With this in mind, a group from the University of Zurich performed a randomized clinical trial of a monolith zirconia crown versus a porcelain-fused-to-metal (PFM) crown, both on a titanium-zirconium reduced-diameter implant (Straumann Roxolid, Tissue Level, Standard Plus, diameter 3.3 mm, regular neck). They report their interim results 3 years after surgery.
Dr. med. dent. Konrad Meyenberg
Experience with reduced-diameter implants (3.5 mm or smaller) for single-tooth implants in the molar region is limited.
A total of 76 patients were randomized, and 59 attended the 3-year follow-up (27 with the monolith zirconia crown and 32 with the PFM crown). At a mean follow-up of 36.9 months, the implant survival rate was 80%, with 14 implants lost – two in females and 12 in males. Notably, three implants in the PFM group were lost with no signs of inflammation, and six PFM crowns and five monolith zirconia crowns were lost due to implant fracture.
Prosthetic complications were reported only with the PFM crowns and comprised four instances of minor ceramic fractures. Occlusal wear was more common with PFM crowns (n=18) than the monolith zirconia crowns (n=9); proximal contacts were missing mesially at 5 implant crowns and distally at 4 implant crowns. Patient satisfaction scores were high in both groups.
Dr. med. dent. Konrad Meyenberg
The present study questions the following 3 paradigms: 1. the geometric design of the implant itself is not so important, the more important is the material (pure titanium versus titanium-zirconium); 2. overloading as a reason for implant loss does not exist; 3. individual molars can be replaced with diameter-reduced single tooth implants.
The results of the study were somewhat unexpected. The survival rate of the implants was lower than reported in the literature, and the authors suggest this is because reduced-diameter implants are more usually placed in the anterior region. Traumatic occlusal forces have been suggested as leading to aseptic implant loss. However, the follow-up observations in the study did not collect information to support or reject this theory. Regarding the implant fractures, the authors note the fractures occurred below the internal screw channel in all cases. As the fractures occurred more often in males than females, biomechanical overload is a possible cause.
Dr. med. dent. Konrad Meyenberg
The authors of the study conclude by pointing out that manufacturers should fulfill their obligation to carry out appropriate clinically relevant simulation tests before approving the indication of an implant type. In this way, ethically questionable studies can ultimately be avoided.
Due to the low survival rate, the authors cannot recommend the use of reduced-diameter implants in molar sites. Although a limitation of this study is the clinically short observation period, the final follow-up at 5 years should yield further valuable information on the posterior use of reduced-diameter implants.
