Several advantages of zirconia have prompted its use by increasing numbers of dentists and patients: its strength is similar to that of other metals, its color and translucency are identical to those of teeth, and it exhibits high biocompatibility, which reduces the risk of inflammation because dental plaque is unlikely to accumulate. However, the strength of zirconia abutments is not as well understood as that of titanium abutments. Some studies have investigated the strength of zirconia abutments, but none of them considered the limit strength and interface damage mode of zirconia abutments that can cause a peri-implant bone to fail over time. Therefore, the main purpose of this study was to determine the maximum deformation and failure forces of titanium implants with titanium-alloy and zirconia abutments under different simulated bone-loss conditions.
Many clinical failure modes has been reported for dental implants[20, 21]. In addition to infection soon after dental implant insertion and poor oral hygiene, another major reason for such failures is poor osseointegration, which refers to the interface condition between the implant and the surrounding bone. Osseointegration may be poor due to overloading or primary instability[2, 23, 24] of the implant. Some marginal bone loss is generally observed after dental implant insertion, which begins at the neck of the implant and can spread to the first thread of the implant body or to the first contact between the bone and the rough surface of the implant. Sunden Piknér and Gröndahl reported that 2.3% of implants demonstrated a marginal bone loss ≥3 mm within the first year, with this proportion increasing to 7.0% after 9 years. Rocci et al. observed 97 implants with turned surfaces in 46 maxillae that were used for single or partial rehabilitation, and found an average surrounding bone loss of 1.5 mm after 3 years of prosthetic loading. Various factors such as the implant design in the cervical region, abutment design, biologic width, and platform-switching concept influence bone loss, and the reported range of marginal bone loss in the first year is 0.4 to 1.6 mm, with an annual rate of 0 to 0.2 mm thereafter. The experiments performed in the present study exposed the top 3.0 and 1.5 mm of the implant, which was designed to simulate the typical clinical conditions after 5–7 and 20 years, respectively, thereby examining the influence of implant marginal bone loss on both medium- and long-term bases.
Yildirim et al. used static tests to compare the fracture load of zirconia and aluminum-oxide abutments to 30-degree oblique loading, and found that the zirconia abutments (737.6 N) were stronger than aluminum-oxide abutments (280.1 N). Kerstein et al. applied 40-degree oblique loading to measure the fracture strength of two brands of zirconia abutment: an Atlantis abutment constructed from zirconia, and a Nobel Biocare Procera AllZirkon abutment. They found that the fracture strength was significant larger for the Atlantis abutment (831 N) than for the Nobel Biocare Procera AllZirkon abutment (740 N). However, for both types of zirconia abutments the failure load exceeded the maximum human bite force. The results of the mechanical tests of the strength of zirconia abutments in the present study may differ from those of the previous studies due to differences in the experimental setups, particularly the angle of the applied load, the implant and abutment sizes and shapes, and the design of the upper loading member. The experimental apparatus used in the present study was designed based on the IS0 14801 standard, with only the distance at the top of the exposed implant—which was the main research focus of this study—adjusted to allow comparisons.
Ferrario et al. found that the single-tooth bite forces in healthy young male adults were 150 and 140 N for the central and lateral incisors, respectively. The physiological maximum incisor biting forces may up to 290 N depending on facial morphology and age. The average maximum failure forces for all groups in the present study exceeded the mentioned bite forces in anterior areas, as well as for all specimens tested, for which plastic deformations first appeared under oblique compressive loads exceeding 480 N. Zirconia abutments can therefore be considered a valid alternative to titanium-alloy abutments in anterior areas. However, zirconia abutments must be further evaluated in the posterior area of the jaw bone because the occlusal force is substantially larger than that of the anterior teeth.
Calderon et al. found that the maximum bite force varied from 656.1 to 108.9 N in females with bruxism (mean maximum bite force: 395.6 N), and from 999.3 to 262.8 N in males with bruxism (mean maximum bite force: 584.5 N). These results suggest that in patients with bruxism and serious marginal bone loss (>3.0 mm) surrounding the implant, both zirconia and titanium-alloy abutments should be applied with extreme care.
Applying oblique loading to a dental implant without marginal bone loss will result in stress concentration at the implant–abutment interface, which is the fulcrum (pivot) of the structure. The energy associated with the applied force would pass through the abutment and onto the implant–abutment interface and abutment screw, and finally to the bottom of the implant and surrounding host material. However, the energy associated with applying an oblique loading to an implant with marginal bone loss would pass through the abutment and onto the implant–abutment interface and abutment screw, to the exposed part (upper side) of the implant, and finally to the bottom of the implant and surrounding host material. In Groups A1 and A2 (with 3 mm of bone loss), the bottom of the abutment screw was located near the fulcrum, and thus the upper abutment screw could enhance the structural stiffness of the exposed part of the implant. The implant–abutment structure was weakest in the fulcrum, which was 3 mm below the top of the implant. Therefore, if the applied loading exceeded the strength of the implant, the implant would begin bending, the thread would then crack, and finally the implant would fracture. In Groups B1 and B2 (with 1.5 mm of bone loss), the lower part of the abutment screw was located in the unexposed part of the implant, which was embedded in the specimen holder. In addition, the upper (cylinder) portion of the implant had a larger structural stiffness than the thread of the implant. The stress would be concentrated at the implant–abutment interface, similar to the case without marginal bone loss. Therefore, most of the applied energy would act at the implant–abutment interface, resulting in serious damage.
Human occlusal forces are dynamic. However, a dynamic loading test was not used to simulate the aging process in this study, due to previous studies indicating that the aging process does not significantly affect the fracture strength of zirconia specimens[28, 32, 33]. Basically, the survivability of zirconia depends on its ability to withstand occlusal forces. The experimental results indicated that the maximum deformation forces in Groups A2 and B2 were 531.8 and 907.3 N, which are much smaller than normal human bite forces. In a very small number of clinical cases the patient’s bite force could be larger than these maximum deformation forces and the marginal bone loss could be larger than 3 mm, resulting in the failure of the implant–zirconia abutment component or fracture of the implant body, although the zirconia abutment itself should remain intact.
Some limitations of this study should be considered. First, the sample size was small, though this was also the case in previous studies[34, 35]; future studies should investigate larger numbers of specimens. Second, this study focused on static loading rather than dynamic loading. Although previous studies[28, 32, 33] have indicated that the use of static or dynamic loading would have no significant affect on the strength of zirconia specimens, further fatigue experiments are needed to confirm this. Third, only a single type of implant was used in this study, and so the effect of different implant sizes and implant–abutment connection types should be also investigated in future studies.