The purpose of this study was to examine and discuss the deterioration of bone mechanical properties as a function of bending forces before and after implant placement in order to seek an answer to the question, whether implant placement can weaken the bone structure.
The three-point bending tests, reported in the literature, were performed only with intact bones [21, 28, 29] and not pre-drilled and implanted ones, as in this work.
Static load tests showed significant differences between the groups tested. In the case of intact bone samples, the load curves shown in Fig. 2 are continuous, and the sudden reduction in force associated with fractures is observed only over a large deformation of ~ 6.6 mm. The maximum force observed for the intact bones is in the range of 200–800 N with an average maximum force of 299 ± 31 N. Typically, the maximum force values were achieved with 1.5 to 3 mm deflection.
For the drilled samples, the resistance force maximum (170–390 N) decreased relative to the control samples, which is well observed in Fig. 2. Most measurements show single or gradual fractures in the 2.4–5 mm deflection range, well below the damage limit of the intact bones. The maximum force observed was 287 ± 26 N. The reduction of the damage limit clearly indicates the weakening of the bone’s resistance to force, which is partly due to the decrease in the effective bone thickness in the drilled region.
According to our static load tests, filling the pre-drilled nest with implants did not improve the mechanical resistance of the bones. For the implanted samples the maximum force measured was in the range 175–380 N, the mean maximum force decreased to 280 ± 28 N. The deflection values corresponding to the first partial fracture are in the range of 1.6–4.5 mm, which is smaller compared to the intact and drilled bone values. Partial cracks were observed between the two middle implants during the load. The appearance of a crack was often accompanied by a sound effect. The earlier cracks appear to be due to the fact that the holes are filled with harder material than the spongy bone, consequently local stresses at the implant-bone interface are exerted during loading.
If the local stress value is greater than the strength of the cortical bone, a crack appears [24], but the macroscopic fracture of the bone does not occur [25]. As the deflection increases, the force–deflection curve shows small breaks, indicating the appearance of new cracks. The local fractures provide stress relaxation, resulting in a higher deflection values for appearance of macroscopic fracture at 7.3–9.5 mm compared to the drilled bone. Due to this phenomenon, the toughness of the implanted specimens will be higher than that of the drilled specimens.
For fatigue tests, the same temporal function of deflection was applied throughout the experiments. To achieve the same deflection at a higher cycle number, a lower force was required for each sample, as shown in Fig. 5. Initially, the decrease in the force values is greater, and with higher cycle numbers, the reduction of the force slows down. This phenomenon shows the weakening of the mechanical structure due to bending cycles. Each cycle causes reduction of bone stiffness [26]. However, macroscopic fractures did not occur at the set deflection values and cycle numbers.
For all fatigue tests, the force required for a pre-set deflection was the highest for intact bone and the lowest for drilled bone. This significant weakening is due to the reduction of local bone volume.
In the case of implanted bones, the maximum force values for a given deflection are between the values of the intact and the drilled bone. Initially, the difference compared to intact bone is greater, but with a higher number of cycles this difference disappears.
Overall, the results of our mechanical examinations showed that the placement of the holes in the bone significantly reduces the stiffness and mechanical strength of the bone, which leads to the appearance of macroscopic fractures even at smaller deformations. The implants partially restore the integrity of the bone and increase the load-bearing capacity against the macroscopic fracture compared to the drilled samples. However, the implanted bone does not reach the mechanical strength of intact bone.
This topic was explored by finite element analysis, and many studies have been conducted on the relationship between the bone and implants under the All-on-four protocol. According to Sannino, distal implants placed at 15, 30, and 45 degrees, with a greater angle at the implant-bone interface, exert the greatest stress, but this mechanical stress value is still lower than what the implant and bone can withstand [27].
Our static load result shows that the toughness is less in the case of drilled bones but not statistically significant. The measured maximum force values also showed no statistically significant difference during the static load. However, during the fatigue load the drilled bones showed significant difference compared to the control samples. The control and the ALL-ON-FOUR implanted samples showed very similar measured force values after the 9000th cycle.
It is important to note that these measurements were performed on non-osseointegrated samples. In the event when osseointegration occurs, mechanical properties are expected to improve further. However, our experiment shows that local mechanical stresses appear at the bone-implant interface, which reduces the force required to cause fractures. A limitation of our study is that the bending forces applied in the tests occur only in extreme cases in clinical circumstances. However, the cyclicity and the magnitudes were in accordance with physiologically observable chewing movements. A further limitation of our research is that the applied protocol does not allow the implant-bone interface to be investigated in a direct way, unlike with the finite element analysis tests.