Introduction
The success of the All-on-4 technique has been widely established [
1‐
4], particularly for cases involving severe atrophy of the mandibular bone, where there is insufficient vertical bone height in the posterior region. However, clinical scenarios frequently present themselves with patients exhibiting significant atrophy on one side of the mandible, while the other side possesses adequate alveolar bone height for implant placement. In such cases, a crucial decision arises: should the traditional All-on-4 technique be employed, or is it more advantageous to opt for conventional vertical implant placement on the side with sufficient bone volume?
The cantilever is currently considered to be the main factor influencing the success of implant-supported fixed prosthetic restorations, and it is widely recognized that the length of the cantilever of the upper restoration has a positive effect on the reduction of stress in the peri-implant bone [
5]. Due to anatomical limitations, the classic All-on-4 may still produce a cantilever that increases the probability of mechanical complications in the restoration such as loosening of the abutment and screw and fracture of the upper bracket; moreover, the bending moment caused by the load applied to the cantilever can increase the force on the implant by 2-3 times, which is directly related to overloading of the peri-implant bone tissue [
6,
7].
For patients with edentulous jaws, the alveolar bone heights on both sides of the jaw are often inconsistent. In clinical practice, such patients are often encountered: the absorption degree of the posterior dental area on both sides of the mandible is different, and the vertical bone volume of the posterior dental area on one side is sufficient, while the bone volume of the other side is insufficient, whether to choose the traditional All-on-4 technique or the option of adding vertical implant placement at the end of the cantilever on the side with sufficient bone is a dilemma that most implantologists will encounter in these cases. There have been no studies on the finite element aspects of performing 5-implant-supported fixed restorations in mandibular edentulous jaws with insufficient bone in the unilateral posterior region. And to date, the extent of the difference in load between vertically placed and angled implants in the case of distal bone abundance has been unclear [
8‐
10].
This study compared the biomechanical aspects of vertical implant placement technique on the side with sufficient bone volume with the standard All-on-4 treatment concept by evaluating the stresses on the implants, and prosthetic components and strains in peri-implant bone in unilateral models of severely atrophied mandible treated with these techniques.
Discussion
Previous findings affirm the well-documented success of the All-on-4 technique in cases of severe mandibular atrophy [
1‐
4,
25,
26]. However, when faced with patients exhibiting unilateral atrophy and sufficient bone volume on the opposing side, our simulations suggest that conventional vertical implant placement may offer distinct biomechanical advantages. The three-dimensional finite element analysis provided insights into stress distribution patterns around implants and the surrounding tissues, aiding in the evaluation of the mechanical efficacy of each treatment modality. The decision between the traditional All-on-4 technique and vertical implant placement on the side with ample bone volume should be made on a case-by-case basis, considering the individual anatomical and clinical factors.
For the “All-on-4” operation on the mandible with edentulous mandible, whether the inclined implant at the end will be overloaded at the stress concentration site is a problem that we need to consider, and the influence of the stress on the surrounding bone tissue is also controversial. Some scholars [
27,
28] believed that the cantilever design would increase the stress of the bone tissue around the distal implant, resulting in occlusal overload. Relatively speaking, the non-cantilever design of implant-supported fixation could better disperse the stress of the implant and the surrounding bone tissue [
29,
30]. In this study, a case with severe unilateral mandibular alveolar bone atrophy was selected to extract the model, the advantage of this approach is that it accurately reflects the characteristics of the current clinical case. The differences in the biomechanical rows of the vertical implant placed at the cantilever on the side with sufficient bone were compared with the All-on-4 technique to provide biomechanical insights for making the most rational decisions when encountering such cases in the clinic.
The success of Three-dimensional finite element analysis (3D FEA) techniques has been reported to be related to the proportion of elements and nodes in the prepared mathematical model [
31]. In this study, the All-on-4 model contained a total of 301,847 nodes and 2,272,953 elements, which is a sufficient number of nodes and elements to maximize the sensitivity of the analysis compared to similar studies [
32,
33].
Compared to the All-on-4 technique, vertical implant placement with the lowest stresses and strains under three different forces represented the best-case condition in this study. The analysis results of all models showed that the maximum von Mises stresses of the implants were mainly concentrated in the neck of the distal implant on the loading side, which were consistent with the results of Sarrafpour [
34] and Mahony et al [
35]. Under different loads, the stress values of the implants and framework in both three models were lower than the yield strength of titanium (960 ∼ 1180MPa) [
36]. In addition, the maximum stress values on the implants in all models were observed on the most distal implant in the All-on-4 group, whereas the All-on-5-o versus All-on-5-v models avoided the concentration of stress in the neck of the implant at position 35 due to the addition of the vertical implant placement, which may be attributed to the elimination of one side of the cantilever by the placement of the vertical implant, minimizing the negative biomechanical benefits.
Several studies have shown that long-term bone remodeling or bone resorption is a process of adaptation of biological systems to a mechanical state [
37,
38]. The maximum strain values of the bone around the neck of the left distal-most implant were higher than the bone resorption threshold reported by Sugiura et al [
14] (equivalent strain value of 3.6 × 10
3µε)when subjected to a unilateral vertical or oblique force of 200 N by the conventional All-on-4 technique, and the maximum strains in the peri-implant bone at position 35 in the All-on-5-o and All-on-5-v groups were less than this value (Table
4). This shows that vertical implant placement technology can effectively reduce the risk of bone tissue absorption and better for long-term bone remodeling. It can be concluded that one-piece unilateral non-cantilevered restorations supported by a sufficient number of implants can optimize the transfer of force between different structures and tissues when there is sufficient bone volume on one side of the mandible.
In implant-supported fixed prosthetics, the clinical placement of implants was basically symmetrically distributed, and the technique of vertical implant placement proposed in this experiment resulted in an asymmetric distribution of implants in the mandible, which, according to the results, led to a more balanced distribution of stresses on all implants and strains on the surrounding bone tissues with no concentration of stresses on a single site. However, since there are fewer clinical efficacy studies related to the effect of symmetry on fixed restorations in edentulous jaws, more clinical studies should be conducted for further corroboration.
Some studies have found that the stresses under oblique load are significantly higher than that under vertical load, even up to 3.5 times [
39]. The oblique load of 200 N was used in this study, and the stresses detected on each structure were significantly higher than the vertical load of 200 N. Therefore, the direction of the additional force also has a certain influence on the generation of stresses. The lateral force generated in the oblique load will form a lateral lever, and the resulting stresses are more likely to lead to the occurrence of related mechanical complications.
It is worth noting that an implant length of 11.5 mm was chosen for this study, mainly due to the sufficient bone volume on the left side of this mandibular model, and a longer implant would provide a larger implant-bone contact area, thus improving the implant’s stability. However, in practice, this may present some challenges, such as the need for more supportive bone volume and the possibility of compromising important anatomical structures. Therefore, although an 11.5 mm implant was selected, it may not be suitable for all clinical situations. In addition, in order to make the models more realistic and to improve the accuracy of the results, we set up the inhomogeneity of the jaw. However, in order to simplify the models and improve computational efficiency, this study simplified the partial design of the finite element model and set more idealized tissue and structural properties, such as neglecting the influence of the masticatory muscles, constraining the motion of the bilateral condyles and rostral processes, indeed, the mandible is surrounded by strong masticatory muscles attached to it, including the occlusal, temporal, intrapterygoid, and extrapterygoid muscles, which may have an impact on the biomechanical behavior of the mandible; Assuming that the mandible is linearly elastic and isotropic, in fact, the mandible has certain viscoelasticity and anisotropy [
40]; As in other studies [
41,
42] it was assumed that the implants were 100% osseointegrated with the bone. Although histologic studies have shown that osseointegration between the bone-implant interface has not been entirely materialized [
43]. Our study was based on CT data of a patient with typical bone morphology characteristics. Therefore, the results of this study can only be used as a reference for patients with similar mandibular morphology. Also, in order to further confirm its validity and applicability, our results need to be validated in a larger group of patients.
Our study contributes valuable biomechanical insights to guide clinicians in making informed decisions tailored to the specific needs of patients facing such challenging scenarios. Future research and clinical trials are warranted to validate these simulated findings and further enhance our understanding of optimal treatment strategies for individuals with varying mandibular bone conditions.
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