Dental implantation is considered optimal treatment for the replacement of a lost tooth due to its high success rate. However, treatment planning and the surgical/impression approaches in implant treatment require high precision. ¹ , ² , ³ , ⁴ , ⁵ Factors such as implant site, prosthetic design, attachment type, diameter and length of implant, loading protocol, and prosthetic material. Moreover, the type of framework can all affect the long-term success of implants. ⁶ , ⁷ , ⁸ , ⁹

Placement of long implants should not compromise the esthetics of the final restoration. This used to be particularly important when implants had machined surfaces and the placement of longer and wider implants was the conventional method of increasing the bone-implant contact area. Long and wide implants are usually associated with a high success rate. The success rate would be even higher if these implants are placed symmetrically in the jaw. ¹⁰ , ¹¹ However, the posterior maxilla may be an exception to this rule due to several parameters, ¹² including limited access, less visibility, inadequate interarch distance, bone loss after tooth extraction, ¹³ , ¹⁴ , ¹⁵ and low quality of bone (mainly Type IV). In the posterior areas, a thin layer of cortical bone often surrounds low-density trabecular bone, which decreases the success rate of implants placed in these areas. ¹⁶ , ¹⁷

More recent studies have reported similar clinical success rates for short and long implants, ¹⁸ , ¹⁹ , ²⁰ and it appears that short implants may be a suitable alternative to long implants in resorbed alveolar ridges. ²¹ Moreover, placement of short implants is often technically easier and safer than long implants due to lower risk of interference with anatomical structures like the maxillary sinuses.

Placement of short implants is also advantageous in cases with insufficient mandibular bone height where reconstruction surgery is not feasible. However, the ability of short implants to tolerate high loads at these areas (in comparison to long implants) is questionable. Finite element analysis (FEA) is a popular mathematical simulation in dentistry that can assess the stressed sites. ²² , ²³ This study sought to assess comparatively the stress distribution pattern around short and long implants in mandibular overdenture using FEA, to check whether similar stress distributions would be observed around both implants under different loads.

This in silico simulation study evaluated stress distribution patterns through below steps.

Modeling

Abaqus CAE version 6.10 software (ABAQUS Inc., Pawtucket, RI, USA) was used for this purpose. Each mandibular model (retromolar pad to retromolar pad), mandibular removable denture, two cylindrical implants, and an implant-supported bar were separately designed. In the mandibular model, cortical and spongy bones were defined. A simplified mandibular model was used with the average dimensions of a normal human mandible. In the simplified model of the mandible, implants were designed as homogeneous cylinders with 4-mm diameter and 6 mm and 10 mm lengths. A 16-mm bar was also designed attaching the two implants at canine areas. Mandibular overdenture was also designed according to the dimensions of the mandibular model and was attached to the bar and implants Figure 1. Figure 1

The created model.

Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11show the patterns of stress distribution in peri-implant bone following load application to the overdenture. Various levels of stress are marked with different colors. In each Figure, areas bearing the same level of Von Mises stresses are marked with the same color. Figure 2

(a and b) Pattern of stress distribution in bone around short (6 mm) implants following the application of 35N vertical load.

It was found that stresses might be heavier around long implants, although the difference might not be considerable. Placement of short implants is advantageous for both patients and clinicians. From the patient's point of view, the use of short implants eliminates the need for additional surgical procedures for autogenous bone grafting or nerve transposition. Thus, donor site morbidity and sensory disturbances of mental nerve following nerve transposition would be prevented. Moreover, unnecessary costs would be avoided, surgical time would decrease, and patient discomfort would be less. From the clinician's point of view, the placement of short implants is technically easier in the oral environment. Furthermore, short implants make implant treatment possible for patients who are not qualified for receiving long implants.

The current study evaluated the maximum level of von Mises stresses distributed in the crestal bone at four areas around implants and found no difference in this respect between short and long implants following load application to overdenture. Lower levels of stress were noted around short implants compared to long implants; however, this difference was not considerable. Thus, considering the advantages of short implants, they may be used for implant-supported rehabilitation and also for mandibular overdentures.

In contrast to our findings, Hasan et al. ³³ reported higher levels of stress around short compared to long implants and demonstrated a less homogeneous pattern of stress distribution around short implants. However, it should be noted that in both their study and ours, the range of stress was quite higher than the physiological stress threshold suggested by the researchers. ³³ The maximum levels of physiological stress and strain are reported to be 100 MPa and 3000 microstrains in cortical and spongy bones, respectively. Considering these values, it is assumed that short and long implants placed in the current study may be associated with a risk of overloading in some cases. Although some studies have reported the risks of placement of short implants and lower success rates compared to long implants, some others have confirmed their high success rate and favorable prognosis, with an overall success rate of about 90%–100%. ³⁴ , ³⁵ , ³⁶

The assessment of load distribution in bone around short and long implants by FEA in the current study revealed that the placement of short implants in cases with inadequate bone height can yield results similar to those of long implants. In the current study, no major difference was found in load distribution around short and long implants. The survival rate of short implants was reported to be within the range of 90%–100%. ³⁶ Thus, in the clinical setting, when the placement of long implants is contraindicated, clinicians can still consider using short implants as suitable alternatives (although with slightly higher risks of failure). ³⁶ Evidence also confirms the advantages of short implants and their equal efficacy to long implants. Renouard and Nisand ³⁷ reported that short implants could be used for reconstruction of the severely resorbed maxilla as an alternative to complex surgical techniques. ³⁷ Moreover, in a study by Grant et al., ³⁸ the survival rate of 8 mm implants placed in the mandible was reported to be 99% from the first-stage surgery to 2 years after functional loading. They also stated that short (8 mm) implants could be placed as an alternative to bone grafting in atrophic posterior mandible with inadequate height. ³⁸

Esposito et al. ³⁹ found no significant difference in terms of the rate of failure or complications between short and long implants. ³⁹ Moreover, Maló et al. ⁴⁰ evaluated the outcome of the placement of 7 mm implants for the rehabilitation of atrophic posterior maxilla and mandible at 1 year after loading and reported relatively good survival rates for short (7 mm) implants. They also suggested the placement of short implants as a suitable alternative for the rehabilitation of resorbed areas. ⁴⁰

Quantity and shape of the mandibular residual ridge may dictate the position and number of required implants. In severely resorbed mandibular ridges, only short implants can be placed. Short implants provide limited implant-bone surface area. In other words, a small volume of bone must resist loads transferred through the implants. Increasing the number of implants increases the implant-bone surface area and enhances the load-bearing capacity of bone.

Bone is a complex, porous, anisotropic, composite structure with different physical properties at different sites. ⁴¹ The results obtained in the current study are probably attributed to the stress transfer mechanism of the bone-implant complex. ⁴² Mechanical distribution of stress occurs primarily at the bone-implant interface. ⁴³ First, the occlusal stresses are transferred through the implant to the cervical bone. Limited amounts of the residual stress in trabecular bone are transferred to the apical region. Furthermore, a high level of strain is transferred to the cortical bone in contact with the implant because its modulus of elasticity is higher than that of trabecular bone and it also has high-stress transfer capacity. ⁴² , ⁴⁴ In cortical bone, stress distribution is limited to areas in close contact with the implant; whereas, in trabecular bone, stress is distributed to more distant areas. ⁴⁵ Misch in 1990 reported that the percentage of bone-implant contact in the cortical bone was significantly greater than that in the trabecular bone. ⁴³ Moreover, the cortical bone is dense and more resistant to deformation. These findings have been confirmed by some in vitro studies. ⁴⁴ , ⁴⁶ , ⁴⁷ Thus, inappropriate loading results in overaccumulation of stress and eventual bone loss. Stress distribution plays an important role in bone loss during the implant treatment period. The stress must be uniformly distributed around implants to decrease the risk of bone loss and improve treatment prognosis.

Although bone loss is a fundamental concept in dental rehabilitation, its exact mechanism has yet to be fully understood. Moreover, due to the complexity of bone structure and its heterogeneous mechanical properties, a specific safe threshold for stress or strain cannot be determined. Since microdeformation of bone is an important factor compromising the survival of implants, assessment of the level of stress and strain in bone is critical. ⁴⁸

The stress values reported in the previous studies do not necessarily indicate the behavior of materials in the clinical setting. ⁴⁴ On the other hand, it has not been specified what proportion of biological changes like bone resorption or deposition occurs in response to stresses created in the clinical setting. ²⁸ , ⁴⁴ Nonetheless, FEA has yielded beneficial information in many studies and thus, is now extensively used for prediction of the behavior of materials in the clinical setting. For this reason, FEA was used in the current study to assess stress distribution in bone around short and long implants in a mandibular overdenture. FEA provides a numerical scale to determine the level of stress and deformation in structures with different geometries. In this method, a structure is divided into tiny blocks known as elements, and the behavior of structures is evaluated through simple equations. Type, order, and number of elements affect the accuracy of the results. Moreover, successful modeling of implants depends on accurate simulation of the geometry and surface structure of implants, structural characteristics of implant and bone, load application conditions, support, and bone-implant interface properties. ⁴⁹ FEA is superior to the photoelastic method used in some previous studies. Despite the advantages of FEA, it has some shortcomings in the simulation of the behavior of biological systems in response to load application, similar to the photoelastic and load measurement methods. ⁵⁰ The main problems against an accurate simulation of the mechanical behavior of dental implants include modeling of human bone and its reaction to mechanical load application. ⁵¹ , ⁵² All structures modeled in the current study were considered homogeneous with a linear modulus of elasticity. However, these structures, particularly the vital ones, may show totally different behavior in the clinical setting. For instance, it has been discussed that the mandibular cortical bone has some nonhomogeneous areas in its transverse cross section. ⁵³ Thus, some differences may exist between the function and performance of different materials under in vitro and in vivo conditions. This decreases the generalizability of in vitro results to the clinical setting. ⁵⁴

Long-term clinical studies are required to assess the effect of stress distribution on the performance and success rate of short and long implants. Furthermore, the effect of bolus position during mastication on the pattern of stress distribution around implants must be investigated in future studies. Comprehensive evaluation of factors related to stress distribution around short and long implants in an overdenture may help improve the prognosis and prolong the clinical service of implants.

This study was limited by some factors. It was better to test more models, with various lengths and diameters of implants, to draw more comprehensive knowledge. Furthermore, although simulation studies provide valuable information, their results are limited to the very controlled virtual simulation, and cannot be generalized to clinical findings where ever-changing forces and stresses are presented in the mouth. Therefore, our results should be verified using in vitro and later indirectly using clinical studies. Finally, lack of statistical assessments disallows conducting any null hypotheses.

Within the limitations of this in silico simulation, the following results were obtained. The difference between maximum levels of stress distributed in bones around short and long implants was small. Basic stress values were slightly lower around short implants. Thus, the placement of short implants in cases with inadequate bone height might yield results similar to those of long implants.

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.