The use of endosseous dental implants to replace missing or hopeless teeth has become routine clinical practice over the past three decades. Implant-supported fixed prostheses are often considered the treatment of the first choice. Clinical success largely depends on the biomechanical behavior of implants in terms of stress and strain transfer to supporting bone. Long implants were initially preferred, despite early finite element analyses indicating that major stress transfer to surrounding bone is primarily limited to the first 3–5 threads.^[1] However, in the posterior mandible, alveolar bone resorption may limit the use of long implants (>8 mm)^[2,3] due to the proximity of the mandibular neurovascular bundle.^[4,5] A minimum of 2 mm of bone height should remain undisturbed above this vital structure to avoid nerve damage.^[6,7] Avoiding the mental nerve is also a consideration at mandibular bicuspid sites.^[8,9] In additional, lingual mandibular bone concavities may increase the risks of fenestrations or perforations of the lingual cortical plate.^[10] Short (6–8 mm) or even ultrashort (<6 mm) implants often allow effective treatment.

Short-threaded implants had a mixed history in the past,^[11] but substantial evidence now supports their use with proper technique and implant design.^[12] Most implant manufacturers now offer short implants for use in the posterior mandible.

Current short implant designs feature moderately rough surface textures to increase surface contact with bone.^[13] Due to the high crown/implant ratios associated with short/ultra-short implants, prosthesis design should ensure favorable occlusal load distribution.^[14] Splinting short implants helps distribute occlusal stresses among connected implants,^[12] and increasing the number of short implants in a splinted prosthesis further aids stress distribution per unit area.^[15] Tabrizi et al.^[15] reported that increasing the number of short implants in splinted prostheses reduces marginal bone loss.

A noninvasive way to predict in vivo stress distribution with dental implants is through computerized modeling.^[16] Finite element analysis (FEA) is widely regarded as a suitable method for predicting three-dimensional (3D) stress and strain patterns around dental implants.^[17,18] However, 3D FEA studies on optimal load distribution with implant-supported fixed prostheses in the posterior mandible are limited. This study aimed to assess the effect of the number of short, splinted implants, and prosthesis designs on load distribution.

This study employed FEA, a computational technique in biomechanics for analyzing hard tissue modeling. Ethical approval was obtained from the Ethical Committee, with the approval code IR.IAU.KHUISF.REC.1398.27. A 3D finite element model was developed to calculate the maximum von Mises stress, shear stress, von Mises strain, and shear strain values around splinted short implants placed in the posterior mandible. Implants (SIC invent AG, Basel, Switzerland) measured 6 mm in length and 4 mm in diameter (abutment platform: 4 mm). Loads of 100 N were applied vertically and obliquely (30°).

Table 3 shows the highest and lowest stress and strain values in all eight FEA models. The highest stress and strain values under vertical and off-axial loadings were observed in the model with two short implants and two pontics. Conversely, the lowest values were noted in the model with four splinted short implants.

Models II and III showed reduced stress and strain values and more homogeneous distribution patterns in models with three implants at sites #4, #6, and #7 with a pontic at the second premolar site compared to three implants at sites #4, #5, and #7 with a pontic at the first molar site. Off-axial loads generally resulted in higher stress and strain values than vertical loads. Maximum stresses were noted in the implant neck regions, while maximum strains occurred apically. Cortical bone recorded the highest stress values, and trabecular bone recorded the highest strain values.

This study evaluated the effects of the number of short implants on stress and strain distribution in the posterior mandible using 3D FEA. Bone constantly remodels in response to mechanical loads, preserving its mechanical properties.^[19] Stress induces strain, causing deformation. A strain of 1,000 με equates to a 1% change in bone length. Excessive strain can lead to fatigue fractures, while insufficient strain may result in bone resorption (“disuse atrophy”).^[19,20] Repetitive stresses exceeding 3000 με can cause microdamage and marginal bone loss, adversely affecting osseointegration.^[21]

Bone has a porous structure with complex and tiny micro-structures. It is anisotropic and different parts have different physical properties.^[22] Around dental implants stress transfer from occlusion occurs at the bone-to-implant interface in cortical bone primarily at the most coronal implant threads. Strain is highest here because cortical bone has a lower modulus of elasticity than cancellous bone.^[23]

Unless certain precautions are taken,^[22-26]

Marginal/crestal bone loss may occur post-implant placement to re-establish biological width,^[27] but significant further loss is not anticipated with proper hygiene and absence of risk factors.^[28]

In vitro studies suggest that excessive biomechanical stresses due to incorrect occlusal design,^[29] or framework misfit^[30] of the implant-supported restoration can adversely affect stability of marginal bone. Since too much stress can lead to unwanted MBL, choosing the appropriate number of implants to restore an edentulous space is crucial. Excessive MBL may lead to microbial infection of exposed implant surfaces leading to inflammation-induced bone resorption and peri-implantitis.

Waskewicz et al.^[31] reported that peri-implant stress generation begins following prosthesis placement, and can be decreased with appropriate prosthesis design.^[29-31]

FEA is a valuable tool for studying stress distribution in implant-supported prostheses.^[32-34] In this study, vertical and off-axial loads were applied to assess the impact of implant number on load distribution. Results indicated that increasing implant numbers in the posterior mandible reduces stress and strain values, aligning with Tabrizi et al.^[15] and Gümrükçü and Korkmaz’s findings.^[35] The optimal configuration for restoring posterior mandibular sites with short implants appears to be three implants with one pontic at the second premolar site.

Increasing the number of splinted short implants in the posterior mandible decreases stress and strain values under both vertical and off-axial loads. Maximum stress was observed in cortical bone, whereas maximum strain was recorded in trabecular bone. The configuration of three implants at sites #4, #6, and #7 with one pontic at the second premolar site provided the most uniform stress and strain distribution.