Photoelastic analysis was used in the current study to evaluate the stress occurred in the mandibular overdenture retained by three implants submitted to load exerted by the maxillary denture or axial single force in the first molars. The different loads and force intensities caused different stresses in the mandibular overdenture retained by three implants; therefore, the study hypothesis was accepted.

The biomechanical behavior of implant-supported prostheses during the occlusal loading was verified in studies using the photoelastic model method. Some of these studies investigated the following variables: Screw-and cement-retained implant prostheses [13], stress in oral obturator prosthesis [14], stress of bar-clip and short coping overdenture attachments [15], number of supporting implants [16], different prosthetic connections [17] and different attachment systems and soft relining [18].

The current study showed that different occlusal loads with different intensities caused different stresses in the alveolar ridge region of the photoelastic mandibular models similarly manufactured. For occlusal load, similar stress occurred in the apex of the median implant and in the anterior region of the mandibular model for the three load intensities (Figure 2). Stress occurred in the apex of the lateral implants and in the posterior region of the mandibular model with the three loads, and increased with the load increase in both sides of the photoelastic model (Figure 3 and 4). For single load in the molar, stress in the apex of the left implant and in the posterior region of the mandibular model with the three loads, and increased with the load increase (Figure 5). Stress occurred between the median and right implants, and the posterior region of the mandibular model did not show stress concentration (Figure 6).

Stress around and in the apex indicates that the implant acts as a stress concentrator. In addition, the concentrated stress intensity would depend of the location and slope of the applied force. The oblique loading increased the number of photoelastic fringes in all models; however, the internal hexagon implant exhibited better biomechanical behavior for the 3-unit implant-supported fixed partial prosthesis [17].

A previous study showed that different loading locations promoted  significant differences among the overdenture attachment systems. In general, the moment and force on the implant were greater when the load was directly applied over the implant or between implants in the mid-anterior region [19]. On the other hand, attachment systems promoted different stress distribution, showing greater stress concentration surrounding the implants and homogenous distribution in the photoelastic model without implants [18].

These aforementioned reports seem to corroborate the findings of the current study in relation to the force intensity and consequent different stress concentration in the overdenture supported by three implants. Similar stress concentration occurred in the mandibular overdenture with the three occlusal loads. In this case, the occlusal force exerted by the maxillary denture in maximum intercuspation with the mandibular overdenture was not able to increase the stress concentration level with load increase.

Different load types and force intensities promoted different stress concentrations in the alveolar ridge of the photoelastic models. The shear stress (T) and fringe order (N) increased with the occlusal load intensity (Table 1). Similar shear stress (T) and fringe order (N) values occurred with single load in the left or right molar for the three loads (Tables 2 and 3).

A photoelastic study showed that the number of implants had no significant effect in the stress value around the implants when the axial force was exerted in the central fossa of the first right molar [16]. This result seems to indicate that the stress resulting of the axial force on the first molar would have greater difficulty reaching regions farther from the implant, regardless the implant number. In mechanical terms, this condition would be more damaging to the long-term survival of the implant and consequent alveolar bone loss when compared to the forces distributed homogeneously with greater extension in alveolar bone. The 3-D in vivo force measurements in mandibular implant supported overdentures showed that chewing and grinding resulted in lower vertical forces compared to maximum biting [20].

Besides, there is no evidence to support the assertion that differences exist in the marginal peri-implant bone loss between single implant prostheses and multiple screw-retained prostheses [8]. By other hand, it cannot be affirm that the stress occurred in the overdenture of the current study would not result in some damage to the osseointegration process over the long term. In addition, axial single load applied to the first right or left molar caused different stress levels in each posterior side of the photoelastic model (Figure 5 and 6). The cause of this undesirable result would be verified in studies using finite element analysis to clarify the different stresses between the left and right sides of the photoelastic model.

A previous report showed that stresses were observed on the loaded side of the photoelastic model, and the lowest photoelastic stress was found with the Locator and bar attachments that transmitted little or no discernible stress around the implants [21]. Moreover, overdenture retained by implants provide stronger bite force causing hydrostatic stress within the mucosa and bone resorption of the residual ridge [22]. It was also claimed that the hydrostatic pressure in the mucosa signifies a critical indicator and correlated with clinically measured bone resorption, pointing to severe posterior mandibular ridge resorption in implant-retained overdenture [23].

On the other hand, an increment in implant number tends to cause lower stresses and seems to be a safe and sufficient solution for the treatment of mandibular edentulism with overdenture [24]. In addition, a finite element study showed that the cortical and cancellous bone regions are the least stressed, while ball attachment produced higher stress [25].

The number and the implant position submitted to diferente biting forces influence the movement of the mandibular implant-supported overdenture; however, overdentures retained with three or more implants showed minimal movement [26], and the stress in mandibular overdenture under various loading and dislodging conditions showed that three implants promoted the highest lateral resistance force [27]. Rotational movement of mandibular overdentures is also influenced by the implant numbers, anchoring location and mastication region. During chewing with anterior teeth, the use of 2 implants increased the denture base rotation compared to 1 or 3 implants, and the horizontal movement was small compared to the vertical. However, denture movement under the occlusal force in the molar region was smaller than that in the anterior region [28]. This earlier study showing that the occlusal force in the molar region was smaller compared to anterior region seems did not corroborate with the photoelastic images obtained in the current study with higher stress concentration in the lateral of the models (Figure 3 and 4) compared to anterior region (Figure 2).

Additionally, the existence of a possible inequality between the two sides of the photoelastic model would be considered as a study limitation. Therefore, it would be timely that further studies verified the correlation between the variables of the current investigation and the biomechanical failures in overdenture retained by three implants.