## 4 Discussion
However, in a real patient situation, chewing forces can be greater than in this study [44; 45], especially in subjects with natural teeth in the maxilla and in those with bruxing habits. Due to the fact that microstrains increase under higher loading forces, higher chewing forces may elicit higher strains in a real patient situation compared with this study, especially when keeping in mind that the magnitude of chewing forces is inversely correlated with proprioception [44]. Moreover, in a real clinical situation, the attached mucosa is not of uniform thickness and consistency. Various mucosal thicknesses can be found at different alveolar ridge sites in the same subject. Sometimes even a flabby ridge can be present. All these complicate denture micromovements under loads, and thus the direction and distribution of the transferred forces. Therefore, higher strains can be elicited in a real-patient situation compared with those recorded in the present study, or strains can be repeated more frequently. Additionally, implant inclination (parallel in this study) may be different in a real-patient situation due to the morphology of the alveolar bone, and all this can influence the distribution of loads and consequent strains in the peri-implant and posterioredentous area bone. The bone density in real patients can be lower than the D2 density in this study, which would lead to a larger difference in the elasticity modulus between the implant and the bone, as the low-density bone has a lower elasticity modulus than the cortical bone. We mimicked a near-ideal patient situation. Therefore, the aforementioned facts represent the study's limitations and should be addressed in future research. Because high strains were recorded during anterior loading in the one-MDI model, even under small forces, the utmost precaution should be taken in such clinical situations due to the possibility of implant overloading under higher forces and more unfavorable alveolar ridge morphology, lower bone density or mucosa thickness. In subjects with two MDIs, the highest strains were recorded under unilateral 150 N loads. Strains may be even higher under higher OD loading forces and in less favorable clinical situations (uneven mucosal heights and consistency, lower bone density, etc.). Therefore, it is very important to advise patients to chew bilaterally, especially when a low number of implants is present. In the three- and four-MDI models, peri-implant and dedentulous area strains were far from those that could interfere with the bone reparatory mechanisms. Therefore, three or four Ti-Zr mini-implants can be used clinically for retention of mandibular OD without too much fear of failure. Additionally, the dimensions of implants [56; 57], mucosa thickness, height and consistency, residual ridge morphology, bone density, overdenture characteristics, and extent and direction of chewing forces [44; 45; 46] play an important role in stress and strain distribution. Therefore, future studies are needed to complete the overview of mandibular overdentures retained by Ti-Zr mini-implants. The limitation of the study is that the models represent only an average case scenario of possible situations in a \"real\" mouth. However, higher loading forces, different thicknesses and consistencies of the mucosa and inclination of MDIs could lead to different results and should be studied further. We also could not fully control all parameters, e.g., glue thickness, as SGs were manually bonded. Different bone densities could also influence the results and should be studied further. Additionally, the elastic modulus of a metal plate transferring loads to ODs could influence the results. The strength is that this study is the first one to analyze the new mini-implant system made from the Roxolid(r) (Ti-Zr) dental alloy with a new innovative retention mechanism (Optiloc(r) Retentive System) when varying the number of inserted implants for retention and support of mandibular ODs.