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Evaluation of median mandibular flexure values in dentulous and edentulous subjects by using an intraoral digital scanner

PURPOSE Mandibular flexure is a crucial phenomenon that may affect the success of rigid bilateral mandibular prosthetic treatment. The aim of this study was to determine the amount of median mandibular flexure (MMF) that occurs during mouth opening from anterior to posterior mandible in seven different regions, in different age and gender groups of both dentulous and edentulous subjects, using an intraoral digital scanner. MATERIALS AND METHODS In this study, the mandibular arch of 56 dentulous and 35 edentulous individuals were scanned with an intraoral scanner at both the minimum mouth opening (MnMO) and the maximum mouth opening (MxMO). MMF values were calculated by subtracting the distance value at the MxMO from the distance between the reference points at the MnMO at seven different mandibular regions of dentulous and edentulous subjects. In addition, the left and right side MMF values were measured. All data were analyzed statistically (α = .05). RESULTS MMF value increased linearly from anterior to posterior mandible in both dentulous and edentulous individuals. The differences in MMF values were not statistically significant according to side (left/right), age, or gender (P > .05). No significant differences were found between the mean MMF values of the same region in both dentulous and edentulous individuals (P > .05). CONCLUSION MMF was seen in different regions of the mandibles of both edentulous and dentulous individuals measured at the MxMO. Mandibular flexure should be considered for the success and prognosis of the long-span and rigid prostheses.


INTRODUCTION
Mandibular flexure is a multifactorial phenomenon that occurs immediately and simultaneously with mandibular movements and causes changes in the shape of the mandible. 1 Researchers have shown that the lateral ptery-during mouth opening from anterior to posterior mandible in seven different regions, in different age and gender groups of both dentulous and edentulous subjects, using an intraoral digital scanner. MMF amounts were evaluated and patterns were schematized. The null hypotheses were that: (1)  The mandibular arches were scanned with an intraoral scanning system (TRIOS; 3Shape, Copenhagen, Denmark) and digital images were obtained. Before the scanning process, incisal tips of canine teeth, buccal cusp tips of 1st premolar and 2nd premolar teeth, mesiobuccal and distobuccal cusp tips of 1st molar and 2nd molar teeth were determined as reference points bilaterally, and marked with a black marker pen (0.1 mm Faber-Castell; A. W. Faber-Castel Cosmetics GmbH, Stein, Germany) in dentulous individuals. The center of 0.1 mm-thick pen mark was used as the reference center.
In edentulous arches, three reference points were determined: (1) the top of the right retromolar pad, (2) the top of left retromolar pad, and (3) the midline corresponding to the lingual frenulum. Before scanning, occlusal surfaces of the mandibular teeth in dentulous individuals and mandibular alveolar mucosa of goid muscles (LPMs) contract and the condyles are pulled towards each other during the opening and protrusion movements of the mandible. 2,3 Apart from the LPMs, the mylohyoid, platysma and superior constrictor pharyngeal muscles, which are the secondary auxiliary muscles, are also responsible for the medial movement of the condyles. 4 During the opening movements of the mandible, the LPMs on both sides and the muscles of the floor of the mouth exert a force of contraction on the mandible. Mandibular flexion is also observed during clenching, occlusion or biting in the mandible. 2,4,5 The deformation of the mandible during flexure is very small, so by some clinicians, it is often disregarded and considered to be of no practical importance. It was suggested that there are four patterns of jaw deformation during mandibular flexure: symphyseal bending, dorsoventral shear, corporal rotation, and antero-posterior shear. 12 The most critical factor that cause mandibular deformation is the contraction of the LPMs during the opening and protrusion movement of the mandible in the frontal plane. 4 The bending force is mainly exerted by the medial component of the obliquely located LPMs. 12  In dentulous individuals with relatively less mouth opening (If 28% of the measured MxMO of an individual was less than 18 mm), only the buccal cusps of the premolar and molar teeth and the incisal surfaces of the anterior teeth with reference points were scanned. Lingual and remaining occlusal areas of teeth were not scanned. All digital occlusal images were recorded automatically and were transferred to the digital software program (Sketch-up Pro2017; Trimble Inc., Sunnyvale, CA, USA) using the "top view" option in the system.
All measurements and evaluations were made independently by two observers. On digital images, the lines connecting the incisal tips and cusp tips on the right and left sides, marked in dentulous arches, were coded as D1, D2, D3, D4, D5, D6, D7 from anterior to posterior mandible (Fig. 1).
In edentulous individuals, two lines were drawn from the reference point on the midline to the right and left tops of the retromolar pad. The area between the tips of the canine teeth and the retromolar pads was divided into eight equal parts. New reference points were determined in order to measure the distance in seven regions where these lines intersect the top of the mandibular crest. The lines connecting the right and left reference points determined on the edentulous crest were coded as E1, E2, E3, E4, E5, E6, and E7 from anterior to posterior, respectively (Fig. 2).
In dentulous individuals, it was seen that the incisal tips of the canine teeth almost corresponded to E1, when the lines between anterior crest tips and the retromolar pad were divided into eight equal parts as in the edentulous mandible. For this reason, it was assumed that E1 passes through the incisal tips of the     Table 2). D1 region was accepted as a reference point.
In edentulous individuals, mean and standard deviation (SD) of MMF values in all regions and also MMF values according to gender variable are given in Table   3. MMF values increased linearly from anterior to posterior mandibular region (Fig. 5A).
MMF values of edentulous individuals are listed  The Journal of Advanced Prosthodontics in Table 4. No statistically significant difference was found between the mean MMF values of right and left sides (Table 4). It was seen that the MMF values increased linearly from anterior to posterior mandible in both the right and left sides (Fig. 5B).
No statistically significant difference was found between mean MMF values of males and females in both dentulous and edentulous individuals (P > .05).
Also, there was no statistically significant difference between mean MMF values of the right and left side in both dentulous and edentulous individuals (P > .05).
As a result, there were no statistically significant differences between the mean MMF values of D1 and E1, D2 and E2, D3 and E3, D4 and E4, D5 and E5, D6 and E6, and D7 and E7 regions, when comparing the dentulous and edentulous groups (P > .05).

DISCUSSION
Evaluation of mechanical properties in the mandible is difficult because of large variations in mandibular size and bone density in the population. In previous studies, in vivo intraoral and extraoral measurement methods were used to examine the amount of mandibular flexure. Traditional intraoral measurements were made using calipers, transducers fixed to tooth surfaces or implants, and strain gauge. 2,3,5,7,10,[12][13][14][15][16] Extraoral measurements were usually made on diagnostic models obtained from impressions taken at various openings of the mandible. 2,17,18 In vitro studies on stress distribution in the corpus of the mandible were performed using photoelastic technique and FEA models. 10 In this study, in the evaluation of MMF,   proper water/powder ratio of plaster material. 27 The dimensional accuracy of digital models obtained by intraoral scanning was found to be higher compared to models obtained by scanning conventional impressions. 28,29 Besides, the pressure applied to the mandible during impression taking with traditional methods may affect the amount of flexure. 19 In this study, the disadvantages of different methods used to obtain study models in previous studies were eliminated by using an intraoral scanner.
It is stated that digital measurements are more controlled, safer and more precise than mechanical measurements. 30 In this study, computer-assisted measurement method was applied. It is a reproducible method that is not affected by the person performing the measurement. Most of the studies on edentulous mandibular arches focused on measuring the amount of strain that develops in the mandible during different jaw movements. [32][33][34] In other studies, the amount of displacement between dental implants was evaluated using a transducer. 7,8 Horiuchi et al . 8 measured the position between two dental implants in the first molar region using a magnetic sensor system during jaw opening and protrusion movements. The displacement of the two implants at MxMO and the displacement during protrusion were found to be between 8 -25 µm and 10 -37 µm, respectively.
In a 3-D FEA study, Choi found the mean MMF values of 0.1685 mm in the 1st molar region and 0.256 mm in the 2nd molar region. 33 Madani et al . 16  In the indirect digital model study by Wolf et al ., 19 the models were overlapped and the MMF was measured separately for each tooth on the right and left sides. The highest mean MMF value in females was 0.232 mm and it was observed in the right 1st molar and 2nd molar regions. The lowest mean MMF value was measured in the right canine region as 0.020 mm. In male patients, the highest mean MMF value was 0.184 mm and was observed in the right 1st molar and 2nd molar tooth regions, and the lowest mean MMF value was 0.011 mm and was observed in the right canine region. In the study of Wolf et al ., 19 the mean MMF on the right side was found to be higher in dentulous patients, which was in line with our study.
The reason for this might be the effect of individuals' unilateral chewing preferences on muscles and bones. 35 Unfortunately, there is no other study in the literature that measures the right and left MMF values of edentulous individuals.
In the current study, the mean age of edentulous individuals was 63 and the mean age of dentulous individuals was 29. There was no significant difference in terms of age within the group in both edentulous and dentulous individuals (P > .05). So, the third hypothesis that age and gender would not affect the amount of MMF was accepted for age parameter.
In this study, the mean MMF values were generally J Adv Prosthodont 2022;14:32-44 Evaluation of median mandibular flexure values in dentulous and edentulous subjects by using an intraoral digital scanner The Journal of Advanced Prosthodontics higher in edentulous individuals. However, no statistically significant differences were found between the mean MMF values of the same region in both dentulous and edentulous individuals (P > .05). Therefore, the fourth hypothesis that there would be no difference between the MMF of dentulous and edentulous subjects was accepted. The rigidity of the mandible depends on the cross-sectional area of the mandible, the cortical thickness, the structure of the cancellous bone, the characteristics of the bone and the shape of the mandible. 12 Following the loss of teeth, alveolar bone loss and resorption are seen in the mandible with age. 36 Age-related bone loss is associated with reduced density and mineral content of cortical and trabecular bone. With increasing age, the bone becomes more porous. With poor bone density, the mandible is more prone to flexure. Moreover, the mandibles, which have smaller symphysis areas and more porous bone structure, are more prone to flexure. 10,37 On the contrary, with age, the flexibility of bone tissue decreases due to the decrease in the proportion of collagen fibers responsible for bone elasticity. 38 However, it should be considered that stronger muscles may be associated with more mandibular flexure. 4 Considering these reasons, it can be concluded that the amount of MMF in dentulous and edentulous individuals is in balance and will not change with age considerably.
In the present study, no statistically significant difference was found between genders in terms of MMF values in both groups of dentulous and edentulous individuals (P > .05). So, as stated before for age parameter, the third hypothesis was also accepted for gender parameter. The results are in accordance with the other studies. 16,24 Nevertheless, the mean MMF values were slightly higher in males. This finding might be due to the larger and more voluminous mandible in males. Additionally, factors such as the mechanical properties of the bone, muscle strength and MxMO might also affect the amount of flexure. 11,13,26,37,39 Generally, the decrease in the mineral density of the trabecular bone is more pronounced in females than in males. Specifically, bone resorption in postmenopausal women was noted to be three times than that of men. 40 Splinting of dental implants and fixed dental pros-theses create a rigid joint in the flexing mandible. Although there is a small amount of flexure in the mandible, stress areas may occur in the bone when rigid and long-span fixed dental prostheses and implants are used. Design of the implant-supported prostheses should allow mandibular flexure without generating stresses at the bone-implant interface. 41 In mandible, when a dental restoration is planned by using multiunit (one piece) mandibular fixed dental prostheses or implant-supported fixed dental prostheses in completely edentulous mandible, mandibular flexure should be taken into account and prostheses should be fabricated as independent short pieces as possible. 32,41,42 Additionally, during digital denture making based on intraoral scanning, the MMF should also be taken into consideration and scanning should be done at MnMO as possible.
On the other hand, the mechanical properties of prosthetic materials may also require changes in the design of prostheses. 43 The modulus of elasticity of the supra-structure material used should also allow a relative degree of flexure compatible with that of the mandible. Otherwise, as a result of flexure caused by mandibular movement, failures such as fracture in the ceramic material or decementation of the fixed dental prostheses may be encountered. When planning mandibular long-span one-piece fixed or removable prostheses, mandibular flexion should be taken into account for patient satisfaction in the short term and for success of the prostheses and survival of teeth and implants in the long term.
Mandibular flexure occurs in 3D direction. In this study, the mandibular flexure in horizontal plane was considered when MMF was calculated. In further studies, mandibular flexure should be measured in 3D direction using digital scanning methods.

CONCLUSION
The results of this study revealed that MMF is seen in all regions of both edentulous and dentulous individuals measured at the MxMO. A linear increase was observed in the MMF values from the anterior to the posterior mandibular regions. The amount of flexure on the right side of dentulous individuals was slightly higher, but there was no significant difference be-tween the right and left mandibular flexure amounts of all individuals. Gender and age did not show a significant difference in terms of MMF values in both groups of edentulous and dentulous individuals.
When a rigid long-span bilateral mandibular fixed dental prosthesis is planned, especially extending to the posterior region of the mandible, the phenomenon of mandibular flexure should be considered for the success and prognosis of a dental prosthesis.