Effect of changing the direction of the force vector of the mandible in relation to the skull base: study in rabbits

The temporomandibular joint has a great capacity for functional adaptation. The aim of this study was to evaluate, bilaterally, the influence of unilateral modification of the direction of the mandibular force vector in relation to the skull base in rabbits. Thirty New Zealand rabbits (Oryctolagus cuniculus L.) were randomly divided into two groups (n = 15/group): test (modification of the mandibular force vector) and control (no modification of the mandibular force vector). The animals were killed at 20, 40, and 60 days postoperatively. Histomorphometric evaluation of the temporal and condylar joint structures bilaterally always showed significant differences (P < 0.05) between the test and control groups on both sides of the TMJ. The results demonstrate that the rabbit temporal bone and mandibular condyle showed significant adaptive capacity as a biological response to mechanical forces on both the operated and opposite sides.


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The 98 With the aid of curved mosquito forceps, the fixation was performed to define its initial 99 position, and with a ½ spherical steel bur four points were defined, corresponding to the four 100 holes of the microplate.

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After horizontal perforations corresponding to plate fixation, cortical bone perforations 102 were made in the mandibular angle region, determining the vertical fracture line to the long axis 103 of the mandibular body, followed by cortical ostectomy with complete continuity solution and 104 fracture under constant irrigation with distilled water using a cylindrical drill (13mm x 6mm).
105 The fractured proximal mandibular segment obtained was composed of the posterior portion of 107 Then, it was removed, and from the approximation of the bone stumps in the basilar 108 region, the definitive position of the plate was determined, locating the proximal screws at 2 109 millimeters in the same direction, in linear continuity of the original position.

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The microplate was fixed with micro screws in the distal segment and then in the 111 proximal segment. In the control group, the bone fixation maintained the original position with 112 2 mm between the bone stumps (Fig. 1B). The new position of the proximal segment provided 113 a sagittal and medial tilt of the articular condyle, displacing it in the anteroposterior direction 114 ( Fig 1C). The condyle now articulates in the region of greater temporal convexity, changing the 115 direction of the mandibular force vector on the TMJ.   252 demonstrate that craniofacial growth can be modified, with TMJ involvement being the most 253 important determining factor [20][21][22].
254 Some theories and hypotheses are useful to explain the nature of TMJ growth 255 modification, with the growth relativity hypothesis and the functional matrix theory being the 256 most advocated in the literature [23][24][25]. In this hypothesis, they discuss equilibrium 257 interactions among five main factors: skeletal, viscoelastic, dental, neuromuscular, and non-258 muscular tissues. This set of factors contributes to TMJ adaptation by promoting increased 259 growth of the condyle-fossa complex, redirection, and ultimately remodeling of TMJ growth 260 [11,25].

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In turn, the adaptive capacity of the TMJ articular surface corresponding to the temporal 262 bone of rabbits is evident since its histological composition is influenced by the different 263 characteristics of the mechanical forces that affect the TMJ during its movements and 264 morphological changes persist in adult rabbits [15,16,18,19,[26][27][28]. Therefore, chondrogenic 265 responses in the temporal bone of the TMJ are more pronounced in regions where mechanical 266 forces are higher while osteogenic responses are observed where forces are lower [29,30].

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The adaptive characteristics of the rabbit TMJ show that there is a correlation between 268 an increase in the thickness of cartilage and a decrease in the number of cartilaginous cells in 269 response to increased forces on the surfaces of the temporal bone [12,31,32]. The remodeling 270 process, on the other hand, occurs through osteogenesis as the bone increases in size and is 271 characterized as a secondary process when bone reorganization occurs induced by deposition 272 and resorption at the endosteal and periosteal surfaces [4,15,16,18,19,33].

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Even though there are particularities in the comparison between human and rabbit TMJ, 274 the morphological and histological characteristics inherent to this animal model make it the best 275 experimental model for the proposed study. Rabbit TMJ exhibits physiological lateral and 276 anteroposterior movements [34,35]. As a particularity of the rabbit, the articular surface of the 277 temporal bone forms a convex eminence anteroposteriorly and concave medio-laterally. As for 278 the articular surface of the condyle, it is known that in rabbits the anterior part is convex, both 279 latero-laterally and anteroposteriorly [36]. The most notable morphological difference between 280 rabbit and human TMJ(s) is the shape of the articular surface of the condyle and the retrodiscal 281 area, since the animals do not have a post-glenoid wall [37]. Regarding histological and 282 histochemical characteristics, similarly to the human condyle, that of the rabbit is covered by 283 secondary cartilage and fibrous tissue [38]. In contrast, the animal cartilage is thicker in the 284 medial region and not in the posterosuperior region. Regarding the arrangement of the 285 cartilaginous cells, there are no significant differences between both ATM(s) [39].

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Therefore, considering the surgical model used, this study confirms what has already 287 been described in the literature, adding proof that the change in the force vector incident on the 288 skull base promotes bilateral effects on the TMJ, demonstrating the adaptive capacity of the