Intrusion of posterior teeth using miniplates: intrusive mechanics is not the same as intrusion force

ABSTRACT Objective: Biologically explain some of the bone mechanisms involved in the intrusion, or intrusive effect, of teeth submitted to skeletal open bite correction using four miniplates. Methods: The results of dental intrusion were measured and compared in 3D reconstructions of cone beam computed tomography scans taken before and after treatment of 20 patients with skeletal open bite, aged between 18 and 59 years. Results: The results allow deducing that the compression and traction forces biologically promoted deformation or deflection of the osteocyte network that controls bone design, and these effects involved the external and internal surfaces of the bone, with the formation of new layers, including the cervical portion of the alveolar bone crest. This helps understanding how dental intrusion occurs in intrusive mechanics, whose forces are of inclination rather than intrusion. The root resorptions caused by the use of miniplates were insignificant, due to the more homogeneous distribution of forces in the several teeth simultaneously involved. Conclusion: Imaging studies in CT scans tend to capture in details the subperiosteal and endosteal phenomena of dental intrusion - before and after the application of intrusive mechanics -, in the form of a set of modifications called dental intrusion or intrusive effect .


INTRODUCTION
Teeth are repositioned in the bone thanks to bone remodeling, which represents a set of events in the periodontium and jaw bones that allow the desired reshaping of bone. 1 Dental movement alone does not justify the esthetic and functional benefits promoted by orthodontics, since orthodontic treatment is the result of an induced movement of teeth and bones.
Previously, it was thought that tooth movement was done through the bone, but advances in knowledge have revealed that teeth and bone continually reposition and harmoniously reshape themselves together.
In clinical practice, pure and specific intrusion forces parallel to the long axis of the root, and perpendicular to the alveolus bottom are not used. 2 Intrusive mechanics can have the effect of moving teeth deeper into the jaws bones, but the forces that promote these effects are not those of intrusion, but rather of Apolinário STMPM, Meloti AF, Silva E, Cardoso MA, Consolaro A Intrusion of posterior teeth using miniplates: intrusive mechanics is not the same as intrusion force 5 It is noteworthy that: 1. Intrusion force: acts on the tooth's longitudinal axis or parallel to it, on the root structure, forming right angles with the tangent that passes parallel to the bottom of the alveolus.

Intrusive effect, or intrusion:
it is the displacement of the tooth to a more intraosseous position than the existent position (Fig 3). This intrusion can be achieved in several ways, with different forces and mechanisms -which, in conjunction, can be called intrusive mechanics 2 . Perhaps the least effective force for this purpose is pure intrusion.

THE BIOLOGICAL FOUNDATIONS OF INTRUSIVE MECHANICS
The effective shock absorber represented by the periodontal ligament and the extracellular matrix, with collagen fiber bundles, is prepared to receive and dissipate forces applied parallel to the long axis of the root. The sustaining periodontium absorbs intrusive forces even if they are intense and complex, such as masticatory forces. Masticatory loads, as intense as they may be, do not compress the periodontal ligament at the dental apex, do not damage the vascular bundle that enters the dental pulp, nor promote its necrosis.   The intrusive effect of inclination forces during the mechanics known as intrusive (Fig 3) can, in part, be explained by regional orthopedic stimulus to reshaping, represented by the forces on the bone, at the expense of the periosteal and endosteal tissues. 1 The periosteum reacts to stimuli or aggressions of low intensity and long duration with the formation of new bone layers on the cortical surfaces; that is, at the periosteum-cortical interface. 1,7,8 This reaction capacity can modify the shape of the bone in ques- Apolinário STMPM, Meloti AF, Silva E, Cardoso MA, Consolaro A Intrusion of posterior teeth using miniplates: intrusive mechanics is not the same as intrusion force 10 In intrusive mechanics, the tooth that entered into the bone space and was repositioned therein, also called intruded tooth, is repositioned in relation to the bone as a whole, and in relation to the other teeth, by the application of stimuli of an orthodontic and orthopedic nature (Fig 3). The height and shape of periodontal tissues can be altered without modifying the biological distances of periodontal tissues and without affecting the biological viability of the pulp tissues.
Intrusive mechanics promotes a spatial rearrangement of the bone in relation to the tooth -by means of phenomena known as orthopedics -and, simultaneously, the tooth repositions itself orthodontically, upon inclination forces. As a result of this synergy, the tooth is in a new position in relation to the bone and other teeth in the dental arch (Fig 3).

TOOTH RESORPTION AND INTRUSIVE MECHANICS
The high risk of root resorption 3 is always mentioned when it comes to intrusion movements, which are mentioned when referring to intrusive mechanics -intrusive mechanics does not mean intrusion force, as we explained before. There are many studies that have revealed that there is no such relationship between tooth resorption and intrusion force 5,[9][10][11][12][13] The forces that become most concentrated and potentiated in points of the periodontal ligament, with death of cementoblasts, are those of inclination, which characterize the intrusive mechanics. 7 The use of specific appliances, with absolute anchorage by means of plates and osseointegrated implants in dogs' teeth, 3 induced imagiologically insignificant root resorption, despite the intrusive effects obtained. The same occurred microscopically after a period lasting between four to seven months. 4,5,6 Clinical trials with important intrusive effects have also revealed a very low or non-existent rate of tooth resorption. 14,15 Appliances with absolute anchorage tend to distribute more homogeneously the forces that are applied, dissipating and eliminating the points of concentration of forces in periodontal tissues. This reduces the possibility of death of cementoblasts along the root, and reduces the chance of root resorptions. 16 Pure intrusive forces in Orthodontics exist only experimentally and, when resorptions occur at the apexes and bifurcations of these teeth, they are detectable only under a microscope, as they cannot be diagnosed by imaging exams. [3][4][5][6] Studies have revealed that when intrusive forces are eliminated, neighboring tissues quickly repair the resorbed areas.

DEMONSTRATIVE EXPERIMENT OF INTRUSIVE EFFECTS
In a research project conducted for a master's dissertation, Apolinário 17 used temporary skeletal anchorage to quantify dental intrusion into the bone in patients with skeletal open bite, using miniplates, as previously suggested. 18,19 Dental Press J Orthod. 2021;26(5):e21ins5 Apolinário STMPM, Meloti AF, Silva E, Cardoso MA, Consolaro A Intrusion of posterior teeth using miniplates: intrusive mechanics is not the same as intrusion force 12 Miniplates are more effective alternatives than mini-implants, as they bear loads of greater intensity and to a greater extent of the dental arches. 20 Consolaro 21 mentioned that skeletal anchorage is necessary for bone remodeling to redefine morphology, esthetics and function in cases of deformities, always supported by orthodontic appliances.
Molar intrusion is difficult to achieve with conventional orthodontics -in these cases, orthognathic surgery is indicated.
However, miniplates in the zygomatic bone region represent an alternative mechanics, 5 as they are predictable, having a success rate of 98.6%. 22 In Apolinário's study, 17

FINAL CONSIDERATIONS
The natural inclination of the roots favors the tooth movements in intrusive mechanics to be of inclination type. There are compression forces in some areas of the periodontal ligament, with deformation or deflection of the osteocyte network that controls bone design. In other areas, there is deformation of the osteocyte network, due to tension forces. These effects (Fig 3)  Refined imaging studies, such as the one reported by Apolinário,17 capture these subperiosteal and endosteal phenomena in high precision computed tomographs taken before and after the application of intrusive mechanics; that is to say, these phenomena represent the final effect called dental intrusion, or intrusive effect.