The Orthodontic therapy with hinge mechanics

Marino Musilli,  private practitioner in Naples, Salerno and Milan, Italy

www.lingualnews.com Vol 3 No. 1

Nowadays thanks to the research and development in orthodontics, our therapies are richer in auxiliary devices, which increase the potentialities and reduce the treatment times.
One of these devices is the hinge mechanics, a well know device that works on the same principles as the door (fig 1).
As a matter of fact, the door is made with an hinge with an hinge axis around which the door revolves. The handle is forced to open or close the door and the small resistance to the movement is made by the air moved or by the friction with the floor.

Fig 1. The hinge auxilary principle: a door

The appliance
Since 1982 Dr. Alain Fontenelle (1, 2, 3) designed an orthodontic device with the same principles of  the door, in order to drive and control a tooth or a group of teeth during their movements around an arc of circle. Dr. Fontenelle’s devices were made with dental alloys, following a methodology of laboratory very similar to the one used to make the Maryland bridge.
The Author has tried to reproduce, with less noble materials (stainless steel wire from 0.9 to 1.5 millimetres of diameter, round tubes, mini bonding pads and orthodontic solder) Dr. Fontenelle’s same structures, with similar characteristics and action principles, although less stiff and less anatomically shaped to the patients, easier to realize and with the same results.
The orthodontic components (fig. 2) of an hinge mechanics are usually made by a lingual arch or a palatal bar connected, from one side, with the tooth we are going to move and, from the other side, with the hinge around which the rotation of this system takes place.
Like the hinge of a door, the orthodontic hinge is high enough to control the dental tip during its movement.
From the orthodontic biomechanic rules we know that we can translate a tooth only if we apply an adequate force to the same level of the Center of Resistance. Using an  hinge mechanic, of course, we do not need to be exactly at the same  level of the Center of Resistance, because this system allows the control of the tip of the dental movement (fig. 3). Likewise a door makes that same movement either if we push it from the top or from the bottom, because the movement is controlled by the hinge.
The inclination of the hinge determines and controls the plane on which the dental movement occurs. That’s why we can realize a translation of a tooth along the occlusal plane or with a small intrusive component of the movement.
So, the most important characteristics of a hinge mechanics are:
1) controlled trajectory of the dental movement, which occurs along an arc of circle;
2) translational movement of a tooth, even if the application point of the force is not exactly on the projection of the Center of Resistance;
3) constant Moment/Force ratio (translation of the tooth sep by step);
4) maintenance of the plane along which occurs the dental movement.
For all these reasons it is very important to define, during the planning steps, where we are going to set the hinge.

Fig 2: The Hinge mechanics appliance

Fig 3: Use of the hinge mechanics eliminates the need to be exactly at the same level of the CRES

How to calculate the hinge position
The best way to do it is “the rule of the bisecting line” (fig. 4): first of all we need to imagine the final position of the tooth we have to move (the goal); then we have to trace a line from a starting point to the ending point of the dental movement. This line is the chord of the arc through which the tooth will move. In the middle of the chord we’ll trace an orthogonal line toward the opposite side of the dental arch and we will place the hinge where this line crosses the dental arch.
The angle of inclination of the hinge axis defines the plane of the dental movement too. So, if  we do not want a dental trajectory with an intrusive or extrusive movement of the tooth, the hinge axis must be orthogonal to the occlusal plane.

How to calculate the amount of the force delivered on the tooth from the appliance
In the hinge mechanic it is possible to calculate, with extreme precision, the force delivered by the appliance on the moved tooth.
If it were possible to pull with our hand on the palatal bar, we would feel a different resistance in moving the tooth, whether  we pull in a point close to the tooth or close to the hinge. In the same way, we feel different resistance in closing a door by pushing near the handle or near the hinge.

Fig 4: Calculating the hinge position by the "rule of bisecting line"

Fig 5: The Hinge appliance is a simple lever arm mechine

The hinge mechanic is a simple machine: it is a lever (fig. 5). The fulcrum is the hinge, the applied force is the coil spring or an elastic chain and the resistance is the tooth to  move. Since the force is applied between the resistance and the fulcrum, we deal with a III type lever, like the dental tweezers. So, in conditions of equilibrium we can apply the following equation R * a = F * b, where "R" is the resistance, "a" is the  distance from the point of application of the resistance and the fulcrum, "F" is the applied force and "b" is the distance from the point of application of the force and the fulcrum (fig. 6). Therefore R=F * b/a.
It is important to remember that, in conditions of equilibrium, the value of "R" also corresponds to the intensity of the applied force on the dental element, even if with opposite direction. So, we can calculate the intensity of the strength delivered on the tooth we have to move.

Opportunities given by the hinge mechanics
Thanks to the characteristics of the hinge system ( 1)a defined trajectory; 2) constant Moment/Force  ratio; 3) maintenance of the plane of the dental movement; 4) force system calculation) if, during the dental movement there are no bad contacts with the opposite dental arch that can disturb the biology of the orthodontic movement, we will be able to realize, as described Dr. Fontenelle, a dental movement "with bone"(2, 3). The tooth will move at the same time with all the periodontium and its alveolar process. This kind of orthodontic movement very seldom occurs during an orthodontic therapy and it allows us to move a tooth trough an edentulous area  with a thin bone, without running the risk of bringing outside the roots from the alveolar bony structure. Besides, this kind of movement compared to the normal dental movements "across the bone", causes no pain to the patients: it means that the dental movement is happening as a "natural" process, like in the dental migrations.

Fig 6: How to calculate the amount of the force delivered on the tooth from the appliance

Orthodontic cases

1) The patient D. M., 23 years old, female, showed the following:

Objective problem list:
• agenesia of the 22; (Fig 7)
• Maryland bridge from 23 one to 23 one; (Fig 10)
• Bad tip of the 23 (mesial position of the root); (Fig 10)
• depressed alveolar morphology on the labial side in the 22 area; (Fig 10)
• Inadequate space between the roots of the 21 and 23 to realize an implant (as asked to the surgeon); (Fig 10)
• previous orthodontic therapy;

Subjective problem list:
• necessity of an implant in the 22 area;
• previous orthodontic therapy with distalization of the 23;
• the patient did not want another orthodontic therapy on labial;
• in case of lingual therapy, a possible limited therapy to the superior dental arch.

Goals of the therapy:
• translation "with bone" of the 23 in the 22 area, avoiding as much as possible occlusal interference with the opposing arch, in order to remodel the bony depression in the 22 area;
• mesial tip of the crown of the 23, to upright it.
• Al the therapy in lingual orthodontics.
 

Fig 7-12: Case 1, pretreatment intraoral photos

The device (Fig. 13):

• palatal bar with hinge mechanic (in yellow;
• fulcrum on the 14;
• a pushing coil (in pink) on the 23 (100 gr.);
• anchorage (in red.)

Clinical phases:
• at the beginning of the movement; (Fig. 14)
• at the end of the dental translation; (Fig. 15)
• during the tipping of the crown; (Fig.16)
• at the end of the dental movement; (Fig 17)
• Last X ray (Fig. 18).

Fig 14-17: Clinical Phase

                                                                                     Fig 18: Post-treatment radiograph

2) The patient, I. G., 37 years old, female, showed the following 

Objective problem list (Figg. 19 – 24):
• 1st upper right molar over eruption (Fig. 21);
• missing teeth in the lower right side of the dental arch;
• not enough space for the prosthesis in the lower right side of the dental arch;

Subjective problem list:
• to resolve the prosthesis problem in the lower right zone (Fig. 21);
• no cutting or extraction of the first upper right molar (over erupted);

Goals of the therapy:
• 1st upper right molar intrusion;
• implants in the lower right side of the dental arch;
• prosthesis on implants

Fig 19-24: Case 2, pretreatment intraoral photos

Fig 25: Palatal bar with hinge device (in yellow); fulcrum on the 26; a pulling coil (in yellow) on the palatal bar near to the 16 (100 gr.); anchorage (in red)

Fig 26-28:Palatal bar with hinge device (in yellow); fulcrum on the 26; a pulling coil (in yellow) on the palatal bar near to the 16 (100 gr.); anchorage (in red).

The amount of the force delivered on the tooth (Fig. 28):
 The coil in green is 100 gr.;
• P is the intrusive component of the force. P is the sinus of C at almost 45° (P = 70 gr.);
• P is applied at 4/5 of the distance between the fulcrum and the tooth to move, so F, the intrusive force, is 4/5 of P (F = 55 gr.)

Clinical phases:
• picture at the beginning of the movement (Fig. 21);
• picture at the end of the dental intrusion (Fig. 32);
• X rays (Figg. 29 and 36);
• Implants (Fig. 43);
• crown on implants (Fig. 44)

                                                            Fig. 29. Pretreatment radiograph

-picture at the end of the dental intrusion (Fig. 30-35);
-X rays (Fig 36);
-Implants and crowns on implants (Fig. 37-38);

Fig 30-35: Case 2 at the end of dental intrusion

Fig 36: Posttreatment radiograph

References
1) Melsen B. Recenti controversie in ortodonzia, pp 235-284 Milano Scienza e tecnica dentistica edizioni internazionali s.r.l. 1991
2) Fontanelle A. Une conception parodontale du movement dentaire provoque : Evidences cliniques. Rev. Orthop. Dento-Faciale 16 :37-53,1982
3) Fontanelle A. Une conception parodontale du deplacement provoque : vers une application clinique raisonee. J. Parodont. 2 :131-155, 1982

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Adult and Lingual Orthodontics
EDITORS:
Dr. Silvia Geron D.M.D., M.Sc
Dr. Rafi Romano D.M.D., M.Sc
Dr. Pablo Echarri D.M.D., M.Sc