“What is essential is invisible to the eye.” Antoine de Saint-Exupery

Historically, orthodontic appliances are described as shape-driven or force-driven.(1) The first group constitutes those with whom the professional expects the tooth to move precisely towards the shape of the arch (eg. straight wire techniques). On the other hand, the force-driven appliances require the clinician to be focused more on the force system than on the shape. I mean, it’s important to have a shape, but it is more important that the shape produces the desired force system. In this approach, the wire shape often does not have a visual correspondence with the forces that will be applied.

The ability to work with force-driven appliances is called creative wire bending.(2) According to Dr. Burstone, this is not the ability to manipulate wire and pliers, but how a wire should be bent to produce a desired force system. An infinite number of shapes or bends can be placed between two brackets in order to produce a specific force system. In addition to a lot of scientific knowledge, this skill requires the use of our sixth sense – the PROPRIOCEPTION, the sensation of body (and its parts) position and movements.

The aim of this post is to clarify, by demonstrating a specific practical example, why you should not “read” the wire when a creative bend or appliance is used.

Let’s take an example of a classic study with loops.(3) Let’s compare the application of a rectangular loop, activated on the vertical axis, in the occlusal (positive activation) and cervical (negative activation) directions, as illustrated in Figure 1.

Figure 1. Force systems delivered by equal rectangular loops, activated with vertical displacements in the gingival direction -negative activation, for extrusion (A), or in the occlusal direction – positive activation, for intrusion (B). Note that the proportions between the moments in the 2 situations are not equivalent to the proportion of a geometry I force system (MA/MB = 1.0), despite the geometric relationship between the brackets constitutes a geometry I.

In situation A, the canine needs to be extruded, and in situation B the canine needs to be intruded. If you compare the angular relationships between the brackets in both situations, notice that both represent an interbracket relationship of geometry I. However, in both situations the released force system will NOT be the same as the force system characteristic of geometry I. Remember that the correspondence between the angular relations and their respective force systems is valid for installing a segment of a STRAIGHT wire. When incorporating bends and loops, with any configuration, the attempt to “read” the effect of the appliance due to its shape (eg. geometry between the segments of a loop) will result in a mistaken prediction, with errors commonly greater than those predicted when we try “reading” a straight wire, depending on the appliance evaluated.

 

Figure 1 is also a good example to illustrate that the term “the six geometries” actually has two meanings in the orthodontic literature: (2-4) 1) the relationship of the angles formed by the axes of the slots with the inter-bracket axis . Literally, it is a geometric relationship between two brackets. It is said, this is a “class X geometry relationship”; and 2) the system of forces produced for those geometric relationships tested. In this case, we could say, this is a “class X geometry force system”. In the example, rectangular loops without any pre-activation, installed between two brackets with an interbracket relationship of the same class (geometry I) evoked two different force systems. In situation A, the force system was similar to the one released by geometry II (MA/MB=0.8). In situation B, the system was similar to one located between geometry III (MA/MB =0.5) and geometry IV (MA/MB=0). In addition, anteroposterior forces not considered in the geometry force systems will be present in this scenario of extrusion and intrusion with rectangular loops without pre-activations (that is, without specific bends to obtain a specific force system).

 

If you cannot rely on the geometric relationship between the brackets to predict the forces delivered by different types of appliances (except a straight wire), which guidelines could help the clinician in the appropriate use of these tools? The first step when using a loop or bend is to know the force system delivered by it. This knowledge may come from a scientific article, based on mathematical models or experimental and clinical evaluations. For example, the application of a V-bend at specific points can produce specific force systems correspondent to those from the classic Burstone’s geometries,(2) and this knowledge can be used to apply V-bends with a force-driven approach. The use of different appliances or loops can further complicate the achievement of the desired forces, but again, experimental and clinical studies can help us with this proposal. In the situations A and B of the previous figure, for example, in which the desired force systems would be the application of only a single force at the active units, specific activations of the loop would be able to achieve this goal. (Figure 2)

Figure 2. Force systems delivered by rectangular loops activated according to a force-driven approach.  Loop configurations for extrusion (A) and intrusion (B) are different*, but in both situations the force system released are correspondent to a geometry IV force system (MA/MB = 0). (C) angles of the loop; *For extrusion, angles 1, 2 and 4 were opened; For intrusion, angles 1, 3 and 4 were closed.

 

The force systems applied in Figure 2 were checked by a clinical procedure called trial activation. A detailed description of this method is beyond the scope of this post, but we recommend reading the reference about it. (5) Briefly, the method represents the visualization and PROPRIOCEPTION of the forces and moments that will be applied by a specific orthodontic appliance. Proprioception training in this test is essential for those who wish to master the creative wire bending techniques, and we hope to come back to this topic in the next posts. For now, we end with the advice: “Do not trust everything you see. In force-driven orthodontics, the essential is invisible to the eye.”

 

References

  1. Burstone CJ. Charles J. Burstone, DDS, MS. Part 2: Biomechanics. Interview by Dr. Nanda. J Clin Orthod. 2007;41(3):139-47.
  2. Burstone CJ, Koenig HA. Creative wire bending–the force system from step and V bends. Am J Orthod Dentofacial Orthop. 1988;93(1):59-67.
  3. Koenig HA, Vanderby R, Solonche DJ, Burstone CJ. Force systems from orthodontic appliances: an analytical and experimental comparison. J Biomech Eng. 1980;102(4):294-300.
  4. Burstone CJ, Koenig HA. Force systems from an ideal arch. Am J Orthod. 1974;65(3):270-89.
  5. Burstone CJ, Koenig HA. Precision adjustment of the transpalatal lingual arch: computer arch form predetermination. Am J Orthod. 1981;79(2):115-33.