Indirect anchorage is an established form of anchorage provided by orthodontic miniscrews. Although there are different ways to set up the mechanics, rigid indirect anchorage offers the greatest biomechanical versatility but is more difficult to install than conventional, nonrigid indirect anchorage or direct anchorage. The purpose of this article was to introduce readers to the concept of rigid indirect anchorage and provide guidelines as to its use.
Highlights
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For indirect anchorage, anchorage elements should be aligned with the line of action of the orthodontic force.
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Rigid anchorage allows for greater freedom of insertion site selection.
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Basic engineering concepts can aid in understanding the preferred setup of indirect anchorage.
Over recent years, studies have reported much improved success rates with orthodontic miniscrew implants, compared with the early reports of success rates that were mediocre at best. It seems therefore that some factors affecting success and how to control them have finally been identified—the dark days where high failures were simply accepted as fact seem to slowly be left behind us, despite still not fully understanding the phenomenon of how screws fail.
However, a successfully inserted and long-term stable miniscrew is only part of the formula for clinically successful miniscrew use. The other important component is the coupling of the miniscrew to the dentition. If this connection were to fail, even a stable miniscrew will not deliver the desired anchorage.
It is generally accepted that there are 2 main methods of connecting miniscrews to the patient’s dentition: one resulting in direct loading of the miniscrew, the “direct anchorage” approach, and the other resulting in indirect loading of the miniscrew, the “indirect anchorage” approach.
Direct anchorage mechanics comprise a setup where an elastic module spans from the miniscrew to the tooth, or group of teeth, that are supposed to be moved. The miniscrew serves a different purpose when used with indirect anchorage mechanics. Here, a nonelastic element spans from the anchorage screw to the tooth unit that ideally should remain stationary: the traditional “anchorage segment,” preventing reciprocal tooth movement resulting from conventional orthodontic mechanics. Whereas this approach as we have used and described it has applications in practice, it has never been explored in greater detail in the literature, and scientific guidelines for the proper installation of indirect anchorage based on the underlying mathematics have not yet been published.
The purposes of this article were to explain the mechanics related to the use of rigid indirect anchorage biomechanics by applying physics and bioengineering principles and to derive some clinical guidelines for the proper installation of rigid indirect anchorage.
Indirect anchorage options
Indirect anchorage refers to a setup in which the miniscrew is used to prevent tooth movement in the “anchorage segment.” This can be accomplished with either a nonrigid coupling element, such as a steel ligature ( Fig 1 ), or a rigid coupling element such as a stainless steel wire segment ( Fig 2 ).
From an engineering viewpoint, the coupling elements described above can be classified as either struts or ties. The Table gives an overview of the properties of struts and ties for better understanding, but the major difference is that ties are tension loaded and aim to keep 2 objects together, whereas struts are compression loaded and serve to keep 2 objects apart ( Fig 3 ).
| Struts | Ties |
|---|---|
| Part of a framework | Part of a framework |
| Provide outward-facing support | Provide inward-facing support |
| Keep 2 objects apart | Keep 2 objects together |
| Loaded lengthwise | Loaded lengthwise |
| Load: compression force | Load: Tension force |
| Rigid structure | Rigid or nonrigid structure |
Nonrigid indirect anchorage is typically achieved by running a tightly wound steel ligature tie from an undercut in the miniscrew head to the tooth one aims to stabilize. This may be a popular option because of the simplicity of the setup; however, despite the simplicity, one should be aware of the proper ways to install this anchorage option to avoid negative outcomes.
First, due to the nonrigid nature of steel ligatures, they can only be tension loaded and hence are structurally classified as ties. This means that their purpose is to keep the miniscrew and the tooth together. When this rule of thumb is kept in mind, it becomes apparent how the setup is best constructed.
Clearly, the screw should be inserted in a position that allows the ligature to span in the direction of the undesired tooth movement. Only then can the ligature wire serve its purpose to resist the applied force and maintain the distance between the screw and the tooth in the plane of the force. In other words, if anteroposterior tooth movement is supposed to be prevented, the ligature tie should also run in an anteroposterior direction, with the screw obviously placed on the opposite side of the force application ( Fig 1 ).
Improper installation of nonrigid indirect anchorage carries the risk of anchorage loss.
Using rigid coupling elements from the screw to the dentition allows the installation of rigid indirect anchorage. Because the connectors are rigid, the miniscrew can be placed on either side of the tooth, disregarding the side to which force is applied. Hence, these coupling elements can function either as struts if the screw is placed on the side of the force application since they are compression loaded or as ties if the screw is placed opposite the side of force application; as a result, they are tension loaded, similar to the way steel ligature ties are loaded.
A tremendous advantage of the rigid indirect setup is that there is more freedom when choosing the insertion site. Since the same element can serve structurally as both a strut and a tie, biomechanical considerations are less important. One can put more emphasis on ideal anatomic parameters for the insertion site vs being forced by the indication to select a certain spot for the screw insertion.
However, because of the coupling element’s double function, the setup guidelines are slightly more complex and require a deeper understanding of the element’s static properties and its strength. A better understanding of the thought process required to determine the best design of the anchorage setup can be gained by using some clinical examples.
When attempting to retract anterior teeth en masse after first premolar extraction while maintaining the molar position with an orthodontic miniscrew, one may chose a Nance appliance-inspired setup. Although it should be clear to the informed reader that the proper insertion site for the screw is the anterior palate with a paramedian screw position, the question arises if it is more favorable to have the stabilizing wire run more horizontally, for example to the second molars ( Fig 2 ) or more vertically to the second premolars ( Fig 4 ). The mathematics to answer this question will explain how to best construct this indirect anchorage setup.
For the horizontal wire direction to the second molars, to calculate the internal forces of the stabilizing wire when acting only as a strut, the assumption needs to be made that it is part of a framework in which all joints are pin jointed and frictionless, the wire is not deformed under a force, the effect of gravity is negligible in this situation, and the system is in static equilibrium, which means that the sum of the forces acting on a body must be zero (Newton’s first law). Also, we will project the setup into the sagittal plane (2 dimensional) as seen on a cephalometric radiograph to simplify the mathematics resulting in what is known as a free-body diagram.
We will first calculate the internal forces for the scenario in which the connecting wire runs from a miniscrew in the anterior palate to the second molars. Force application in the plane of the wire is assumed to be at a physiologic level of 1.5 N (F 1 ), and the angle between the archwire and the stabilizing wire will be 30°, a clinically realistic assumption. Using the free-body diagram, Figure 5 is a schematic of the joint resulting from our clinical setup. To understand the internal forces in the stabilizing wire, we apply Newton’s laws and the rules of trigonometry.
Because the system is in static equilibrium, according to Newton’s laws, there cannot be any unbalanced applied forces on the wire; therefore, the following holds true.
This can be expanded as follows.
where Fx indicates the force acting on the x-axis direction and Fy indicates the force acting on the y-axis direction. Using Figure 5 , the following equations can be written.
F 2 = −1.73 N (negative indicates that the force acts in the opposite direction from the originally chosen direction, which is a compression force in this situation).
By introducing a strut that is not solely in the plane of the force application, a vertical force component R (reaction force) is created according to Newton’s third law. Using Newton’s laws again, the following equation can be written.
R = −0 .86 N ( negative indicates downward direction )
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