Bracket placement and arch wire selection are areas that clinicians tend to focus on during treatment, however how the archwire creates force through deflection is often overlooked. The process of moving teeth, developing arches and ultimately designing smiles comes down to force; specifically, how to create a force system and control it for optimal results. In self-ligation, archwire activation is a key factor that affects treatment outcome and efficiency.
With 25 years of clinical experience and more than a decade of using self-ligation, Dr. Lisa Alvetro has amassed critical expertise on perfecting treatment outcomes. Here, she explains various questions surrounding archwire sequence, active vs. passive systems, the advantage of lightweight wires and more.
Q: What is the difference between passive and active self-ligating systems?
A: There is a lot of conversation surrounding passive and active self-ligating in our field regarding the merits of each and the key differences between the two.
In its most basic form, active self-ligation employs a flexible clip that pushes against the archwire to hold in the bracket slot. This method produces force from both the active clip and the deflection of the archwire.
Passive self-ligation uses a rigid component such as a door to enclose the wire and create a tube system. 3M™ Clarity™ Ultra Self-Ligating Brackets, for example, are passive self-ligating brackets.
Both active and passive self-ligation can be used during treatment. Certain stages, such as arch development and alignment, are well-suited for passive methods, and other stages of treatment may require the friction to perform movements such as correcting a midline and incisor control during Class II correction. The tie wings on the Clarity Ultra Bracket are a feature of the bracket design that allows us to introduce friction and create a more active bracket on demand.
Q: How should wire sequencing for self-ligating brackets be approached?
A: Many people will switch to self-ligating brackets but continue using the same archwire protocol they are using with their ligated brackets. They become disappointed because they are missing out on one of the biggest benefits of self-ligation. The ability to complete a case with low force flexible wires, fewer archwire changes and longer time intervals between appointments brings a new level of satisfaction from both clinicians and patients alike.
I like to think about archwire progression based on the stages of treatment. Pick your favorite wire for each stage. The first stage is aligning. Choose a very small, flexible, round wire to correct rotations and eliminate crowding. We routinely use a .014 SE NiTi or in malocclusion with severely crowded or rotated teeth we use a .012 SE NiTi. Stage two is leveling. Once the rotations are corrected we need to start to fill the slot and produce enough force to start to level the arches. Options we have are adding additional archwire and use tandem arches. We can use a larger round wire or a small rectangular archwire. We commonly we use a .016x.022 SE NiTi to complete rotation correction and initiate leveling. In stage three we will be working on the occlusion so we need an archwire rigid enough to support the occlusal plane during Class II or Class III correction. In our clinic an .019x.0255 SE NiTi or .019x .025 Beta Titanium Is used. In finishing a case my preference is to reposition brackets, so we can use the existing .019x.025 SE NiTi since it is flexible enough to be inserted into most repositioned brackets.
Q: Can you discuss the importance of proper activation of the archwire?
A: It’s the combination of the bracket, the wire and the deflection of the archwire that produces the force required to move a tooth. Too often, we get caught up placing all our focus on the brackets when using a new bracket system and end up neglecting the archwire selection including sequencing, size and materials. Only change those archwires that are passive or when you are moving to the next stage of treatment where different characteristics such as stiffness are required. To me, if any deflection exists in a wire and I am unable to slide it freely I explain to the patient that there still is "energy" in the wire due to the deflection, so it is still working. I recommend that clinicians think back to load deflection charts and the force that is produced by archwires of different sizes and different alloys. A very flexible and small dimension archwire can produce a significant force when deflected. In passive self-ligation one can use a very small wire to create an ideal force system because force is not needed to overcome friction within the system. We are often surprised by the amount of movement accomplished with an .012 NiTi wire. When choosing larger archwires or an archwire of a stiffer alloy consider the amount of deflection. Larger, stiffer archwires can produce significant forces when even deflected a small amount. For me I want minimal to no deflection of stiff rectangular wires. If I will need a .019x .025 Beta Titanium or Stainless Steel, I first place a .019x .025 SE NiTi and allow it to remain in place until it is passive and slides freely within the arch. Then I can insert the stiffer wire without deflection and with minimal force generated.
Q: What are the key considerations in arch development?
A: I consider several factors with first being the existing arch form and its relationship to facial and smile esthetics. I also consider the position of the dentition within the arch in both the anterior and posterior segments. Are the teeth tipped lingual, tipped buccal or are they in an upright position? How much do I need to develop the arch for alignment and how much of a modification of the existing arch form is acceptable? My objective is to obtain an esthetically pleasing arch form that allows for alignment and positions the dentition in the alveolar ridge with adequate periodontal support and stability. This objective is easier for me to accomplish using self-ligation. We have found using light force archwires associated with self-ligation, especially our Clarity Ultra Brackets, allow for arch development and control in tipping.
Q: What are the advantages of low force and lightweight wires?
A: With lower forces patients report less discomfort. As a clinician, appointments are easier for me too. Avoid the temptation to change the wires at every appointment or at 8-week intervals. Change it only if it has become passive. I challenge people to see how small of a wire, and how few wire changes, they can use to achieve movement. Patients also enjoy experiencing fewer wire changes during treatment, so utilizing the low force options is an efficient route to see results.
Choosing the optimal archwire is the first part. The second part is knowing when to make the archwire active or passive. I explain to patients that we leave the wires until they are passive, or there is no deflection. This also allows us greater scheduling flexibility, as we only need to have patients come in when the wires become passive.
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