Should we change space closing wires every visit?

While orthodontists tend to disagree quite often regarding planning decisions and the mechanics that we use, most of us tend to close extraction space on large dimensional steel wires. Some engage the working base wire passively for some weeks before commencing space closing. I don’t think this is necessary. Because I feel that the slots should be sufficiently aligned to start space closing on the same day.

I typically see patients on a 6-weekly basis, try not to extend my space closing mechanics to the terminal tooth in the lower arch (where possible), and think it is vital to keep forces light. Furthermore, I regularly take the arch-wire out to remove debris and flatten the wire (mainly if there are apparent kinks or evidence of local intrusion or transverse issues).

However, should we replace the wire during space closure due to the possible effects of intra-oral cycling? We might do this because this cycling could lead to increased friction caused by a build-up of debris and notching of the wire. These issues may be an impediment to tooth movement.

What did they do?

They conducted a split-mouth randomised controlled trial with an additional laboratory-based element as follows:

Methods:

Participants:

Age 16 years or above with proclined maxillary upper incisors, mild upper arch crowding who required loss of upper first premolars. Participants all had normal vertical skeletal proportions (MMPA 22 to 32 degrees).

Intervention:

A complete upper fixed appliance was placed, including the maxillary second molars. Alignment was undertaken with a 0.014-inch, 0.018-inch, 0.016 X 0.022-inch, and 0.019 X 0.025-inch NiTi archwires before engagement of a 0.019 X 0.025-inch stainless steel (SS) rectangular archwire. This wire was engaged passively for four weeks. One side of the arch was randomly assigned to non-replacement (non-replacement side), and the other was allocated to be replaced every month (replacement side)

Then, the authors sectioned the upper working wire in the midline. They then linked the new and older halves in the midline before reinserting them to continue space closure. In the replacement group, they changed the wire portion at each monthly visit.  Space closing mechanics were reactivated monthly over the 3-month study period. They used an elastomeric chain to close the space from the second molar to the second molar.

Outcome:

The rate of space closure (mm/month) over the three months. They measured this with 3D software after scanning the physical study models. The authors took readings at the contact points and the gingival level.

Finally, they used the retrieved brackets and wires to evaluate the frictional resistance using a bespoke model.

The authors did a sample size calculation, used pre-prepared randomization, and concealed the allocation of the sides in sealed envelopes.  The patient and data recorder were blinded to the treatment allocation.

What did they find?

The authors were able to analyse data from 23 of the 28 participants. They found no difference in the rate of space closure with new or cycled wires (0.8mm on the non-replacement side and 0.69mm monthly on the replacement side). More space closure occurred on the non-replacement side over the study period (2.41 vs 2.07mm). However, the difference was not clinically or statistically significant. The rates of space closure were broadly comparable with previous studies. The clinical findings were slightly at odds with the lab-based evaluation. Higher friction was observed on the non-replacement side (3.58N) compared to 2.43N on the replacement side.

What did I think?

I think that this is an interesting study. The authors were very inventive in their methodology, going to great lengths to use a split-mouth design to reduce possible confounding effects. However, I must admit that I have never thought about the routine replacement of stainless steel wires during space closure from a clinical perspective. It is, therefore, not going to change my practice.

The mean rate of space closure observed (approx. 0.7 to 0.8mm per month) was generally in keeping with other studies. Interestingly, I note that the authors used an elastomeric chain for space closure. I continue to feel that space closure with an elastomeric chain is just as efficient as with NiTi alternatives. I also think that the forces needed to space close most efficiently continue to be poorly understood. It may be that constant forces provided by NiTi coils are unnecessary and that the force decay associated with elastomeric materials is less problematic than we may consider it to be.

I liked the idea of including a laboratory-based element in the study. Unfortunately, as is so often the case, the laboratory findings seem to bear little or no resemblance to the clinical situation. This finding reminds me very much of some of the preliminary data published around the potential benefit of self-ligating brackets relating to alignment efficiency and the rate of space closure. As we know, the promise of these laboratory studies was not borne out in clinical research. Therefore, the authors are commended for including both components in their research study.

Improved study design?

A split-mouth design made optimum use of a small sample of patients. However, we must consider if splitting and joining the archwire influenced space closure? It would be interesting if these results were found in a larger group of participants randomly allocated to non-replacement and replacement interventions. Nevertheless, until investigators carry out this research, we should consider that the present findings have validity.

What can we conclude?

Based on this small split-mouth study, the routine replacement of stainless steel wires during space closure is not required. Therefore, I will try to continue to inspect my stainless steel wires at recall visits and be vigilant for possible distortion and unwanted intra- or inter-arch changes. However, it is reassuring to know that we do not need to replace these routinely.