Canine retraction: Chain or coil?

Space closure and canine retraction is often a time-consuming treatment phase, particularly in the absence of pre-existing crowding and where anchorage control is at a premium. Sliding mechanics is the most popular approach to space closure, with the typical choice being between the use of elastomeric or NiTi coils. We know that elastomerics degrade over time and require reactivation. But are they inferior to NiTi coils? The authors of this new paper aimed to shed further light on this question.

This study was carried out in Cairo and published in the Angle Orthodontist. I have been to Cairo once and enjoyed it, but I did not have a chance to visit the Pyramids. I hope to right this wrong once the pandemic subsides.

What did they ask?

They did the study to find out if Ni-Ti coil or elastomeric chain was the best method for extraction space closure.

What did they do?

They carried out a split-mouth randomised controlled trial involving 32 participants as follows:

Participants: 

• Full permanent dentition undergoing bilateral first premolar extractions
• Maximum anchorage requirement in the maxillary arch

Interventions:

Either a 6mm NiTi Coil or elastomeric chain.

They set force levels at 150g. A stainless steel ligature was added to extend the auxiliary if the force level exceeded 150g. Importantly, they used temporary anchorage devices in all quadrants, with the auxiliary extending from the TAD posteriorly to a power arm (8mm in length) on the canine. They started Space closure within two weeks of extraction following the posterior segment’s alignment in 17×25-inch SS.

Primary outcome(s):

Rate of canine retraction over six months

Secondary outcomes:

Side effects of canine retraction, including rotation, tipping, root resorption, anchorage loss and pain.

The researchers took alginate impressions (7 per participant) monthly over the six months and used them to generate digital models to evaluate space closure. They superimposed digital models on the mesial aspect of the third palatal rugae to evaluate anchorage loss. Medium-field CBCTs were taken before and following canine retraction to assist with evaluating canine angulation and root integrity. The participants recorded their perceived pain on a continuous scale over ten days following each activation.

What did they find?

They found no significant difference in canine retraction rate with a mean rate of 0.79mm per month with coil spring and 0.86mm per month with elastomeric chain. Over the study period, they measured canine retraction of 4.44mm and 4.33mm with NiTi Coils and elastomeric chain for the mandibular arch. The corresponding figures were slightly higher in the maxillary arch (4.73mm with NiTi coils and 5.17mm with chain). Based on the CBCT data, the authors found similar amounts of canine tipping and rotation at six months with 1mm of root resorption on both sides. They found no difference in pain intensity between the groups, although pain appeared to last slightly longer on the chain side.

What did I think?

I think that this was a simple study. While there was relatively little novelty (with the authors highlighting eight similar studies in their literature review), it does provide further information confirming that the rate of space closure is similar with NiTi coils springs and elastomeric chain.

From a clinical perspective, the rate of space closure observed was generally expected to be of the order of 0.8mm per month. As they used TADs posteriorly, they did not plan reciprocal space closure. This was reflected in the finding of minimal posterior anchorage loss based on the superimposition.

Adverse effects were minimal with limited tipping or rotation, and clinically insignificant amounts of root resorption observed. Again, I was not surprised to see these findings. The authors refer to this holistic evaluation as a strength of the study. I agree that they have done a broad assessment. However, the decision to take repeated CBCTs at short intervals is questionable (and indeed topical).

In terms of sample size, they did provide a sample size calculation.   Unfortunately, this is incomplete, with no account for the split-mouth design or description of mean rates of space closure or differences that the authors regarded as being of clinical significance. Nevertheless, the numbers appear to be reasonable, and the observed between-groups difference (of less than 0.1mm) suggests that a lack of statistical power was not overly problematic.

The split-mouth design has potential benefit by controlling for possible general, skeletal and occlusal features, including age, gender, vertical facial proportions, overbite and physiological differences. Importantly, they minimised tooth movement’s potential impact on contralateral sides by using sectional appliances with individual retraction of canines. Pain experience may, however, be a little difficult to discern.

The authors activated space closing mechanics on a 4-weekly basis to apply 150g of force per side at each activation. This approach is consistent with many practitioners. My own purely anecdotal feeling is that this reactivation protocol may be excessive, and less frequent reactivation and lower force levels may be more helpful during space closure. Equally, some practitioners might feel that less frequent reactivation might be appropriate with NiTi-based mechanics and not with elastomerics.

What can we conclude?

There does not appear to be a difference between NiTi coils and elastomeric chain in terms of canine retraction rate. It may be that the present study has erased any possible doubt that there is any meaningful difference between the efficiency of space closure or canine retraction with NiTi coils or elastomeric chain.

Perhaps we can now consider other related questions, including the ideal frequency of reactivation with space closing mechanics? In doing so, we can shift the emphasis to perhaps where it ought to lie- the decisions we make, not the tools we use.