How much relapse occurs more than 15 years after orthodontic treatment?

Relapse and retention are two of the great unknowns of orthodontic treatment. Investigators have devoted much time attempting to identify the most stable treatment methods and the best retainers. While this work has provided us with helpful information. Most orthodontists and patients’ hopes and dreams may not be realised regarding long-term stability. Our only answer seems to be that our patients should wear their retainers indefinitely. Unfortunately, we all know they will not do this, and relapse is inevitable.

Furthermore, many studies evaluate the short-term effects of different retainers on relapse. Unfortunately, there are few long-term studies because they are incredibly challenging. Therefore, I was very interested in these two long-term studies that the authors published last month. They managed to recall patients who had been treated in the 1970s and 1980s.

At this time, I was an introverted teenager who spent many hours listening to early Pink Floyd music. Those who know me may say that not much has changed! It is also 50 years since the release of Dark Side of the Moon and I skipped school to buy the album. I have chosen relevant images to reflect the echo of that distant time.

As with all long-term studies, some things could be improved. Nevertheless, this is the best that we can do at present.

What did they ask?

This study aimed to;

“Compare long-term incisal irregularity and arch dimension changes in subjects treated with four premolar extractions more than 30 years after retention”.

What did they do?

The team did a retrospective study.

The inclusion criteria for the sample was that the patients had Class I or Class II malocclusions treated with fixed appliances with the extraction of 4 first premolars or upper first premolars and lower second premolars. Postgraduate students at the university dental school treated the patients in the 1970s and 1980s.

At the end of treatment, the patients wore an upper Hawley retainer for at least one year and had a lower canine-to-canine bonded retainer for 1-5 years post-treatment.

The patients had to have complete records at the start and end of treatment.

The team then recalled the patients who attended between October 2017 and October 2019.

They recorded dental measurements from scans of the casts. These included Little’s Index (LLI) to measure relapse.

Finally, they divided the patients into two groups. One had mild crowding, defined as LII of less than or equal to 6mm. Severe crowding was defined as an LLI of greater than 6mm.

What did they find?

They collected records for 16 patients with mild and 25 with severe crowding. The follow-up times were 38 years for the mild and 37 years for the severe crowding groups.

The team presented a large amount of data to which they applied several statistical tests. As usual, I attempted to identify the most critical variables, and I do not have the space to go into detail here. However, their general finding was that the treatment corrected the incisal irregularity.

When they looked at relapse, they found that;

• The mandibular irregularity returned to pre-treatment values in the long term for the mild group. Before treatment, LII was 3.3 mm and relapsed to 3.67mm at the end of 38 years.
• In the severe group, the irregularity did not return to the pre-treatment value. For example, pre-treatment LLI was 8.4mm; after 37 years, it was 4.6mm.
• The post-retention irregularity was mild for both groups, and the results were satisfactory in the long term.

Their overall conclusion was;

“The mild crowding group has proportionally more relapse than the severe crowding group. The correction of severe crowding with the extraction of four premolars showed satisfactory results”.

What did they ask?

They asked;

“What is the effect of arch widths and their changes during treatment on the long-term stability of dental arches”?

What did they do?

They did a retrospective study that analysed the records of 103 patients.  One specialist orthodontist in Texas treated the patients during the 1970s and 1980s.

The patients had Class I and Class II malocclusion treated with fixed appliances with and without extracting first permanent premolars. They had to have complete records before and after treatment with a post-retention period of at least five years.

His retention protocol was a wrap-around retainer in the upper arch and fixed canine-to-canine mandibular retention for at least three years. He removed all the retainers after three years of retention. Notably, he did interproximal reduction for the lower canine segment.

They scanned dental casts taken at the start and end of treatment and at least five years post-retention.

The outcome measures were dental arch widths and LII for the upper and lower incisor relapse.

What did they find?

They analysed the records of 73 female and 30 male patients. The average post-retention time was 17.2 years. Extractions were done in 55 (54.4%) patients, and 48 (46.6%) were treated non-extraction.

At the start of treatment, there were significant differences in the LII between the extraction and non-extraction groups.

They found the following when they looked at the cases at the end of the long-term recall.

• In the extraction group, the mean LII for the upper arch was 1.69, and for the lower arch, this was 2.05,
• In the non-extraction group, the mean LII for the upper arch was 1.10, and for the lower arch, it was 1.52.
• These are not clinically significant differences. Furthermore, these differences show that the treatment was stable.

In the discussion, the authors pointed out that the operator attempted to keep the teeth in the basal bone, limiting proclination to 3 degrees and limiting the expansion of intercanine width to a maximum of 1mm. This led them to suggest that “the high degree of stability was due to the operator making good diagnostic and treatment decisions”.

The authors came to the following conclusion.

“There was good stability in the long-term post-retention period”.

What did I think?

These were two interesting papers that came to similar conclusions. The data suggests that a conservative approach to expansion and judicious use of extractions results in relatively stable cases. Consequently, long-term stability is possible with “sensible” treatment planning.

These findings are clinically relevant, although controversial. Many readers may disagree with my interpretation. Furthermore, some may dismiss the results by suggesting that contemporary techniques allow us to grow bone with our new appliances. They may even suggest that we can split sutures and expand dentitions as we have never been able to before. Nevertheless, this data persuades me that we need to be cautious in following the “expand all-the-time approach” that cookbook orthodontists promote, and some follow like sheep.

Finally, I should consider the problems with these studies. The most important is that they are both retrospective studies. This, of course, means that they are full of selection bias. Importantly, we do not know the direction of this bias. Furthermore, one practitioner and a university dental school department treated the patients. As a result, these findings need more generalisability.

Nevertheless, we must consider whether improving this study design is possible when we interpret these studies. This is not possible because of the long-term nature of the follow-up. So, this is the best we can do; we cannot set the controls for the heart of the sun. One of these days, someone will obtain this data, perhaps, in a long-term cohort study.

It is up to you to interpret these studies with your understanding of the uncertainty inherent in the study design.