Which type of retainer lasts the longest?
We all have our favourite retainer. But which lasts the longest? This new study may help us decide.
Retention is one of the “hot topics” in orthodontics. Unfortunately, there is only a moderate level of evidence that may help us choose our retainers. As a result, our choices are often based on operator and patient preferences. The EJO published this new study A team from New Zealand carried out this research. I had the pleasure of meeting them when I went to New Zealand for four days last year to speak at their conference. This is a nice picture of me that the residents showed when they thanked me for my lectures.
Carrol Jin et al
European Journal of Orthodontics: On line
They did this study to answer this question:
“What are the survival times of different retainers and what causes their failures”?
What did they do?
They did a retrospective study. You may now be thinking “What is going on, he does not usually write about retrospective stuff”? You would be correct as I do tend to avoid reading retrospective studies because of the inherent selection bias. Nevertheless, sometimes it is worth reading this type of study because it may be best long term evidence that we may have. I will return to this later. So lets cast a critical eye over this study.
They collected data from the files of the Department of Orthodontics in Otago, New Zealand. The data collection period was from 2003 to 2014. Orthodontic residents had treated the patients and fitted the retainers.
They got 591 retainer records from 309 patients. These included 199 Hawleys, 34 Vacuum formed retainers (VFRs), 278 Lingual fixed retainers (VFRs) and 80 combinations.
They recorded the year of fitting, survival time and reason for failure for each retainer from the patient record. In addition, they recorded standard demographic data on each patient.
Finally, they used survival analysis and regression to evaluate the survival time and failure rate for each type of retainer.
What did they find?
They found that the retainers with the longest median survival times, in days, were the LFRs (1604), Hawley (1529) and the shortest survival times were for the Combinations (258) and Vacuum formed (VFRs) 105 days.
There was no statistically significant differences between the LFR and Hawleys. However, the Combination and VFRs were different from the Hawley and LFRs.
When they looked at the reasons for failure. The mean failure rate across all the retainers was 43% (I thought that this was high!). The Hawley was the most likely to be lost (52%), LFR most likely to debond (63%) and fracture was the most likely cause of VFR failure (43%).
What did I think?
I think that this provided us with some useful information. Nevertheless, we need to be rather cautious when we interpret the data. This is because of the retrospective design leading to two major flaws.
Firstly, we do not know the actual number of retainer fitted during the study period. The “red flag” that I spotted was that the total numbers of retainers they included was from 309 patients over an 11 year period. This struck me as being rather low and suggests that there must be selection bias in the study.
Secondly, we do not know how many retainers failed and the patients did not return to the clinic. This means that there maybe an unknown amount of missing data.
We also need to consider that we do not know the exact effect of these factors on the data. All I can conclude is that there is likely to be a high level of uncertainty in the information that we can derive from this paper.
Nevertheless, we should not totally dismiss this study without considering whether other types of study can provide us with better information. Ideally, we could do a trial or a prospective cohort. This should reduce the selection bias. However, if we were to run this type of study over a long period, then the drop out rate would be high and we would run into problems of compliance bias. There is no easy solution to this problem and all I can suggest is that we need to be cautious when interpreting these studies.
In summary, I feel that we can only conclude that there are differing retainer survival rates and these may be marked. I think that I shall stick to VFRs because I like them and they are easy to replace when they break or are lost.
Emeritus Professor of Orthodontics, University of Manchester, UK.