In North West England, 7.6% of orthodontic treatments that were started in 2008 for patients aged ≥10 resulted in discontinuation and 5.2% resulted in residual need. The proportion of patients with residual need among those who discontinued and completed treatment was 16.3% and 5.4%, respectively. It is likely that early discontinuation has a much larger effect on residual need than discontinuation near the end of a course of treatment. However, our results show that residual need is evident for some patients even when a course of treatment is completed.
NHS expenditure on treatments that resulted in discontinuation amounted to £2.3 m, and £1.6 m was expended on treatments that ended with residual need (for treatments that were either discontinued or completed). These figures highlight the need to increase the cost-effectiveness of NHS orthodontic care.
Moreover, previous studies of NHS orthodontic treatment outcomes [6,7,8,9,10] have tended to report higher rates of poor treatment outcomes compared to those identified in our study. For example, a study in England of 144 patients who were treated at several hospitals and a primary care practice found that 43% failed to complete their treatment (the most common reasons being poor oral hygiene, multiple missed appointments, and orthodontic appliance breakages) . However, most of the patients were hospital patients, and treatment provision may also have been affected by the three orthodontic clinicians involved being aware that the discontinuation rates would be published. Another study compared the differences in residual need rates (as measured using the IOTN, PAR index, and ICON) for 130 patients who were treated in hospitals in the North of England . The study found that different occlusal indices indicated differing levels of residual need, for example, 20.1% of patients had residual need according to their IOTN DHC scores and 17.2% according to their ICON scores . In contrast to our study, the study involved hospital patients (as in the abovementioned study), so the sample may represent a more complex case mix than those treated in primary care practices, and the skills and training of the clinicians involved would differ from those of primary care orthodontic clinicians.
We also found that lower SES was associated with discontinuation and residual need after completing treatment, indicating SES-related inequality in outcomes. However, there was no association between SES and residual need among the patients who discontinued treatment, which suggests that ceasing treatment early does not contribute to SES-related inequality in residual need.
Previous UK studies have also indicated that there are associations between low SES and poor treatment outcomes. First, a study in England and Wales of 1431 patients (based on a 1990–1991 Dental Practice Board data set that covered all patients who discontinued treatment and 2% of those who had completed treatment) found a larger percentage of those from the more deprived groups (using multiple area-level SES measures) discontinued treatment . A study in England of 135 12- to 16-year-olds treated for one year with fixed orthodontic appliances in the hospital dental service found that deprivation (based on characteristics of parental employment) was associated with high improvement in occlusion (defined as a score equal to or higher than the sample median ICON improvement score of a modified version of the ICON) . However, other elements of SES, namely parental education and employment status, were not associated with treatment outcomes . Moreover, a study in North West England of 144 9- to 19-year-olds reported that SES (measured using Townsend scores) was not associated with discontinuation . However, this study largely involved patients treated in hospitals, who (like the patients in the abovementioned study) may have had a different treatment experience compared to those treated in primary care.
SES may be linked with discontinuation because patients in the more deprived groups have been reported to be more likely to miss orthodontic appointments  and discontinuation can be increased by orthodontic practices having a strict policy on discontinuing treatment for patients who miss appointments . It is likely that the more deprived groups are more affected by prohibitive transport costs and the potential impact of lost pay for the accompanying parents . In addition, the SES associations with treatment outcomes may be due to the effect of SES on the patient’s development of self-efficacy  (i.e., the strength of one’s belief in one’s ability to complete tasks and reach goals ). Self-efficacy influences health behaviours , and health behaviours (e.g., patient compliance with treatment instructions such as to regularly replace intraoral elastics) are associated with orthodontic outcomes [26,27,28]. Studies have shown that low SES is associated with some of the elements of low patient compliance, such as poor oral health practices [29,30,31].
The data set provided comprehensive individual-level data on all NHS primary care orthodontic treatments provided under NHS contracts in North West England, and the IMD score for each patient. While the results reflect orthodontic outcomes in North West England, they may not be generalizable to other populations if there are regional variations in practitioner processes and patient preferences. However, practitioner processes should largely be uniform across England given that the same standards and procedures exist across NHS England. Also, patient preferences are likely to be generalizable, and approximately 13% of individuals aged ≥10 in England resided in North West England in 2011 , so the analyses were conducted on a significant percentage of the population of England. Additionally, North West England has a diversity of individuals from different SES backgrounds, with all five IMD quintiles being represented (though the lower SES groups are overrepresented, with approximately a third of North West LSOAs being in the most deprived quintile in England ).
We used the IMD (which is an area-level measure that takes into income, employment, health, education, crime, housing and services, and living environment) because there were no individual- or household-level data on income, occupation, or other indicators of SES. The IMD is the most commonly used measure of SES in the UK; however, one limitation of the use of the IMD is that not everyone living in a deprived area is deprived and not all deprived people live in deprived areas. This implies that there can be misclassification error, which could bias the SES-related associations. IMD scores are typically reported at the LSOA level, which represents the smallest area for reporting UK census data, with population sizes of 1000–3000 individuals. As the population size of small-area deprivation measures decreases, the risk of misclassification error decreases. Although individual- or household-level SES indicators would help to avoid misclassification error, collecting individual-level data on, for example, self-reported income, increases the likelihood of non-response. In addition, despite the risk of misclassification error, small-area SES measures can help to identify areas with higher proportions of deprived households, so they are useful for planning and targeting healthcare services .
Another major limitation of the study is the missing outcome data, i.e., 18.9% of treatments (associated with an NHS expenditure of £5.9 m) ended without an outcome record being submitted and 19.4% were associated with incomplete IOTN outcome fields (£5.7 m). This inevitably led to underestimation of the proportions and NHS expenditure associated with discontinuations and residual need.
In addition, higher SES was associated with incomplete IOTN outcome fields among patients who completed treatment (but not among those who discontinued treatment), for reasons that are unclear. In contrast, lower SES was associated with missing outcome records. If missing outcome records were partly reflective of discontinuations (e.g., if dentists did not submit records because patients discontinued treatment and the dentists were initially unsure whether the patients would return), the association between lower SES and discontinuations would be attenuated.
Another limitation of the study relates to criticism regarding the ability of the IOTN to measure outcomes. The developers of the ICON argue against the use of the IOTN to investigate outcomes on the basis that it was ‘developed and validated to assess treatment entry and exits as separate phenomena, when they are clearly part of the same clinical process. This requires additional training and duplicates the effort of measuring what are often similar occlusal traits’ . Nonetheless, the IOTN was used on the basis that it indicates the degree of residual normative need at the end of active treatment. Moreover, the IOTN remains the principle index used to assess individuals’ need for NHS orthodontic treatment need and the prevalence of malocclusion in the population [36, 37], and using the same index to establish both a baseline assessment of need and the treatment outcome is practical when assessing the effects of treatment. However, the IOTN outcome scores were measured at the end of active treatment, but there can be relapse after active treatment has finished , particularly if there is poor compliance with retention instructions. From this perspective, both the proportion of treatments that end with residual need and the expenditure on these treatments are underestimated. Lastly, the IOTN outcome scores were not independently validated, which may also have led to bias in the data, and thus an underestimation of residual need.
Regarding the implications of the findings, the quality of the NHS activity data could be increased by ensuring that outcomes are reported (and validated) for all treatments (except in cases where the patients fail to return, when only discontinuations, rather than IOTN scores, can be reported), and monitored by the service commissioners. This is in line with 2015 guidance on the delivery of NHS orthodontic care in Wales, which highlighted the importance of the development of local health board policies to ensure that treatment outcomes in terms of completions and discontinuations are reported for each patient . Monitoring outcomes more closely could help service commissioners to determine which providers provide the best value for money.
In addition to recording information on discontinuations and IOTN outcome scores, there is a contractual requirement for dentists to monitor the outcomes of 20 patients plus 10% of the remainder of their patients using the PAR index , though these data are not collected using the NHS activity forms . Implementing a contractual requirement to monitor the PAR scores of consecutive patients and to utilise independent third parties who are calibrated in the use of the index may help to reduce bias . The British Orthodontic Society have noted that NHS commissioners may make participation in a peer review process a contractual requirement , and NHS guidance on commissioning has stated that ‘PAR scoring will in future be undertaken within a managed orthodontic clinical network…under a peer review mechanism’ .
Payment by Results (PbR) remuneration is used in many areas of the NHS , and could potentially help to improve orthodontic treatment outcomes . However, a difficulty with implementing this policy is that there is a variation in the case mix (e.g., different percentages of patients from more deprived groups) between orthodontic clinicians, which would influence the percentages of patients who discontinue treatment and have residual need. Further, a PbR approach may generate perverse incentives for orthodontists to reject referrals of patients from disadvantaged backgrounds, widening inequalities further. In addition, although contract penalties for discontinuations may help to reduce the SES-related inequity in discontinuation, care would need to be taken to ensure that treatment was not continued in cases where there could be risk of harm to the patient (i.e., if compliance with oral hygiene advice was poor).
Key areas to be explored include how factors related to patient compliance underlie the associations between SES and orthodontic outcomes, and why higher SES was associated with incomplete IOTN outcome fields among patients who completed treatment.