The first aim of the study was to identify patient groups who were prone to nonadherence. People with paid employment had higher adherence than those without paid employment, which may be because employed people spend more time away from home, where adherence is often higher than at home . Furthermore, the level of adherence was higher among people with current or previous foot ulcers, suggesting that active foot complications may serve as a wake-up call to patients  and that additional clinical attention should be paid to people without personal experience of foot ulcers. However, differences in adherence were small between those with and without paid employment and between those with and without ulcer experience (Table 1). This may also explain why previous studies with smaller sample sizes have not found employment [7, 9] or previous foot ulcer experiences [7, 9, 24] to be significantly associated with adherence. Previous amputations were associated with adherence in the univariate analysis but not in the multiple regression analysis. This result may be explained by the fact that most amputations are preceded by a foot ulcer , and thus, the ulcer and amputation variables may have overlapped. The majority of the respondents in this study were men, which is consistent with several studies that have found men to be at higher risk than women for developing foot ulcers . Sex, age, education level and diabetes type were not associated with adherence. This finding is consistent with most previous research [4, 6, 7, 9, 24, 27] and suggests that basic demographic and diabetes-related characteristics are not useful for identifying nonadherent patient groups.
The second aim was to identify modifiable factors that are associated with adherence to wearing therapeutic footwear. The greatest proportion of variance was explained by the Strategies for footwear use domain. This domain explained 28% of the variance, which is more than the variance explained in a previous study on footwear adherence . The secondary analyses demonstrated substantial differences in adherence between people who stored their footwear in different ways, between people with high and low self-efficacy, and between people who did or did not make consistent choices about footwear. Although these aspects have not previously been investigated in quantitative studies, the results are consistent with qualitative studies that have found that the formation of new habits is important for adherence to diabetic foot self-care and footwear use [28, 29]. The final regression model consisted of four variables (self-efficacy, conventional footwear storage, therapeutic footwear storage and habitual footwear choice) that explained a moderate proportion of the variance in adherence. This finding supports the notion that adherence is a multifactorial phenomenon [8, 28, 30]. It also indicates that the observed variables are of importance but that there still is a substantial amount of unexplained variance in adherence to therapeutic footwear. This unexplained variance may be due to other independent variables that were not measured in the present study as well as to errors in the measurement of adherence. Thus, future research should investigate additional variables, such as body mass index  and patients’ acceptance of their disease and need of therapeutic footwear , as well as use objective measures of adherence, such as temperature and activity monitors .
Studies on footwear adherence have been criticized for not defining the conceptual framework, resulting in high heterogeneity, and for focusing on a narrow range of factors, typically those related to the patient, therapy and health condition . This study included a wide range of factors and is the first study to use the entirety of the Health Belief Model to study the predictors of footwear adherence. However, perceived benefits of therapeutic footwear, self-efficacy and cues to action were the only factors from the model that were significantly associated with adherence in the multiple regression analysis. This would suggest that the model may need to be revised or replaced by another model to better understand footwear adherence.
Although the conclusions are preliminary, the study has some clinical implications. First, patients should be advised to store their conventional footwear out of sight to eliminate the visual cue (temptation) to wear it and increase the effort to choose it by needing to go and get it from somewhere else. Second, patients should be advised to keep their therapeutic footwear visible at home to provide a visual cue (reminder) to wear it and reduce the effort to choose it. Third, clinicians should discuss with patients how to form new footwear habits, that is, how therapeutic footwear can become the new default option that is chosen without conscious effort. Strategies to create such new habits is an important avenue for future research and may include advice on how to store footwear and patient education to support self-efficacy , which was the factor that most strongly correlated with adherence in this study. Other suggestions for clinicians to improve adherence are to follow-up therapeutic footwear prescription and educate patients on how peripheral neuropathy increases the risk of ulcerations. Additionally, patients should be educated with the aim of strengthening their belief in the efficacy of therapeutic footwear in healing and preventing ulcers. For instance, education could include a visualization of plantar pressure measurements to compare the amounts of pressure when one wears therapeutic footwear, wears conventional footwear and walks barefoot . Finally, clinicians prescribing therapeutic footwear should acknowledge that improving certain footwear attributes (e.g., fit, pain and walking difficulties) may improve adherence. However, these suggestions are preliminary, and future research is needed to explore this further.
This is the largest study on therapeutic footwear adherence to date, and the results revealed that adherence was explained to the largest extent by variables that have not previously been investigated. Thus, a strength of the study was that a wide range of potential predictors were included. Some limitations of the study were that it was observational and cross-sectional, which limited the possibility of inferring causality. For example, certain ways to store footwear, high self-efficacy and the habitual use of therapeutic footwear may improve adherence, but it is possible that these variables reflect adherence rather than cause it. Thus, future studies should use interventions to modify these variables to investigate whether they actually are causes of adherence and potential targets for clinical interventions to improve adherence. In addition, studies should be conducted in other countries to test the generalizability of the results. Other limitations were that we only had data on sex and age for the non-respondents, which means that we cannot know if the sample was representative of the full population. Furthermore, all data were self-reported, and the questionnaire’s content validity and psychometric properties were not investigated.