Strengths and limitations of this study
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Comprehensive search strategy was used to gather evidence in this mixed-methods systematic review.
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Consolidated Framework for Implementation Research (CFIR) was used for the data extraction.
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Confidence in the papers was limited due to moderate quality of the papers.
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The included studies were not excluded based on their quality, as they contributed relevant information to this systematic review.
Introduction
Severe and multiple disadvantaged (SMD) populations are individuals who have experienced homelessness, substance use, offending or a combination of all three.1 They experience disproportionately high levels of poor physical and mental health along with high levels of occupational deprivation,1 which results in isolation and difficulty in accessing healthcare services.2 There is also an added burden of stigma that affects their access and engagement.3
Among people experiencing SMD, oral health problems have been highlighted as one of the major unmet needs.4 This is aggravated by high levels of smoking, substance and alcohol use and poor diet (high intake of sugar).4 5 Elevated tobacco use make them more susceptible to periodontal disease, tooth loss, oral lesions and oral cancer.5 6 Research also shows that they do not meet the daily nutritional requirements and have high levels of sugar consumption.6 7 Oral health has an overall impact on physical and mental well-being.8 It is, therefore, important to address not only oral health concerns in people experiencing SMD but also related health behaviours such as smoking, alcohol and substance use, and poor nutrition.1 9
Previous papers focus on intervention design and outcomes, and none focus on the implementation approach of these interventions especially in people experiencing SMD.10–12 Hence, there is a need for evidence on interventions addressing these health challenges, with a specific focus on ways to improve implementation and long-term sustainability of interventions. Frameworks are used to apply a theoretical underpinning to our understanding of why implementation of interventions succeed or fail. The Consolidated Framework for Implementation Research (CFIR), which is composed of five domains, was used as a theoretical framework to identify the facilitators and barriers that influence implementation.13 14 This framework, therefore, assists with bridging the gap between research and practice, as well as reducing the challenges of implementing these interventions.15
To investigate how we can improve implementation and sustainability, we conducted this systematic review to synthesise various factors such as acceptability, settings and potential adverse effects of interventions that improve oral health and related health behaviours of adults with SMD.
Methods
The research protocol was pre-registered and published registered with the Prospective Register of Systematic Reviews (PROSPERO) (reg. no: CRD42020202416).16 17 The review was reported according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.18
Search strategy
The search strategy (see online supplemental file) was formulated and conducted with an information specialist within the research team. The following electronic databases—MEDLINE (Ovid), EMBASE (Ovid), CINAHL (Ebsco), APA PsycINFO (Ovid) and Scopus—were searched for relevant qualitative, quantitative and mixed-method studies from inception to February 2023. Grey literature searches were conducted using Google Incognito and selected charity organisation websites such as Fulfilling Lives, Crisis and Groundswell, which were informed by the expertise of the research team. Forward and backward citation search of the included studies were also conducted.
Supplemental material
Study selection
The search results were downloaded and deduplicated using EndNote V.20.4.1 and the uploaded into Covidence, an online tool for managing the whole systematic review process.19 Title, abstracts and full texts were independently screened by two reviewers. In case of a discrepancy, consensus was reached after consultation with a third reviewer. Table 1 presents the inclusion criteria used during screening.
Eligibility criteria used to select the studies
Data extraction and quality appraisal
The data extraction and quality assessment for all the included studies were conducted by one reviewer and cross-checked by a second reviewer. Included studies were critically appraised to guide how much confidences could be placed on the findings. Qualitative studies were appraised using the Critical Appraisal Skills Programme (CASP) qualitative checklist.20 Quantitative studies were appraised using Cochrane’s Risk of bias for randomised controlled trials (RCTs).21 For cross-sectional studies, the National Institutes of Health (NIH) Study Quality Appraisal Tool was used.22 Qualitative studies were rated as good, moderate or low quality, which was informed by a scoring system: scores 9–10 was of high quality, 7.5–9 was of moderate quality and <7.5 was of low quality.23 The scoring was informed by the quality checklists. Studies were not excluded based on their quality; poor reporting is not always reflective of poor methodology.24 Studies were included on whether they contributed data relevant or novel data to this review.24 Moreover, including all studies allowed gathering the global evidence related to the review questions.
Data synthesis
Abstracts and data from the results of included studies were uploaded on to NVivo software (QSR International, Melbourne, Australia, V.12, Release 1.6.1). Narrative synthesis was undertaken. Deductive codes based on the CFIR framework were used to initially code the findings followed by a three-step inductive synthesis process which involved coding the text, identifying the themes and creating the subthemes. To maximise thematic yield, data reported in different papers but from the same study were individually coded. The developing themes and subthemes were discussed with the other reviewers and consensus was reached regarding these.
Results
Seventeen articles (12 individual studies) met the inclusion criteria and were included in this systematic review. Figure 1 presents the PRISMA flowchart for the included studies. Table 2 presents the descriptive summaries of the included studies. The papers were published between 1995 and 2022, and were related to interventions targeting oral health,25–32 substance use33–39 and smoking, and none on diet.
Descriptive summaries of the 17 included studies, including quality appraisal (high quality/moderate quality/low quality)
PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flowchart for the search results.
SMD groups in the studies found in this review included young adults, single mothers, veterans and adults with co-occurring conditions of severe mental illness. Based on the information reported in the studies, most of the interventions were focused on adults who were experiencing homelessness and substance use issues,33–40 but did not explicitly report on whether they included those who had repeated involvement with the criminal justice system.
Quality appraisal
Of the 12 articles reporting qualitative findings, 2 were of low quality due to lack of detailed findings and methodology not being reported adequately,31 36 and 5 were of moderate quality,26 29 33 37 38 due to reporting bias and five high quality.25 28 32 40 41 The risk of bias was assessed for the five articles reporting quantitative findings; among the two RCTs, one had a high risk of bias because of attrition and reporting bias,34 and the other article had a low risk of bias,35 the remaining three cross-sectional studies were of moderate quality.27 30 39 The findings reported in this review are mostly from high or moderate quality articles, with the inclusion of insights from low quality articles employed strategically for completeness of reporting evidence available and to supplement findings from the adequately reported articles.
Synthesis of qualitative findings
Table 3 presents the themes, subthemes, codes and quotes from individuals experiencing SMD and frontline staff and stakeholders.
Themes and subthemes from qualitative synthesis of findings along with the relevant codes and quotes
Synthesis of 12 papers with qualitative findings25 26 28 29 31–33 36–38 40 41 identified three overarching themes in relation to the aims of this review. The three themes are (1) intervention settings, (2) intervention delivery and (3) ways to enhance engagement and participation.
Theme 1: intervention settings
Eleven papers identified issues related to the settings of interventions which can play a role in the delivery of interventions targeting oral health, substance use and smoking.25–28 32 33 36–38 40 41
Physical settings
Physical settings involved the environment in which the intervention took place. The wider physical environment has been found to have an impact on the intervention experience,41 with privacy being the key factor for improving physical settings.32 33 41 Communal homeless shelters and busy teaching hospitals lack the space and privacy to deliver interventions involving discussions about difficult and sensitive topics.32 33 41 Contrastingly, stable housing with the necessary privacy allowed people experiencing SMD to focus on their recovery journey, while also creating a space in which residents could spend time away from peers who were sometimes perceived as having a negative peer group influence.36 40
Psychological aspects of settings
Psychological aspects related to the less visible parts of the interventions were identified across 10 papers.25 26 28 29 32 33 37 38 40 41 First, it was reported that relationships between people experiencing SMD and service providers played a vital part in the delivery of interventions. Through good communication,26 28 32 41 trust building,28 29 32 41 familiarity of working with a vulnerable population25 32 and mentorship,33 37 interventions were able to form a ‘safe and respectable environment’.28 32 37 41 Second, papers discussed the importance of peer support as a way of increasing the effectiveness of interventions.32 37 38 There were also reports of the impact negative peer influence could have on the recovery process. For example, smoking and drinking were linked to socialising with others, which could increase the urge to smoke or drink.40 41
Accessibility
Accessibility of interventions was one of the factors found to be important related to implementation of interventions among people experiencing SMD.25 26 32 41 First, accessible and spacious meeting points within the services were reported to help with their participation in the intervention, especially in the case of oral health interventions that were delivered either in a community setup (open space) or a mobile dental van .26 Second, geographical proximity could act as a barrier as rural and remote areas lack the facilities and resources, which could influence the access of people experiencing SMD.25 32 Lastly, it was reported that access could become an issue when service users move to more stable housing as weather conditions, distance, work and other appointments tend to make it challenging to attend the intervention sessions.41
Theme 2: intervention delivery
Nine papers discussed aspects such as information availability, resources and perceived risks of working with a vulnerable population that could be important for roll out and delivery of interventions addressing oral health, smoking and substance use.25 26 28 29 31–33 37 41
Improved awareness
Awareness and information availability were discussed in papers focusing on improving oral health, smoking and alcohol use.28 29 32 41 Sharing information between service providers and SMD groups was identified as an important issue across the papers as it created opportunities to promote involvement and behaviour change.28 29 41 It was reported that easily understandable information encouraged people experiencing SMD to view healthier behaviours as important (eg, tooth brushing) and helped to signpost them to necessary services.28 41 Clear and simple explanations of treatment options available was seen to help them in decision-making.32 Service providers also felt that they learnt more about healthy behaviours and were able to pass their newly gained knowledge to their clients.29
Resources
Five papers discussed the importance of having necessary resources to enable interventions to run efficiently and effectively.25 31–33 37 The majority of these highlighted the importance of distribution of workloads among staff because of difficulties in implementing interventions with competing duties and work within the organisations.25 33 37 Funding and resources such as volunteers and materials were identified in oral health interventions as an important issue that impacts implementation and long-term sustainability.31 32
Perceived risks working with a vulnerable population
Papers reported on the perceived risks of delivering interventions to vulnerable populations as challenging at times by service providers.25 26 32 There were concerns about safety of service providers while interacting with clients who were seen to be ‘unpredictable’. The need for training and being better equipped to work in this environment and setting boundaries between service providers and clients was repeatedly mentioned by service providers.25 26 32 37 The papers also highlighted the importance of training opportunities that provide service providers with the necessary skills to handle volatile and difficult situations.25 37
Theme 3: ways to enhance participation and engagement
Ten papers identified factors such as interest and motivation levels, adaptability and long-term support that could help to improve outcomes and create sustainable interventions by enhancing engagement and participation.25 26 28 29 31–33 37 40 41
Interest and motivation
Nine papers highlighted that the interest and motivation levels of both staff supporting SMD groups and people experiencing SMD play an important role in the implementation of interventions. Disinterest was sometimes observed among service providers, due to concerns about the complexity of delivering the intervention,25 29 31 lack of engagement with third sector organisations,26 poor uptake of the intervention by the target populations25 29 31 and preconceived notions of improper behaviour by SMD groups.41 Interestingly, interventions were met with similar feelings of indifference by people experiencing SMD if the intervention did not address their specific and complex needs such as housing and financial problems.25 26 32 Two papers on oral health interventions found that younger adults and families with children were more eager to engage compared with single men.28 29 Papers discussing the same smoking intervention illustrated that an awareness of health benefits and risks played a part in motivating people in engaging with the intervention.40 41
Adapting to specific circumstances
Adaptability of interventions was noted as an essential feature among four papers.29 32 33 40 Tailoring the interventions to address their specific needs at the time such as housing and employment was noted to increase participation and better outcomes.29 40 Service users of a community dental service also suggested flexible and longer dental appointments would be helpful and in the long-term these adaptions would help reduce missed appointments.32 Another paper reported that people experiencing SMD were keen to have more face to face interactions rather than digital, which highlights the drive to more personalised care.33
Long-term support
Four papers identified sustained and long-term support as a factor that could contribute towards better intervention outcomes.29 32 33 37 Service providers expressed a need for interventions which allowed people experiencing SMD to continue with services/programmes despite missing appointments or not completing treatment within the required delivery timeframe especially because of the transitionary nature of SMD groups.32 33 Similarly, for a substance reduction intervention, there is a preference for a long-term intervention, which allowed and supported them to gradually integrate into the new stage of their lives.33 37 Two papers on oral health interventions suggested that drop-in services offered flexibility in seeking advice or seeing a practitioner and helped to reduce anxiety surrounding accessing treatment for dental health.29 32
Synthesis of quantitative findings related to retention and implementation
Four papers reported quantitative findings on retention and programme attendance,30 34 35 39 as indicators of uptake and sustained implementation of interventions.
Three papers on substance use interventions reported high levels of retention in their intervention groups.34 35 39 Two studies among them delivered the interventions along with housing services but the findings were mixed and limited on whether retention was significantly associated with the housing services or not.34 35 39
There was no difference in the attendance levels in the studies related to substance use interventions.34 35 The attendance level for an oral health promotion intervention delivered in community settings was high (85%); however, it varied across community centres and was dependent on timing of appointment and dental treatments offered. More non-attendance was seen for afternoon appointments and complex dental treatments (eg, surgical and prosthodontic treatments).30
Additionally, workplace beliefs and practices among service providers, such as knowledge, intention and goals, were reported to influence implementation behaviours.27
Discussion
This review synthesised different factors that could influence the implementation and sustainability of interventions related to improving oral health and related health behaviours of people experiencing SMD. Evidence suggested that psychological aspects of intervention settings such as building trust and communication form an integral part in the creating a safe environment and that these are just as essential as the structural components of settings such as physical environment. Review findings further suggest that adequate staff capacity, funding and equipment would ease the delivery of interventions by reducing the immense pressure faced by service providers supporting the interventions. It was also suggested that implementation is dependent on the interest and motivation of not only people experiencing SMD but also on that of service providers in delivering difficult and complex interventions.
Most of the included studies were related to oral health and substance use (drug and alcohol). There was a lack of evidence on diet and smoking interventions among this population. Previous evidence has shown that tobacco use and poor diet, often due to limited choice available while experiencing homelessness and related disadvantages, result in a range of adverse short-term (nutritional deficiencies) and long-term health outcomes (cancer, diabetes, heart disease).42–44 Food insecurity is often linked to elevated tobacco use, mental health issues and an increased risk of substance misuse.45–47
While most of the papers mainly focused on the perspectives of people experiencing SMD, the limited data from service providers brought light to some of the challenges faced during implementation. This supports the notion that intervention implementation needs the coordination and collective effort of everyone involved. All the interventions included were designed focusing on service provision,25–28 30–35 37–41 48 except for one study that focused on a training intervention for service providers.29 Limited evidence was available on the long-term sustainability of interventions, which highlights another evidence gap that needs to be addressed.
Our review findings suggest that the retention in interventions may depend on the type of treatment offered, which at times can be influenced by the availability of housing provision. Timing and type of treatment may also influence attendance rates; for instance, morning appointments might be more beneficial, especially for individuals struggling with alcohol addiction, as they may be less intoxicated compared with later in the day. Our review findings also complement our systematic review about the effectiveness of interventions that improve oral health and related health behaviours in SMD groups—the effectiveness review found that interventions that integrated health with the individual’s wider needs (eg, housing, employment, mental health) were more effective than usual care.49 The findings we have are very limited regarding retention and attendance, more effort needs to be taken to understand how to improve reach and retention among SMD groups so that they can access and use the interventions efficiently.
A systematic review on access to dental care among individuals experiencing homelessness in the UK identified similar findings around awareness, accessibility and organisational issues (lack of financial resources and collaboration between sectors) having an influence on implementation.50 This was also similarly identified in another review on smoking cessation among homeless populations in high-income countries.51 The importance of continued engagement in services was highlighted in a review on substance use support for young people (ages 12–24) experiencing homelessness, which was also reflected in our findings.52 Existing literature on interventions targeting health conditions such as HIV and hepatitis C in this population have shown that improved health outcomes are linked to increased awareness, establishment of positive relationships with service providers and integrated treatment involving other health behaviours.53–55
Some findings from our review on aspects related to intervention settings and intervention delivery aligned with CFIR constructs of inner and outer settings domains.14 Subthemes in our findings on ways to enhance engagement aligns with both individuals and implementation process domains.14 The use of CFIR framework helps us understand the impact of intervention settings, delivery methods and engagement on the implementation process. It also provides a comprehensive approach for guiding the development of interventions targeting SMD groups and improving their efficacy in practical settings.
Strengths and limitations
This systematic review is novel in that it assesses the implementation and sustainability of interventions on oral health together with co-occurring and related health behaviours in people experiencing SMD. It addresses an evidence gap on interventions targeting these health challenges and identifies ways to overcome implementation issues faced by these specific interventions. Another strength of this review lies in its comprehensive search strategy and use of a published tool (ie, CFIR) to make sense of the results. It also highlights gaps in the evidence base on interventions related to diet, as well as studies that include repeat offenders. However, the confidence in the evidence from this review is limited as most of the papers were of moderate quality. Studies lacked detailed data collection methods and standardised evaluations which influenced their quality. Another limitation of this work is that intersectionality was not considered explicitly during the analysis of the data. Furthermore, the findings may not be generalisable to all contexts since the included papers were from high-income countries.
Implications
These findings offer valuable insights for enhancing existing interventions by paying attention to settings, delivery and engagement opportunities. Evidence from this review points to the need for additional research on interventions targeting smoking and diet. These areas hold significant value due to their direct links with general and oral health. It is also important for interventions to address not only individual behaviours but also overlapping behaviours of substance use, smoking and poor diet. This could help reduce the strain on resources and improve engagement. Furthermore, higher quality research that focuses more on sustainability and intersectionality is warranted to further investigate and refine interventions focused on SMD groups.
Data availability statement
No data are available.
Ethics statements
Patient consent for publication
Ethics approval
Not applicable.
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