External facilitation for implementing complex interventions: a realist synthesis

External facilitation (EF) is a key measure for strengthening the implementation of complex interventions. However, limited evidence exists on how EF works across different contexts. This study aims to explain how, why, under what circumstances, and for whom EF supports the implementation of complex interventions in healthcare settings. An initial theory of EF was developed by a monitoring committee comprising researchers, research staff, clinicians, and patient partners, using a participatory and iterative realist synthesis approach. This initial theory, comprising constructs from existing theories and frameworks in implementation science, guided the collection and analysis of data from empirical studies identified through a previous scoping review on the role of external facilitators in the implementation of complex healthcare interventions. Additional sources on these complex interventions were also included. The data (primary studies and additional sources) were coded to identify context-mechanism-outcome configurations, informing the refinement and development of the EF theory. The review included 36 primary studies examining EF in the implementation of 34 complex interventions, as well as 105 additional sources. The analyses identified 14 context-mechanism-outcome configurations grouped into four demi-regularities: sensemaking and coherence-building; capacity-building and self-efficacy; development and maintenance of good relationships; and engagement and ownership. The refined theory of EF highlighted new mechanisms such as sensemaking, coherence-building, team cohesion, and external facilitators’ capacity to transform implementation barriers into opportunities. The findings provide guidance for optimizing EF in the implementation of complex interventions. The next phase will test and refine the theory through a realist evaluation.