## 5 Conclusions and Future Perspectives  
The findings of this systematic review indicate that, although multiple studies report improvements in the ROM of the PIPJ following various therapeutic interventions, the lack of methodological rigor in most of these studies prevents drawing solid conclusions regarding the actual effectiveness of the treatments. The vast majority of the analyzed studies present significant methodological limitations, including the absence of standardized protocols, inadequate controls, data collection biases, small sample sizes, and insufficient follow-up. Additionally, the lack of essential data, such as baseline, intermediate, and final ROM values, along with their standard deviations, as well as the absence of control over co-interventions, hinders study comparisons and raises doubts about the validity of their conclusions. This review highlights the urgent need to improve methodological quality in research on PIPJ treatment. The issue is not merely developing new interventions but ensuring that future studies follow rigorous, transparent, and replicable protocols, allowing for the generation of knowledge that is truly evidence-based. To achieve this, it is essential that future research implements methodologically robust designs, with well-defined inclusion and exclusion criteria, standardized outcome measures, and adequate controls to ensure the validity of the findings. It is also crucial to standardize data collection by using validated measurement tools and ensuring precise reporting of clinical results, including ROM values and their progression, to enable reliable comparisons between studies. Furthermore, efforts must be made to eliminate biases in result interpretation, ensuring that reported improvements are contextualized within a robust methodological design that accounts for external factors or potential placebo effects. Another key priority is the development of long-term follow-up protocols to assess the sustainability of interventions and prevent the recurrence of stiffness after treatment. Additionally, it is critical to integrate patient-centered outcomes, such as QoL and treatment satisfaction, to evaluate the real impact of interventions on patients' daily lives. In conclusion, this review has demonstrated that the existing literature lacks adequate methodologies to rigorously assess the effectiveness of PIPJ treatments. Until these deficiencies are addressed, any intervention applied without a solid methodological framework cannot be considered evidence-based science but rather an unsupported practice. To advance the treatment of PIPJ stiffness, future research must adopt stricter methodological criteria, standardized protocols, and a rigorous approach, ensuring the development of reliable scientific knowledge applicable to clinical practice. Conceptualization, V.P.-I. and A.C.; methodology, V.P.-I. ; software, V.P.-I. ; validation, V.P.-I. and A.C.; formal analysis, V.P.-I. ; investigation, V.P.-I. ; resources, A.C.; data curation, V.P.-I. ; writing--original draft preparation, V.P.-I. and A.C.; writing--review and editing, A.C.; visualization, V.P.-I. ; supervision, A.C.; project administration, V.P.-I. ; funding acquisition, V.P.-I. All authors have read and agreed to the published version of the manuscript. This research was funded by Funding from Hand Therapy Barcelona Physical Therapy and Clinical Investigation Center. Not applicable. This review highlights the urgent need to improve methodological quality in research on PIPJ treatment. The issue is not merely developing new interventions but ensuring that future studies follow rigorous, transparent, and replicable protocols, allowing for the generation of knowledge that is truly evidence-based. To achieve this, it is essential that future research implements methodologically robust designs, with well-defined inclusion and exclusion criteria, standardized outcome measures, and adequate controls to ensure the validity of the findings.