## 1 Introduction  
Proximal interphalangeal joint (PIP) stiffness is a condition that significantly affects hand function, limiting the ability to perform essential activities and reducing patients' quality of life (QoL). Its clinical management is complex due to the variety of factors that can contribute to its development, including direct trauma, prolonged immobilization, postoperative complications, and various associated pathologies. These factors progressively reduce the joint's range of motion (ROM), potentially leading to persistent functional disability and, in advanced cases, permanent deformities [1].  
To address this condition, various therapeutic strategies have been developed, which can be broadly categorized into conservative and surgical interventions. Among conservative options, the use of orthoses or splinting is one of the most commonly employed tools in clinical practice, along with other approaches such as therapeutic exercises, manual therapy, and joint mobilization techniques [2; 3; 4]. Orthoses, in particular, can be classified into static, static progressive, dynamic, and torque transmission devices, each with specific mechanisms of action aimed at promoting joint mobility and preventing stiffness progression [4; 5; 6].  
Within this field, our research group has developed and evaluated digital neoprene orthoses with elastic tension as an alternative for the progressive recovery of joint ROM. Our studies have demonstrated their potential to provide controlled and sustained force, gradually facilitating the extension of the affected joint [7]. These devices have generated increasing interest in clinical practice, as they offer an accessible and less-invasive alternative compared to other interventions. Analyzing their applicability within the broader set of available therapeutic strategies is essential to understanding their impact in various clinical settings.  
Despite the growing interest in these strategies, a more in-depth analysis is still needed to understand how they are implemented in clinical practice, the criteria used for their selection, and the methodological frameworks that support their application [8]. Beyond their effectiveness, it is crucial to understand the rationale and methodology behind each intervention to establish a more structured therapeutic approach. In this regard, [PERSON] [9], in one of the most comprehensive Cochrane systematic reviews on the topic, concluded that stretching does not have clinically relevant effects on joint mobility. This finding underscores the necessity of documenting and analyzing the methodologies used in different therapeutic strategies.  
Therefore, the present study aims to examine the current state of knowledge on PIPJ stiffness by documenting existing therapeutic strategies and analyzing the methodological approaches employed in their study. To ensure the quality and reproducibility of findings, this review will adhere to widely recognized methodological standards, such as the PRISMA guidelines, which provide a structured framework for conducting systematic reviews and meta-analyses [10]. This approach will enable a rigorous synthesis of available information, facilitating a better understanding of the methodologies applied in clinical practice and contributing to the development of evidence-based therapeutic strategies.  
This study aims to analyze the methodology used in research on the treatment of PIPJ stiffness to assess its homogeneity and detect inconsistencies that may affect the validity of evidence-based clinical recommendations. Through a systematic review, the study seeks to identify patterns in study design, intervention selection criteria, and the quality of reported data, with the goal of providing a more structured foundation for future research and improving decision-making in clinical practice.