The Reconstructive Pelvic Anatomy classification system : a valid and reliable classification system for pelvic reconstruction.

Aims: Pelvic reconstruction in the setting of significant bone loss has become increasingly complex, with advancements such as custom 3D-printed prostheses now used in both large oncological resections and revision total hip arthroplasty (rTHA) with massive bone loss. Surgeons must consider how to best reconstruct the pelvis to optimize patient outcomes. The Reconstructive Pelvic Anatomy (RPA) classification system is proposed to aid in communication, reconstructive planning, and developing research cohorts suitable for modern pelvic reconstructions. This study aimed to 1) describe the development of the RPA classification as a novel classification system for pelvic reconstruction; 2) demonstrate the content and construct validity of the system; and 3) investigate its interobserver and intraobserver agreement. Methods: A group of orthopaedic surgeons and researchers, based in Sydney, Australia, developed the system's four domains, which together form the overall classification. A total of 23 pelvic reconstruction surgeons provided an assessment of each domain's clarity and relevance, producing an Item-Content Validity Index (I-CVI). They then applied the classification to the surgical plan and postoperative radiograph of 20 pelvic reconstructions twice, at a minimum of two weeks apart. Construct validity was assessed by agreement between observers and authors. Interobserver and intraobserver agreement was assessed using the kappa statistic. Results: A total of 23 surgeons completed the assessment of content validity, and 18 surgeons completed both surveys. Each domain demonstrated content validity for both relevance and clarity (I-CVI > 0.78). Authors and observers had moderate agreement on the classification supporting construct validity (k = 0.6) with a median agreement of 90% for individual domains. Interobserver agreement was moderate overall (k = 0.54), with individual domains having substantial or better agreement (k = 0.73 to 0.82) except for anterior fixation (k = 0.57). Intraobserver agreement of individual domains was substantial or almost perfect (k = 0.76 to 0.84) with moderate agreement for the overall classification (k = 0.6). Conclusion: This study establishes the content and construct validity of the RPA classification system as a novel classification system for pelvic reconstruction, and its reliability to support its use as a communicative tool, and in research and surgical planning.