A novel description of dynamic organ-specific trajectories in pelvic organ prolapse based on magnetic resonance imaging traces

BACKGROUND: The dynamic motion trajectories of pelvic organs during prolapse remain incompletely characterized. Detailing these trajectories could enhance the understanding and treatment of pelvic organ prolapse (POP). This study aimed to generate and visualize generalized dynamic trajectories for the bladder, uterus, and rectum. METHODS: This observational study enrolled 63 symptomatic patients with Stage III or worse POP (57 with cystocele, 35 with uterine prolapse, and 29 with rectocele). Patients underwent dynamic pelvic MRI scans during three consecutive maximum Valsalva maneuvers. Pelvic organs were segmented by contouring to calculate their centroids. Centroid displacement was tracked with a coordinate system based on the sacrococcygeal-inferior pubic point (SCIPP) line. Trajectories were analyzed using generalized additive mixed models (GAMMs) to generate composite fitting curves, followed by segmented linear regression. RESULTS: The fitted trajectories revealed distinct, organ-specific motion patterns. Based on the angles between the centroid motion trails and the SCIPP line, the bladder formed an inverse C-shape (three breakpoints, with slopes decreasing from 99.3° to 63.5°), the uterus exhibited an inverse S-shape (four breakpoints, with slopes fluctuating between 56.3° and 93.3°), and the rectum followed a near-linear path (slopes ranging from 62.3° to 67.0°). Furthermore, measurement approaches utilizing these organ-specific trajectories quantified prolapse severity more accurately than conventional, one-size-fits-all methods. CONCLUSIONS: Generalized POP trajectories are organ-specific, nonlinear, and segmented, challenging the clinical utility of universal reference lines. The heterogeneous motion patterns of these organs highlight the need for organ-specific MRI assessment protocols and tailored surgical strategies to optimize clinical outcomes.