Why the “Y” Becomes a “Trans-Y”: Validation of the “Number Y” for Implant Height Prediction in Gender Affirming Breast Augmentation Using Anatomical Implants

Abstract Background Breast augmentation is the most frequently performed gender-affirming surgery (GAS) for transfeminine individuals. Due to anatomical differences between transfeminine and cisgender women, implant selection requires tailored planning. The “Number Y” is an established method for implant height selection in cisgender patients. Since its adoption for transfeminine patients at our institution in 2016, its reliability in this population has not been validated. Objectives To validate the “Number Y” in transfeminine patients, identify anatomical factors influencing implant height prediction, and propose an adjusted “Number Trans-Y” to improve surgical planning. Methods This retrospective study included 21 transfeminine patients undergoing breast augmentation as part of GAS. Preoperative implant height predictions using the “Number Y” were compared with actual intraoperative implant choices. Implant shapes were categorized as vertical-elliptical, circular, or horizontal-elliptical. Patient and implant characteristics were analyzed for concordance. Results Mean patient age was 39.62 years, BMI 25.42, and mean Y-Number 4.31, higher than reported in cisgender cohorts. Implant height prediction was accurate in 16 patients (76.19%). Discordant cases (23.81%) had significantly smaller thoracic perimeters (9 cm difference, p=0.01) and lower Y-Numbers (3.96±0.11 vs. 4.42±0.28; p<0.001). Most concordant cases received low-height implants, whereas discordant cases were predicted to require middle-height implants but ultimately received low-height implants. Conclusions The “Number Y” has limited predictive accuracy in transfeminine patients due to distinct thoracic anatomy and higher Y-Factors. An adjusted “Number Trans-Y” is proposed to optimize implant selection and surgical outcomes in this population.