Why the "Y" Becomes a "Trans-Y": Validation of the "Number Y" for Implant Height Prediction in Gender-Affirming...

Breast augmentation is the most frequently performed gender-affirming surgery (GAS) for transfeminine individuals. Because of 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. The aim of this study was 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. 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. The mean patient age was 39.62 years, BMI of 25.42, and the mean Number Y of 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 = .01) and lower Number Y (3.96 ± 0.11 vs 4.42 ± 0.28; P < .001). Most concordant cases received low-height implants, whereas discordant cases were predicted to require middle-height implants but ultimately received low-height implants. The Number Y has limited predictive accuracy in transfeminine patients because of distinct thoracic anatomy and higher Y factors. An adjusted Number trans-Y is proposed to optimize implant selection and surgical outcomes in this population. Level of Evidence: 4 (Therapeutic).