A Geometric Framework for Predicting Donor Area Thinning After Follicular Unit Excision

Background Safe donor harvesting in follicular unit excision (FUE) is traditionally estimated using donor density alone, despite substantial variation in donor appearance among patients with similar densities. Objective To develop a geometric framework for estimating donor coverage and predicting visible donor thinning based on individual hair characteristics. Methods A mathematical model was developed incorporating hair shaft diameter, hair length, exit angle, and donor density. The projected scalp coverage of each hair shaft was approximated geometrically, allowing calculation of minimum residual hair density and maximum safe harvestable graft number. Clinical examples were used to illustrate model behavior. Results The model demonstrated that safe donor capacity is highly sensitive to variations in hair caliber, length, and exit angle. Patients with identical donor densities showed markedly different predicted harvesting limits based on hair geometry alone. Increased hair caliber and hair length substantially improved coverage, whereas steeper exit angles reduced donor yield. Conclusion This framework provides a quantitative, geometry‐based approach to donor assessment and may assist individualized donor planning and reduction of overharvesting risk.