Geometry of the fusiform excision for skin lesions

Abstract Objective The fusiform excision technique is commonly used by surgeons to remove round skin lesions to minimize "dog-ears" at the ends of the incision. We propose a geometric analysis to easily design the fusiform incision and consequently standardize the surgical procedure. Background The classic ellipse is formed by tracing 2 arcs of a circle on the skin. The arcs, which are symmetrical with respect to the midline axis separating them, intersect at their ends to form a convex shape and classically result in a 1:3 width-length ratio between the short and long axes of the ellipse. Methods Using basic geometry rules, namely Pythagorean theorem and the ratios of the angles of right triangles, we first calculated the ratio between the radius of the lesion and the radius of the arcs of the circle of the fusiform incision and then the distance between the center of the lesion and the intersection between the line perpendicular to the axis of the fusiform excision and the tangent to the arcs of the circle. Results The ratio between the radius of the lesion and the radius of the arcs of the circle of the fusiform incision is 5 and the distance between the center of the lesion and the intersection between the vertical axis and the tangent is 2.25 for a fusiform incision with a width-length ratio of 1:3. We then generalized the formulas. Conclusions Our approach provides an introduction to the geometry of dermatologic surgery to students in order to standardize the surgical procedure.