Predicting penile size during erection.

The aim of this prospective study was to identify clinical and engineering parameters of the flaccid penis for prediction of penile size during erection. Dorsal and ventral penile lengths, as well as base and tip circumferences were measured in flaccid states, gently stretched states and at full erection resulting from intracavernosal injection of prostaglandin E1 in 55 patients. The forces required to stretch the penis were measured by a specially designed gauge and regression relationships of the measured dimensions were calculated. An engineering model was developed to analyze differences between results obtained during stretching and erection, as well as to approximate the optimal force values which should be applied during the stretching part of the clinical evaluation of penile size. The ratio between the flaccid to stretched penile lengths was shown to be the best predictor for the ventral elongation from flaccid to erect penile lengths. The engineering analysis predicted that a minimal tension force of approximately 450 g during stretching of the penis is required to reach the potential erection length. The stretching forces exerted by the urologist in the clinical setting were experimentally shown to be significantly (P<0.01) less than this value. The values of the relative and absolute elongations of the stretched penis at its ventral aspect provide reliable estimations of its potential maximal elongation during erection. The model designed for this study may obviate the use of intracavernosal injections for estimating penile length during erection.