A three-dimensional theoretical model of the relationship between cavernosal expandability and percent cavernosal smooth muscle.

Introduction: Percent corporal smooth muscle content, a traditional predictor of corporal veno-occlusive function, is invasive and clinically assessed by histomorphometric analyses of erectile tissue biopsies. Cavernosal "expandability" which may be a more physiologically relevant parameter is a measure of work performed to achieve penile erection, and as a consequence, an indicator of the ability to approach maximum penile volume at low intracavernosal pressure.

Aim: To demonstrate that cavernosal "expandability" determined by noninvasive methodology can replace the determination of percent smooth muscle. To predict Young's modulus for the corpora cavernosa in rabbits and, this by inference, in humans; the latter facilitates the comparison of resistance to penile expansion presented by the tunica vs. cavernosal tissue.

Main Outcome Measure: A refined three-dimensional formula for cavernosal expandability, defined as the negative reciprocal of the cavernosal bulk modulus in the semierect state, was derived as a function of percent corporal smooth muscle content, using principles of engineering mechanics of materials. The model included Young's modulus, E, for the corpora cavernosa as an unknown parameter.

Methods: Volume-pressure data obtained from three groups of New Zealand white rabbits: (i) control group (N = 7); (ii) hypercholesterolemic group (N = 5) on 0.5%; (iii) atherosclerotic group (N = 8), was plotted, and compared with the model.

Results: Data points of mean cavernosal expandability (0.012-0.017 (mm Hg)(-1)) vs. percent trabecular smooth muscle content (33.9-45.4%) for the three groups of rabbits were analyzed. The revised model formula was fitted to the existing rabbit experimental data points producing a value of Young's modulus equal to 0.01 (MPa).

Conclusions: Rabbit cavernosal expandability can predict percent smooth muscle content. Cavernosal Young's modulus can be predicted. Further clinical research efforts to provide human data are needed.