The prostatic urethra is a bent tube, and the clinical significance of the prostatic urethral angle (PUA) is poorly understood. We investigated the effect of the PUA on the urinary flow rate in men without benign prostatic obstruction.

In a theoretical model, the urinary flow rate at the end of the prostatic urethra was formulated according to some principles of fluid dynamics, including the Navier-Storkes equation, Bernoulli's equation, and the loss of coefficient at bends. The change in the urinary flow rate according to the PUA was also calculated through mathematical simulations. In addition, the relationship between the PUA and the peak flow rate was evaluated in 65 healthy men without benign prostatic obstruction (age 50 to 59 years, prostate volume less than 30 cm³, peak flow rate greater than 15 mL/s). The prostate volume and PUA were determined by transrectal ultrasonography.

Using the fluid dynamic model, the urinary flow rate increased in proportion to the urethral diameter and was inversely associated with the PUA. The mathematical simulations showed that the urinary flow rate decreased by more than 27% as the PUA increased from 35° to 90°. In the clinical setting, the peak flow rate was significantly associated with the PUA by linear regression analysis (R = 0.34, β = −0.342, P = 0.005), but it was not associated with the total prostate volume (R = 0.18, β = −0.181, P = 0.150).

The PUA was inversely associated with the urinary flow rate in this theoretical model and preliminary clinical study. This relationship could be an important factor in the pathogenesis of benign prostatic hyperplasia/lower urinary tract symptoms.