Two-dimensional computational fluid dynamics study of urine flow in the prostatic fossa after benign prostatic hyperplasia surgery: a single-patient model.

Background: Transurethral enucleation techniques such as holmium laser enucleation of the prostate (HoLEP) have become increasingly popular as an alternative to conventional transurethral resection of the prostate (TURP). Yet enucleation leaves a wider prostatic fossa than tissue-sparing TURP, and whether a larger post-operative lumen consistently yields better voiding remains uncertain. To address this question, we used a single-patient, two-dimensional computational fluid dynamics (CFD) model to investigate how prostatic fossa geometry modulates urinary flow and energy loss during mid-voiding. Methods: Sagittal prostate magnetic resonance imaging (MRI) from a 63-year-old benign prostatic hyperplasia (BPH) patient was segmented in COMSOL Multiphysics 6.1 to construct an idealized two-dimensional CFD model of the lower urinary tract. The prostatic urethra was approximated as a curved ellipsoid and urethra as an S-shaped tube, the relative angle was measured from the mid-sagittal image. Eight experimental groups were designed with prostatic urethral diameters ranging from 0.8 to 2.2 cm at 0.2 cm intervals (prostatic urethral diameter ratio, defined as prostatic urethral diameter divided by 0.6-cm urethral diameter; range, 1.33-3.67). A transient simulation (0-2 s) was run with a fixed inlet velocity of 0.25 m/s and outlet pressure of 0 Pa using the realizable k-ε turbulence model. Outlet velocity at the external urethral meatus and inlet pressure at the bladder neck were recorded, and turbulent kinetic energy (TKE) and turbulent dissipation rate were sampled along a horizontal cut line at the midpoint of the prostatic urethra. Results: Outlet velocity at the external urethral meatus increased minimally with prostatic urethral diameter (0.5152-0.5180 m/s, relative change 0.54%), a difference of doubtful clinical relevance. Under the constant inflow condition of this study, prostatic urethral diameter therefore has almost no effect on outlet velocity. Mean inlet pressure followed a U-shaped curve, reaching a minimum of 292.5 Pa at a prostatic urethral diameter ratio of 2.33. Turbulence, identified from elevated TKE and dissipation rate, appeared in the low-velocity zone at the dorsal aspect of the median lobe when the prostatic urethral diameter was ≥1.4 cm. Larger cavities were associated with higher turbulence intensity, greater TKE and dissipation rate, and longer stabilization time. Conclusions: In a single-patient, two-dimensional CFD model, turbulence emerged in the prostatic fossa once the prostatic urethral diameter exceeded 1.4 cm, and the inlet pressure required to maintain flow was minimal at a diameter ratio of 2.33. These findings suggest that an over-widened fossa is associated with higher inlet pressure and increased turbulence. Whether this translates into clinically meaningful differences in voiding requires validation under physiologic conditions, which were not simulated in this study. As a preliminary, constant-inflow, single-patient simulation, the results require prospective validation in larger cohorts with patient-specific three-dimensional reconstructions and physiologic pressure conditions.