Laser treatment of female stress urinary incontinence: optical, thermal, and tissue damage simulations

Abstract

Treatment of female stress urinary incontinence (SUI) by laser thermal remodeling of subsurface tissues is studied. Light transport, heat transfer, and thermal damage simulations were performed for transvaginal and transurethral methods. Monte Carlo (MC) provided absorbed photon distributions in tissue layers (vaginal wall, endopelvic fascia, urethral wall). Optical properties (n,μa,μs,g) were assigned to each tissue at λ=1064 nm. A 5-mm-diameter laser beam and power of 5 W for 15 s was used, based on previous experiments. MC output was converted into absorbed energy, serving as input for ANSYS finite element heat transfer simulations of tissue temperatures over time. Convective heat transfer was simulated with contact cooling probe set at 0 °C. Thermal properties (κ,c,ρ) were assigned to each tissue layer. MATLAB code was used for Arrhenius integral thermal damage calculations. A temperature matrix was constructed from ANSYS output, and finite sum was incorporated to approximate Arrhenius integral calculations. Tissue damage properties (Ea,A) were used to compute Arrhenius sums. For the transvaginal approach, 37% of energy was absorbed in endopelvic fascia layer with 0.8% deposited beyond it. Peak temperature was 71°C, treatment zone was 0.8-mm-diameter, and almost all of 2.7-mm-thick vaginal wall was preserved. For transurethral approach, 18% energy was absorbed in endopelvic fascia with 0.3% deposited beyond it. Peak temperature was 80°C, treatment zone was 2.0-mm-diameter, and only 0.6 mm of 2.4-mm-thick urethral wall was preserved. A transvaginal approach is more feasible than transurethral approach for laser treatment of SUI.

Characteristics of Non‐Ablative Resurfacing of Soft Tissues by Repetitive Er:YAG Laser Pulse Irradiation

“…On a long temporal scale, the overall thermal effect of a Δ t = t s long sequence of short Er:YAG laser pulses is to heat up the tissue approximately to the same temperatures as if the laser energy was delivered to the tissue in a single t p = t s long pulse with the same pulse fluence as delivered during the sequence ( F p = F s ). Additionally, as the Er:YAG laser wavelength is limited to its very shallow optical penetration depth, it has been hypothesized that instead of having to rely on the pulse sequence heat‐pumping technique, devices with a deeper penetration depth and longer pulse durations may represent a more suitable means for deep thermal remodeling [ 58 ]. However, the published clinical results using the smooth‐resurfacing technique [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ], suggest that the superficial heat shocking resulting from individual short laser pulses within a SMOOTH sequence may represent an additional, indirect mechanism of action for regenerating epithelial and deeper‐lying connective tissues [ 36 , 37 , 38 , 39 , 40 , 41 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 ], which is complementary to the conventional direct slow stimulation of fibroblasts [ 68 ].…”

“…Additionally, as the Er:YAG laser wavelength is limited to its very shallow optical penetration depth, it has been hypothesized that instead of having to rely on the pulse sequence heat‐pumping technique, devices with a deeper penetration depth and longer pulse durations may represent a more suitable means for deep thermal remodeling [ 58 ]. However, the published clinical results using the smooth‐resurfacing technique [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ], suggest that the superficial heat shocking resulting from individual short laser pulses within a SMOOTH sequence may represent an additional, indirect mechanism of action for regenerating epithelial and deeper‐lying connective tissues [ 36 , 37 , 38 , 39 , 40 , 41 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 ], which is complementary to the conventional direct slow stimulation of fibroblasts [ 68 ].…”

Variable heat shock response model for medical laser procedures

“…Since the Er:YAG laser wavelength is limited to its very shallow optical penetration depth, it has been hypothesized that instead of having to rely on the SMOOTH mode heat pumping technique, devices with a deeper penetration depth may represent a more suitable means for deep thermal remodeling [64]. However, the excellent clinical results obtained with the Er:YAG laser wavelength, with a small number of reported complications, warrant further investigation of the potential mechanisms of action that make this laser so effective not only for ablative but also for non-ablative resurfacing.…”