Novel Anatomical Findings in Robot-Assisted Prostatectomy and Optimal Dissection Layer Selection for Preserving Urethral Support Structures.

Background: The role of preserving the superior fascia of the pelvic diaphragm in the context of urinary function remains unclear. This study aimed to investigate the anatomical relationship between the superior fascia of the pelvic diaphragm and the external urethral sphincter in terms of preventing postoperative urinary incontinence. We hypothesized that the external urethral sphincter would be supported by the fascia, smooth muscle, and levator ani muscle.

Methods: Three cadavers were used, and the pelvis was dissected to explore the superior fascia of the pelvic diaphragm, levator ani muscle, and external urethral sphincter. Tissue samples underwent wide-range serial sectioning, Masson's trichrome staining, and immunohistochemical staining for smooth muscle actin. Serial histological sections were reconstructed three-dimensionally.

Results: Macroscopic examination revealed the superior fascia of the pelvic diaphragm as a fibrous membrane covering the levator ani muscle. Histology identified interposing smooth muscle tissue between the levator ani muscle, superior fascia, and the external urethral sphincter. Three-dimensional reconstruction revealed this smooth muscle filling the space between the superior fascia and external urethral sphincter, extending medially and laterally, and connecting with surrounding structures.

Conclusions: This study clarified the anatomical details of smooth muscle tissue interposed between the superior fascia of the pelvic diaphragm, external urethral sphincter, and levator ani muscle. This smooth muscle, continuous with the superior fascia, likely forms a supportive structure stabilizing the external urethral sphincter, playing a crucial role in urinary continence. During robot-assisted radical prostatectomy, selection of the dissection plane should consider preservation of these supportive structures to maintain postoperative urinary function.