Pelvic floor muscles (PFM) are innervated by branches of the pudendal nerves arising from the S2–S4 nerve roots of the sacral plexus, which has an important role in maintaining continence and pelvic support. Also, PFM strength can be one of the factors to improve urinary incontinence after Radical Prostatectomy. Our group previously reported that the reliability of PFM strength assessments for measuring anorectal pressure during PFM contractions in men has been confirmed [1]. To date, there have been several magnetic resonance imaging (MRI) researches on the anatomy and features of the male pelvic floor including levator ani muscle thickness, and membranous urethral length (MUL). A previous study stated that the changes in puborectalis muscle thickness and the position of the bladder neck may contribute to continence after radical prostatectomy [2]. However, the relationships between morphological parameters and PFM strength remain understudied in men. This study aimed to investigate a quantitative evaluation of the morphological features of PFM using MRI and anorectal pressure in men.

Forty-nine men with prostate cancer who had undergone Robotic-Assisted Laparoscopic Prostatectomy (RARP) (age: median 70, range 52-82 years old; body mass index: median 23, range 17-32 kg/m2) volunteered for the current study. Pelvic T2-weighted MRI and PFM strength were performed during the preoperative periods. The expert radiologist (K.F.) reviewed MRI blinded to any clinical data, in consensus. The following MRI parameters were obtained: (1) the thickness of the levator ani muscle in coronal (R/L), (2) the distance at the most convex part of the levator ani muscle (R/L): the distance at the most convex part from a straight line connecting the origin and insertion of the levator ani muscle, (3) the thickness of puborectalis muscle, (4) MUL in coronal, (5) MUL in sagittal (6) the position of the bladder neck in relation to the pubic bone, (7) the thickness of obturator internal muscle (R/L) (8) urethra wall thickness. A manometer with an anal sensor (PeritronTM cat 9300A; Laborie, Canada) was used for quantitative PFM strength assessment. All participants were lying on the bed in a lateral position and a pillow was placed under the head. An assessor used standardized instructions to “squeeze and lift or tighten and pull up the PFM as hard as you can” to assess the maximum strength of the PFM. This study was approved by our institutional ethics committee. Statistical analysis was performed using GraphPad Prism 8.4.3 (San Diego, USA). Data distribution was assessed with the Shapiro-Wilk test for continuous variables. We used the Pearson correlation coefficient with normally distributed data and the Spearman's rank correlation coefficient with not normally distributed data for measuring the strength of a linear association between two variables. For between-group comparisons, we used the Mann–Whitney U test with not normally distributed. The significance level was set at p<0.05.

Pelvic MRI parameters and anorectal pressure are listed in Table 1. There was a significant negative correlation between PFM strength and the distance at the most convex part in both right (median 0.88, range -7.96 - 6.88 mm, r=-0.306, 95% confidence interval -0.540 to -0.027, p=0.033) in figure 1 and left (median 1.32, range -8.12 - 9.76 mm, r=-0.297, 95% CI -0.534 to -0.017, p=0.038) in figure 2. There were no significant differences between PFM strength and other pelvic MRI parameters. The distance at the most convex part on the right side in the aged <65 years old group was significantly higher than in the aged ≥65 years old group (median 3.325, range -3.760 - 6.880 mm and median 0.690, range -7.960 - 5.120 mm, respectively). Also, the distance at the most convex part on the left side in the aged <65 group was significantly higher than in the aged ≥65 group (median 3.350, range -4.860 - 9.760 mm and median 1.120, range -8.120 - 5.310 mm, respectively).

To our knowledge, this is the first study to investigate the relationships between a quantitative evaluation for the morphological features of PFM using magnetic resonance images (MRI) and anorectal pressure in men. We found that there was a significant negative correlation between PFM strength and the distance at the most convex part in both the right and left sides of the levator ani muscle. The higher PFM strength showed the lower distance at the most convex part from a straight line connecting the origin and insertion of the levator ani muscle. Our findings suggest that the distance at the most convex part of the levator ani muscle can be a surrogate parameter of PFM strength. Moreover, the morphological features of the levator ani muscle are associated with aging. We found levator ani muscle in ≥65 years was more downward compared to < 65 years. A previous study also showed the distance at the most convex part in < 65 years was significantly higher than in ≥65 years (mean 3.6 range −8.1 – 11.6 mm and mean 0 range −10.5 – 11.9 mm) [3]. The results of the current study agreed with previous studies.

The results of the current study showed that pelvic parameters measured by MRI may be useful to estimate PFM strength in men.

Neurourol Urodyn. 2020;39(5):1464–71
J Magn Reson Imaging. 2014;39(6):1431–5
Geriatr Gerontol Int. 2019;19(8):834–7

Continence 2S2 (2022) 100296
DOI: 10.1016/j.cont.2022.100296