Three-dimensional ultrasonography study of female defecation disorder caused by changes of the puborectalis muscle

Objective To investigate the correlation between pelvic floor dyssynergia-type constipation and the puborectalis muscle using in a lithotomy position). Results (1) The puborectalis angles of the observation and control groups were not significantly different at rest [(85.664 ± 1.926)°, (85.813 ± 1.500)°] and during squeezing [(87.478 ± 2.125)°, (86.960 ± 1.751)°] (P > 0.05) but were significantly different when straining [(80.389 ± 2.268)°, (94.382 ± 1.540)°] (P < 0.05). The difference in the puborectalis angles of the two groups between rest and straining [(5.275 ± 1.236)°, (-8.569 ± 1.209)°] was statistically significant (P < 0.05). (2) The puborectalis thickness of the observation and control groups was not significantly different at rest ([3.994 ± 0.128) mm, (3.983 ± 0.091) mm] and during anal squeezing [(4.082 ± 0.154) mm, (4.126 ± 0.113) mm] (P > 0.05) but was significantly different when straining [(4.630 ± 0.199) mm, (4.296 ± 0.121) mm] (P < 0.05). The differences in the puborectalis thickness at rest and during straining in the observation and control groups were (-0.636 ± 0.217) mm and (-0.316 ± 0.089) mm, respectively, resulting in a statistically significant difference between the two groups (P < 0.05). Conclusion The puborectalis angle in patients with pelvic floor dyssynergia-type constipation was smaller than that in healthy volunteers during straining. The puborectalis thickness at the 6 o’clock position with the patient in a lithotomy position in patients with pelvic floor dyssynergia-type constipation was larger than that in healthy volunteers. Moreover, larger differences between rest and straining are associated with a more severe degree of constipation. detection angle, 360° and mm.


Introduction
Chronic constipation is a common disease and is usually divided into primary and secondary constipation, with a prevalence rate of 10-15% [1] . It can significantly affect the quality of life of patients.
Primary constipation is divided into outlet obstruction constipation and slow transit constipation [2] . A recent study reported that the occurrence of primary chronic constipation is correlated with changes in pelvic floor muscles [3] . The development and progress of modern imaging technology has led to increased study of pelvic floor dysfunction diseases, which has drawn increasing attention to "pelvic floor dyssynergia-type constipation" [4] . Defecography can only provide an evaluation of the contour, course and relationship with the surrounding organs and the motility function of the large intestine [5] . Magnetic resonance imaging (MRI) defecography (MRId) can provide morphological and functional images of the pelvic floor structures with good contrast with soft tissue, but patients with metal pacemakers and intrauterine contraceptive devices (IUDs) are contraindicated to this examination [6] . Benacerraf et al. [7] stated that three-dimensional ultrasonography imaging is the first choice for the study of the uterus, endometriosis and ovaries in women.
Three-dimensional ultrasonography can simultaneously display three cross-sections (sagittal, transverse and coronal sections) that are perpendicular to one another to obtain a complete threedimensional image of the pelvic hiatus. The reconstructed threedimensional images can be used to evaluate the intactness, thickness and volume of the levator ani of the patient [8] . Rotenberg et al. [9] suggested that transvaginal ultrasonography has certain limitations for pelvic floor examination because the ultrasound probe can cause pelvic floor dilation, deformation and muscle stretching while being placed in the vagina. This study used three-dimensional perineal ultrasonography to observe the relationship between pelvic floor dyssynergia-type constipation and changes in the puborectalis angle or puborectalis thickness at the pelvic floor at rest and during straining and squeezing.

II. Instruments and methods
A colour Doppler ultrasonographic diagnostic scanner (Pro  Tables 1 and 2. This study showed no significant difference in the puborectalis angle and puborectalis thickness between the observation and control groups during rest and anal squeezing (P > 0.05). The changes in the puborectalis angle and puborectalis muscle thickness were significantly different between the observation and control groups (P < 0.05). The differences in the puborectalis angle and puborectalis thickness between rest and straining between the observation and control groups were statistically significant (P < 0.05), and a greater difference was associated with an increased degree of difficulty of defecation (Figures 3 and 4).

Discussion
In recent years, the study of the relationship between pelvic floor dysfunction and functional constipation has become a focus of research [10] . In the past, pelvic floor dyssynergia-type constipation was attributed to rectal protrusion and rectal intussusception and collectively called pelvic floor dysfunction syndrome. According to clinical experience, the postoperative effect in most patients is still not satisfactory due constipation recurrence [11] , regardless of position of rectal mucosa suspension, range of tissue removal or enhanced repair of local muscle tissue. Due to the continuous development of three-dimensional ultrasonography techniques, this method has been increasingly applied to pelvic floor imaging [12] . The structures of the puborectalis muscle can be accurately assessed by three-dimensional perineal ultrasonography, which is convenient and non-invasive. The anatomical structures can be measured in the pubic symphysis, bilateral pubic ramus and medial and posterior edges of the puborectalis muscle [13,14] . Based on many clinical observations and data measurements, this study has shown that the puborectalis thickness at the 6 o'clock position (with patients in a lithotomy position) is an important factor. This study suggests that changes in the puborectalis angle and thickness may be important contributors to pelvic floor dyssynergia-type constipation.
The puborectalis muscle is a striated muscle comprised of stripshaped muscular fibres. The puborectalis muscle originates from the lower part of the pubic symphysis and is adjacent to the pubis. It extends posterior-inferiorly around the vagina or the prostate and merges to form an intact muscle behind the anorectal junction to retract the anorectal junction anteriorly to form an anorectal angle.
The puborectalis muscle is a powerful "U"-shaped "sling" and the most powerful part of the levator ani muscle [15,16] . The puborectalis pubococcygeus, and iliococcygeal muscles form the majority of the levator plate [17] . The puborectalis muscle is more important than the pubococcygeus muscle and the iliococcygeal muscle. Previously, the anorectal angle and levator plate were considered to be good parameters to evaluate the pelvic floor structure [18] , which plays an important role in defecation. However, this study suggested that the puborectalis muscle plays a decisive role in the evaluation of the pelvic floor structure. In normal defecation, the puborectalis muscle relaxes (i.e., increased puborectalis thickness), and the anorectal junction moves to the dorsal side in the sagittal plane to enlarge the anorectal angle and complete defecation. In contrast, in patients with pelvic floor dyssynergia-type constipation, the muscle movements are in the opposite direction [19] . In the 68 patients in the observation group, three-dimensional ultrasonography showed that the puborectalis angle was larger at rest than during straining. In the 67 patients in the control group, the puborectalis angle was smaller at rest than during straining, and only one patient had a larger puborectalis angle at rest than during straining. The difference in the puborectalis thickness between rest and straining was smaller in the observation group than in the control group.
Although the puborectalis angle is larger at rest than during straining, the puborectalis thickness at the 6 o'clock position is smaller at rest than during straining.
In females, the pelvic floor hiatus may be injured during vaginal delivery [20] . In the observation group, three-dimensional ultrasonography showed a hypoechoic area in the right aspect of the puborectalis muscle in multiparas, which may be the consequence of changes or avulsion in pelvic floor tissue caused by obstetric procedures. Studies have shown that nearly half of women with a history of vaginal delivery have anatomical changes in the puborectalis muscle [21,22] . A study by Serdar et al. [23] found that maternal overwork, oversized head circumference and an overweight of foetus can lead to puborectalis muscle injury, which causes changes in the pelvic hiatus, deformation of muscular fibres and muscle tone decline. In the observation group, threedimensional ultrasonography showed that the puborectalis muscle is discontinuous in some patients. This discontinuation is more common in the junction of the pubic symphysis and the right aspect of the puborectalis muscle [24] . Thus, the left and right sides of the anorectal junction were unevenly affected during defecation. The left aspect of the puborectalis muscle produces excessive retraction at the anorectal junction. This prevents the puborectalis angle from expanding, and the puborectalis muscle is unable to be completely relaxed. As a consequence, varying degrees of pelvic floor dyssynergia-type constipation may occur [10] . This study suggests that pregnancy or changes in puborectalis muscle continuity can

Abbreviations
Magnetic resonance imaging MRI Magnetic resonance imaging defecography MRId intrauterine contraceptive devices IUDs