Our study showed that the TRM can be significantly improved after vaginal repair and will gradually become thinner over time. The ultrasound pattern of the uterine scar was represented by a strongly echoic area and transferred to a highly echoic area, finally to the normal echo muscle texture at 3 months after surgery. Vaginal repair caused a change in the PSV value of the uterine artery, while the PI, RI and S/D values of the uterine artery showed little change.
The progression and healing of injury is a complex process. Immediately after an injury, the cells at the break of the muscular layer necrosis and the hematoma began to fill in the fissure of ruptured muscle fibers[21, 22]. At this time, a large number of inflammatory cells arrive at the injured site, and the inflammatory response starts rapidly, which is characterized by typical pain, swelling, redness and local temperature rise. Then, macrophages in circulation are recruited to play a role at the injury site. As the destruction stage gradually subsides, muscular layer healing enters the repair and remodeling stage. Repair and regeneration after injury are accompanied by two simultaneous processes of support and competition, the regeneration of muscle fibers and the formation of connective tissue scars[23]. Inflammatory cells form the early granulation tissue and compose a scaffold, and fibroblasts restore the framework by synthesizing proteins. With the continuous production of collagen, the regenerated muscle fiber matures, the scar tissue shrinks and recombines, and the function gradually recovers[24, 25]. Gradually, the cross-linking and shortening of collagen fibers promotes the formation of tight and strong scars[19].
TRMs significantly increases after vaginal repair, which is consistent with our previous research results[14, 17, 18, 20]. However, we found that TRMs gradually thin with the passage of time after surgery. During the third and final remodeling phases, newly regenerated muscle fibers are completed and the muscle reorganization leads to functional repair and a reduction in lesion size[26]. The thickness of the uterine scar gradually decreases. In the present study, the TRM on the 30th day after repair was 82% of the thickness on the second day and 67% on the 90th day. Within three months after surgery, the TRM was still significantly reduced, which was consistent with the pathophysiology of the healing process. In our previous study, we analyzed 121 patients with CSD, and the indices of the myometrium under ultrasound were stabilized 3 months after surgery[18]. It was further confirmed that the healing of the myometrium was a dynamic process and that the evaluation of postoperative efficacy should be based on three months after surgery.
Injury after vaginal repair is caused by resecting defect tissue and closing the muscular layer edge with a good blood supply again. Excision of the defect tissue can result in the destruction of the muscle continuity structure and damage to the vascular system, which could lead to hematoma formation[26]. Next, the classic inflammatory reaction occurs and results in irreversible tissue damage and cell necrosis[26]. The regeneration reaction causes the optimal reconstruction of damaged tissue and high-quality muscle scarring, restoring normal muscle function[23, 26]. Fibrous reactions lead to poor muscular reconstruction and inelastic scars. In this study, the uterine scar gradually evolved from a strong echoic area (the second day after the operation) to a highly echogenic area with an ill-defined heterogeneous myometrial texture (the first month after the operation) to a low (normal) echoic area, which is a normal healing process. However, fibrotic scars continue to show high echo areas, and there is a thinner TRM. Fibrosis indicates tissue dysfunction, which is characterized by changes in pain, inflammation and internal tissue stress. Some studies even suggest that fibrosis after injury may be the cause of recurrent muscle tears[27, 28]. Such healing results could lead to progressive loss of muscular function and a higher risk of uterine rupture during subsequent pregnancy or delivery.
After vascular repair, a complex angiogenesis process occurs. Vascularization or capillary regeneration of the injured area plays a very important role in injured muscle regeneration[23, 29, 30]. The reconstruction of a normal blood supply is the first sign of regeneration in the injured area and is also a necessary condition for the recovery of injured muscle morphology and function. New capillaries grow from the main blood vessel to the injured area and provide sufficient oxygen to the area[30]. Aerobic metabolism is the main energy pathway for muscle fiber growth. The arterioles in the injured area can expand and increase the blood flow to the injured area[26]. In our research, the PSV value of the uterine artery significantly increased from 50.35 on the first postoperative day to 53.92 on the 2nd day after repair, and the PSV value in the first postoperative month was significantly increased in the group with a menstruation duration of no more than 7 days. The change in PSV was reversible. There was no difference in PSV between the two groups at 3 months after the operation. The results showed that patients with good menstrual recovery (duration of menstruation no more than 7 days) had higher PSV values in the first month after surgery, which may be related to the higher blood flow rate of the uterus. More oxygen and nutrients were transported to support the growth of the myometrium and endometrium and to restore their normal function. However, the PSV between the two groups was almost the same in the third month after surgery, indicating that the change in blood flow was reversible. During this period, the PSV seemed to be stable regardless of whether the patient had a satisfactory duration of menstruation. Andrea Tinelli et al. prospectively evaluated vascular perfusion and healing of the myometrium after laparoscopic myomectomy using Doppler ultrasound and Doppler velocimetry in 149 women with uterine fibroids or leiomyomata[19]. They found that the PI and RI of the uterine arteries were reduced after laparoscopic myomectomy. However, in our study, except for PSV, the other blood flow parameters, such as the RI, PI and S/D value, did not change after surgery. Due to the formation of new blood vessels in the process of healing, the process is complex and phased[23, 26]. We found that the PSV value changed during the dynamic observation of myometrial healing and may be related to the recovery of the menstrual period after surgery. The combined use of drugs that increase the blood flow velocity of the uterine artery may be a method to improve the healing of defects after surgery.
The strengths of the study are summarized as follows. First, it involved prospectively collected data. Routine and dynamic ultrasound Doppler examinations were performed preoperatively and postoperatively. Second, this was the first study to use ultrasonic Doppler ultrasound and Doppler velocimetry to adequately monitor uterine scar healing after CSD vaginal repair. Third, only one professionally trained doctor measured the uterine artery blood flow and avoided measurement bias.
The uniqueness of the surgical methods was a limitation of our study. The current study only analyzed the myometrial healing and uterine artery blood flow after vaginal repair. However, other types of CSD repair methods have not been studied. In addition, the subsequent pregnancy outcome has not been well studied. This work was a preliminary investigation, and further prospective and mega data population studies are needed.