Fecal incontinence (FI) is a non-fatal condition characterized by the involuntary discharge of liquid or solid stool, or both, without the individual's control. It is the most prevalent complication faced by patients with this condition.^1,2 Epidemiological studies have shown that FI affects 2%–15% of the population,^3,4 with prevalence increasing with age and being notably higher among women (13%–23%).⁵ More than 70% of patients with FI avoid discussing their condition to protect their employment, suggesting that the reported prevalence is lower than the actual rate. Beyond the physical symptoms, FI can have devastating psychological effects, including shame, job loss, loss of social relationships, and ultimately, a decrease in self-confidence.^6,7 The etiology of FI involves various factors, including dysfunction of the anal sphincter, sensory dysfunction in the rectal and anal areas, alterations in the passage speed of colon contents, changes in rectal capacity, and neurological and cognitive disorders.⁸ Dysfunction of the external anal sphincter (EAS) can be due to congenital factors such as anorectal anomalies⁹ or trauma from events like childbirth or anal fistulas.¹⁰ These issues can lead to decreased mechanical strength of the sphincter and, ultimately, an inability to control the anal canal. The primary treatment for FI related to anatomical defects of the sphincter is sphincteroplasty surgery.¹¹ Unfortunately, this surgery is often not very effective, particularly over the long term.^12,13 Alternative therapies, such as artificial sphincters and bulking agents, also present challenges due to the risk of device failure, embolism, and granuloma formation.^1,14,15 Therefore, therapeutic strategies such as stem cell injections, which promote the repair of sphincter muscle tissue,^16,17 can be considered a definitive treatment with satisfactory long-term results for patients with FI due to anal sphincter defects. Another promising treatment strategy is the use of low-level laser (LLL) therapy in muscle tissue repair. The effectiveness of LLL in repairing muscle tissue is attributed to the activation of satellite cells embedded in the basement membrane of muscle cells (muscle fibers),^18,19 anti-apoptotic properties,²⁰ anti-inflammatory properties,²¹ and stimulation of the secretion of cytokines involved in myogenesis (such as fibroblast growth factor and insulin-like growth factor-1).²².²³ In animal models, LLL has been shown to increase both resting and maximum squeeze pressures in the anal sphincter and to treat FI by promoting angiogenesis, reducing collagen synthesis, enhancing the expression of actin and myosin genes, and ultimately, promoting myogenesis.²⁴ Considering the efficacy of LLL in repairing muscle tissue, the aim of the present clinical trial was to evaluate the effect of LLL on repairing the EAS and treating FI.

Study design

The present study was a double-blind randomized controlled clinical trial conducted between 2016 and 2018. It aimed to investigate the effect of LLL therapy on the repair of the EAS and the treatment of FI in 30 patients with EAS defects. These patients were referred to Hazrat Rasoul Akram Hospital in Tehran, Iran, in accordance with the Consolidated Standards of Reporting Trials guideline (Fig. 1). All participants underwent sphincteroplasty and were consecutively enrolled in the study. The patients were divided into two groups: one underwent sphincteroplasty alone (control group), and the other received sphincteroplasty combined with laser therapy (LLL group). The stages of this clinical trial received approval from the Ethics Committee of Iran University of Medical Sciences (IR.IUMS.REC.1395.28976) and were registered in the Iranian Registry of Clinical Trials (IRCT) (IRCT2016092830033N1). Throughout the study, researchers adhered to the ethical principles outlined in the Helsinki Declaration, and all participants provided written informed consent before enrollment. All authors had access to the study data, and reviewed and approved the final manuscript.

Figure 1. Consolidated Standards of Reporting Trials flow diagram of the present study.

Patients and grouping

This study involved 30 patients with FI due to an EAS defect. FI was confirmed in these patients through physical examination and Wexner score assessment.²⁵ The presence of an EAS defect was further verified using both 2D and 3D endorectal ultrasound with a 360-degree probe (BK Pro Focus type 2202; GE HealthCare). The patients were randomly assigned to one of two groups, each comprising 15 individuals: Group A, the control group, underwent sphincteroplasty, and Group B, the laser group, received both sphincteroplasty and laser therapy. Eligible participants were over 18 years old, had a Wexner score (indicating the severity of FI) of 8 or higher, had an EAS defect confirmed by endorectal ultrasound, and had provided personal consent. Exclusion criteria included pregnancy or lactation, a history of artificial sphincter use, vaginal delivery within the past six months, chronic diseases, autoimmune diseases, and mental disorders that could interfere with the study process. Randomization was conducted using a permuted block method (block size 3) via a computer program. The randomization and enrollment of patients into the study groups, along with data collection and interpretation, were carried out by separate specialist physicians who were blinded to the study assignments.

Sphincteroplasty

The classical method of sphincteroplasty was performed as follows: The wound was opened, and all scar tissue was completely removed. The two ends of the sphincter were then exposed—due to the annular nature of the sphincter, releasing two ends is necessary following amputation from trauma. After rinsing with normal saline, the ends were aligned without stretching the sphincter. They were sutured using 3-0 polydioxanone in an end-to-end fashion. Internally, the repair site was covered with an endorectal flap, while externally, it was left open to prevent infection, allow for monitoring, and ensure adequate drainage. The site was padded with normal saline-soaked dressings, with daily changes and washes until the wound fully closed. No antibiotics or antiseptics were used during the procedure. All surgeries were conducted by the same individual, a colorectal surgery fellow, under consistent conditions (time of day, operating room environment) for both the control and laser groups.

Intervention (laser therapy)

In this study, a continuous wave diode laser with a wavelength of 660 nm and a power of 100 mW (ME-TL10000-SK; Heltschl Medizintechnik GmbH). The laser was mounted on an adjustable metal arm, maintaining a fixed distance of 2 cm between the radiation source and the lesion site. Immediately following sphincteroplasty, the lesion was exposed to laser irradiation for 90 seconds. Subsequently, laser treatment was administered daily for 2 weeks.

Outcomes

Wexner score

To evaluate muscle function and the severity of FI, the Wexner score of patients was recorded at three time points: immediately after sphincteroplasty, and then three and six months post-surgery. This assessment, based on patient history and the Wexner table,²⁵ was conducted by another colorectal surgeon. The scores range from 0 to 20, where a score of zero indicates complete control of defecation, and a score of 20 signifies complete FI.

Endorectal sonography

To assess the integrity of the sphincter and identify the presence of muscle or collagen at the sphincteroplasty site, endorectal sonography was conducted three months post-surgery. During this procedure, three images of the lesion were captured: the first from the deepest part, the second from the middle, and the third from the superficial part. The proportion of muscle (represented by white pixels) was quantified using ImageJ/Fiji 1.46 software. This was done by calculating the ratio of the area covered by white pixels to the total area of the lesion, then multiplying by 100. The average of these ratios was computed for each patient, based on the three sonography images, and subsequently for each group.

Electromyography

Three months post-sphincteroplasty, electromyography (EMG) was conducted by a neurologist (who was blind to the study) using the Synergy on Nicolet EDX System (Natus Medical Inc.). The procedure utilized a disposable needle (gauge: 30, diameter: 0.3 mm, length: 25 mm, and recording area: 0.02 mm²; Ambu). The sweep setting of the device was adjusted to 10 ms/cm, and the sensitivity was set between 100 mV and 200 mV. During the EMG, patients were positioned on their left side with their knees and hips bent. The EMG needle was inserted to a depth of 4 mm at the junction of the skin and anal mucosa and positioned within the EAS. To evaluate motor unit action potentials, patients were instructed to contract and then relax their sphincter, allowing for the assessment of spontaneous activity.

Sample size calculation

The strategy for determining the sample size was based on studies comparing the mean outcome of a standard treatment method with that of a new treatment method. Previous research indicated that the standard deviation for the area of the injured muscle was 5.1 in the control group and 3.2 in the group treated with laser therapy. The average area of injury was 14 mm² in the control group and 8.5 mm² in the laser-treated group, respectively.²⁶ Considering an alpha of 0.05, a power of 90%, and an anticipated dropout rate of about 20%, the required sample size was calculated to be 15 patients per group, totaling 30 patients.

Statistical analysis

Data analysis was conducted using Stata 14.0 software (Stata Corp.). Quantitative data were presented as mean and standard deviation, while qualitative data were expressed as frequency and percentage. Due to the non-normal distribution of the data, non-parametric methods were employed. The Fisher’s exact test was utilized to compare qualitative variables between the groups, and the Mann-Whitney U test was applied to assess differences in age and the percentage of muscle at the injury site post-intervention. Analysis of covariance was used to compare the Wexner scores post-intervention, as there were variations in the Wexner scores before the intervention across the groups. A P-value of less than 0.05 was considered statistically significant.

Demographic data

In this clinical trial, 30 patients were randomly assigned to either a control group or a laser treatment group, as depicted in Fig. 1. The average age was 45.7 ± 7.8 years in the control group and 43.9 ± 8.4 years in the laser group (P = 0.533). Of the participants, 29 were male. In terms of body weight, 12 patients (80%) in the control group and 14 patients (93.3%) in the laser group were classified as overweight or obese (P = 0.763). There were no significant differences between the groups regarding educational level (P = 0.265), history of pelvic surgery (P = 0.690), abscesses (P = 0.682), hemorrhoidectomy (P = 0.483), diabetes (P > 0.999), asthma (P > 0.999), smoking history (P > 0.999), or alcohol consumption (P > 0.999). Anal sphincter ultrasound revealed that 13 patients (7 in the control group and 6 in the laser group) had experienced sphincter ruptures. Anal fistulas were observed in 2 patients from the control group and 4 from the laser group. Notably, 6 patients in the control group and 5 in the laser group presented with both rupture and fistula simultaneously. The ultrasound findings were similarly distributed between the two groups (P = 0.796). The frequency of bowel movements per week (P = 0.592), the number of FI episodes per week (P = 0.801), and the distribution of preoperative complications (P > 0.999) did not differ significantly between the groups. Regarding the Wexner score, which measures the severity of FI, 13 patients in each group (86.7%) had scores ranging from 6 to 10. Additionally, 2 patients in each group scored between 11 and 15 (P > 0.999). Table 1 provides a comparison of the distribution of these variables before surgery between the two groups.

Table 1 . Baseline Characteristics of the Patients.

Variable	Control (n = 15)	Laser (n = 15)	P-value
Age (yr)	45.7 ± 7.8	43.9 ± 8.4	0.533
Sex
Male	14 (93.3)	15 (100)	> 0.999
Female	1 (6.7)	0 (0.0)
BMI (kg/m2)	28.7 ± 5.2	29.0 ± 5.1	0.864
Normal (18.9–24.9)	3 (20.0)	1 (6.7)	0.763*
Overweight (25–29.9)	8 (53.3)	9 (60.0)
Obese (≥ 30)	4 (26.7)	5 (33.3)
Educational status
Illiterate	1 (6.7)	1 (6.7)	0.265
Primary school	1 (6.7)	4 (26.7)
Under diploma	6 (40.0)	4 (26.7)
Diploma	1 (6.7)	3 (20.0)
Graduate	5 (33.3)	3 (20.0)
Master	1 (6.7)	0 (0.0)
Normal vaginal delivery	1 (6.7)	0 (0.0)	> 0.999
Past medical history
History of pelvic surgery	5 (33.3)	4 (26.7)	0.690
Colostomy	0 (0.0)	0 (0.0)
Abscess	3 (20.0)	4 (26.7)	0.682
Fistula	2 (13.3)	0 (0.0)
Rectum cancer	0 (0.0)	0 (0.0)
Hemorrhoidectomy	2 (13.3)	0 (0.0)	0.483
Diabetes	1 (6.7)	0 (0.0)	> 0.999
Asthma	1 (6.7)	0 (0.0)	> 0.999
Urinary incontinence	0 (0.0)	0 (0.0)
Previous treatment for FI	0 (0.0)	0 (0.0)
Drug abuse
Current smoker	3 (20.0)	3 (20.0)	> 0.999
Ex-smoker	1 (6.7)	0 (0.0)	> 0.999
Pack-year smoking	4.5 ± 3.5	4.5 ± 3.3	0.996†
Alcohol use	1 (6.7)	1 (6.7)	> 0.999
Opium consumption	0 (0.0)	0 (0.0)
Current disease characteristics
Defecation frequency per week	14.7 ± 5.4	13.7 ± 4.7	0.592†
FI frequency per week	1.0 ± 1.6	0.9 ± 1.3	0.801†
Anal sphincter sonography
Rupture	7 (46.7)	6 (40.0)	0.796*
Fistula	2 (13.3)	4 (26.7)
Rupture and fistula	6 (40.0)	5 (33.3)
Pre-surgery complications
Bleeding alone	8 (53.3)	7 (46.7)	> 0.999*
Incomplete defecation	0 (0.0)	1 (6.7)
Mucus defecation	0 (0.0)	1 (6.7)
Bleeding and incomplete defecation	0 (0.0)	1 (6.7)
Bleeding and perineal pressure	6 (40.0)	5 (33.3)
Perineal pressure and mucus defecation	1 (6.7)	0 (0.0)
Pre-surgery Wexner score	9.1 ± 1.2	9.0 ± 1.0	0.87†
0–5	0 (0.0)	0 (0.0)	> 0.999
6–10	13 (86.7)	13 (86.7)
11–15	2 (13.3)	2 (13.3)
16–20	0 (0.0)	0 (0.0)

Values are presented as mean ± standard deviation or number (%).

BMI, body mass index; FI, fecal incontinence.

*Based on the Fisher’s exact test.

^†Based on the Mann-Whitney test.

Patterns of Wexner score changes in the two groups

Prior to surgery, the Wexner score of 13 patients in both groups ranged from 6 to 10. Additionally, two patients in each group had Wexner scores between 11 and 15 (P > 0.999). The day after surgery, all 30 patients in both groups achieved Wexner scores between 0 and 5, indicating a 100% success rate for the treatment as a result of the surgery in both groups (P = 0.993). This 100% improvement in Wexner scores was also observed in the second week of follow-up (P = 0.819). Three months post-surgery, the laser group continued to show improvement, whereas the Wexner scores worsened in two patients in the control group. Notably, at the 3-month follow-up, the mean Wexner score in the control group (3.9 ± 1.5) was significantly higher than in the laser group (1.1 ± 1.2) (P < 0.0001). At the 6-month follow-up, the recovery rate in the laser group remained at 100%, while the recovery rate in the control group dropped to 46.7%. The mean Wexner score at this time was 5.6 ± 1.7 in the control group and 1.3 ± 1.3 in the laser group (P < 0.0001) (Table 2 and Fig. 2). A non-parametric test for trend analysis indicated that the 6-month recovery process of Wexner scores in the laser group was significantly better than in the control group (P < 0.0001).

Table 2 . Trends in the Wexner Score During the Study.

Wexner score	Control (n = 15)	Laser (n = 15)	P-value*
Pre-surgery	9.1 ± 1.2	9.0 ± 1.0	> 0.999
0–5	0 (0.0)	0 (0.0)
6–10	13 (86.7)	13 (86.7)
11–15	2 (13.3)	2 (13.3)
16–20	0 (0.0)	0 (0.0)
Post-surgery	1.5 ± 2.2	1.2 ± 1.7	0.993
0–5	15 (100)	15 (100)
6–10	0 (0.0)	0 (0.0)
11–15	0 (0.0)	0 (0.0)
16–20	0 (0.0)	0 (0.0)
Two weeks	1.3 ± 1.2	1.8 ± 1.6	0.819
0–5	15 (100)	15 (100)
6–10	0 (0.0)	0 (0.0)
11–15	0 (0.0)	0 (0.0)
16–20	0 (0.0)	0 (0.0)
Three months	3.9 ± 1.5	1.1 ± 1.2	< 0.0001
0–5	13 (86.7)	15 (100)
6–10	2 (13.3)	0 (0.0)
11–15	0 (0.0)	0 (0.0)
16–20	0 (0.0)	0 (0.0)
Six months	5.6 ± 1.7	1.3 ± 1.3	< 0.0001
0–5	7 (46.7)	15 (100)
6–10	8 (53.3)	0 (0.0)
11–15	0 (0.0)	0 (0.0)
16–20	0 (0.0)	0 (0.0)

Values are presented as mean ± standard deviation or number (%).

*Based on two-way repeated measures analysis of variance (ANOVA).

Figure 2. Trends in the Wexner score in the surgery-alone and surgery + laser groups during follow-up. (A) Individual data and mean ± standard deviation of the Wexner score in the patients. Two-way repeated-measures analysis of variance showed a significantly lower Wexner score in the surgery + laser group than in the surgery-alone group at 3 months (P < 0.001) and 6 months (P < 0.001) after surgery (df: 4, 112; F = 17.2; P < 0.0001). (B) Comparison of the success rate in the surgery-alone and surgery + laser groups at different time (per week) points. The non-parametric test for trend showed a significantly higher success rate in the patients who received surgery + laser treatment (P < 0.0001). The success rate was defined as at least a 1-category improvement in Wexner score (see the Methods section).

Complications after surgery in the two groups

Complications observed during the study included pain and burning, FI, constipation, flatus, watery stools, hard stools, and recurrence. The most common complications observed in both groups during the second week were pain and burning (86.7% in the control group vs. 73.3% in the laser group), FI (73.3% in both groups), and flatus (73.3% in both groups). The prevalence of these complications was similar in both groups. However, by the third month of follow-up, the prevalence of complications in the laser group was significantly lower than in the control group. All 15 patients in the control group complained of FI in the third month, compared to 53.3% in the laser group (P = 0.006). Flatus was observed in all 15 patients in the control group, whereas it was reported in 46.7% of the laser group (P = 0.002). Watery stools were also noted in six patients (40.0%) in the control group and one patient (6.7%) in the laser group (P = 0.080). Only one recurrence of the disease was observed in the control group 3 months after surgery. Six months after surgery, the most common complications in the control group were FI (100%), flatus (100%), and watery stools (80%). In contrast, only 66.7% of patients in the laser group suffered from FI and flatus, and one patient complained of watery stools. The prevalence of FI (P = 0.042), flatus (P = 0.042), and watery stools (P < 0.001) in the laser group was much lower than in the control group (Table 3).

Table 3 . Length of Stay and Complications During Follow-Up.

Outcome	Surgery alone (n = 15)	Surgery + laser (n = 15)	P-value
Length of stay (day)	6.0 ± 2.0	3.4 ± 1.9	0.001*
Two weeks
Pain and burning	13 (86.7)	11 (73.3)	0.651†
Fecal incontinence	11 (73.3)	11 (73.3)	> 0.999†
Constipation	0 (0.0)	0 (0.0)
Flatus	11 (73.3)	11 (73.3)	> 0.999†
Liquid stool	0 (0.0)	3 (20.0)	0.224†
Solid stool	0 (0.0)	0 (0.0)
Recurrence	0 (0.0)	0 (0.0)
Three months
Pain and burning	0 (0.0)	1 (6.7)	> 0.999†
Fecal incontinence	15 (100)	8 (53.3)	0.006†
Constipation	0 (0.0)	0 (0.0)
Flatus	15 (100)	7 (46.7)	0.002†
Liquid stool	6 (40.0)	1 (6.7)	0.080†
Solid stool	2 (13.3)	0 (0.0)	0.483†
Recurrence	1 (6.7)	0 (0.0)	> 0.999†
Six months
Pain and burning	0 (0.0)	0 (0.0)
Fecal incontinence	15 (100)	10 (66.7)	0.042†
Constipation	0 (0.0)	0 (0.0)
Flatus	15 (100)	10 (66.7)	0.042†
Liquid stool	12 (80.0)	1 (6.7)	< 0.001†
Solid stool	0 (0.0)	0 (0.0)
Recurrence	0 (0.0)	0 (0.0)

Values are presented as mean ± standard deviation or number (%).

*Based on the Mann-Whitney test.

^†Based on the Fisher’s exact test.

Comparison of muscle mass and EMG between the study groups

The results showed that the area occupied by muscle in the laser group was significantly larger than that in the control group (P = 0.008). EMG from the lesion site in the control group revealed minimal action potential, suggesting a high presence of fibrotic tissue and low muscle tissue. Conversely, in the laser group, EMG demonstrated a substantial amount of action potential, indicating a higher proportion of muscle tissue and lower fibrosis at the sphincteroplasty site (Fig. 3).

Figure 3. Endorectal sonography and electromyography findings 3 months after surgery. The ratio of the area occupied by the muscles. (A) Surgery alone and (B) surgery + laser; the area marked with a yellow line indicates the location of the lesion. The number of white pixels representing muscle tissue is higher in the laser group than in the control group. (C) Percentage of area occupied by the muscle in the surgery group and surgery + laser group (P = 0.008). (D) Sample electromyography of the surgery + laser group and surgery alone group.

Sphincteroplasty, as the primary treatment for FI, yields satisfactory short-term results; however, after 2 years, patients’ FI symptoms tend to recur.^13,27 Because the fibrous tissue that develops at the site of the sphincter lesion is not as functional as the muscle tissue, replacing the fibrous tissue with muscle tissue may lead to better and more permanent long-term results. Due to the ability of LLL to promote myogenesis,¹⁸.¹⁹ this clinical trial was conducted on patients with FI resulting from EAS injury to achieve lasting therapeutic effects. In this trial, 660 nm laser irradiation was applied at the injury site as a treatment method.

The effectiveness of LLL therapy, which uses wavelengths ranging from 600 nm to 1,070 nm, varies based on the damage's location and depth. In our study, the patients had damage not only to the sphincter, resulting in FI, but also to superficial tissues such as the skin and mucosa. Research by Chung et al²⁸ has suggested that the optimal wavelength for repairing surface tissues is between 600 nm and 700 nm. Many studies have considered 660 nm to be the best laser wavelength for muscle repair,^29–31 because this wavelength promotes anti-inflammatory responses, angiogenesis, fibroblast proliferation, and cytokine secretion, which are not observed at higher wavelengths, such as 940 nm and 808 nm.³² Therefore, we selected a 660 nm wavelength for our study. The results indicated that in the laser therapy group, compared to the control group, there was a significant increase in muscle bulk post-sphincteroplasty at the lesion site. Furthermore, the increase in the Wexner index, indicating a worsening of FI, observed in the control group was not only absent in the laser therapy group but also significantly lower at the same time points.

The resting and squeeze pressures in the anal canal, which are maintained by the contractile tone of the EAS and internal anal sphincter (IAS), play a crucial role in controlling defecation. The EAS, being a voluntary (striated) muscle, has a more significant role in controlling excretion than the IAS, an involuntary (smooth) muscle.³³ Therefore, in our study, damage to these muscles, especially the EAS, in patients in both groups, led to a sharp increase in the Wexner index. Immediately after sphincteroplasty (before LLL therapy) in both groups, the Wexner index decreased significantly compared to the measurements before surgery. In the control group, this significant decrease was not stable for more than 2 weeks; thus, by the sixth month, the Wexner index had gradually increased to such an extent that the FI status was almost similar to that before sphincteroplasty. This upward trend in the Wexner index (indicating a worsening of FI status) confirms the short-term efficacy of sphincteroplasty in the treatment of FI and the lack of long-term results. An explanation for this short-term efficacy is that after sphincteroplasty in the anal canal, a mechanical (not functional) barrier is created by the formation of fibrous tissue rich in collagen fibers at the site of injury to the sphincter. Over time, the degeneration of collagen fibers causes the fibrous tissue to lose its strength and mechanical properties. In contrast, in the laser therapy group, the improvement in stool control and the reduction in the Wexner index observed post-sphincteroplasty were maintained at the second week, third week, and sixth month post-surgery, with a significantly lower Wexner index than that of the control group at the same time points. Endorectal ultrasound analysis showed a significant increase in muscle bulk in the laser group compared to the control group, likely due to the effect of LLL in replacing fibrous tissue with muscle, which unlike fibrous tissue, has contractile function. EMG findings in the laser group also confirmed muscle contractility and presence by recording the action potentials of motor units.

The role of LLL therapy in muscle regeneration is underscored by its stimulation of satellite cell proliferation and differentiation,³⁴ as well as increases in myofibrils,³⁵ antiapoptotic properties,³⁴ anti-inflammatory properties²¹ and angiogenesis.¹⁹ Our results further confirm LLL’s efficacy in muscle repair by showing increased muscle mass in the laser group compared to the control group. Additionally, LLL can modulate collagen content in injured areas through nuclear factor kappa B (NF-κB) gene expression, rearrangement of collagen I and III³⁶ and reduction of collagen content in skeletal muscle.³⁷ This ability to reduce and rearrange collagen fibers also contributes to the improved ability of the EAS to control stool and to the improved Wexner index in our study.

One limitation of this study is its execution in a single treatment center, which limits the generalizability of the results. Therefore, it is recommended that future studies be designed and conducted across multiple centers. In this study, the trend of FI was evaluated using the Wexner index based on patient statements. It would have been beneficial to include manometric analysis to record the contractile tone of the EAS; thus, future studies should consider this in their design. Based on our findings, we suggest that different protocols for laser irradiation, including variations in the duration per session or adjustments in wavelength, be explored in future experiments.

The findings of this study indicate that two weeks of low-power laser therapy can significantly increase muscle mass at the site of EAS injury in patients with FI undergoing sphincteroplasty. This intervention is safe, with no side effects reported, and effectively leads to significant, long-term improvements in FI. Therefore, it may be considered as a treatment protocol for these patients.

This study was supported by a grant from the Iran University of Medical Sciences (grant number: 95-02-182-28976).

The datasets used during the current study are available from the corresponding author on reasonable request.

The authors would like to thank Dr. Naser Amini of Iran University of Medical Sciences for kindly revising the article.