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Factors to Consider in Augmentation Urethroplasty with Oral Mucosa Graft or Penile Skin Flap for Anterior Urethral Stricture: A Systematic Review and Meta-analysis
Oral mucosa graft (OMG) and penile skin flap (PSF) are common substitutions in urethroplasty; however, the recommended substitution for anterior urethral strictures remains uncertain.
Objective
To compare the efficacy of OMG and PSF in anterior urethral strictures in terms of success rate and prevalence of postvoid dribbling based on current studies.
Evidence acquisition
A systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) and registered at PROSPERO (ID: CRD42022313879). All publications until March 1, 2022, were searched in the PubMed, EMBASE, and Cochrane Library databases without any restriction. Studies that focused on patients with anterior urethral strictures undergoing single-stage augmentation urethroplasty with OMG and PSF, and reported comparable success rates between the two substitutions were included.
Evidence synthesis
Thirteen studies involving a total of 1216 patients were included in the screening procedures, and 12 studies were eventually included in the meta-analysis. No significant difference in success rates was identified between OMG and PSF (odds ratio [OR]: 1.41, 95% confidence interval [CI]: 0.96–2.07, p = 0.08). No significant difference was observed in the comparison of success rates in penile urethral strictures (OR: 0.95, 95% CI: 0.53–1.70, p = 0.86) and in the comparison of postvoid dribbling (OR: 0.59, 95% CI: 0.31–1.11, p = 0.10). However, a subgroup analysis suggested that OMG had a higher success rate than PSF in studies with the top 50% sample size (six studies, OR: 1.678, 95% CI: 1.055–2.668, p = 0.029) and the top 50% follow-up period (five studies, OR: 2.279, 95% CI: 1.193–4.352, p = 0.013).
Conclusions
OMG provides the same success rate and postvoid dribbling as PSF. However, based on the existing evidence, OMG is more likely to perform better in a cohort with long-term follow-up and a relatively large sample size. More studies on the two substitutions are necessary to evaluate the factors of urethroplasty success rate, performance of substitutions in penile urethral strictures, and indicators of quality of life.
Patient summary
In this research, we compared the outcomes of oral mucosa graft (OMG) and penile skin flap for urethroplasty in anterior urethral stricture patients in 13 studies. We found that these were similar in terms of success rate and postvoid dribbling. However, OMG could probably provide a higher success rate when the studies had more patients or a longer follow-up period.
Urethral strictures are among the most common urological conditions. This refers to the process of fibrosis and scarring of the urethral mucosa and surrounding spongiosum, resulting in a narrow segment of the anterior urethra. The etiology of urethral strictures varies between different regions, and this includes socioeconomic factors, exposure to health care interventions, infrastructure, genetics, and environment [
Urethral strictures can be divided into anterior and posterior according to their location. Anterior urethral strictures accounts for a high percentage between 46.2% and 92.2% in different centers [
]. Anterior urethral strictures can further be classified into meatal, penile, bulbar, and panurethral strictures. Augmentation urethroplasty, which is a common treatment for penile and bulbar strictures, requires high-level surgical skills and brings better prognosis and a lower stricture recurrence rate [
]. Penile skin flap (PSF) and oral mucosa graft (OMG) are the most frequently used techniques, and these are both recommended therapies in the guidelines because no current evidence suggests that a certain substitution is better than the other [
]. PSF has been considered to perform stably as a mature substitution. On the contrary, OMG is used in most bulbar stricture cases, as there can be some postoperative complications such as fistula formation and long-term complications such as sacculation in cases using PSF [
]. However, few studies have compared the efficiency of the two techniques in penile urethral strictures; therefore, the recommended substitution in penile urethral strictures remains unknown. Thus, more studies are still required to explore which of the two methods is better in anterior urethral strictures, especially penile urethral strictures.
We have focused on penile and bulbar strictures, and observed that previous primary and secondary evidence exists to evaluate OMG and PSF in urethroplasty for anterior urethral strictures. However, while surgeons pay more attention to the success rate of surgery, patients are most worried about their feelings, functional improvement, and quality of life, including postvoid dribbling and sexual function. These indicators strongly influence the selection of different substitutions [
]. Therefore, the purpose of this meta-analysis and systematic review was to compare the efficacy of the two substitutions in anterior urethral strictures, including the success rate, which consists of the whole success rate and the success rate in penile urethral strictures, and the prevalence of postvoid dribbling based on current studies. Furthermore, potential influencing factors of success rates were also taken into consideration, including study type, publication date, sample size, definition of success or failure, and follow-up period.
2. Evidence acquisition
2.1 Literature search and eligibility criteria
The systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines and registered at PROSPERO (ID: CRD42022313879). We searched articles in three electronic databases: PubMed, EMBASE, and Cochrane Library. The search keywords used were “mouth mucosa,” sed were “mo,” sed were “mout,” sed were “mo,” sed were “mouth muc,” and were “mouth mucosa.” Detailed search strategies are provided in the Supplementary material. All English publications until March 1, 2022, were searched without any restrictions on countries or article types. The reference lists of all selected articles were screened independently to identify additional studies left out during the initial search. The inclusion criteria were as follows: (1) patients with an anterior urethral stricture who underwent single-stage augmentation urethroplasty with OMG and PSF, and (2) comparable success rates between the two substitution groups. The exclusion criteria included reviews, meta-analyses, systematic reviews, and case reports. Patients with a posterior urethral stricture or hypospadias, patients who underwent multistage urethroplasty, and studies focusing on children were also excluded from the analysis.
2.2 Data extraction and research quality evaluation
Data extraction was performed by three authors (Y.C., C.L., and R.Z.). Any disagreement was resolved by discussion until a consensus was reached or by consulting the fourth author (L.S.). Data including patient-based information (age), surgery-related indicators (stricture length and etiology), follow-up period, and postoperative complications (postvoid dribbling, sex function, and stricture recurrence) were extracted from the articles. Literature quality was assessed independently by two evaluators (C.L. and R.Z.) using the Cochrane risk of bias 2.0 (RoB 2.0) tool and Newcastle-Ottawa Scale (NOS). Randomized controlled trials (RCTs) were assessed using RoB 2.0 in terms of the randomization process, deviations from the intended interventions, missing outcome data, measurement of the outcome, and selection of the reported result. Studies with a high overall bias were excluded. Cohort studies were assessed using NOS, where scores of 7–9 indicate high-level quality, 5–6 indicate moderate-level quality, and <5 indicate low-level quality. Low-quality studies were excluded.
2.3 Meta-analysis
The main outcome of this systematic review was the success rate. Additional outcomes included the operative success rate in penile urethral strictures and the incidence rate of postvoid dribbling. Review Manager 5.4 software (Cochrane) and Stata 17.0 software (StataCorp, College Station, TX, USA) were used for data synthesis. I2 test was conducted to evaluate statistical heterogeneity. If I2 was >50%, heterogeneity was considered obvious, and a random-effect model was adopted. Otherwise, a fixed-effect model was used. To further explore the causes of heterogeneity, a subgroup analysis was conducted. Results were considered statistically significant at p < 0.05 (confidence level of 95%). The odds ratio (OR) was used as the effect measure for each outcome. Forest plots were produced to present the results. A sensitivity analysis was conducted to assess the stability of the results. Funnel plots and Eggers cond Begg’s test were used to evaluate the publication bias.
3. Evidence synthesis
3.1 Results
Using a rigorous search strategy, 308 relevant studies were identified from PubMed, EMBASE, and Cochrane database (Fig. 1). After applying the screening procedure, a total of 13 studies were included in the review. Among these, two studies were highly probable to have overlapping samples, so the one with the smaller sample size was excluded from the meta-analysis. Finally, we included 12 studies that compared the success rates of applying OMG and PSF for anterior urethral strictures; three studies compared the success rates of the two substitutions applied in penile urethra strictures; and four studies compared the occurrence rates of postvoid dribbling. More detailed information regarding the included studies is presented in Table 1.
Fig. 1Flow chart of the search for studies. PRISMA = Preferred Reporting Items for Systematic Reviews and Meta-analyses.
A comparative study of long-term results of buccal mucosal graft and penile skin flap techniques in the management of diffuse anterior urethral strictures: first report in Iran.
Comparative retrospective outcome analysis of 62 patients who underwent one-stage repair of penile urethral strictures in a single referral center experience.
Dorsal onlay urethroplasty using buccal mucosa graft versus penile skin flap for management of long anterior urethral strictures: a prospective randomized study.
(1) Maximum flow rate >12 ml/s during the latest follow-up; (2) no urethral fistula or diverticulum at the surgery site; and (3) the patient reported no dysuria and had no need for dilatation or catheterization
Outcome of dorsal onlay buccal mucosal graft versus ventral onlay local penile skin flap in complex anterior urethral strictures; a prospective randomized study.
Management of long segment anterior urethral stricture (≥ 8cm) using buccal mucosal (BM) graft and penile skin (PS) flap: outcome and predictors of failure.
The results of RoB 2.0 and NOS of the included studies are reported in the Supplementary material. There were five RCTs and eight cohort studies. All the RCTs accessed by RoB 2.0 have been evaluated as concerns because all the studies were unable to keep participants and people delivering the interventions unaware due to limitations in the study itself. All enrolled cohort studies scored above 7, indicating high quality. Owing to the specificity of the study, the selection section scored high, especially in the ascertainment of exposure. Comparability was the main source of variation in final scores. A subset of studies controlled for factors that could potentially affect outcomes, such as stricture site, stricture length, and patient age. However, others did not.
Twelve studies comprising a total of 855 patients were included in the comparison of success rates. Overall, there was no significant difference between OMG and PSF (OMG vs PSF, OR: 1.41, 95% confidence interval [CI]: 0.96–2.07, p = 0.08; Fig. 2A). Heterogeneity of the overall synthesis was not significant (I2 = 0%, p = 0.67). No significant publication bias was detected through Eggerion bias t = –0.21, p = 0.560), Beggough Eggez = 0.42, p = 0.677), and the funnel plot (Fig. 2C). Three studies comprising 307 patients were included to compare the success rates at the penile urethra. In the overall synthesis, no significant difference was identified (OMG vs PSF, OR: 0.95, 95% CI: 0.53–1.70, p = 0.86; Fig. 3A). Heterogeneity of the overall synthesis was not significant (I2 = 0%, p = 0.40). The incidence of postvoid dribbling was also compared in the present study. According to the included four studies comprising 257 patients, there was no significant difference between the two techniques (OMG vs PSF, OR: 0.59, 95% CI: 0.31–1.11, p = 0.10; Fig. 4A). Heterogeneity of the overall synthesis was also not significant (I2 = 0%, p = 0.57).
Fig. 2(A) Forest plot, (B) sensitivity analysis, and (C) funnel plot of the comparison of the success rate between OMG and PSF. CI = confidence interval; df = degree of freedom; M-H = Mantel-Haenszel; OMG = oral mucosa graft; OR = odds ratio; PSF = penile skin flap; SE = standard error.
Fig. 3(A) Forest plot and (B) sensitivity analysis of the comparison of the success rate at penile urethra between OMG and PSF. CI = confidence interval; df = degree of freedom; M-H = Mantel-Haenszel; OMG = oral mucosa graft; PSF = penile skin flap.
Fig. 4(A) Forest plot and (B) sensitivity analysis of the comparison of postvoid dribbling between OMG and PSF. CI = confidence interval; df = degree of freedom; M-H = Mantel-Haenszel; OMG = oral mucosa graft; PSF = penile skin flap.
A heterogeneity test was conducted across all enrolled studies (I2 = 0%, p = 0.63). The fixed-effect model was chosen because of the heterogeneity of the test results. A sensitivity analysis also ruled out the possibility that the results were influenced by a single study. Postvoid dribbling, which is a common postoperative complication, was mentioned in four of the 13 selected studies. Owing to the low heterogeneity (I2 = 0%, p = 0.56), a fixed-effect model was employed. To explore this further, a sensitivity analysis was conducted, which ultimately concluded that individual studies had no effect on the combined effect size. There is still no secondary evidence of the superiority of OMG and PSF in the treatment of penile urethral strictures. Three studies mentioned the success rates of penile urethral strictures treated with OMG or PSF. Heterogeneity was evaluated using the Q test, and a sensitivity analysis across these three studies was conducted. Heterogeneity between the OMG- and PSF-treated penile urethral stricture groups was low (I2 = 0%, p = 0.48); therefore, a fixed-effect model was employed. The results of the sensitivity analysis showed that the conclusion was stable and not altered by the exclusion of any study.
In the subgroup analysis (Table 2), significant differences were found in the success rates between the two substitutions in the studies with the top 50% sample size (six studies, OR: 1.678, 95% CI: 1.055–2.668, p = 0.029) and the top 50% follow-up period (five studies, OR: 2.279, 95% CI: 1.193–4.352, p = 0.013). There was no statistically significant difference in the success rates between OMG and PSF in studies in the last 5 yr (p > 0.05). Both RCT and non-RCT studies showed statistically similar success rates between the two substitution techniques (p > 0.05). In studies with different definitions of success, the success rates were also statistically similar (p > 0.05).
Table 2Subgroup analysis of stricture-free survival rate
Subgroup
No. of studies
Pooled OR for success rate
Heterogeneity
OR (95% CI)
p value
I2 (%)
p value
Study type
RCT
4
1.584 (0.677, 3.707)
0.289
0.0
0.937
Not RCT
8
1.334 (0.870, 2.047)
0.186
15.9
0.305
Publication date (recent 5 yr）
Yes
7
1.403 (0.894, 2.201)
0.140
25.8
0.232
No
5
1.328 (0.643, 2.740)
0.443
0.0
0.925
Sample size
Top 50% (>70)
6
1.678 (1.055, 2.668)
0.029
19.7
0.285
Last 50% (<70)
6
0.883 (0.440, 1.771)
0.725
0.0
0.993
Definition of failure
Qmax <15 ml/s
2
1.812 (0.730, 4.500)
0.200
0.0
0.745
Stricture retreatment
3
1.054 (0.462, 2.406)
0.901
0.0
0.504
Composite definition
5
1.540 (0.886, 2.677)
0.126
34.6
0.191
NR
2
–
–
–
–
Follow-up period
Top 50% (>24 mo)
5
2.279 (1.193, 4.352)
0.013
16.6
0.309
Last 50% (<24 mo)
5
1.062 (0.510, 2.211)
0.872
0.0
0.771
NR
2
–
–
–
–
CI = confidence interval; NR = not reported; OR = odds ratio; Qmax = maximum urinary flow rate; RCT = randomized controlled trial.
Median of sample size is 58.5; median of follow-up is 24 mo.
As a frequently used treatment for anterior urethral stricture, augmentation urethroplasty has many choices of material. Common choices include OMG, PSF, penile skin graft, scrotal skin flap, and intestinal mucosa graft. Among these, OMG and PSF have become the most commonly used techniques [
]. Although several studies have suggested that OMG could result in fewer complications, there is currently no solid evidence to decide which one is better in the treatment of anterior urethral strictures [
Follow-up times are reported in different ways such as mean follow-up period, follow-up range, and so on. Of all the studies, five reported both the mean follow-up period and the follow-up range [
A comparative study of long-term results of buccal mucosal graft and penile skin flap techniques in the management of diffuse anterior urethral strictures: first report in Iran.
Comparative retrospective outcome analysis of 62 patients who underwent one-stage repair of penile urethral strictures in a single referral center experience.
Dorsal onlay urethroplasty using buccal mucosa graft versus penile skin flap for management of long anterior urethral strictures: a prospective randomized study.
Outcome of dorsal onlay buccal mucosal graft versus ventral onlay local penile skin flap in complex anterior urethral strictures; a prospective randomized study.
Management of long segment anterior urethral stricture (≥ 8cm) using buccal mucosal (BM) graft and penile skin (PS) flap: outcome and predictors of failure.
]. Owing to the lack of the uniform reporting form of follow-up, the long-term stricture-free survival rates of urethroplasty cannot be compared effectively. According to our analysis, OMG and PSF provided similar efficacy in the success rate of single-stage urethroplasty (OMG vs PSF, OR: 1.41, 95% CI: 0.96–2.07, p = 0.08), which is consistent with the findings of a previous study [
] reported that the long-term stricture-free survival rate of urethroplasty was worse than previously appreciated (5 yr for 76.6%, 10 yr for 58.7%, and 15 yr for 45.0%). Therefore, this study compared the effect of the length of follow-up on the prognosis of augmentation urethroplasty. A subgroup analysis showed that studies with the top 50% follow-up period (>24 mo) indicated that OMG could result in a higher success rate than PSF (p = 0.013). This means that OMG probably offers advantages in terms of stricture recurrence after a rather long urethroplasty period. We speculate that the higher success rate of long-term follow-up in OMG may be due to the fact that the thicker epithelium and thinner lamina propria in OMG provide satisfactory survival conditions. It should also be noted that focusing on stricture recurrence on longer follow-up of OMG and PSF may be a research direction to determine which should be chosen by surgeons in augmentation urethroplasty for anterior urethral strictures.
Similarly, success rate difference also exists when it comes to larger sample size. Based on the results of the subgroup analysis, OMG had a higher success rate than PSF in studies with the top 50% sample size, which was speculated to be related to the surgical proficiency of the surgeon and medical center. OMG, as a relatively new substitution, is not so familiar to surgeons, which means that it requires experience and cases accumulated in the early stage of the learning curve to obtain stable and reliable outcomes. Studies with the top 50% sample size tend to suggest that clinical centers or surgeons have a larger operation quantity; therefore, surgeons are more likely to be skilled. An increasing number of surgeries have been performed by skilled surgeons. Skilled surgical techniques contribute to the success rate of surgery.
The definition of surgical success varies depending on the specific study, and this variety makes comparison between studies difficult. There are five common definitions of urethroplasty failure: (1) stricture retreatment, (2) anatomical recurrence on cystoscopy, (3) peak flow rate <15 ml/s, (4) weak stream mentioned in the questionnaire, and (5) failure by any of these measures [
]. Of the 12 studies included in the meta-analysis, ten reported the definition of surgery success or failure (presented in Table 1). Therefore, subgroup analyses were conducted to discover the impact of different definitions of success on outcomes. The results showed no significant difference between the success rates of OMG and PSF. Therefore, although the success rate of anterior urethroplasty depends highly on the definition of success, the success rates of the two substitutions tend to be unaffected by the differences in definition of surgical success, which is consistent with previous studies and guidelines [
Owing to the distinctive characteristics of each segment of the anterior urethra, the choice of treatment and prognosis for each segment of a urethral stricture varies. The recommended substitution for bulbar strictures in urethroplasty is OMG. Bulbar strictures can be divided into “short” and “long” bulbar strictures. “Short” bulbar strictures refer to those amenable to stricture excision and subsequent tension-free anastomotic repair, and all others are “long” bulbar strictures [
]. Free graft urethroplasty, especially OMG, is the most used material in the treatment of “long” bulbar strictures because of its low incidence of complications. PSF is another alternative substitution. As a pedicle skin flap, the PSF consists of skin and vascular pedicles. The vascular pedicle connects the skin island and the vessel in the flap donor site, guaranteeing an adequate blood supply to the flap. However, an excessively long vascular pedicle will significantly increase surgical difficulty because of the long distance between the donor site and the recipient site [
]. Therefore, it is not appropriate to choose PSF for the treatment of bulbar stenosis, which is distant from the PSF donor site.
The recommended substitution in urethroplasty for penile urethral strictures remains unclear. In this review, the respective success rates after urethroplasty for bulbar and penile urethral strictures were reported in only one of the 12 included studies [
Comparative retrospective outcome analysis of 62 patients who underwent one-stage repair of penile urethral strictures in a single referral center experience.
]. The number of studies focusing on penile urethral strictures is too small to generate convincing secondary research evidence. We expect more studies focusing on penile urethral strictures in the future. According to the meta-analysis, we found that OMG and PSF offered the same success rates in penile urethral strictures (OMG vs PSF, OR: 0.95, 95% CI: 0.53–1.70, p = 0.86). We also observed that the operative success rates for the urethra at individual segments have not been reported in detail separately in most previous studies. Thus, more prospective randomized trials focusing on the comparison between OMG and PSF in penile urethral strictures alone are warranted.
Patients’ feelings and satisfaction also received attention in our meta-analysis. According to the results, there was no significant difference between OMG and PSF in postvoid dribbling (OMG vs PSF, OR: 0.59, 95% CI: 0.31–1.11, p = 0.10). In addition, it is worth noting that few studies comparing OMG and PSF have reported changes in sexual function after single-stage urethroplasty for penile urethral strictures. Specifically, erectile and ejaculatory functions should be assessed and reported separately because these are regulated by different physiological mechanisms. Theisen et al [
] reported that ejaculatory dysfunction after anterior urethroplasty was associated with postvoid dribbling. Moreover, previous studies have overlooked the patient’s recommendation and satisfaction, which are extremely significant to the patient’s comfort and surgeon’s choice of different substitutions. Patient-reported outcome measures (PROMs) are questionnaires aimed at measuring the outcomes from a patient’s perspective and are well suited to assess patient satisfaction [
]. Questionnaires and scales investigating substitution donor sites are also necessary. Therefore, it is difficult to obtain reliable comparative data to identify differences between these two substitutions. More high-quality and comprehensive studies are required and are necessary in the future.
With the current results and available information, we consider it possible for future relevant studies to pay attention to the following aspects. First, focus should be given not only on the “success rate” itself, but also on follow-up period, sample size, definition of success, and other factors that have potential to influence the success rate and prognosis. Second, the comparison between OMG and PSF in anterior urethral strictures in the penile segment rather than the bulbar segment, especially for prospective clinical trials, should also be given attention. Third, the patients’ subjective feelings and satisfaction, in terms of postvoid dribbling, postoperative sex function, and patient recommendation, should be considered. PROMs and other scales or questionnaires can be suitable tools for these.
Our study has some limitations. First, concrete forms of the oral mucosa, such as the buccal, lip, and lingual mucosa, were not considered. The site (ventral and dorsal) and repair method (onlay and inlay) were not discussed in detail as well owing to insufficient reports of existing primary studies. Second, the stricture-free survival rate in different segments of the anterior urethra did not provide adequate comparisons for the same reason. It also led to difficulties in focusing our analysis on the penile segment urethral strictures, the substitution choice of which is most controversial. Third, more refined criteria for the included studies were failed to be developed because of the inconsistent reporting of the available studies on the definition of follow-up time and surgical success. To sum up, these need to be refined further in our future work, and we also expect more high-quality and comprehensive studies to complement and refine the existing issues in these areas.
4. Conclusions
In conclusion, OMG provides the same success rate and postvoid dribbling as PSF. In penile urethral strictures, there was no significant difference between the OMG and PSF groups. However, based on the existing evidence, OMG is more likely to perform better in a cohort with long-term follow-up and a relatively large sample size. More studies on the two substitutions in the future are necessary to explore the factors influencing the success rate of urethroplasty, performance of substitutions in penile urethral strictures, and indicators of the patient's quality of life.
Author contributions: Lujie Song had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: Song, Zhang.
Acquisition of data: Chen, Lu.
Analysis and interpretation of data: Zhang, Lu.
Drafting of the manuscript: Zhang, Chen, Lu.
Critical revision of the manuscript for important intellectual content: Song, Zhang.
Statistical analysis: Chen, Lu.
Obtaining funding: None.
Administrative, technical, or material support: Song, Zhang.
Supervision: Song.
Other: None.
Financial disclosures: Lujie Song certifies that all conflicts of interest, including specific financial interests and relationships and affiliations relevant to the subject matter or materials discussed in the manuscript (eg, employment/affiliation, grants or funding, consultancies, honoraria, stock ownership or options, expert testimony, royalties, or patents filed, received, or pending), are the following: None.
Funding/Support and role of the sponsor: None.
Acknowledgments: We appreciate Dr. Jing Zhang at Shanghai Jiao Tong University School of Public Health for her support in statistical analysis techniques in this study.
Appendix A. Supplementary data
The following are the Supplementary data to this article:
A comparative study of long-term results of buccal mucosal graft and penile skin flap techniques in the management of diffuse anterior urethral strictures: first report in Iran.
Comparative retrospective outcome analysis of 62 patients who underwent one-stage repair of penile urethral strictures in a single referral center experience.
Dorsal onlay urethroplasty using buccal mucosa graft versus penile skin flap for management of long anterior urethral strictures: a prospective randomized study.
Outcome of dorsal onlay buccal mucosal graft versus ventral onlay local penile skin flap in complex anterior urethral strictures; a prospective randomized study.
Management of long segment anterior urethral stricture (≥ 8cm) using buccal mucosal (BM) graft and penile skin (PS) flap: outcome and predictors of failure.
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