Introduction
Pelvic organ prolapse (POP) refers to downward descent of female pelvic organs into or through the vagina. POP is a common condition and prevalence varies on whether prolapse is defined using objective or subjective measures. Rates of POP are up to 50%, when defined using objective measures such as vaginal examination(1) and around 3% when defined as symptomatic bulge(2, 3). POP surgery prevalence ranges from 6 to 18% with over 300,000 surgeries performed annually in the US alone(1, 4).
If a hysterectomy is performed for prolapse, recurrence rates of vault prolapse are estimated at 11.6%(5). Prolapse of the anterior vaginal wall or cystocele is the most common POP detected(6) and is strongly associated with apical prolapse(7). Current indications for sacrocolpopexy (SCP) include apical or multicompartment prolapse. Although SCP is an extensively studied and highly efficacious procedure there is significant heterogeneity in the surgical technique. This was illustrated in a systematic review by Moroni et al., which included 22 RCTs and assessed procedure standardization. In this review, most studies failed to describe the procedure steps in detail and there was heterogeneity in materials used for attaching mesh to vagina and sacrum, length of vaginal dissection from only dissecting apex to full vaginal length. Outcomes studied could be potentially influenced by technical choices intraoperatively(8).
Other characteristics affecting outcome can be mesh weight, pore size and material used. Human and animal studies have shown that heavier mesh may be associated with more chronic inflammation and poorer remodelling of vagina as compared to light weight mesh(9). In 2018, Askew et al. compared anatomic failure rates between ultralight mesh (≤20 g/m2) with heavy mesh (≤35 g/m2). Ultralight weight mesh had twice the hazard of failure within 3 years compared to heavy weight mesh (p=0.03) with a lower mesh exposure rate (1.6% vs 6.0%, p= 0.01)(10).

Aim of study
To assess long-term anatomic and subjective outcomes following sacrocolpopexy based on the weight of mesh used.

This is a prospective cohort study conducted at two tertiary hospitals in Melbourne, Australia. We identified 358 patients who underwent SCP for symptomatic pelvic organ prolapse between January 2000 and December 2019.  The primary outcome was Patient Global Impression of Improvement (PGI-I) at follow up post SCP using ultralight (UL) mesh (≤20 g/m2, Restorelle® Y) with heavier (HM) mesh (≥25g/m2, UpsylonTM, UltraproTM, InteproTM, Gynemesh®, Vypro®).  
Secondary outcomes were patient satisfaction assessed using Australian pelvic floor questionnaire(11) and composite failure.  Composite failure was defined as at least one of ≥ stage 2 apical prolapse, anterior or posterior wall beyond hymen, complaining of on bulge on questionnaire or retreatment.
To account for different lengths of follow-up, we calculated the incidence rate of composite failure per 1000 persons per month of follow up in the two groups. Effect measure estimates were calculated as the incidence rate ratio of composite failure comparing the use of ultralight with the heavier mesh group. Crude and adjusted incidence rates ratios (IRR), controlling for age and body mass index at follow-up, parity, smoking and the presence of advanced prolapse prior to surgery, along with their respective 95% confidence intervals (95% CI), were obtained using univariable and multivariable Poisson regression models. This study was approved by the local ethics committee (RES-19-0000330A)

Of the 358 patients who underwent sacrocolpopexy, 220 (61%) attended a later follow up appointment. Of these, 95 (43%) had ultralight weight mesh (UL) and 125 (57%) had heavier mesh (HM). 
Table 1 displays baseline demographics, preoperative anatomic and subjective data using Pelvic Organ Prolapse Quantification (POP-Q) system and Australian Pelvic Floor Questionnaire (APFQ).
The median follow-up for UL mesh and HM was 36 (Interquartile range [IQR] 22 -42) and 63 (IQR 48 -87) months, respectively (p <0.001). Total length of follow up for UL mesh was 3249 months and 8884 months in HM. For PGI-I 90.4% (85/94) in the UL group reported to be ‘very much better’ or ‘much better’ compared to 74% (94/127) in HM group (p = 0.002). The difference remained significant after accounting for difference in follow up time (IRR 2.5,95% CI 1.9 – 3.4, p<0.001).

Due to the temporal differences in the use of heavier and ultralight meshes, the group with heavier mesh had a significantly longer median follow-up than the ultralight mesh group (Table 2). Accounting for differences in follow-up time, there was no statistically significant difference between the incidence rates of composite failure between the ultralight and heavier mesh groups (9.8 per 1000 persons per month in the ultralight mesh group, 7.3 per 1000 persons per month in the heavier mesh group; IRR 1.35, 95% CI 0.85 – 2.09, p = 0.17). After adjusting for age and body mass index at follow-up, parity, smoking and the presence of advanced prolapse prior to surgery, the difference in rates between the groups remained non-statistically significant (IRR 1.47, 95% CI 0.92 – 2.35, p = 0.11).

In this study, no significant differences in surgical failure incidence rates were found between ultralight and heavier mesh use in sacrocolpopexy procedures, when accounting for the length of follow-up.