Introducing the M-factor: A ratio to normalize measurements of pubic diastasis with growth

Bladder exstrophy-epispadias complex (BEEC) involves osseous pelvic defects and a wide pubic diastasis (PD). PD widens over time after the initial closure, but the degree and impact of this increase is not well delineated. Using the transverse distance between posterior inferior iliac spines (dPIIS) as an internal comparison, we aimed to: 1) compare dPIIS, PD, and M-factor (PD/dPIIS) measurements made on radiographs to computed tomography (CT) images and between reviewers; 2) assess the measurements at birth and over time for BEEC and for age- and gender-matched controls; 3) determine how the M-factor compares with the PD over time.

This was a single institution, IRB-approved retrospective study of patients with BEEC who underwent a complete primary repair of bladder exstrophy (CPRE) or repair of epispadias or cloacal exstrophy with bilateral osteotomies between 5/1/2013 and 3/1/2020. PD and dPIIS were measured on cases and controls at 4 timepoints: preoperative, intraoperative, early post-operative (6-8 weeks) and late post-operative (greater than 6 months). For statistical analysis intra class correlations were used to assess inter-rater reliability. Mixed model two-way ANOVA was done with subjects as random effect, and group (patient/control), exam time as the two fixed effects.

Forty-three cases (33 male) ranging in age from 1 day to 129 months (IQR 15.25 months) during the time points were evaluated. There was excellent agreement for measurements among the 6 raters. Following BEEC repair with osteotomies, PD and the M-factor decreased significantly. In the entire cohort, due to early post-operative spreading of the bones, there was no difference between pre-operative and early post-operative PD (35.2 ± 8.6 mm to 34.1 ± 9.8 mm, p = 0.08), however there was a significant difference in the M-factor between those time points (0.89 ± 0.2 to 0.73 ± 0.2, p < 0.01). With gradual increase in PD over time, there was a significant difference between early and late post-operative PD measurements (35.2 ± 8.6 mm to 39.8 ± 9.9 mm, p < 0.01), but not in the M-factor (0.73 ± 0.2 to 0.75 ± 0.2 (p = 0.45).

PD in patients with BEEC changes over time due to surgery as well as growth, while a ratio of PD to an internal reference (dPIIS), does not change. This M-Factor ratio can better determine the initial extent of PD within the context of the size of the child, as well as standardize assessment of changes after surgery and during growth and development.