Understanding the mechanical behavior of intrauterine devices during simulated removal

Abstract

Objective: To evaluate differences based on intrauterine device (IUD) frame geometry in force, and stress, and strain at the stem/arms junction during simulated IUD removal.

Study design: We manufactured injection-molded frame models for three Nova-T IUDs (Mirena [model M]; Liletta [model L]; Kyleena [model K]) and a Tatum-T IUD (Paragard [model P]) at two-times scaling. We created a custom fixture to simulate the uterus and used a screw-driven machine to pull models at various displacement rates through the 10 cm fixture cavity to measure force and strain and calculate stress at the IUD stem/arms junction. We tested models at 30 mm/min and higher displacement rates for exploratory analyses. We used Mann-Whitney U test for statistical testing.

Results: We completed testing at 30 mm/min using five of each Nova-T model and nine model P samples. Resistance against the cavity walls created significantly more force on model P (11.83, interquartile range [IQR] 11.61-12.31) than any Nova-T model samples (p < 0.001). The smaller model K created slightly more median stress (MPa) than the larger model M (0.36 [IQR 0.33-0.38] and 0.79 [IQR 0.76-0.80], respectively, p = 0.008); model P samples generated significantly more median stress than other models (1.70 [IQR 1.67-1.77], p < 0.001). Strain plots demonstrated permanent deformation for some samples during IUD removal simulation. We tested 20 samples at various higher displacement rates up to 2500 mm/min, with stress notably increasing for model P samples with increasing rates. No fractures occurred.

Conclusions: Force and stress at the stem/arms junction are higher with Tatum-T-shaped compared to Nova-T-shaped IUD models under the same testing conditions, and a higher speed of extraction causes more stress.

Implications: Sharp corners create vulnerability under static and fatigue loading in structural components due to increased local stresses. Our findings suggest that IUDs with Tatum-T frames should be removed slowly to minimize the stress at the stem/arms junction. Future studies can provide more information if performed with commercially available products.

Keywords: Force; Fracture; Intrauterine device; Strain; Stress.