Design and validation of an automated dual‐arm instrumented intravaginal dynamometer

Abstract

Aims: (1) To present the design of a novel intravaginal dynamometer (IVD) capable of measuring vaginal closure force on both the anterior and posterior arms, (2) to use bench testing to validate the force, speed of arm opening, and positional accuracy of load measurement along the IVD arms, and (3) to present in vivo force measurements made with this device, comparing forces measured by the anterior and posterior arms. Methods: IVD load measurements were validated against an Instron® Universal Tester, arm opening speeds were validated using video analysis, and position-load accuracy was validated against calibration weights. In vivo IVD data were acquired from female volunteers during passive opening and pelvic floor muscle contraction tasks. Anterior and posterior IVD arm force outcomes were compared. Results: Forces measured by the IVD and Instron® exhibited a strong linear relationship with excellent model fit. The speed control system was valid when tested under physiological loading conditions, however smaller anteroposterior opening diameters (25 and 30 mm) exhibited some error. The loading position along the IVD arms had no effect on force outcomes. In vivo data exhibited differences between force outcomes measured at the anterior and posterior aspects of the vagina during active contraction and passive elongation of the pelvic floor muscles. Conclusions: This IVD design demonstrates valid load measurement and speed control during bench testing. Active and passive forces measured are consistent with the literature. With dual instrumented arms, this device allows for further investigation into the source of measured vaginal closure forces.

The state of pelvic floor muscle dynamometry: A scoping review

“…From the 101 records included, 23 PFM dynamometers from 15 research groups were identified. PFM dynamometers were grouped into two categories: clinical PFM dynamometers ( n = 20, from 12 research groups), 7–26 meant for research settings, and personal PFM dynamometers ( n = 3, from three groups), which were commercially available and intended for patient use 27–29 …”

“…Outcomes : rate of force development, number of contractions, peak force and normalized area under the curve between 10‐60s (Intraclass correlation coefficient) 12 …”

“…Dynamometers were made of a wide range of materials. Specifically, nine dynamometers were made of aluminum, 7,8,16,19–21,23–25 one was made of plastic externally and steel internally, 17 two were made of stainless‐steel, 9,18 one was made of metal, 26 three out of plastic, 13–15 two out of polylactic acid, 11,12 one was made of acrylonitrile butadiene styrene and polycarbonate 10 and one was of unknown material (not reported) 22 . Of interest, the material used for the research PFM dynamometers appears to have shifted over time from aluminum and stainless‐steel to 3D‐printed plastic (polylactic acid, acrylonitrile butadiene styrene, and polycarbonate), which allows for more possibilities in device design.…”

“…There were differences in the structure of the intra‐vaginal dynamometers with one type being more prominent. Fifteen dynamometers were “speculum shaped” 7–13,19–26 and made of two branches, four were cylindrical‐shaped probes, 15–18 and one had four‐leaf spring branches attached to a cylinder 14 . Interestingly, while most speculum‐shaped dynamometers opened their branches in parallel to each other, seven opened in an angle, 9,10,13,23–26 which causes a higher opening pressure applied to deeper vaginal tissues.…”

“…Dynamometers varied in their force sensor number and arrangement. Most used multiple sensors ( n = 11) 7–9,12,14–16,19–22 while some used one force sensor only ( n = 9) 10,11,13,17,18,23–26 . Furthermore, only two reports mentioned the use of differential arrangement of strain gauges, 8,19 which is important to ensure the force is measured independently of its exact site of application on the dynamometer branch 63 .…”