  Functional luminal imaging probe (FLIP) is used to measure cross-sectional
area (CSA) and pressure at sphincters. It consists of a catheter surrounded by
a fluid filled cylindrical bag, closed on both ends. Plotting the pressure-CSA
hysteresis of a sphincter during a contraction cycle, which is available
through FLIP testing, offers information on its functionality, and can provide
diagnostic insights. However, limited work has been done to explain the
mechanics of these pressure-CSA loops. This work presents a consolidated
picture of pressure-CSA loops of different sphincters. Clinical data reveal
that although sphincters have a similar purpose (controlling the flow of
liquids and solids by opening and closing), two different pressure-CSA loop
patterns emerge: negative slope loop (NSL) and positive slope loop (PSL). We
show that the loop type is the result of an interplay between (or lack thereof)
two mechanical modes: (i) neurogenic mediated relaxation of the sphincter
muscle and (ii) muscle contraction proximal to the sphincter which causes
mechanical distention. We conclude that sphincters which only function through
mechanism (i) exhibition NSL whereas sphincters which open as a result of both
(i) and (ii) display a PSL. This work provides a fundamental mechanical
understanding of human sphincters. This can be used to identify normal and
abnormal phenotypes for the different sphincters and help in creating
physiomarkers based on work calculation.
