Tissue Characterization for Improved External Penile Occlusive Device Design

[+] Author and Article Information
Gerald W. Timm, Robert E. Hampton

Department of Urologic Surgery, University of Minnesota, Minneapolis, MN 55455

David R. Wulfman, Seoggwan Kim, Samuel Will, John DiCosimo, Arthur G. Erdman

Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455

J Biomech Eng 127(6), 956-963 (Jul 15, 2005) (8 pages) doi:10.1115/1.2049335 History: Received April 05, 2005; Revised July 15, 2005

This study is motivated by the need for quantitative data on the material properties of the penis in order to develop an optimal design for an external penile occlusive device (EPOD) for the treatment of urinary incontinence.

Copyright © 2005 by American Society of Mechanical Engineers
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Figure 1

Exploded view of penis cradle assembly. The measurement device is a circular ring structure (item 2) in which are mounted four diametrically paired pneumatic cylinders (See Ref. 19) spaced 90° from one another about the cylinder’s circumference. The piston throw is directed toward the ring’s center. The measurement protocol prescribes control of actuating pressure common to all cylinders, and measurement of resultant displacement of each piston individually.

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Figure 2

Cradle index positions 0, 30, and 60deg, which allow 12 distinct measurements. Three different measurement sets are conducted on a subject at actuator ring rotational positions of 0°, 30°, and 60°. This produces twelve distinct pressure vs displacement profiles, spaced at 30° increments about the circumference of the penis. Subsequent data analysis refines the data collected into tissue reaction force vs displacement profiles for the different rotational positions.

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Figure 3

Concept schematic of hydraulic measurement system. Water is used as the measurement media, and its effect on the pneumatic actuators is considered negligible. Each actuator is attached to a 1∕16in. i.d. PTFE tube which is connected to the manometer subsystem. The indenter head displacement is determined from the change in height of the water in the columns of the manometer measurement device.

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Figure 4

(Color) Hydraulic measurement subsystem; (a) Measurement cradle; (b) Water columns. The actuators are pressurized by an 861.8kPa air reservoir. At each of the three index positions, a series of pressures (0.00, 55.2, 68.9, 82.7, 96.5, 110.3, 124.1, 137.9, 151.7, 165.5, and 179.2kPa) is applied to the actuators. At each pressure the change in column height on the manometer is manually recorded. The change in water height in the column is directly proportional to the change in extension of the actuator.

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Figure 5

Schematic of piston actuator. A force balance of this simplified but representative system allows for the determination of the reactive force of the tissue. This is accomplished by expressing the displacement of the spring as a function of the change in column height of the water.

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Figure 6

(Color) Simplified finite element model for the indentation of penis. Utilizing geometric symmetry, only a quarter of the penile shaft, along the longitudinal direction, was modeled to increase computational efficiency. It was observed that the direction of tissue deformation due to an actuator occurs almost exclusively in the direction of piston movement. This observation allows the division of the FEM model into pie shaped sections.

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Figure 7

(Color) Raw output data compared to unloaded piston spring curve. Data points shown are those taken directly from a subject at the different indexed points of the measurement instrument (0°–330° in 30° increments). The straight line in each plot is the linear curve fit of the internal spring loaded piston indentor working against no external load. It is inferred that the piston head is in contact with tissue at subject data points that deviate positively from the unloaded piston indentor data.

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Figure 8

(Color) Force and dimension adjusted data. The plots above result from two operations made on raw data shown in Fig. 7. For each subject data point taken, the force contribution of the piston is calculated as a function of the extension recorded from the instrument and then subtracted from the raw force recorded for the same point. The force adjusted points are then surveyed to locate that point that deviates positively from zero. This point (the zero point) is considered the first record where contact is made between the piston head and penile tissue. Extension values shown are extensions beyond the zero point.

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Figure 9

(Color) Compiled adjusted data. The data shown here are the data from Fig. 8 compiled into a single plot. Significant observations are that the tissue associated with 180°, that closest to the urethra, appears more stiff than others for this subject. There also appears to be a degree of stiffness asymmetry which can also be seen in Fig. 1.

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Figure 10

(Color) Adjusted Data: Tissue reaction force as a function of angular position of the penis along lines of constant extension. Significant observations include the significantly greater force required to indent the region about the urethra (180°). Also there appears to be a stiffness asymmetry in this subject about the vertical axis.

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Figure 11

(Color) Graphical results of the finite element analysis of the indentation test. (a) Transverse cross section; (b) Closure of the urethra view in the longitudinal cross section. Results from the FEA of the indentation test using a linear elastic and homogeneous material are presented. As mentioned earlier, it can be observed that the direction of tissue deformation due to an actuator occurs almost exclusively in the direction of piston movement and also that the urethra is closed as the bottom indenter moves upward.

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Figure 12

Force-displacement curves from finite element analysis. An average force displacement curve has been calculated from the linear curve fit of every force displacement curve from the experiment. And an elastic modulus of 0.014MPa was obtained by matching the simulated force displacement curve with the average force displacement curve from the experiment. The initial portion of the force displacement curve shows nonlinear behavior, even with a linear elastic and homogeneous material, due to the change of the contact area between the penis and the indenters as the indenters move toward the center of the penis.

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Figure 13

(Color) Effect of bladder pressure on occlusion of urethra (longitudinal cross section of penis). Part (a) illustrates that a device displacement of 13mm is sufficient to completely close a 14mm longitudinal section of the urethra without bladder pressure. When a bladder pressure of 120cmH2O (which is a typical bladder pressure induced by a cough) is applied, however, the urethra is completely reopened as shown in part (b). This tells us that the device must close the urethra enough to prevent the reopening of the urethra by the urine fluid pressure acting on it, an important guideline for the design of an occlusion device.

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Figure 14

(Color) Effect of the tissue stiffness of each pie section on the closure length of urethra (stiffness of each section doubled, 0.014–0.028MPa). This figure shows the effect of the tissue stiffness of each pie section of the penis on the closure length of urethra and consequently can provide clinical information leading to the selection or even unique design of a more effective EPOD for the treatment of urinary incontinence.

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Figure 15

(Color) Device geometries used to determine successful closing of urethra. For the optimum design of a device, it is important to know the effect of the device geometry including its cross section. The effect on the longitudinal closure length of the urethra, with the applied force of 0.3N, by the different geometries shown in this figure is investigated and its results are shown in Table 1. Again a homogeneous material with elastic modulus of 0.014MPa was assumed. The study shows that urethral closure length in addition to device displacement is highly dependent upon device geometry.



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