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Optimization of Design and Surgical Positioning

of Inflatable Penile Prostheses

A. GEFEN,1 J. CHEN,2 and D. ELAD1

1Department of Biomedical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israeland 2Sexual Dysfunction Clinic, Department of Urology, Tel Aviv Sourasky Medical Center, Tel Aviv 64239, Israel

(Received 10 December 1999; accepted 23 May 2000)

AbstractThe interaction between the cylinders of an inflat-able penile prosthesis IPP and the surrounding tissues duringIPP-aided erection may result in local elevated stresses. Thesestresses may reach values that can obstruct penile blood vesselsand cause ischemia and/or stimulate nerves around the opera-tion site, thereby inducing sensations of pain. A new numericalmodel was used to analyze penile stresses postimplantation of

different IPP types, in order to optimize prosthesis design andsurgical positioning by enabling minimal stress transfer to dor-sal blood vessels and nerves. The results suggest that intralu-minal pressures should be maintained at low levels about 80kPa while cylinder thickness and stiffness should be kept justhigh enough approximately 15% of the radius and 1000 MPa,respectively to eliminate deleterious cylinder-tissue contactstresses. Smaller prosthetic cylinders, i.e., occupying about45% of the cavernosal space, may be advantageous in terms ofreducing dorsal stresses, but lower penile rigidity should beexpected. A significant decrease of dorsal stresses can also beachieved by encouraging the surgeon to position the cylinderstoward the lower part of the corpora. The numerical simula-tions indicate that circular cylinders may allow greater biome-chanical compatibility of the IPP with the penis structure than

elliptic ones, and this should be a subject for clinical investi-gations. 2000 Biomedical Engineering Society. S0090-6964 00 01106-1

KeywordsErectile dysfunction, Impotence, Numerical

model, Finite element method.

INTRODUCTION

The inflatable penile prosthesis IPP was first intro-

duced in the early 1970s as an interventional treatment

of impotence.14 The IPP is composed of three compo-

nents: a reservoir that is located in the perivesical space,

two inflatable cylinders that are surgically inserted alongthe erectile bodies of the penis i.e., corpus cavernosa ,

and a pump that is implanted within the scrotum. When

erection is desired, the penis can be distended to a near

normal erectile condition by repeatedly squeezing the

pump, thereby transferring fluid from the reservoir to

inflate the cylinders. Manual release of the valve at the

lower portion of the pump will cause the fluid to flow

back into the reservoir, returning the penis to a flaccid

state. Since this mechanism of operation mimics the nor-

mal erectile process, it is generally accepted that the IPP

provides the best physiological results and, accordingly,

numerous successful IPP implantation procedures were

performed during the last two decades.13,18 However, a

variety of postoperative complications have been en-

countered in up to 20% of the patients, including infec-

tions, mechanical failures of the prosthesis, severe pain

associated with its operation, prosthesis protrusion

through the glans penis, and aneurysms.4,12,15

The mechanical interaction between the artificially in-

flated prosthetic cylinders and the surrounding penile tis-

sues induces mechanical stresses within the penis, which

may include sites of localized, highly elevated stresses.

Destruction of some penile tissues during the surgical

procedure exposes nerve endings on the internal surfacessurrounding the cylinders. Many researchers now agree

that under these conditions, these nerve endings develop

hypersensitivity alodynia and, therefore, their being

stimulated by excessive stretching and/or compression of

adjacent tissue are perceived by the patient as painful

sensations.2,10,16 These sensations could also be aug-

mented by obstruction of some dorsal penile blood ves-

sels due to intensive compression, i.e., ischemic pain.

These effects of local intensified stresses could be the

underlying causes for penile pain during inflation of the

prosthetic cylinders, which was clinically observed to be

the most common cause for dissatisfaction with the

device.6 Therefore, the biomechanical compatibility

of an IPP can be characterized by the stresses developing

around the primary nerve roots and blood vessels due to

inflation of the cylinders. Gefen et al.3 recently intro-

duced a computational model of the penis/IPP complex

that allows for quantitative analysis of the distribution of

stresses within the different penile tissues during IPP-

aided erections. In the present study, we utilized this

computational model to optimize the stresses within the

Address correspondence to Professor David Elad, Department of

Biomedical Engineering, Faculty of Engineering, Tel Aviv University,

Tel Aviv 69978, Israel. Electronic mail: [email protected]

Annals of Biomedical Engineering, Vol. 28, pp. 619628, 2000 0090-6964/2000/28 6 /619/10/$15.00Printed in the USA. All rights reserved. Copyright 2000 Biomedical Engineering Society

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penile tissues during IPP-aided erections that had been

obtained by using different IPP structural designs with

different surgical positions of the cylinders within the

cavernosal spaces. Our aim was to minimize stress trans-

fer to primary nerves and blood vessels and thereby

alleviate some of the complications sometimes produced

by these devices.

METHODS

The Penis/IPP Structural Model

The methodology used to build a two-dimensional

2D computational model for analysis of structuralstresses in the penile tissues during IPP-aided erection is

described in detail in Gefen et al.;3 its essential compo-

nents relevant to the present report are given later.

The symmetrical geometry of a typical 2D transverse

cross section of the human penis was extracted from an

anatomical scheme. The model includes the following

elements: IPP cylinders, compressed corpus cavernosa,

tunica albuginea, skin, dorsal blood vessels, and the ure-

thra Fig. 1 a . The corpus spongiosum, whose cross-

sectional area is usually significantly smaller than that of

the corpus cavernosa, was excluded. The prosthesis bulk

and penile soft tissues were assumed to be made of

homogenous, isotropic, and linear elastic materials,

whose mechanical properties are detailed in Gefen et al.3

The stress distribution was determined for each configu-

ration by employing a commercial finite element analysis

software package ANSYS which was used to solve the

general equilibrium equations for plane stress. Automatic

meshing was used to generate optimally converging

meshes of 25003000 quadrilateral and triangular ele-

ments that described the cross-sectional geometry Fig.2 . The meshes were determined by a converging process

in which the mesh density was gradually increased until

the deviation in the produced stress values did not ex-

ceed 5%. During the meshing process, special attention

was paid to avoid extreme transitions in sizes of adjacent

elements, which may induce local inaccuracies in the

numerical solution. The boundary conditions included an

intralumenal device pressure ( P i) applied to the interior

FIGURE 1. Biomechanical model of thehuman penisIPP complex and its adap-tation to different prosthesis designs,orientations and positioning: ad cir-cular cylinder cross sections and eh elliptic cylinder cross sections.

620 GEFEN, CHEN, and ELAD

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profiles of the cylinders, as well as four constraints on

the lateral and dorsal-plantar aspects of the penis which

allowed its expansion but not rotation in response to

inflation of the cylinders. The finite element analysis

provided the structural stress distribution in terms of von

Mises (v .M.) equivalent stress

v .M. 1

2

22

121/2

, 1

which weighs both principal tension (1) and principal

compression (2) effects.

In order to characterize the biomechanical perfor-

mances of a given IPP, we examined the averaged values

of the stresses transferred to the dorsal nerve roots and

blood vessels. Since the deformation of the penis during

IPP-aided erection varies among different cases, we de-

fined the average stress at this region as

1S 0

Sv.M.d, 2

where the linear course of length S originates at the

center of the dorsal face of the penis cross section above

the dorsal vein and artery , crosses the dorsal nerve roots

and the nerves of the tunica albuginea, and terminates at

the apex of the corpus cavernosum Fig. 1 b .

IPP Design Optimization

The model was implemented to simulate the interac-

tion of soft tissues of a typical penis with a series of

inflatable prostheses that differ by their cross-sectional

geometry, cylinder material properties, and intraluminal

operational pressures. In addition, we tested different po-

sitioning of the cylinders within the cavernosal spaces

which is also likely to play an important role in deter-

mining the distribution of stresses. Values for these pa-rameters were selected to conform to the design param-

eters of commercial IPPs.

Cylinder Geometry

Two types of cylinder cross-sectional shapes, circular

and elliptic, are presently commercially available at dif-

ferent lengths 1028 cm flaccid , diameters 1018 mm flaccid , and wall thickness. When elliptic cylin-

ders are considered, alignment of their principal axes

with respect to the corporal anatomy during surgery is

also important and was taken into consideration in this

study.The following penis/IPP configurations were selected

for the present optimization analysis Fig. 1 : a large

and dorsally positioned thick circular cylinders, which

are defined as a reference case for the comparative

analysis; b large and dorsally positioned thin circular

cylinders; c large and ventrally positioned thick circular

cylinders; d small and ventrally positioned thick circu-

lar cylinders; e large and dorsally positioned thick el-

FIGURE 2. Examples of meshes of thepenisIPP model geometry: a and b cir-cular cylinder cross sections see Figs.1b and 1c for respective model con-figurations and c and d elliptic cylin-der cross sections see Figs. 1f and 1hfor respective model configurations.

621Optimization of Design and Positioning of Penile Prostheses

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liptic cylinders with an aspect ratio of 0.9 and the long

axes directed toward the dorsal-lateral and ventral-medial

aspects of the penis; f small and dorsally positioned

thick elliptic cylinders with an aspect ratio of 0.9 and the

long axes directed toward the dorsal-lateral aspect of the

penis; g small and centrally positioned thick elliptic

cylinders with an aspect ratio of 0.7 and the long axes

aligned with the medial-lateral direction of the penis; and h small and centrally positioned thick elliptic cylinders

with an aspect ratio of 0.7 and the long axes aligned with

the dorsal-ventral direction of the penis.

Based on measurements of commercial cylinder walls,

thickness of a thick cylinder wall was taken as being

15% that of the internal radius, whereas for the thin

cylinder, it was taken as being 5% of the radius. Large

cylinders were built to occupy 60% of the cavernosal

space, while small cylinders occupied 45% of it. The

earlier set of eight geometrical configurations enabled

analysis of various IPP designs by altering the material

properties of the cylinder walls and the intraluminal cyl-

inder pressures, as detailed in the following paragraphs.

Cylinder Material Properties

Inflatable prostheses are generally made of thermo-

plastic elastomers. Silicone rubber and Bioflex polyure-

thane a polymer which is more durable and less elastic

than silicone are the most common materials in current

use.7 Since this group of materials demonstrates a variety

of highly nonlinear stress-strain relationships, it is diffi-

cult to establish any generalized method for determining

characteristic material stiffness. In the present study, the

generally accepted characterization of the Young modu-

lus as an initial slope of the stress-strain curve was used,

yielding elastic moduli in the range of 101000 MPa.11

Consequently, for the present analysis, we defined soft

(E10 MPa , medium (E100 MPa , and hard (E

1000 MPa material types. The Poisson ratio of the

cylinders was set as 0.4 in all cases.

Cylinder Intraluminal Operational Pressures

Pescatory and Goldstein12 conducted in vitro experi-

ments with several commercial IPPs and measured mean

intraluminal cylinder pressures between 80 and 173 kPa

at maximum inflated volumes 1 kPa equals 7.5 mm Hg .

These pressure values are significantly higher than the

physiological cavernosal pressure during normal erec-tion, which is approximately 13.3 kPa.17 This difference

appears to be mostly due to the fundamental differences

between the behaviors of the natural anatomy of the

penis and its structure post-IPP-implantation. In the natu-

ral penis, internal stresses during erection are borne not

only by the main structural framework of the penis, the

tunica albuginea,1,5 but also by the intracavernous struc-

ture of fibromuscular columns which penetrate the cor-

pora and attach to the intracavernosal vasculature as

well.12 During IPP implantation, most of this tissue is

removed to provide space for the cylinders which be-

come the main structural support. In order to examine

the role of the cylinder intraluminal pressure, we simu-

lated conditions of low ( P i80 kPa and high ( P i173 kPa inflation pressures.

RESULTS

The model was utilized to study penile stress distri-

butions following implantation of an IPP in each of the

cases shown in Fig. 1. Results are presented during full

IPP-aided erection and in terms of von Mises equivalent

stress Eq. 1 . Figure 3 demonstrates the resulting stress

distributions during IPP-aided erections for dorsally po-

sitioned thin and thick circular cylinders subjected to low

and high intraluminal pressures. The results for each case

are shown for only one half of the symmetric transverse

cross-section: the cylinders made of the hard material are

shown on the left side and those of the soft one appear

on the right. In all cases, the tunica albuginea, which is

considered the structural framework of the penis,5 is

shown to be the most stress-loaded biological compo-

nent. With the decrease in stiffness of the prosthetic

cylinders, i.e., from hard to soft material characteristics,

the load carried by the tunica albuginea dramatically

increases by a factor of approximately 8 Fig. 3 a .

Obviously, some of the elevated tunical stresses are also

transferred to the dense network of nerves that is inter-

laced within this structure.

The combination of soft cylinders and high inflation

pressures not only causes further loading of the ventral

aspect of the tunica around the urethra , but also inducesa region of concentrated compression stresses at the cen-

tral part of the dorsal aspect of the penis Fig. 3 b in

the vicinity of the dorsal artery. When thin and soft

cylinders are used, these phenomena become even more

pronounced, as elevated compression stresses spread at

the dorsal aspect of the penis, reaching up to the skin

surface if intraluminal pressures are kept high Figs. 3 c

and 3 d / . The thin cylinders also tend to lose their

original circular cross section during inflation, eventually

adopting an egg-shaped cross-sectional area whose local

small-radii edge is compressed against the dorsal nerve

roots and blood vessels. Table 1 details the results of a

quantitative performance analysis for the circular cylin-ders when implanted at the dorsal aspects of the corpus

cavernosa. According to the criterion of min , IPP

cylinders ideally should be thick, made of a hard elas-

tomer, and inflated to low intraluminal pressures. Reduc-

tion of the cylinder thickness by 65% increased the mean

stress values by a factor of about 2.1. Reduction of the

Young modulus of the cylinders by one order of magni-

tude increased the average stress values by a factor of


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about 2.3. Alteration of the intraluminal pressure from

low to high values also raised the stresses by a factor of

about 1.4.

In order to examine if the biomechanical compatibil-

ity of circular cylinders could be enhanced by position-

ing them as far as possible from the dorsal nerve rootsand blood vessels, we analyzed cases of large and small

cylinders positioned lower within the corpora cavernosa,

toward the ventral aspect of the penis Figs. 1 c and

1 d . Analysis of the stress distributions that had re-

sulted due to inflation of the large cylinders showed that

positioning of the cylinders close to the ventral aspect of

the corpora only about 4 mm lower than in Figs. 1 a

reduced stresses at the vicinity of the dorsal nerves ()

by as much as 20%40%, compared with the reference

case Figs. 4 a and 4 b . Such positioning, however,

also increased the stresses around the urethra. Use of

cylinders with smaller diameter Figs. 4 c and 4 d not

only effectively eliminated this problem, but also mini-

mized the dorsal stresses to negligible levels Table 1 . Itshould be borne in mind that small-diameter cylinders

will also have relatively less penile rigidity during IPP-

aided erection.

In the following stage, interaction of the penis with

elliptic cylinders was analyzed and compared to perfor-

mances of circular cylinders. Since the combination of

thick walls, hard material, and low inflation pressure was

shown to provide the best results for circular cylinders,

FIGURE 3. Distribution of vonMises stresses during IPP-aided erection for dorsally po-sitioned circular cylinders witha thick walls subjected to lowintraluminal pressure; b thickwalls subjected to high intralu-minal pressure; c thin wallssubjected to low intraluminalpressure; and d thin wallssubjected to high intraluminalpressure. For each of the ear-lier cases, three diagrams areshown: the left one is thestress distribution for the hardcylinders, the middle one is forthe soft cylinders, and the rightone presents curves ofstresses along the line S loga-rithmic scale for both the hardsolid line and soft dashedline cylinders.


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all elliptic cylinders were constructed using the same set

of design parameters. Hence, the analysis of elliptic cyl-

inders was focused on the effect of prosthesis dimen-

sions, position, and orientation, aiming to further im-

prove the performances of circular cylinders. The

resulting stress distributions and prosthesis deformations

during IPP-aided erections that were obtained by using

the elliptic cylinders are depicted in Fig. 5. The results of

the performance analysis for these cylinders based on

the criterion of min ) are detailed in Table 2. These

results clearly demonstrated that all the elliptic cylinderstested in this study generated significantly higher stresses

at the vicinity of the dorsal nerve roots and blood ves-

sels, compared with circular cylinders of identical thick-

ness, material properties and intraluminal pressure.

In an attempt to minimize stresses in the vicinity of

dorsal nerve roots and blood vessels, large and small

elliptic cylinders were prepositioned with the large-radii

curve of the ellipse facing the dorsal-central surface of

the penis Figs. 1 e and 1 f . This approach was shown

to be unsuccessful since the poles of the elliptic cross

section are inverted due to the nonhomogeneous resis-

tance of the surrounding tissues during inflation of the

cylinders. Thus, at the end of the inflation process, asartificial erection is achieved, the small-radii boundaries

of the cylinders are directly facing toward the dorsal-

central surface of the penis, loading the dorsal nerve

roots and blood vessels Figs. 5 a and 5 b . This phe-

nomenon is well reflected by the values of obtained

for these two cases Table 2 . For instance, for the

large inclined elliptic cylinders is as much as three times

higher than the corresponding value obtained for the

large, dorsally located thick circular cylinders with iden-

tical material properties. Use of small, laterally medially

orientated elliptical cylinders limited the inversion of the

ellipse poles Fig. 5 c . This configuration, however,

induced highly elevated stress sites at the medial, ventral,

and dorsal aspects of the penis, thereby providing the

poorest results among the set of elliptic cylinders in

terms of Table 2 . Rotation of the cylinders so that

their long axis aligned with the dorsal and ventral direc-

tions moderately reduced stresses at the dorsal aspect of

the penis, but inversion of the poles during inflation

caused stress concentrations to appear at both lateral

aspects.

DISCUSSION

It is commonly accepted that adequate mechanical

interaction between the artificial implant and the sur-

rounding biological tissues in vivo plays an important

role in the success of insertion of a penile prosthesis. It

has been shown that mechanical factors such as cylindercollapse or tissue stiffening are significant causes for

severe penile pain due to stress concentrations and ex-

cessive local deformations during IPP-aided erection.3,8

Elevated local stresses and deformations may also accel-

erate cylinder wear and malfunction in the form of an-

eurysms or tubing fluid leaks that were reported to occur

in 1%4% of the cases.9,15 In view of the earlier circum-

stances which eventually require a traumatic and often

expensive revision procedure to replace the faulty im-

plant, it is highly recommended that both the design of

the IPP and the surgical positioning of its cylinders

within the penis be aimed towards minimal stress trans-

fer to nerves and blood vessels during IPP-aided erec-tions.

In order to optimize the in vivo performances of IPPs

by reducing the stresses transferred to the penile soft

tissues as much as possible, we utilized a recently devel-

oped structural model of the penis/IPP complex. The

model was adapted to analyze not only the effect of the

IPP design on its biomechanical compatibility, but also

the outcomes of surgical decisions of positioning and

alignment of the cylinders within the corpora. The simu-

lation of stress distributions at various postimplantation

conditions indicated that most of the load during IPP-

aided erection is carried by the dorsal and lateral aspects

of the tunica albuginea, which are also the most vulner-able sites since they contain the penile nerve roots and

major blood vessels. Our objective was to minimize

stresses at these regions, and the results of the present

analysis suggest that an ideal IPP could be designed to

more closely mimic the structural behavior of the human

penis by achieving functional rigidity at lower cylinder

pressures and preferably at pressures that approach the

physiological value of about 13.3 kPa . The results also

TABLE 1. Average von Mises stresses through a

representative section A A in the region of primary penile

nerves and blood vessels for dorsally and ventrally

positioned circular cylinders.

Average stress transferred to nerves

and blood vessels (kPa)

Dorsally positioned cylinders

Thin cylinder wall Thick cylinder wall

Cylinder

material

E (MPa)

Low

pressure

High

pressure

Low

pressure

High

pressure

10 176 246 128 160

100 71 103 46 65

1000 58 74 16 23

Ventrally positioned thick cylinders

Large cylinders Small cylinders

Low

pressure

High

pressure

Low

pressure

High

pressure

1000 10 19 2 4


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indicate that both thickness and stiffness of the cylinders

should be kept sufficiently high 15% of the radius and

E1000 MPa, respectively in order to eliminate large

cylinder deformations during inflation, the result of

which may be substantial contact stresses between the

cylinders and the cavernosal tissue. Smaller prostheticcylinders, i.e., occupying about 45% of the cavernosal

space, may be advantageous in terms of reducing dorsal

stresses, but relatively less penile rigidity should be ex-

pected. Insofar as this trade-off relation may have little

negative effect on sexual satisfaction while it may be

highly efficacious in avoiding the development of penile

pains during prosthetic-aided erection, further investiga-

tion is warranted.

Significant decreases of up to 40% relative to the

reference case in the mean stress values at the dorsal sensitive part was shown to result from the selection of

a 4 mm lower position for the cylinders. Current surgical

techniques do not allow for very highly accurate manipu-

lation of cylinder positioning, mainly because the dilata-tors used to clear intracavernosal space for the cylinders

during the operation are not designed to provide precise

control of the dilatation positioning. However, based on

the simulation results, we believe that the implanter

should aim toward the ventral aspect, considering that

even minimal lowering of the cylinder position would be

highly advantageous in terms of decreasing the stresses

within the dorsal penile tissues. This and other possible

FIGURE 4. Distribution of vonMises stresses kPa duringIPP-aided erection for ventrallypositioned and thick-walledcircular cylinders: a large cyl-inders subjected to low intralu-minal pressure; b large cylin-ders subjected to highintraluminal pressure; c smallcylinders subjected to low in-traluminal pressure; and dsmall cylinders subjected tohigh intraluminal pressure. Foreach of the earlier cases, three

diagrams are shown: the leftone is the stress distributionfor the hard cylinders, themiddle one is for the soft cyl-inders, and the right one pre-sents curves of stresses along

the line S logarithmic scalefor both the hard solid lineand soft dashed line cylin-ders.


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implications of the present findings on the surgical tech-

nique and tools for IPP implantation should now be

experimentally evaluated in animal models in order to

test their validity in vivo.

Based upon the set of configurations tested in the

framework of the present study, it was clearly demon-

strated that circular cylinder cross sections allow fargreater biomechanical compatibility of the IPP with the

penis structure compared with elliptic cylinders. Inver-

sion of the elliptic poles was observed in three out of

four configurations containing elliptic cylinders due to

nonhomogenous resistance of the surrounding tissues to

inflation. This phenomenon, which not only causes dif-

ficulties in predicting the final steady state of deforma-

tion of the cylinders in vivo, but also induces undesired

stress concentrations which compress nerves and blood

vessels in the regions of the inverted poles, suggests that

an elliptic cross section is less suitable for IPP cylinders.

Nevertheless, clinical studies comparing performances of

commercially available prostheses of both types in ad-

equate numbers of patients are still required before spe-

cific manufacturing recommendations could be estab-lished.

Undesirable alteration of the cross-sectional shape of

the prosthetic cylinders toward a more elliptic profile

increases the risk for localized collapse under compres-

sion conditions, mainly due to the effect of buckling.

Buckling collapse commonly induces sharp geometry

transitions within the deformed prosthesis, as demon-

strated by magnetic resonance imaging MRI by

Moncada et al.8 These sharp geometry transitions are

clearly capable of inducing sites of concentrated me-

chanical stresses within the surrounding penile tissues, in

which mechanical failure could appear in the form of

microtears. If some contamination of the cylinders occursduring the implantation procedure, infection will develop

within these sites, leading to pain. Since the present

study deals with a sterile model, the issue of infection is

not applicable, and the discussion in the following para-

graph is limited to the structural behavior of the penis/

prosthesis complex.

Buckling of an inflatable penile prosthesis during coi-

tus may be the result of axially applied forces associated

with initial vaginal penetration or axial compressive and

lateral constraining forces induced by contact with the

vaginal walls during continued intercourse. The Euler

formula for buckling provides the critical force which

induces buckling of a column, FbEI/L 2, where E isthe Young modulus, L is the column length, and I is the

second moment of inertia of the column cross section.

The buckling force is, therefore, linearly proportional to

the moment of inertia of the column cross section. In the

case of a circular column, the moment of inertia is IcD4/64 where D is the column diameter , while in

the case of an elliptic column, the moments of inertia are

Ixa3b/64 and Iyab

3/64 where a and b are the

TABLE 2. Average von Mises stresses through a

representative section AA in the region of primary penile

nerves and blood vessels for thick-walled and hard elliptic

cylinders positioned at different alignments and subjected to

low intraluminal pressure Pi80 kPa.

Cylinder type and alignment (kPa)

Large and inclined cylinders [Fig. 1(e)] 48

Small and inclined cylinders [Fig. 1(f)] 38

Small and laterally medially aligned cylinders[Fig. 1(g)]

72

Small and dorsally ventrally aligned cylinders

[Fig. 1(h)]

35

FIGURE 5. Distribution of von Mises stresses during IPP-aided erection for elliptic and thick-walled hard cylinderssubjected to low intraluminal pressure: a large and inclinedcylinders Fig. 1e; b small and inclined cylinders Fig.1f; c small and laterally medially aligned cylinders Fig.1g; and d small and dorsally ventrally aligned cylindersFig. 1h. The dashed, white line marks the neutral cylindergeometry for each case flaccid mode.


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dimensions of the ellipse on its long and short axes,

respectively . It can be shown that for any given pair of

circular and elliptic columns, Ic is always greater than

min(Ix ,Iy , provided that the cross-sectional area of both

columns is identical. Accordingly, elliptic cylinders will

normally buckle in response to smaller loads compared

with circular cylinders of the same cross-sectional area.

Since the present study showed that cylinders of circularcross-sectional shape could adopt a more elliptic profile

due to inflation, the risk of buckling and localized col-

lapse in these cases would need to be considered.

In order to deal with the structural complexity of the

penis and, especially, with the lack of experimental data

on the nonlinear behavior of its tissues, assumptions

were made for the purposes of simplification, and this

should be kept in mind while interpreting the results.

Penile tissues were assumed to be isotropic and linearly

elastic. This assumption is highly likely to be adequate

for analysis of the tunica albuginea, which is a dense

parallel-fibered collagenous tissue5 and is, therefore, ex-

pected to produce a stress-strain curve with a consider-

ably long linear part. Since the tunica albuginea was

shown to be the main load-bearing structure in the penis,

predictions of the present analysis could be considered

realistic. Nevertheless, in order to estimate the effects

of some possible nonlinear elasticity of the tunica on the

simulation results, we replaced the tunical elastic modu-

lus with a nonlinear constitutive law of a ligamentous

tissue,11 which is of similar biological structure. The

resulting stresses were greater by 10%30%, depending

on the prosthesis type and location. In view of the ear-

lier, experimental data that reveal the nonlinear charac-

teristics of the penile tissues are needed. After these datahave become available, a quasilinear viscoelastic ap-

proach could be useful to obtain a more accurate repre-

sentation of the structural behavior of the penis.

The computational methodology of this study presents

powerful biomechanical tools for optimal design, devel-

opment, and examination of penile implants to improve

postimplantation outcomes. Implementation of this or

similar approaches as an integral part of the engineering

design process of an IPP may not only enhance its per-

formances, but could also yield important surgical guide-

lines in terms of the preferred position and alignment of

the prosthesis within the penis. In the future, the presentapproach could be further expanded into a pre-operative

routine for evaluation of surgical procedures for indi-

vidual patients. In such cases, the real cross-sectional

anatomy of a specific patient acquired by ultrasound or

MRI will be directly incorporated in a finite element

model. The surgical procedure can then be individually

planned and its biomechanical consequences can be

simulated before undertaking the actual intervention.

CONCLUSIONS

The present numerical simulations demonstrate that

optimal selection of engineering designed parameters for

the IPP e.g., cylinder geometry and stiffness can elimi-

nate substantial cylinder-tissue contact stresses, thereby

reducing the likelihood for postoperative complications.

By allowing control of anatomical structural parameters,

as well as those of the prosthesis, the methodology ofthis study could also be applied to test the biomechanical

implications of different surgical positions of the pros-

thetic cylinders. Hence, the use of the present modeling

approach in the analysis of penile prosthesis implantation

procedures significantly increases clinical and research

opportunities by delineating what are the experimental

studies required for the establishment of specific manu-

facturing and medical recommendations.

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