DEVELOPMENT OF AN ELECTRICAL STIMULATION DEVICE

FOR OSSEOINTEGRATED AMPUTEES

A Novel Approach for Expediting Skeletal Attachment and Rehabilitation

Brad Isaacson

1,2

, Jeroen Stinstra

, Rob MacLeod

2,3

and Roy Bloebaum

1,2,4

Department of Veteran Affairs, Salt Lake City, UT, U.S.A.

Department of Bioengineering, Univerisity of Utah, Salt Lake City, UT, U.S.A.

Scientific Computing Institute, University of Utah, Salt Lake City, UT, U.S.A.

Department of Orthopedics, University of Utah, Salt Lake City, UT, U.S.A.

Keywords: Osseointegration, Electrical stimulation, Osteogenesis, Percutaneous, Amputation.

Abstract: The projected number of American amputees is expected to rise to 3.6 million by 2050. Many of these

individuals depend on artificial limbs to perform routine activities, but prosthetic suspensions using

traditional socket technology can prove to be cumbersome and uncomfortable for a person with limb loss.

Moreover, for those with high proximal amputations, limited residual limb length may prevent

exoprosthesis attachment all together. Osseointegration technology is a novel operative procedure that

allows integration between host tissue and an orthopaedic implant and has been shown to improve clinical

outcomes by allowing direct transfer of loads to a bone-implant interface. However, the associated surgical

procedures require long rehabilitation programs that may be reduced through expedited skeletal attachment

via electrical stimulation. To determine optimal electrode size and placement, we have developed a system

for computational modeling of the electric fields that arise during electrical stimulation of residual limbs.

Three patients with retrospective CT scans were selected and three dimensional reconstructions were

created using customized software (Seg3D and SCIRun). These software packages supported the

development of patient specific models and allowed for interactive manipulation of electrode position and

size; all variables that could affect the electric fields around a percutaneous osseointegrated implant.

Preliminary results of the electric fields at the implant interface support the need for patient specific

modeling in order to achieve the homogenous electric field distribution required to induce osteoblast

migration and enhance skeletal fixation.

1 INTRODUCTION

Osseointegration implant technology is a novel

surgical procedure that provides direct skeletal

attachment between an implant and host tissue and

can significantly increase the quality of life for

amputees (Albrektsson & Albrektsson, 1987;

Branemark, 1983). However, one challenge with

using natural biological fixation is attaining a strong

skeletal interlock at the implant interface, a

prerequisite for long-term implant function

(Albrektsson, Branemark, Hansson, & Lindstrom,

1981). Therefore, we propose a means to accelerate

osteogenesis through external electrical stimulation

and present a simulation approach for which to

develop such a system.

Veterans with combat related injuries form an

especially relevant population that requires the

development of new tools to enhance the success of

osseointegration, due to their limited residual limbs

caused by explosive devices. Improvements in

medical care and evacuation strategies have led to an

increase in survival rates, resulting in an elevated

number of veterans with amputations that require

follow-up care and extensive rehabilitation. The

relative youth and otherwise good health of these

amputees make them an ideal population for

aggressive rehabilitation but also reveal the

limitations of current technologies of prosthetic

attachment (Hagberg & Branemark, 2001). Physical

limitations with warrior amputees using sockets

include heat/sweating in the prosthetic socket, skin

irritation and inability to walk on challenging terrain

(Hagberg & Branemark, 2001). In addition, a

significant number of returning service men and

178

Isaacson B., Stinstra J., MacLeod R. and Bloebaum R. (2009).

DEVELOPMENT OF AN ELECTRICAL STIMULATION DEVICE FOR OSSEOINTEGRATED AMPUTEES - A Novel Approach for Expediting Skeletal

Attachment and Rehabilitation.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 178-185

DOI: 10.5220/0001511501780185

 SciTePress

women have short residual limbs for which socket

technology is not suitable.

Utilizing metallic implants as a means of

biological fixation has been the objective of

orthopedic surgeons over the past two centuries

(Williams, 1982). However, controlling osteogenesis

at the implant interface, which is essential for

providing strong skeletal fixation, remains

challenging. Regulated electrical stimulation has

proven effective in fracture healing and non-

traumatized bone models (Brighton, 1981;

Friedenberg, Zemsky, Pollis, & Brighton, 1974), but

has not been investigated in a percutaneous

osseointegrated implant system. One advantage of

this patient population is that an orthopedic implant

protrudes from the residual limb functioning as an

exoprosthesis attachment and a potential cathode for

an external electrical stimulation device.

By understanding the method of current injection

with varying electrode size and placements, an

electric field on the magnitude of 1-10 V/cm may be

established at the implant interface, capable of

inducing osteoblast migration and improving

skeletal attachment (Ferrier, Ross, Kanehisa, &

Aubin, 1986). An electric field of this degree may

increase the quantity and quality of bone at the

implant interface, and thus would improve the

prospects for accelerated rehabilitation and skeletal

fixation for an amputee. Figure 1 contains a

schematic of a stimulation apparatus that we

envision.

Figure 1: Representative model of the electrical

stimulation device proposed for amputees with a

percutaneous osseointegrated implant.

The initial approach to develop an electrical

stimulation osseointegration system is to simulate

electric field strength on a patient specific basis

using a computational model. Because variations in

electrode size and placement could inhibit or

expedite bone growth, the modeling approach

includes both the electrode placement for

stimulation at the skin as well as patient specific

anatomy to determine an accurate estimate of the

underlying electrical simulation fields.

The objective of our research is to develop an

alternative means of prosthetic suspension superior

to the traditional socket method for patients with

amputations. This goal is strongly motivated by the

growing rate of amputations occurring annually

from vascular occlusive diseases, diabetes, and

traumatic injury. While an amputation can be a life

saving surgical procedure, many interpret the

operation as a significant loss, which alters “the

perception of an individual of his or her degree of

physical, psychological and social well-being and

the effects that illness and treatment have on daily

life” (Hagberg & Branemark, 2001). We

demonstrate here the feasibility of a computational

approach to developing such a system.

2 METHODS

2.1 Overview

In order to simulate the electric field that is present

at the bone-implant interface, three patient specific

models were created. A volume conductor model

was developed to compute realistic electric fields

from computer tomography (CT) scans of the limbs

of these patients by assigning tissue conductivities

during segmentation. Using this model and

assuming that the electrical field can be calculated

using a quasi-static approach, the electrical potential

was computed by solving Laplace’s equation for

each tissue type.

∇

⋅

∇

(1)

In this model the boundary conditions were

formed by the electrodes that injected currents and

the guideline that current remained within the body.

Since the electrodes and the implant had a much

larger conductivity than the surrounding tissues, it

was assumed that the implant (cathode) was at a

constant potential, likewise the surface electrodes

were modeled with a constant potential difference

from the percutaneous implant.

Numerical simulation was used to compute the

electric potential through the CT scans of the

patient’s residual limb. To evaluate the efficacy of

electrode configuration and sizing, patient specific

models were developed and the electrical potential

around the implant interface was used to determine

localized electric field strengths.

Medullary Canal

DMM

Cathode

Anode

Power

Supply

Residual Limb

Muscle

Implant

Polyurethane foam saturated with saline solution

Electrode

DEVELOPMENT OF AN ELECTRICAL STIMULATION DEVICE FOR OSSEOINTEGRATED AMPUTEES - A Novel

Approach for Expediting Skeletal Attachment and Rehabilitation

179

2.2 Image Acquisition

CT images were obtained retrospectively from the

radiology department at the University of Utah in

accordance with Institutional Review Board (IRB)

approval. Femoral slice thicknesses ranged from 600

μm to 1 mm and of the 50 patients examined, 3 were

selected based on predetermined demographics and

absence of metallic implants, which cause image

artifacts. Table 1 lists patient specifics.

Table 1: Patient Demographics.

To determine the variability amongst patients,

only one amputee was selected from the population

(Patient 3). The remaining two models were

generated from subjects in the general population

who were made into “artificial amputees” from

segmentation using computer software. To account

for natural anatomical differences in patient limbs,

wide variation in age (SD = 26.2), height (SD =

17.8) and weight (SD = 18.7) were selected.

Establishing accurate tissue differentiation was

performed using the Seg3D (www.seg3d.org)

software. The tissue boundaries of the bone, bone

marrow and adipose tissue were generated by

thresholding the CT images interactively. The

musculature was obtained by manually setting seed

points inside the muscle tissue and using a

confidence connected filter to find all the tissue

connected to the seed points. Finally, the skin, which

was impossible to discern reliably from the CT

images, was generated by dilating the outermost

tissue 2 millimeters based on average skin thickness

to produce a layer of homogeneous thickness that

surrounded the full model (Tortora & Nielsen,

2008). Segmentations were manually inspected,

corrected to ensure accuracy and combined in a

hierarchy into a single label map required for finite

element analysis. An example of a segmentation,

consisting of skin, adipose tissue, muscle, bone and

bone marrow is depicted in Figure 2.

2.3 Electrode Placement & Design

Since there are no preliminary results using such a

system for a percutaneous osseointegrated implant,

we selected four widely variable electrode

configurations for testing. Small electrodes were

designed with patient compliance in mind, since the

device should not restrict ambulation or daily

activity.

Figure 2: Hierarchical label maps constructed in Seg3D

for Patient 2. The completed femoral mapping was

composed of skin (A), adipose tissue (B), musculature

(C), bone (D) and bone marrow (D).

The SCIRun (software.sci.utah.edu) software

package was utilized for electrode design because it

supports interactive electrode placement and

simulation. The configurations selected consisted of

a one patch electrode, two patch electrodes, one

continuous band and two continuous bands (Figure

3). External electrode bands were applied on the

residual limb of the patient and were 1.6 cm in

thickness. Electrode patches were placed as a strip

covering approximately half the diameter of the leg

and were 3 cm in thickness. Lastly, the percutaneous

implants were set to match the size of the cavity of

the bone marrow to represent proper implant fit and

fill, since gaps in excess of 50 μm may lead to

fibrous encapsulation without bone ingrowth

(Bloebaum, Bachus, Momberger, & Hofmann, 1994;

Hofmann, Bachus, & Bloebaum, 1993).

Figure 3: Electrode configurations modeled to determine

optimal performance for amputees. The electrode

configuration is shown from patient 2. A two patch setup

was changed to a one patch setup by removing the medial

electrode (A). A double band electrode was altered to a

single band configuration by removing one band and

centering it amongst the implant area (B). Electrode size

and position are illustrated with arrows.

Patient Sex Amputee Age

Height

[cm]

Weight

[kg]

1 M No 60 185.4 79.9

2 F No 28 157.5 50.1

3 F Yes 80 152.4 45.5

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2.4 Finite Element Analysis

In order to predict electric fields from exogenous

voltage potentials, label maps and electrodes were

modeled using a hexahedral mesh that consisted of

approximately 1.7 million elements. The elements

were treated as piecewise homogenous, ohmic and

isotropic, and were assigned conductivities using

previously published values and estimations by the

investigators (Chiu & Stuchly, 2005; Gabriel, Lau,

& Gabriel, 1996; Stinstra et al., 2007) (Table 2).

Because tissue behaves inherently electrolytic,

treating the models with DC conductivities was

considered to be an important factor (Grimnes &

Martinsen 2008).

Electrodes were incorporated in the finite element

meshing and assigned a constant potential difference

of 9 volts between the skin electrode and

osseointegrated implant, a selection based on

expected tissue resistivity. Using an iterative solver,

the potentials in the finite element models were

computed for the three patients and four electrode

configurations.

Table 2: Conductivity values assigned to segmentations.

Tissue Type Conductivities [S/m]

Skin 0.26

Muscle 0.25

Adipose 0.09

Organ 0.22

Cortical Bone 0.02

Bone Marrow 0.07

2.5 Data Analysis

In order for an electrode configuration to be deemed

acceptable, a uniform homogenous field around the

implant interface was required for bone growth.

Therefore, histograms were computed for about

6000 elements in the immediate area encompassing

the bone implant interface. The results were

analyzed using maps of electric field strength that

show the variability of the electric field within the

leg.

3 RESULTS

Interactive placement of electrodes allowed for

various computational simulations. Figure 4

illustrates one example of the differences between

patients. The figure depicts cross sections for three

different patients, where the color scale indicates the

strength of the local field. The corresponding

histogram (right) represents the electric field

strength of the 6000 elements surrounding the

implant site and shows the homogeneity of the field.

The histograms showed a broad variation among

patients, some producing normal Gaussian

distributions and others with broad or skewed peaks

and thus a higher than random level of homogeneity.

The complete set of histograms used for analysis is

listed subsequently in Figure 5.

4 DISCUSSION

The necessity for patient specific models with a

percutaneous electrical stimulation device was

confirmed in the study. The distribution of electrical

potentials at the implant-bone interface varied across

subjects due to variations in anatomy and the

presence of an amputation. While creating “artificial

amputees” using a segmentation program was

straightforward and permitted robust computation,

histograms of electric field strength confirmed that

electrical metrics changed dramatically when

compared to a known amputee. While the shape of

the histograms tended to be bell shaped, the position

of the peaks were often skewed depending on the

electrode configuration and patient. The results

clearly showed that the 1 patch electrode generates

the smallest electric field in the bone-implant

interface, while the 2 band electrode configuration

generated the highest field for the same applied

potential, suggesting that proper electrode placement

could improve efficiency.

Due to the limited quantity of patients in the

study, a strong correlation was not established

relating patient demographics with voltage

potentials. However, the highest voltage gradients

mapped during simulations were consistently from

subject 2, a patient who was in the best physical

condition. The increased electrical field was likely

caused by the reduction in the diameter and

thickness of adipose tissue in the subject’s residual

limb, since adipose tissue would raise resistivity and

impede current flow.

While an optimal electrical configuration may

not have been established for the patient population

collectively, two bands appeared to produce the

most homogenous electrical field distributions

between 1-10 V/cm. Minor adjustments would be

required if the device were used clinically to account

for the varying anatomy of patients, spatial location

of topical electrodes and may be confirmed with CT

files and computational modeling.

DEVELOPMENT OF AN ELECTRICAL STIMULATION DEVICE FOR OSSEOINTEGRATED AMPUTEES - A Novel

Approach for Expediting Skeletal Attachment and Rehabilitation

181

ELECTRIC FIELD

Figure 4: Sample distributions of the electric field strength surrounding the implant. The color map reflects the strength of

the electric field in a cross section through the lower part of the limb. External electrode placements were illustrated as

rectangular objects on the outside of the residual limb. The results were for an 18 centimeter percutaneous implant that was

interactively inserted into the medullary canal of adult patients. The histograms on the right represent the distribution of the

electric field strength in the volume next to the implant.

While the initial target of the exogenous

electrical stimulation system utilizes an orthopedic

implant as a functional cathode, it may also reduce

the potential for superficial and deep infections by

preventing additional surgical procedures to remove

implanted devices as seen with older fracture

healing models (Lavine & Grodzinsky, 1987).

However, the success of the system is dependent on

numerous factors including hydration levels,

quantity of soft tissue and the material type selected

for the orthopedic implant. In order for this novel

technology to be beneficial, a balance must be

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Electrode Type Patient 1 Patient 2 Patient 3

1 Band Electrode

2 Band Electrodes

1 Patch Electrode

2 Patch Electrodes

Figure 5: Comparison between patients and electrode configurations. The results confirm the requirement for individual

patient modeling. Distributions of electric fields were not homogenous in each case and would require manipulation of the

applied voltage potential to attain uniform bone ingrowth.

attained between obtaining a desired electric field

and the host tissue reaction which may occur with

varying implanted metals. Titanium alloy was

selected as the cathode in this model (3x10

S/m)

since it is regarded as the one of the most

biocompatible material type for total joint

replacements and has low thermal conductivity to

protect tissue necrosis from heat generation (Agins

et al., 1988; Beder & Eade, 1956; Emneus &

Gudmundsson, 1967). However, if clinicians and

engineers require an altered rate of electron

propagation, the material can be exchanged

(Grimnes & Martinsen, 2008) or porosity altered but

careful attention should be paid to ensure the

material does not illicit a foreign body response.

Utilizing electrical stimulation for older

amputees is a critical aspect which must be explored

as well. Bone mass is maximum a decade after

skeletal growth ceases but decreases significantly by

the eighth and ninth decade (Buckwalter, Glimcher,

Cooper, & Recker, 1995). As long bones change

confirmation with age, the endosteal diameter tends

to increase more rapidly than the periosteal diameter

which could lead to implant loosening (Lane &

Vigorita, 1983). This problem coupled with the

reduction of strain on bones by weaker muscles may

DEVELOPMENT OF AN ELECTRICAL STIMULATION DEVICE FOR OSSEOINTEGRATED AMPUTEES - A Novel

Approach for Expediting Skeletal Attachment and Rehabilitation

183

contribute to debilitating diseases such as

osteoporosis and osteopenia (Lane & Vigorita,

1983) and require additional treatment options.

However, controlled electrical stimulation and

mechanical loading may act as a synergistic catalyst

of bone ingrowth (Spadaro, 1997) and maintain host

bone bed integrity with elderly patients using an

osseointegrated electrical implant system.

Establishing tools for enhancing skeletal

attachment may assist with reducing the length of

rehabilitation required for an osseointegrated

procedure. Current programs require 2 to 24 months

of rehabilitation (Branemark, Branemark, Rydevik,

& Myers, 2001), a lengthy time period to ensure

uniform ingrowth. Because of the slow biological

process of skeletal attachment, loading at the

implant interface would be restricted for several

months after the operative procedure (Hofmann,

Bloebaum, & Bachus, 1997). However, using an

electrical stimulation system may enhance ingrowth

and allow patients to return to earlier ambulation.

4.1 Limitation

Since the conductivity of a titanium implant

significantly exceeded that of cortical bone, the

current densities at the implant interface should be

modeled to ensure localized tissue heating does not

occur, which may lead to patient discomfort or

potential tissue necrosis. Computational modeling of

current density fields are attainable with the given

software package and will be utilized in future work.

Additional efforts with be paid to altering the

porosity of the titanium implant and determining the

effect on the predicted electric fields since porosity

and conductivity are inversely related and may

affect the model as well (Ke et al., 2007).

5 CONCLUSIONS

The simulations developed for the proposed

biomedical device may have the capabilities of

expediting skeletal attachment by increasing

osteoblast migration. Computation modeling has

effectively shown that 1-10 V/cm electric fields may

be generated using the implant as a functional

cathode and topically applied anode band and

patches. Implementing computational models may

be the first step to resolving the classic problem with

electrical stimulation which is the inability to define

current pathways in the human body (Chakkalakal &

Johnson, 1981; Noda & Sato, 1985).

Patient specific modeling was effective for

attaining values that may be osteogenic at the

implant site, but wide variations in electric field

distributions shown in histograms reaffirm the need

to evaluate each case specifically. However, in order

to determine the accuracy of finite element analysis,

the quantity of subjects will be increased in the

future work to determine if an electrode

configuration could be optimized for patients with

percutaneous osseointegrated implants. Additional

model validation of electrically enhanced

osseointegration will be assessed using a small in

vivo animal model based on computational evidence

in future work.

ACKNOWLEDGEMENTS

This material is based upon research supported by

the Office of Research and Development,

Rehabilitation R&D Service, DVA SLC Health Care

System, Salt Lake City, Utah; the Albert & Margaret

Hofmann Chair and the Department of

Orthopaedics, University of Utah School of

Medicine, Salt Lake City, Utah. Additional technical

support for the simulations was provided by the

Center for Integrative Biomedical Computing of

Scientific Computing and Imaging Institute and was

made possible in part by software from the

NIH/NCRR Center for Integrative Biomedical

Computing, P41-RR12553-07.

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