Magnetic field strength properties in bone marrow during pulsed electromagnetic stimulation

doi:10.4236/jbise.2010.312150

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J. Biomedical Science and Engineering, 2010, 3, 1156-1160

doi:10.4236/jbise.2010.312150 Published Online December 2010 (http://www.SciRP.org/journal/jbise/

JBiSE

Published Online December 2010 in SciRes. http://www.scirp.org/journal/JBiSE

Magnetic field strength properties in bone marrow during

pulsed electromagnetic stimulation

Hir oyuki Tamaki1, 2, Kengo Yotani3, Atsumu Yuki4, Hikari Kirimoto1, Kazuhiro Sugawara1, 2,

Hideaki Onishi1, 2

1Institute for human movement and Medical Sciences, Japan;

2Department of Physical Therapy, Niigata University of Health and Welfare, Niigata, Japan;

3Department of Physiological Sciences, National Institute of Fitness and Sports, Kanoya, Japan;

4Department of Physical Education, Aichi University of Education, Kariya, Japan.

Email: [email protected]

Received 4 October 2010; revised 7 October 2010; accepted 8 October 2010.

ABSTRACT

We clarified the characteristics of pulsed electro-

magnetic field (PEMF) strength in marrow cavity

with bone marrow in long bones based on actual

measurements taken during pulsed magnetic stimu-

lation (PMS). Measurements were made under 810

different conditions of stimulation intensity, distance,

and position. Significant and strong linear correla-

tions were observed between PEMF strength and

stimulation intensity. PEMF strength in marrow cav-

ity during PMS showed an exponential decay de-

pending on coil-sensor distance, with a breaking

point at approximately 30 mm. PEMF strength dis-

tributions in bone showed geometric differences be-

tween 3 types. These findings suggest that PEMF

strength in bone depends on stimulation intensity,

distance and horizontal position. Our actual meas-

ured data could be useful in determining stimulation

programs and estimating the in vivo efficacy of

PEMF in marrow cavity for research and clinical use.

Keywords: Magnetic Stimulation; Bone Tissue; Focality;

Geometry; Intensity

1. INTRODUCTION

Pulsed electromagnetic field (PEMF) stimulation has

been used clinically to treat bone disorders and report-

edly promotes osteogenesis, in part through direct ac-

tions on osteoblasts. However, such clinical success

contrasts with negative reports regarding the effects of

pulsed magnetic stimulation on cellular proliferation,

differentiation and mineralization of osteoblasts in vitro

[1,2]. Differences in response to biophysical factors

seemed to depend on the specific conditions of pulsed

magnetic stimulation, e.g., stimulation intensity, fre-

quency, exposure time and stage of osteoblast matura-

tion [3]. In particular, stimulation intensity depends on

stimulator output, distance and position [4,5]. Only a

few in vivo studies have shown PEMF effects on bone

formation processes in tissue, protein and mRNA levels

[6-8]. An accompanying problem is the difficulty in

determining magnetic field strength inside bone in vivo,

as characterization of magnetic field strength in bone

during PEMF stimulation under different conditions

remains incomplete. A measurement of PEMF strength

in marrow cavity into which a magnetic sensor probe

has been inserted would enable us to obtain the data

applicable to in vivo studies [5,8]. This study clarified

the characteristics of PEMF strength in marrow cavity

with bone marrow in long bones during pulsed mag-

netic stimulation under 810 different conditions of

stimulation intensity, distance, and positions.

2. METHODS

The end of a magnetic sensor probe (length, 101.6 mm;

sensor location, 8.5 mm from bone end; active area,

3.8-mm diameter) was placed into the distal metaphysis

of a femur removed from male Wistar rats at 30-33

weeks old (n = 6). Three-dimensional micro-CT images

of long bones (Skyscan 1076®, Skyscan, Aartselaar,

Belgium) were obtained to confirm the positions of sen-

sor and wire in the marrow cavity (Figure 1) [8].

Bones were set on a precision horizontal stage that

was able to be manually moved in 3 dimensions (X, Y

and Z axes). A 9-cm-diameter magnetic coil was held in

a clamp horizontally and placed with the center of the

coil right above the active area of the sensor probe. The

magnetic coil and sensor probe were connected to a

magnetic stimulator (Magstim 200; The Magstim Com-

pany, Whitland, UK) and a gaussmeter (5180; FW Bell,

Orland, USA), respectively. The PEMF was generated

H. Tamaki et al. / J. Biomedical Science and Engineering 3 (2010) 1156-1160 1157

Figure 1. A schematic of the experimental setup. Measure-

ments were performed at 9 horizontal positions (A-I) on the

precision stage (X-Y axis) and at 9 coil-sensor distances. Sen-

sor position was confirmed with μCT scanner and a laser beam

so as to be centred (E) on the 3  3 grid points at 10-mm inter-

vals before the experiment.

by a commercially available, clinically approved mag-

netic stimulator (Magstim 200; The Magstim Company)

which was able to generate a maximum magnetic field

of 2 T. Stimuli were delivered at 10 stimulation intensi-

ties from 10-100% in increments of 10% of the maximal

stimulator output. A total of 81 different measurement

positions were set to determine a distribution map of

individual magnetic field strengths in marrow cavity

using the 3-dimensional drive precision stage, that is, at

9 coil-to-sensor distances from 10-90 mm with intervals

of 10 mm (Z axis) and at 9 horizontal positions on 3  3

grid points with intervals of 10 mm (X-Y axes) (Figure

1). Peak PEMF strength (magnetic flux density) during

PEMF stimulation was measured 3 times for each condi-

tion using the gaussmeter (5180; FW Bell) [9] with an

analog output of 100 kHz sampling rate, ±0.75% accu-

racy and DC-30 kHz bandwidth. Signals from each

measurement were checked using an oscilloscope and

subsequently digitized at a sampling frequency of 100

kHz using a 16-bit A/D converter (Power Lab; AD In-

struments, Tokyo, Japan) and stored on a personal com-

puter for later analysis. Each instrument was calibrated

immediately before data collection. The same procedure

for measurement was followed without the bone at the

end of a magnetic sensor probe as a control. Repeatabil-

ity was assessed on 2 separate days in a pilot study under

conditions of 30 mm magnetic sensor-coil distance and

position E as in Figure 1 on the X-Y axis at stimulation

intensities of 10-100%.

Precision of measurements for peak PEMF strength

was calculated as the percent coefficient of variation

(CV%) using data from 100 repeated measurements un-

der each individual condition. Bland-Altman analysis

[10] was used to evaluate systemic bias and the limits of

agreement between measured values obtained on

test-retest. Linear regression analysis was used to deter-

mine correlations between stimulation intensity and

PEMF strength values. Non-linear equations were used

to describe the relationship between PEMF strength and

sensor-coil distance. Analysis of variance (ANOVA) was

used to compare PEMF strength as a function of stimu-

lation intensity, sensor-coil distance and position. Data

are presented as mean ±standard deviation (SD), with

values of P < 0.05 considered statistically significant. All

study protocols were approved by the ethics committee

of the National Institute of Fitness and Sports and con-

ducted in compliance with the Declaration of Helsinki.

3. RESULTS

CV% for repeated measurements and Pearson’s correla-

tion coefficient for the test-retest of PEMF strength mea-

surements in bone during PEMF stimulation were 0.19-

0.97% and r = 1.00 (P < 0.0001), respectively. Bland-

Altman analysis showed a mean difference (bias) of 0.1

mT and 95% limits of agreement of ±1.21 mT, demon-

strating excellent agreement between test-retest values.

A significant strong linear correlation was seen be-

tween PEMF strength and stimulation intensity at each

position and coil-sensor distance (r2 = 0.99-1.00, P <

0.0001) (Figure 2). PEMF strength in marrow cavity

during PEMF stimulation showed an exponential decay

depending on coil-sensor distance at each stimulation

intensity, with a breaking point at approximately 30 mm

distance, except for at positions C, F and I (r2 =

0.97-0.99, P < 0.0001) (Figure 3). Significant reverse

correlations were noted between PEMF strength and

coil-sensor distance in positions C, F and I under all

stimulation intensity conditions (r2 = 0.84-0.96, P <

0.0001). Horizontal positions were classified into 3

groups exhibiting the same trends of regression lines and

curves according to horizontal position, with the highest

values for positions A-D-E, middle values for B-G-H,

and lowest values for C-F-I. For instance, mean PEMF

strength for each position at 50% stimulation intensity

from 30 mm distance were 163.7 ± 4.8 mT, 128.3 ± 2.1

mT and 90.0 ± 2.9 mT for high, middle and low values

groups, respectively (Figure 4). Intra-group comparison

of each regression slope did not reveal any significant

differences (P > 0.36) among regression lines for posi-

tions A, D and E, and likewise for the B-G-H and C-F-I

groups. For inter-group comparison between position

A-D-E, B-G-H and C-F-I groups, significant differences

H. Tamaki et al. / J. Biomedical Science and Engineering 3 (2010) 1156-1160

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Figure 2. Relationships between peak PEMF strength and stimulation intensities in marrow cavity and control conditions. Note the 3

types of geometric difference in PEMF strength correlating with stimulation intensity and distance, with the highest values for posi-

tions A-D-E, middle values for B-G-H and lowest values for C-F-I. Positions A-I are the same as in Figure 1.

Figure 3. Relationships between peak PEMF strength and coil-

sensor distance at 50% stimulation intensity and at 9 positions

(A-I). Figure 4. Relationships between peak PEMF strength and

horizontal positions (A-I) at 50% stimulation intensity and 30

mm coil-sensor distance.

were shown between each regression slope (P < 0.0001)

at a distance of 10-70 mm. Values of regression slopes

and those differences among groups decreased exponen-

tially with increasing distance. The ratio of slope values

in B-G-H and C-F-I groups to those in the A-D-E group

were 0.72 and 0.26 at 10 mm distance, 0.86 and 0.71 at

50 mm distance, and 0.93 and 0.88 at 90 mm distance,

respectively. These ratios exceeded 0.8 at a distance of

>60 mm. Comparison of the fit of non-linear curves re-

vealed one curve for all data set in each position group.

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H. Tamaki et al. / J. Biomedical Science and Engineering 3 (2010) 1156-1160 1159

For control conditions, no significant differences in

PEMF strength were observed under any conditions.

4. DISCUSSION

The present study demonstrated: 1) intensity dependency

(i.e., PEMF strength correlated perfectly with stimula-

tion intensity); 2) distance dependency (i.e., these char-

acteristics diminished with increased exponentially

coil-sensor distance); and 3) position dependency (i.e.,

PEMF strength distributions in bone were classified into

3 groups according to horizontal position).

The strong linear relationship seen between PEMF

strength and stimulation intensity indicates stimulation

intensity as a very accurate predictor of PEMF strength

in marrow cavity with bone marrow in long bones.

However, 3 types of geometric difference in regression

line slopes and absolute values of PEMF strength were

identified, with higher stimulation intensity obviously

showing larger geometric differences in PEMF strength.

For the condition of 50% stimulation intensity from a

distance of 30 mm, for example, our results indicate that

PEMF strength was attenuated by approximately 32%

and 45% at positions B-G-H and C-F-I compared to po-

sitions A-D-E, respectively. In addition, distance-de

pendent declines in regression slopes and reductions in

geometric differences suggest that a distance factor also

influences the relationship between stimulation intensity

and PEMF strength. The relationship between electric

field strength and coil-cortex distance at 5-30 mm also

reportedly shows exponential decay as a steep decrease

with increasing stimulation distance [4]. In the present

study, the relationships between PEMF strength and

coil-sensor distance (10-90 mm range) showed exponen-

tial decay with a breaking point at a coil-sensor distance

of around 30 mm. This result suggests that the longer

coil-sensor distance, the smaller the geometric difference

in PEMF strength, particularly in terms of the uniformity

of the stimulated area (i.e., a decrease in focality) at dis-

tances >40 mm. Testing the influence of PEMF stimula-

tion on bone tissue in vivo [6,7,11], stimulation condi-

tions with lower spatial focality would be preferable to

achieve approximately equal PEMF strength throughout

the whole marrow cavity with bone marrow in long

bones. To the best of our knowledge, this is the first

study investigating PEMF strength based on the meas-

urements in marrow cavity in long bones under 810

stimulation conditions. Our results might be helpful in

determining stimulation programs and estimating the in

vivo efficacy of PEMF in marrow cavity in long bones

for research and clinical use. The characteristics of

PEMF strength presented here also suggest the potential

for in vivo application of our data to other tissues [12] by

manipulating focality and stimulation intensity. Further

studies are needed to clarify magnetic field strength

properties in other tissues and animals.

5. ACKNOWLEDGEMENTS

This work was supported in part by a Grant-in-Aid for Scientific Re-

search (C, project nos. 18200512 and 22500611) from the Japan Soci-

ety for the Promotion of Science in 2010, and by a Grant-in-Aid for

Developed Research (B, project No. H22B19) from the Niigata Uni-

versity of Health and Welfare in 2010.

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