J. Biomedical Science and Engineering, 2009, 2, 144-154
Published Online June 2009 in SciRes. http://www.scirp.org/journal/jbise
JBiSE
Assessment of bone condition by acoustic
emission technique: A review
Sharad Shrivastava1, Ravi Prakash1
1Birla Institute of Technology and Science, Pilani, India.
Received 14 February 2009; revised 18 March 2009; accepted 21 March 2009.
ABSTRACT
The paper deals with the review of acoustic
emission technique in biomedical field. The re-
view is done with the aim to provide an ov erview
of the use of AE technique in biomedical field,
mainly co ncentrated on the AE beha vior o f bone
under different loading conditions, its depend-
ence on strain rate, in osteoporosis, monitoring
the fracture healing process of bone. The over-
all conclusion from the review was that almost
all the studies in bone indicated that the initial
AE occurs only in the plastic region and just
prior to yield. That means the use of AE tech-
nique for clinical application cannot be consid-
ered as a safe technique, but the early occur-
rence of AE events from callus promises the
application of AE technique for monitoring the
fracture healing process. The negligible effect
of soft tissues on AE response of bone prom-
ises AE to become a non-invasive method for
assessment of bone condition.
Keywords: Acoustic Emission; Assessment; Strain
Rate; Callus; Fracture Healing; Osteoporosis
1. INTRODUCTION
Bone is primary structural element of human body. The
anatomy of human beings is quite well known but the
strength and mechanical properties of bones have not
been investigated thoroughly. The 206 named bones of
skeleton constitutes 18% of the adult human body
weight, only skin and fat (25%) and muscles (43%) be-
ing greater [1]. In biological terms bone is described as a
connective tissue and in mechanical terms bone is a
composite material with several distinct solid and fluid
phases. The mechanical properties of bone have been
more extensively investigated than those of any other
biological tissue materials. Although our understanding
of the mechanical properties and fracture behavior of
bone is continuously improving, as yet it is far from
complete. As pointed by Hayes, W. C. [2] while funda-
mental research is needed on many aspects of the me-
chanical response of the bone, applications of the tech-
niques of analytical and experimental mechanics in this
area are made complicated by the fact that bone is highly
complex living material.
The initial work in the field of bone biomechanics can
be traced back to the 17th century when “attempts to ex-
press biological findings in physical terms” [3] were
made by the scholars at that time. This philosophical
background of the aspect was intensified in the age of
determinism which lasted until the middle of the 19th
century. One of the main aspects of the research work in
that time was to relate the architecture of bone and its
mechanical functions. In the year 1832, Bourgery, J. M.
[4] in his work on anatomy raised the question of rela-
tion between architecture and mechanical functions of
bone. In his book on osteology, Ward, F. O. [5] compared
the proximal end of the human femur with a crane and
he mentioned the compressive and tensile stresses
evoked in the bone by loading. In the year 1867, a more
detailed analysis of the structure of cancellous bone and
its mathematical significance was given by Meyer, G. H.
[6] in association with the famous mathematician Cul-
mann.
The industrial revolution took place in the second half
of the 19th century. It had an impact on the research
works in the bone also. New developments were made in
the field of material testing and the new methods were
developed for mechanical measurements. For a while,
these methods were used to determine the in vitro me-
chanical properties of bone. The bones were tested under
various loading conditions and the ultimate strength of
bone was determined by many investigators [7,8,9,10,11,
12,13,14,15,16,17,18]. Mc Elhaney, J. H. [19] from his
study on the strain rate dependence of the mechanical
properties of bone showed that both the compressive
strength and modulus of longitudinally oriented compact
bone specimens were significantly increased by increas-
ing the strain rate. A critical strain rate for bone has been

S. Shrivastava et al. / J. Biomedical Science and Engineering 2 (2009) 144-154 145
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claimed in compression [19], torsion [20] and tension
[21]. However Wright, T. M. and Hayes, W. C. [22]
found no critical strain rate in tensile tests of bovine
bone over a wide strain-rate range.
In the last few decades attempts were made to use the
newly developed/improved non-destructive testing tech-
niques to find the mechanical properties of in vivo and in
vitro bones. Those include finding the elastic constants
using ultrasonic techniques [23,24,25,26], finding the
mechanical strength of bone specimens by X-ray com-
puted tomography, etc.
Assessment of in vivo bone condition is one of the
research areas, which have attracted many biomedical
engineers and clinical orthopaedicians in recent times.
Presently the radiological examination is widely used for
the assessment of in vivo bone condition [27,28]. In
some clinical problems such as diagnosis of the point of
clinical union of fracture, the manual assessment of sta-
bility is also used along with the radiological examina-
tion. However for many applications the radiographic
technique was found to be suffered from low sensitivity.
For instance, for the evaluation of osteoporosis it re-
quires a minimum loss of 30% or more of bone mineral
content before an unequivocal roentgen logical diagnosis
can be made [29].
Monitoring the fracture healing process is another
area where the currently used techniques failed to give
satisfactory results. Uncertainty regarding the signifi-
cance of the radiographic and clinical findings may re-
sult in unnecessarily long immobilization periods which
can produce discomfort and inconvenience for the pa-
tients, as well as possible joint stiffness and even per-
manent loss of motion especially in the elderly.
In certain long bone shaft fractures the healing process
is modified by the method of treatment so that the clini-
cal assessment of mechanical integrity is impossible and
the interpretation of radiographs may be difficult.
Diaphyseal fractures treated by “rigid” internal fixation
always demonstrated this problem, since the fracture
cannot be tested mechanically and external callus forma-
tion is not seen on radiographs, methods are needed to
assess the mechanical integrity of fracture healing in
such circumstances, or an unreliable and unsafe reha-
bilitation programme may be prescribed.
Mechanical impedance, natural frequency, vibration
analysis, stress wave propagation, ultrasonic- measure-
ments, impact response technique, electrical potential
measurements and mechanical tissue response analysis
are some of the techniques, which have been attempted
by different investigators for the assessment of in vivo
bone condition in the past [30,31,32,33,34,35,36,37,38,
39,40]. However, in all these studies, the intervening soft
tissues, whose quantity and quality changes with individual
to individual, affected the results. Furthermore some meth-
ods were not really non-invasive in nature and some were
not practicable for widespread clinical use because of low
reliability and complicated instrumentation.
The structure of bone is very much similar to engi-
neering composite materials and is therefore advanta-
geous to use a non-destructive testing technique, which
has already proved it usefulness in the field of composite
materials testing. Acoustic emission (AE) technique has
been used very successfully for the non-destructive
evaluation of composites. The relationship between AE
response and mechanical behavior in composite materi-
als has been extensively studied in the past [41]. This
paper deals with the review of AE technique which is to
be used for bone assessment. The review has been
broadly classified as follows (Figure 1).
2. ACOUSTIC EMISSION TECHNIQUE
The AE technique is the sound produced by materials as
they fail. A familiar example is the audible cracking
Acoustic Emission Technique
AE in bone
Historical Background
Behavior of bones in different loading
conditions
Prediction of mechanical
properties
Effect of strain rate on AE properties
on callus-bone
Figure 1. Broad classification of the review study.

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noise from wood. Almost all engineering materials gen-
erate acoustic emissions but unlike wood, the sound is too
faint to be heard without sensitive electronic monitors.
Acoustic emission waves can be detected by means of
remote piezoelectric sensors and their source can be lo-
cated by timing the wave arrival at several sensors. Thus
AE provides a unique method of recognizing when and
where deformation is taking place as a structure is stressed.
The first systematic investigations of AE phenomenon
was made in 1950 by Kaiser, J. [42] at the technical uni-
versity of Munich. In his investigations, the noise emit-
ted by the deformation of materials was examined by
means of electronic equipment capable of detecting in-
audible ultrasonic signals. Kaiser, while working with
polycrystalline specimens concluded that acoustic vibra-
tions originate in grain boundary interfaces and was be-
lieved to be associated with the interaction induced be-
tween interfaces by applied stresses. He noted that, for a
given materials, characteristics spectra of frequency and
amplitude existed. One of the important observations
made in his study was that irreversible processes were
involved with AE phenomenon; an effect later came to
be known as Kaiser Effect. The universality of the AE
phenomenon, as recognized by Kaiser, leads to a very
wide range of applicability. AE has been recorded from
hundreds of materials-metals, composites, ceramics, plas-
tics, glasses, building materials, biological materials in
vitro and in vivo as well as from multi material structures
and joints between different materials [43]. Compared to
other NDT techniques which rely on extraneous energy
for the illumination of defect; AE enjoys the unique fea-
ture that the defect makes its own signal. This leads to a
natural complementary between AE and other methods.
3. ACOUSTIC EMISSION DETECTION
AND SIGNAL PROCESSING
Figure 2 [44] shows the method of detection of acoustic
emission events by remote piezoelectric transducers.
Here, an AE source generates an expanding spherical
Figure 2. Detection of acoustic emission event by remote
piezoelectric transducers (source: 44).
wave packet losing intensity at a rate of r-2. When this
wave reaches the body boundary, a surface wave packet
is created, either Rayleigh or Lamb wave type depending
on the thickness. This method is mainly used for flaw
monitoring in inaccessible areas. The individual signal
has a short duration at the source and a corresponding
broad spectrum which typically extends from zero fre-
quency to many megahertz. The form of the signal at the
point of detection is a damped oscillation, developed in
the structure according to known principles of acoustic
wave propagation. Figure 3 [44] shows the signal
waveform of one acoustic emission event and different
parameters normally measured to characterize the acous-
tic emission source.
4. ACOUSTIC EMISSION IN BONE
Hanagud, S., et al. [45] using bovine femora first dem-
onstrated detectable acoustic emissions from bone. His
work made the way for other investigators to use the AE
technique for characterization of bone and also to ex-
plore the possibilities of using it as a tool for clinical
orthopaedicians to detect bone abnornamilities [46,47,48,
49,50]. Knet-s, I. V., et al. [46] has shown that the char-
acter of the fracture surface depends on the orientation
of the load relative to the direction of the osteons, the
rate of loading, and the geometrical shape of the actual
sample. They concluded that the most promising ap-
proach in testing the internal state of a bone is acoustic
emission, sometimes also known as the method of
stress-wave emission. This approach involves recording
of deformation noise in the material due to the develop-
ment and further propagation of structural defects. These
defects may include dislocations or cracks appearing in
the course of loading. This study was the first work to
visualize the degree of micro cracks development in
bone tissue subjected to longitudinal extension. This
experimental work was only visualized for longitudinal
loading and the deformation rate considered was also
very low (1 mm/min). The study was not conducted for
higher strain rates. In another investigation Hanagud, S.,
et al. [50] conducted AE tests on carefully prepared bone
specimens subjected to bending loads. Their specimen
included femur from cattle and cadavers. They compared
the AE patterns from 60 perfect and defective specimens.
The result clearly indicated that the development of an
effective early diagnostic tool for osteoporosis was pos-
sible by using AE technique.
Thomas, R. A., et al. [51] studied the acoustic emissions
from fresh bovine femora and its clinical applications.
They employed a more sophisticated set up of AE tech-
nique by including both amplitude and pulse width dis-
tribution to investigate whole fresh bovine femora which
were loaded by compression and bending. They found that
both the amplitude distribution and pulse width distribu-
tion results of fresh bone had clearly shown characteristic
spectra which could be used for the early detection of bone
abnormalities such as fracture and osteoporosis.

S. Shrivastava et al. / J. Biomedical Science and Engineering 2 (2009) 144-154 147
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Figure 3. Signal waveform for one acoustic emission event
(source: 44).
Yoon, H. S., et al. [52] developed a new AE technique
for applications to human and animals both non-inva-
sively and non-traumatically. Bones from several differ-
ent species of animals and different kinds in the same
species were tested to obtain AE parameters. Their re-
sults indicated that the AE amplitude distributions of all
the bones are similar, somewhat independent of the spe-
cies of animals and kind in the same species and how-
ever different from those of those materials such as met-
als, ceramics and plastics. The technique was found use-
ful for the diagnosis of micro fractures, such as stress
fractures in the tibia of runners, which were not detect-
able by conventional X-ray technique until they begun to
heal. Moreover conventional techniques would require
introducing some additional stresses in to the part of the
body under examination, which also introduces addi-
tional trauma to the patient. In their technique the low
intensity ultrasonic pulses were injected through an AE
transducer, instead of applying loads to the bone under
tests. The loading as pulses reduced the introduction of
trauma to the live subject. Another receiving AE trans-
ducer was used to collect five types of useful AE data:
per-event- distribution of counts, peak amplitude, energy
and pulse duration, and cumulative counts vs time.
Netz, P. [53] monitored the AE response of canine
femora in torsion at 6 degrees per second. His work
demonstrated that the AE events occur in the non-linear
plastic portion of the load deflection curve. Wright, T.
M., et al. [54] monitored the permanent deformation of
compact bone using AE technique. Uniaxial tension tests
were performed on standardized specimens of bovine
harvesian bone to examine the contributions of mineral
and collagen to permanent deformation in bone and to
monitor the damage mechanisms occurring in permanent
deformation using AE technique. Their results were con-
sistent with a two-phase model for bone in which the
mineral behaves as an elastic-perfectly plastic material
when bound to the collagen fiber matrix. The AE events
occurred just prior to the yield point and continued dur-
ing yielding. Significant AE counts occurred again just
prior to fracture. No emissions occurred in the elastic
region and few occurred in the major portion of plastic
region between yield and fracture. To monitor micro
cracks in the specimen they used AE and plotted graphs.
Figures 4,5 [54] show stress vs. strain and cumulative
acoustic emission counts vs. strain curves for one of the
control specimens and decalcified specimens. These
graphs indicate the similarity between the acoustic emis-
sion data of the bones prior to fracture. Figure 6 [54]
shows stress strain plots based on the mean values from
Ta bl e 1 [54]. The limitation of their work lies in the hy
pothesis that the mineral only exhibit elastic-perfectly
plastic behavior in conjunction with collagen. They con-
ducted experiments on control, decalcified and depro-
tenised groups of specimens with the same hypothesis. The
fact is that the deprotenised groups of specimens behave
in brittle manner. Hence they suggested further studies to
be undertaken to examine the contributions of mineral
and collagen for permanent deformation in the bone. The
two phase model used could be used to study the qua-
sistatic tensile behavior of compact bone but more work
could have been carried out for higher strain rates re-
sponses before coming to any conclusion.
Figure 4. Stress vs strain and acoustic emission counts vs
strain curves for one of the control specimens (source: 54).
Figure 5. Stress vs strain and acoustic emission counts vs
strain curves for one of the decalcified specimens (source: 54).

148 S. Shrivastava et al. / J. Biomedical Science and Engineering 2 (2009) 144-154
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JBiSE
Figure 6. Stress-strain curves for the three test groups con-
structed from the mean values in Table 1. Error bars are shown
for ultimate stress values (source: 54).
Table 1. Mechanical properties of decalcified and partially
deproteinized bovine bones (source: 54).
control decalcified Deproteinized
No of specimens 7 11 10
Yield stress 118(9.8) - -
Yield strain 0.544(0.150) - -
Ultimate stress 128(15.6) 34(7.5) 71(11.5)
Ultimate strain 2.02(0.924) 9.247(1.524) 0.956(0.342)
Elastic modulus 20.6(2.76) 0.37(0.05) 11.3(3.15)
Plastic modulus 0.66(0.357) - -
It is an established fact that the ultimate tensile
strength of bone is dependent on the applied strain rate.
Based on these Fisher, R. A., et al. [55] studied the effect
of using two different strain rates on the AE in bones. In
their work bovine cortical bone was milled in to standard
tensile specimens which were tested at two different
strain rates while being monitored with AE equipment.
They found that the amplitude distribution of the AE
events in bone is dependent on strain rate. Greater num-
ber of events occurred with the slower strain rate but the
events were of lower amplitude than those emitted dur-
ing the more rapid strain rate. Here also the initial AE
occurred well in to the plastic region of the stress-strain
curve near the point of fracture of the tensile specimens.
It was evident from the study that if acoustic emission
technology is to be utilized clinically for the assessment
of fracture healing; careful selection of rate of loading
would be necessary. Furthermore the study indicated that
acoustic emission response was different at different
strain rates. As the emissions did not occur until failure
was imminent, it indicates that Acoustic emission tech-
nology is not suitable for evaluating the integrity of bone.
The limitation included that the specimens taken were of
very large sizes to minimize the stress concentrations
effects of normal bone architecture. They did their work
at only two different strain rates (0.0001/s and 0.01/s);
therefore no conclusion could be made for broad range
of strain rates.
Later on Nicholls, P. J., et al. [56] studied the AE prop-
erties of callus. In their work, rabbits with 45 degree
midshaft oblique osteotomies were strained in shear
while monitoring for AE events. Each fracture remained
essentially quiet until over 50% of load to failure had
been applied. They suggested that since callus formation
during fracture healing takes important role in the heal-
ing process, the AE from callus may have clinical appli-
cations. The limitation of the study lies in the test
method used for evaluating bone. As bone is a non-ho-
mogeneous substance, it is very difficult to evaluate with
instruments which have been designed for homogeneous
substances. A test method should be so designed that
eliminates background noise, such as slippage of speci-
men in the grips and motion of the transducers on the
bone surface. This hampers reproducibility of acoustic
emission patterns. In most of the AE studies of bone the
bone has been tested without the surrounding soft tissues.
But in the case of clinical applications of AE, one cannot
separate the bone from soft tissues and hence the tests
should be performed with soft tissues. Hanagud, S., et al.
[57] studied these phenomena. They used freshly dis-
sected rabbit tibia and femur with soft tissues. Tests were
conducted through bending load. They found that the
soft tissues of 2 to 9mm thickness did not affect the
bone’s AE response.
A study of bone-tissue samples by Martens, M. [58]
used acoustic emission for to study the mechanical be-
havior of femoral bones in bending loading. Ono, K. [59],
provided an insight about the fundamental theories and
equations related to acoustic emission. Stromsoe, K., et
al. [60], worked on bending strength of femur using non
invasive bone mineral assessment.
The work done till this time demonstrated that the safe
use of AE technique for the non-destructive testing of
bone is impossible because the AE events occurred only
after plastic deformation occurred.
Lentle, B. C. [61], University of British Columbia
used acoustic emission to monitor osteoporosis. He de-
vised a method for in vivo diagnosis of patients using
AE technique, which could also predict the severity of
osteoporosis.
Table 2. The predictive capability of acoustic emissions ex-
pressed in terms of the specimen’s fatigue life (source: 71).
Maximum
stress(MPa)
NF Fatigue
life [cycles]
NP Fracture
onset via
AE [cycles]
Predictive
Capability [%
of fatigue life]
Specimen 155 26363 22580 85 %
Specimen 261 35020 33382 95 %
Specimen 366 4737 2989 63 %
Specimen 471 600 402 67 %

S. Shrivastava et al. / J. Biomedical Science and Engineering 2 (2009) 144-154 149
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Table 3. A statistically significant effect of time on these mechanical properties was detected. Within a row, values with differing
letters are significantly different from each other (P<0:05) (Source: 65).
weeks after surgery
Mechanical properties 4(n=8) 4(n=8) 4(n=9) 4(n=9)
Tensile Strength (N/mm2) 36±21a 130±60b 220±32c 510±95d
Tensile Stiffness(N/mm) 0.47±0.18a 1.3±6b 1.8±6b 3.0±2c
Maximum Strain (%) 10±3a 3.7±3b 1.6±0.4b 1.8±0.3b
AE Initiation Load (N) 21±15a 71±3b 150±62c 330±31d
Std. TensileSstrength (N/mm2) 0.12±0.6a 0.33±0.2b 0.55±0.1c 0.82±0.03d
Std. Tensile Stiffness (N/mm) 0.028±0.02a 0.57±0.3b 0.82±0.3b,c 1.0±0.06c
Std. Maximum Strain 5.6±1.4a 2.2±1b 0.90±0.2b 0.86±0.1b
Ta bl e 4 . Ash content was calculated by (ash density/apparent
density  100). A statistically significant effect of time on
these mechanical properties was detected. Within a row, values
with differing are significantly different from each other letters
(P< 0:05) (Source: 65).
weeks after surgery
Mechanical
properties 4(n=9) 6(n=9) 8(n=10) 12(n=10)
Apparent
density
(g/cm3)
0.42±0.07a 0.67±0.07b 1.2±0.1c 1.2±0.08c
Ash density
(g/cm3) 0.14±0.04a 0.40±0.06b 0.82±0.08c 0.88±0.06c
Ash content
(%) 33±4a 59±5b 71±1b,c 73±1c
Acoustic emission was being used to predict changes
in mechanical properties due to fatigue [62,63,64]. Wa-
tanabe, Y., et al. [65] used AE technique to predict
mechanical properties of fractures. Experimentally pro-
duced fractures of femur in rats were tested in tension
and in torsion at 4, 6, 8 and 12 weeks after fracture. AE
signals were monitored during these mechanical tests.
The values for load and torque at the initiation of the
AE signal were defined as new mechanical parameters.
Tensile strength, tensile stiffness, and torsional stiffness
were found to increase with time. They focused on how
AE signals can help a surgeon to remove the external
fixators in the sense that AE signals can be used to
monitor healing of bones. Ta bl e 3 [65] indicates a sta-
tistically significant effect of time on these mechanical
properties. Table 4 [65] indicates the calculated ash
content and the statistically significant effect of time on
the mechanical properties of bone. The data obtained
by them were compared to the original values and were
found out to be almost the same. The study was a first
step towards the establishment of AE testing as a means
of predicting the callus strength. The two parameters
exhibited strong and positive linear correlation with
tensile strength and torque. The linear correlations sug-
gested that it may be possible to use AE technology to
evaluate fracture healing process, following osteotomy
surgery. There were still many issues which needed to
be resolved to make it clinically viable.
Kevin S. C. K., et al. [66] developed an acoustical tech-
nique for the measurement of structural symmetry of hip
joints. Since, these techniques depend very much on the
intensity and quality of sounds emitted from the joints
under investigation. They developed an acoustical tech-
nique for the measurement of relative acoustic transmis-
sion across both hips of the test subjects while they were
subjected to an external vibratory force applied at the
sacrum. The merit of this approach was that it allows
direct comparison of the sound signals transmitted
across both hips regardless of the measure of the input
vibratory force. Simultaneously, other acoustic tech-
niques like scanning acoustic microscopy [67,68] acous-
tic mapping [69,70] was being used to predict and study
mechanical properties of tissues and bone.
Ozan A. [71] worked on a hypothesis that an increase
in micro damage activity during repeated loading of
bone will signal the approaching stress fracture. Inter-
ception with the training regime prior to the incidence of
the fracture as signaled by acoustic emissions would
reduce the time necessary for recuperation. Acoustic
emission was used for real time monitoring of micro
cracks. They used acoustic emission technique to predict
the failure of cortical bone. Table 2 [71] indicates the
predictive capability of acoustic emissions expressed in
terms of the specimen’s fatigue life.
Information was collected on all acoustic events, re-
gardless of whether they originated from micro damage
or somewhere else and then signals originating from the
micro damage were isolated. The rest of the irrelevant
signals were filtered out based on their average fre-
quency, duration, amplitude, and intensity. With the
non-micro damage signals removed from the data, we
were able to determine the number acoustic events re-
lated to bone damage as well as the time at which they
occurred. Specially designed software is yet to be de-
veloped which will segregate the zones of micro damage.
Fracture healing and prediction of healing time of frac-
tures were increasingly being studied. A review by
Browne, M., et al. [72] on acoustic emission’s capability
to monitor bone degradation and bone fatigue provided
us information with latest developments in this field.
In 2004, Franke, R. P., et al. [73] used acoustic emis-
sion for in vivo diagnosis of the knee joint. For the as-

150 S. Shrivastava et al. / J. Biomedical Science and Engineering 2 (2009) 144-154
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sessment of the tribological knee function and by the
probability of fracture of the femur an adapted Acoustic
Emission Measurement System named Bone Diagnostic
System (BONDIAS) was developed. This system makes
the in vivo analysis of the medical status possible. Dif-
ferent mechanisms of cracking were accompanied by
different acoustic emission from human femora as
shown in literature. An acoustic emission signal typical
of crack initiation is shown in Figure 7 [73]. This Figure
is indicative of the acoustic emission from healthy knee
joint cartilage after a sudden change from a two leg
stand to a one leg stand. It is characterized by a very
short rise time and an exponential decrease of the am-
plitudes. From the medical point of view such mechani-
cal loads are regarded as non destructive although there
is already crack initiation in the interface of the compact
and the trabecular system of the bone. These micro
cracks seem to be essential for the physiological bone
remodeling. For the description of the development of
bone strength over time it is necessary to assess both the
threshold of crack initiation and the conditions for crack
propagation. The sudden change in amplitude indicates
high thickness of the cartilage layer. There are several
advantages of the diagnostic procedure by AE when
compared with established conventional methods:
1) No pain is caused by this procedure.
2) This procedure is non-destructive. Mechanical load
even beyond the crack initiation threshold are typical of
day to day life and necessary for the physiological bone
remodeling to avoid the degeneration of the bone and
joint system.
3) There is no health burden through ionizing radia-
tion as is unavoidable with X-ray examination and CT.
4) There is no danger of infection since this is a
non-invasive examination.
5) The time required for the assessment of the acous-
tic emission behavior and analyses of data are of the
order of seconds to minutes.
Figure 7. Acoustic emission from healthy knee joint carti-
lage deformation after the sudden change from a two leg
stand to a one leg stand (source: 73).
6) The expenses for the AE measurement system are
small compared to X-ray systems.
7) The costs per examination including a detailed di-
agnosis are well below costs of other diagnostic proce-
dures and there is no danger of infection leading to fur-
ther costs, as happens with invasive methods, e.g. endo-
scopic examinations.
8) Diagnostic (Real time) monitoring of bone and
joint training of sports professionals becomes possible.
The disadvantage of the measurement system sug-
gested was that the physician will be left with the bundle
of data and the task to evaluate the AE.
Tatarinov, A., et al. [74] proposed multiple acoustic
wave method for assessing long bones. The method was
based on measurement of ultrasound velocity at different
ratio of wavelength to the bone thickness and taking into
account both bulk and guided waves. They assessed the
changes in both the material properties related to poros-
ity and mineralization as well as the cortical thickness
influenced by resorption from inner layers, which are
equally important in diagnosis of osteoporosis and os-
teopenia. More in vivo studies on animals and human
volunteers has to be carried out before the proposed
method could be made clinically usable. The advantage
of the method proposed was that it allowed assessment
of changes in both the material properties related to po-
rosity and mineralization as well as cortical thickness
influenced by resorption from inner layers, which are
equally important in diagnosis of osteoporosis and other
bone osteopenia. The method could also be used for di-
agnosis of bone condition if the contribution of soft tis-
sues and topographical heterogeneity in real bones are
considered. The method had a potential for better detec-
tion of early stage of osteoporosis in long bones. Singh,
V. R. [75] reviewed an acoustic imaging technique known
as acoustic stress wave propagation technique which was
used for bone examination. The technique was developed
with the view to solve the problems encountered with the
conventional technique like X-rays. As bone is a hetero-
geneous, complex and fibrous tissue, determination of
very small abnormalities viz. shape and size of bone
defects, is not usually possible by means of conventional
X-ray technique.
In 2006, Azra Alizad, et al. [76] studied the change in
resonant frequencies of a bone due to change in its
physical properties caused due to a fracture. Experiments
were conducted on excised rat femurs and resonance
frequencies of intact, fractured, and bonded (simulating
healed) bones were measured. These experiments dem-
onstrated that changes in the resonance frequency indi-
cated bone fracture and healing. The fractured bone ex-
hibits a lower resonance frequency than the intact bone,
and the resonance frequency of the bonded bone ap-
proaches that of the intact bone. The graphs are indicative
of the result (Figures 8,9) [76], that the frequency re-

S. Shrivastava et al. / J. Biomedical Science and Engineering 2 (2009) 144-154 151
SciRes Copyright © 2009
sponse of a cut femur is less than the intact femur. The
proposed method may be used as a remote and non inva-
sive tool for monitoring bone fracture and healing process,
and the use of focused ultrasound enables one to selec-
tively evaluate individual bones. The proposed method
offers several advantages over vibrational methods using
external mechanical excitation. The ultrasound can be
applied remotely and directly to the bone under test, thus
avoiding interference of overlaying muscle or other tis-
sues on force distribution. Furthermore, in contrast to
traditional methods in which it is difficult to target small
bones and to access them, the proposed method allows
application of excitation force directly and selectively to
the intended bone. The acoustic method for measuring
Figure 8 . Frequency response of the intact femur A. The plots show the motion of the intact femur vs frequency. These
Fig plots indicate peaks at 925 Hz, 4.2 kHz, and 8.1 kHz. Peaks of motion below 700 Hz were explored and found not
to be related to the femur. Top left: Driven and measured at the end of the femur at frequency range of 100 Hz to 1000
Hz. Top right: Driven and measured at the end of the bone at 1 kHz–10 kHz. Bottom left: Driven and measured at bone
midpoint at 100 Hz–1000 Hz. Bottom right: Driven and measured at bone midpoint at 1 kHz–10 kHz (source: 76).
Figure 9. Frequency response of the cut femur (source: 76).
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152 S. Shrivastava et al. / J. Biomedical Science and Engineering 2 (2009) 144-154
SciRes Copyright © 2009 JBiSE
bone response is suitable for in vivo applications as long
as one take the frequency response of the surrounding
structures in to account. An advantage of acoustic motion
detection method is that it does not require a direct path to
the bone because the acoustic emission produced by the
bone travels easily in every direction, thus the location of
hydrophone is not critical. Further investigations are
needed to demonstrate the applicability of the proposed
method for evaluation of bone quality in human body.
The boundary conditions of bone to body must also be
considered before applying the proposed method.
In 2008, Dipan Bose, et al. [77] studied the effect of
valgus bending and shear loading on knee joint. They
used acoustic sensors to determine the failure timing of
soft tissues attached to femur and tibia. The failure tim-
ing was determined based on the knee injury mechanism
due to valgus loading. At the estimated time injury, the
corresponding values of αvalgus_fail, dshear_fail, Mvalgus_fail, and
Vshear_fail were designated as the failure parameters of the
knee. Numerical methods with accurate geometric and
material properties could be implemented to simulate and
further extend the injury threshold to alternate loading
detection. Elmar K. Tschegg, et al. [78] did stiffness
analysis of tibia implant system under cyclic loading. He
used a bio- mechanical system integrated with acoustic
emission sensors at the screw head. 3 sequences of load-
ing were used to determine when the locking screws
break. Data was obtained from the acoustic sensors onto a
data acquisition board and were processed using a
acoustic emission software. This acquired data was used
to determine as to which screw is bearing the load and as
to when does the screw break.
5. CONCLUSIONS
· Many investigations carried out in the field “Assessment
of bone condition” are mainly in vitro studies. For any
method which is going to be used in clinical practice, a
thorough experimental study with animals and/or a clini-
cal study with human volunteers are very much essential.
·The AE is very much dependent on the strain rate.
·AE technique is highly sensitive to specimen damage
and cracks and detects them even before visual detection.
· Diagonostic (Real time) monitoring of Bone is possi-
ble.
·It is non-destructive and helps us to predict the time
length of the healing process.
·It has no harmful effects unlike X-rays, which have
radiation effects on patients.
·The emissions from the callus during fracture healing,
gives its possibility to be clinically used.
6. FUTURE RESEARCH DIRECTIONS
·Many researchers have used this technique for in vitro
as well as in vivo characterization of bone. However,
the clinical application of the technique was not fully
investigated. The time of occurrence of initial AE and
the AE response of bone under different loading con-
ditions at different strain rates are not well established.
So focus can be on finding the exact time of occur-
rence of initial AE with respect to the stress/strain
curve and the AE behavior under a suitable loading
condition at different strain rates.
·In the case of in vivo studies, the AE response of callus
is not thoroughly investigated. Also it is necessary to
conduct an experimental study with laboratory ani-
mals/volunteers or patients to prove the clinical usage
of AE.
7. ACKNOWLEDGEMENT
The authors wish to acknowledge Mr. Sudeep Mohapatra & Mr.
Kaushik. V. for their help. Financial assistance provided by Depart-
ment of Science and Technology, Government of India is also grate-
fully acknowledged.
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