DUAL-ENERGY X-RAY ABSORPTIOMETRY AS AN

INDICATOR FOR FRAGILITY FRACTURE RISKS

OF THE FEMORAL NECK

Alexander Tsouknidas

, Nikolaos Michailidis

, Kleovoulos Anagnostidis

and Antonios Lontos

Laboratory for Machine Tools and Manufacturing Engineering, Mechanical Engineering Department

Aristoteles University of Thessaloniki, Thessaloniki, Greece

Physical Metallurgy Laboratory, Mechanical Engineering Department, Aristoteles University of Thessaloniki,

Thessaloniki, Greece

3rd Orthopaedic Department ”Papageorgiou” General Hospital, Aristoteles University of Thessaloniki,

Thessaloniki, Greece

Department of Mechanical Engineering, Frederick University, Nicosia, Cyprus

Keywords: Dual-energy x-ray absorptiometry, Femoral neck, FEM, Fragility fracture risks.

Abstract: Osteoporosis is a clinically silent bone pathology usually manifesting in the form of fragility bone fractures.

Due to the high morbidity of the disease, the association of noninvasive imaging techniques to the

implicated risk factors, could serve as a valuable indicator for surgeons. In the present investigations, the

evaluation of 30 patients femurs' bone mineral density was performed in vivo by Dual-energy X-ray

absorptiometry (DXA), while the strength characteristics of the examined specimens were determined ex-

vivo using uniaxial compression experiments. The obtained stress strain curves, reflect the mechanical

properties of the femur while facilitating their correlation to the obtained DXA measurements. FEM

simulations revealed critical stress values within the femoral neck, indicating which DXA values represent

abnormal high fragility fracture risks and thus should be considered for surgical intervention.

1 INTRODUCTION

Osteoporosis is a multifactorial bone disease con-

cerning roughly 4% of the human population

(Melton et al., 1992). As an asymptomatic condition,

osteoporosis fails to exhibit noticeable symptoms,

particularly at early stages and thus is usually

underdiagnosed. Untreated however, this clinically

silent disease, is likely to increase the risk of

fragility fractures (Ettinger, 2008); (Rockwood et al.,

1990); (Cooper et al., 1992). Due to its high

morbidity and global nature, osteoporosis is

considered a pathology with a significant

socioeconomic impact (Ray et al., 1997).

The affected patients' bone mineral density is

drastically reduced, deteriorating the bones'

micostructural characteristics as a result of excessive

bone resorption followed by insufficient bone

formation during remodeling (Frost and Thomas,

1963); (Raisz, 2005). The pathogenesis has been

associated to dietary aspects (Hackett et al., 2009),

immobilization (Minaire, 1989), hyper-

parathyroidism (Dupree and Dobs, 2004), vitamin D

deficiency (Holick, 2004), alteration of biochemical

markers like hormone (Parfitt et al., 1995); (Black et

al., 2003) and aging (Newton-John and Morgan,

1970). Regardless etiology, decreased bone mineral

density renders the skeletal system susceptibility to

fracture, predominantly occurring at the hip (Bohr

and Schaadt, 1985), the vertebral column (Old and

Calvert, 2004) and wrist (Dempster, 2011).

According to the World Health Organization,

osteopenia and osteoporosis are defined by the

patient's bone mass deviation, when compared to

that of an average, young and healthy adult(WHO,

1994) when measured by DXA.

Even though DXA can accurately determine the

minerals and lean soft tissue of the examined area,

the overall accuracy of the measurement is impaired

by the subtraction of the indirectly calculated fat

mass (St-Onge et al., 2004). Furthermore, DXA

results are represented as mass per area, thus not

considering the anisotropy of the bone tissue and are

Tsouknidas A., Michailidis N., Anagnostidis K. and Lontos A..

DUAL-ENERGY X-RAY ABSORPTIOMETRY AS AN INDICATOR FOR FRAGILITY FRACTURE RISKS OF THE FEMORAL NECK.

DOI: 10.5220/0003770000960101

In Proceedings of the International Conference on Bioinformatics Models, Methods and Algorithms (BIOINFORMATICS-2012), pages 96-101

ISBN: 978-989-8425-90-4

 2012 SCITEPRESS (Science and Technology Publications, Lda.)



hence as a quantitative and not qualitative index of

the bone structure(Lochmuller et al., 2000).

Several other methods have been recently

introduced to determine bone mineral density

(Genant et al., 1996, Braun et al.1998), DXA

nevertheless is still widely considered as the method

of choice, as techniques like peripheral quantitative

computed tomography (pQCT) may be accurate in

measuring BMD at peripheral skeletal sites, exhibit

however restrictions that prohibit measurements at

the proximal femur (Augat et al., 1996); (Augat et

al., 1998).

The aim of this investigation is to determine the

correlation of the bone mineral density in the

femoral neck, as measured by DXA, to

experimentally determined strength characteristics

of the bone. This, followed by the introduced FEM

simulation, will facilitate the use of DXA as an

indicator of fragility fracture risk in the hip region,

as there is a consensus throughout literature that hip

fractures involve the most severe consequences of

osteoporotic bone loss.

2 MATERIALS AND METHODS

This study was conducted on femoral neck samples,

harvested from patients undergoing total hip

replacement due to osteoarthritis. In order to

determine the samples' structural integrity, standard

X-rays (anterior- posterior) of the pelvis were taken

preoperatively in all cases. Patients with a sort

femoral neck, large cysts in neck region or previous

surgeries in proximal femur were excluded from the

study.

Overall 30 patients (27 female and 3 male) were

considered as representative candidates for this

study and thus subjected to DXA, to catalogue their

proximal femur bone mineral density. The average

age of these patients was 63.7 years (57- 76 years).

During the surgical procedure and after a 45

osteotomy, femoral heads were removed and stored

at -60oC until evaluation. A plane bone slice with

6mm thickness was harvested from the femoral neck

(see figure 1) as two parallel blades, mounted on a

mechanical saw at a 6mm distance, simultaneously

entered the proximal femur. This ensured similarity

among all specimens while producing parallel piped

specimens, directly employable in compression tests.

Mechanical testing was performed on an electric

INSTRON Testing system. To determine the

specimens’ strength characteristics, all samples were

subjected to uniaxial compression, until failure. A

cross-head traveling speed of 0.6mm/s was selected

and the maximum travelling distance (upon contact)

was set to 5mm in order to avoid contact of the

moving cross-head and the fixed base plate. To

reduce friction, the sample-actuator contact areas

were lubricated. The displacement of the cross-head

was measured by means of an inductive sensor, at an

accuracy of 1 µm.

Figure 1: Considered bone specimen and reverse

engineered model.

The biomechanical parameters were correlated

with BMD using the Pearson correlation coefficient

(r) and a linear regression model.

30 experiments were conducted to determine

both, compressive yield strength and modulus of

elasticity and associate these to the DXA determined

T-scores. The T-score compares the measured BMD

to that of a young adult (at the age of 35) of the same

gender with peak bone mass, while considering

statistical values.

3 RESULTS

A correlation of characteristic and mean values

DUAL-ENERGY X-RAY ABSORPTIOMETRY AS AN INDICATOR FOR FRAGILITY FRACTURE RISKS OF THE

FEMORAL NECK



(BMD and T-score) determined by DXA

measurements, to the corresponding mechanical

properties (yield stress and elastic modulus) of the

examined specimens are reflected in table 1

. These

values are in good coherence with previously

presented data (Keller et al., 1990, Reilly and

Burstein, 1975). The offset in the determined values

can be attributed to the different sampling sites and

techniques of the compared studies.

Table 1: Descriptive values concerning BMD, T-score and

their correlation to the yield stress (σ

) and elasticity

modulus (E).

BMD (g/cm

) T-score σ

(MPa) E (GPa)

Min. 0.4638 -4.47 109.448 12.643

Max. 0.9694 -0.15 218.02 28.536

Mean 0.7248 -2.218 169.996 20.627

S.D. 0.263 24.843 4.129

A significant dependency of the femoral neck’s

yield stress and elastic modulus to the measured T-

score was affirmed. The highest correlation

coefficient was noted for T-score versus maximum

failure load (yield stress) of the samples (r=0.838,

p<0.001) as illustrated in figure 2.

Figure 2: Equivalent T-score values versus yield stress σy

(p<0.001).

A similar tendency can be observed for the

compressive moduli of the samples, which are

calculated based on the linear elastic region of the

determined stress-strain curves (Turner and Burr,

1993), as illustrated in figure 3.

A limitation however of the introduced process,

is based on the assumption of the material’s isotropy

and the determination of universal properties of a

bone segment comprising of both, cortical and

cancellous tissue. This methodology was adopted, as

DXA measurements reflect a combined BMA

encapturing both bone types by default and thus the

assumption of a compound material is beneficiary to

the approach.

Figure 3: Equivalent T-score values versus elasticity

modulus (p<0.001).

4 FEM SIMULATION

In order to associate the ultimate compression

strength of the samples, to fragility fracture risks of

the femoral neck, the geometry of the specimens was

reverse engineered and employed in a linear elastic

simulation of a gait type loading scenario

considering combined multiaxial forces (Jacobs et

al., 1997).

During the simulation, the specimens were once

again considered as a uniform-isotropic material,

comprising of cortical and cancellous bone tissue, to

directly facilitate the correlation of the DXA

measurements to the fracture risk of the femoral

neck.The experimentally determined mechanical

properties were adopted as bulk properties of the

compound material and assigned as such in the

simulation. The Poisson ratio was assigned as 0,3

corresponding to a mean value of cortical and

cancellous bone (Lu and Hutton, 1996, Smit et al.,

1997) regardless DXA value.

The acting loads on the femur, comprised of a

2317N joint force (Sarikat and Yildiz, 2011), evenly

distributed over the femoral head (inclined by 24

the frontal plane and 6

to the sagittal one). This

force was remotely applied on the upper surface of

the reverse engineered specimens at a distance of

46mm corresponding to the mean distance from the

tip of the femoral head at which the specimens were

severed from the femur. This, based on the

coordination system of the model, resulted in a

vector force comprising of Fx= 689N, Fy= 942N

and Fz= 2001N for axis x, y and z respectively.

The abductor muscle was considered as inactive,

as this muscle force acts during the lift up of the

foot, thus loading the trochanter during the

relaxation of the joint force. As the abductor muscle

force has been documented to amount to

approximately 703N, the worst case scenario during

BIOINFORMATICS 2012 - International Conference on Bioinformatics Models, Methods and Algorithms

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normal loading of the femur, relates to the

aforementioned 2317N joint force.

The acting force and boundary conditions were

chosen to mimic the average loading history

encountered during walking of an adult human,

corresponding to 10.000 daily cycles as described by

Sarikat and Yildiz (2011) and are schematically

represented in figure 4.

Figure 4: Applied load and boundary conditions of the

developed FEM model.

There exists skepticism concerning the ability of

compression tests in predicting the hip fracture risk,

as fractures in the hip region are the effect of

complex dynamic force application, comprising of

shear, tension and compression. Based on the

forgoing description of the model, it becomes

evident that the conducted compression experiments

encapture the loading scenario in a realistic manner,

as the compressive strength of the femoral neck

exerts a dominant impact on the structural integrity

of the femur. Furthermore, the compression tests

were identically performed in all cases while the

only variation between samples was based on the

bone mineral density.

A characteristic stress field developing on a

femoral neck sample (T-score=-4.47,

=109.448MPa and E=12.6GPa) is demonstrated in

figure 5.

5 DISCUSSION

DXA scans in the hip region, are conventionally per-

Figure 5: Calculated stress field on a reverse engineered

femoral neck sample.

formed in the trochanter, the Ward’s triangle and the

femoral neck (in an orthogonal area of6 by 10mm).

The aim of our study was to correlate the BMD

obtained from DXA, to the mechanical strength

characteristics of the examined area, as to provide

surgeons with a DXA based risk assessment,

concerning fragility fractures.

The introduced experimental investigation

affirmed the reliability of BMD in predicting the

mechanical properties of the femoral neck. A strong

enslavement of the ultimate material strength to

BMD (r=0.838) was found, while the correlation to

elastic modulus (r= 0.689) was weaker.

There exists a consensus throughout literature,

that bone density can be considered as a strong

independent predictor of failure strength

(Stankewich et al., 1996). Considering the foregoing

FEM simulation is based on static load representing

a daily cycle of at least 10.000 loadings, the

maximum developing stress should be multiplied by

3, to account for the fatigue safety factor. By

overlaying these fatigue stress value with the

experimentally determined fracture strength of the

examined specimens, a correlation between T-score

and fracture risk can be determined as demonstrated

in figure 6.

Even though DXA is a cost efficient BMD

determinant, dominating the preference of surgeons

due to its simplicity, there are some limitations

associated to the method that may affect the

accuracy of the introduced procedure.

As DXA quantifies the bone mass and not the

bone quality of a specific site, micro-fractures in

vertical trabeculae of cancellous bone will maintain

undetected. It is however widely accepted, that

micro-fractures exert an important influence on the

mechanical strength of the bone. Despite this, DXA

can be treated as a macroscopically indicator of bone

strength. Especially in the hip region, where gait like

loading ensures constant remodeling and thus the

DUAL-ENERGY X-RAY ABSORPTIOMETRY AS AN INDICATOR FOR FRAGILITY FRACTURE RISKS OF THE

FEMORAL NECK



Figure 6: T-score as a fracture risk indicator.

probability of micro fractures is considered as rather

low.

Another possible limitation of our study is

associated to the patients, the samples were

harvested from, as all of them were diagnosed with

osteoarthritis. This might have a twofold effect on

the BMD-bone properties correlation.

Primary, it has not been established if the most

common musculoskeletal disorders of the elderly

(osteoarthritisand osteoporosis) may be treated as

independent, studies have shown that the presence of

one disease may act protective against the other

(Solomon et al., 1982, Cooper et al., 1991). The

effect however of this on the presented results, can

be neglected as the selected patients exhibited

significant differences in terms of BMD.

Secondary, osteoarthritis has been associated to

subchondral scleroses in femoral head; the femoral

neck and the trochander region however, are rarely

affected by the condition (Li and Aspden, 1997). In

order to circumvent this aspect, our methodology

considered DXA scansin femoral neck, trochanter

and Ward’s triangle and was determinedas reliable.

Additionally, osteoarthritic patients undergoing total

hip arthroplasty were the only group of patients from

whom, we could receive bone samples from the

femoral neck region.

Studies have indicated that the femur carries a

30% of the applied loads in the subcapital region,

while the base of the neck is subjected by 96% of the

total load (Lotz et al., 1995). This strengthens the

vital role of the femoral neck’s capacity to transmit

the compressive stress from the joint to the shaft of

the femur. Although the etiology of osteoporotic hip

fracture is complex and multifactorial(Melton and

Riggs, 1985, Greenspan et al., 1994), bone quality

is, without a doubt, a major risk factor.

6 CONCLUSIONS

Bone mineral density measured by DXA, regardless

limitations associated to the technique’s ability to

encapture bone quality, is a strong predictor of bone

strength in the femoral neck region. Supported by an

adequate FEM simulation, DXA may be regarded as

a valuable tool during the prediction of BMD

spectrums which present a significant risk of

fragility fractures.

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