Directional Variation of Trabecular Bone in the Femoral Head,

a μ-CT based Approach

Varitis Emmanouil

, Sagris Dimitrios

, David Constantine

and Lontos Antonios

Mechanical Engineering Department, Aristoteles University of Thessaloniki, Thessaloniki, Greece

Mechanical Engineering Department, Technological Education Institute of Serres, Serres, Greece

Department of Mechanical Engineering, Frederick University, Nicosia, Cyprus

Keywords: Femoral Head, Cancellous Bone Anisotropy, μ-CT.

Abstract: The structural characteristics of bone are described by features of high complexity, defining the directional

anisotropy of its mechanical properties. This phenomenon originates in the orientation of collagen fibers and

osteons within the cortical tissue and the trabecular morphology of cancellous bone. the purpose of this

study was the examination of the geometrical anisotropy of cancellous bone in the femoral head. 28 femoral

heads, harvested during hip replacement of 17 women and 11 men, were studied in total. Cylindrical

specimens of 11mm in diameter were extracted perpendicular to the fovea capitis femoris and subjected to

micro Computed Tomography (μ-CT). a 11mm sphere was isolated from all samples and the cross-sectional

area of the sphere was studied for 8 predefined regions, corresponding to planes perpendicular to principal

loading directions of the hip joint. Significant topographical variations of trabecular bone structure in

different subchondral regions were determined. in the superior region, the trabecular bone strength was the

highest, while the inferior region exhibited the lowest bone strength and medial and lateral regions had

intermittent magnitudes. No significant difference in anisotropy was found between male and female

samples, although the absolute values were greater in males. The obtained results cohere with recent

literature data of osteopenetration experiments in these directions.

1 INTRODUCTION

Trabecular bone is a major load bearing tissue of our

musculoskeletal system. When modelling

anatomical sites with large bones (Khosla et al.,

2006) or structures consisting of both, cancellous

and cortical bone (Tsouknidas et al., 2012a) the

simulation is less sensitive on micro-architectural

variations due to the thicker cortices. When

assessing however, therapeutic efficiency, implant

stability or fragility fractures, cancellous bone

remains a major qualitative determinant.

The proximal femur is an anatomical site of

major interest to clinicians as its mechanical failure

is one of the most common reasons for pain and

morbidity (Rockwood et al., 1990); (Cooper et al.,

1992). The primary bearing surface in the hip joint is

the articular cartilage. The subchondral cancellous

bone is located deep inside the articular cartilage and

the percentage of the load carried by this tissue

varies from 4% at the base of the neck to as much as

70% in the subcapital region (Lotz et al., 1995). This

suggests that cancellous bone plays an important

role in the mechanical strength of the proximal

femur and especially of the femoral head.

Cancellous bone however is not homogenous, it is

characterized by a microstructural anisotropy, that

drastically affects its biomechanical response to

loading. There exists a consensus in literature, that

trabecular bone is organized and oriented, in order to

adapt to the mechanical loads it is subjected to, this

process is based on continuous bone resorption

followed by bone formation during remodeling

(Raisz, 2005).

The purpose of this study was to examine the

micro architectural anisotropy of cancellous bone in

the femoral head and detect possible variations

based on the patients’ Bone Mineral Density

(BMD).

2 MATERIALS AND METHODS

Twenty-eight femoral heads from 17 female and 11

male patients, undergoing total hip arthroplasty,

were studied. The mean age of the patients was 75.6

237

Emmanouil V., Dimitrios S., Constantine D. and Antonios L..

Directional Variation of Trabecular Bone in the Femoral Head, a µ-CT based Approach.

DOI: 10.5220/0004327702370241

In Proceedings of the International Conference on Bioinformatics Models, Methods and Algorithms (BIOINFORMATICS-2013), pages 237-241

ISBN: 978-989-8565-35-8

 2013 SCITEPRESS (Science and Technology Publications, Lda.)



years (range, 63 – 88 years). The specimens were

harvested during operation (Tsouknidas et al.,

2012b) and frozen/stored at -60

C, upon receiving

written consent from all patients. None of the

patients had been diagnosed with any type of

metabolic disease or cancer and no bone cysts were

apparent in any of the femoral heads. All cases with

evidence of osteoarthritis, rheumatoid arthritis,

avascular necrosis, osteomalacia or secondary

osteoporosis due to corticosteroids were excluded

from the study. Cylindrical specimens were

extracted perpendicular to the medial region, by

means of a hole saw.

Prior to extraction, the patients’ Bone Mineral

Density (BMD) was measured through Dual-energy

X-ray absorptiometry (DXA) and recorded for

further use. a major strength of this study is reflected

by the wide range of BMD considered, varying in

both gender and patient condition (from healthy to

osteoporotic). This is due to the fact that trabecular

bone is a highly porous tissue differing substantially

not only across individuals but also between

anatomical sites and thus varying BMDs can be used

for studying many applications (Tsouknidas et al.,

2011).

All harvested specimens were immersed in a 200

kHz ultrasonic bath with a 1% enzyme solution by

Alconox to assimilate the proteinaceous tissue and

repeatedly cleaned until completely defatted

(Nauman et al., 1999). Upon drying, the specimen

was weighted with a micro-scale of a 100μg

resolution (Mettler Toledo) and their apparent

density (ρ

) calculated using equation (1).

௔௣

ൌ

ௗ

ߨ݀

ଶ

(1)

where m

represents the weight of the dry specimen

d and h its diameter and height respectively. This

resulted in an apparent density of the specimens

ranging from 2.19 to 3.35 g/cm

, as presented in

table 1.

The specimen was scanned with a μ-CT device

(Werth TomoScope® HV Compact) to reconstruct

its 3D shape. The measurements were conducted at a

spatial resolution of 10μm, a high image resolution

was chosen as there exists a consensus throughout

literature that measurement accuracy directly affects

geometric discretization and model convergence

(Bevill and Keaveney, 2009). Data acquisition was

in accordance to DICOM (Digital Imaging and

Communications in Medicine) this allowed the

conversion of multiple 2D images into a 3D volume.

Interpolation of the obtained measurements ensured

higher representation accuracy, even though this

process did not result in higher resolution of the

sample. the smoother representation facilitated the

distinct removal of the remaining soft tissue, which

was either not visible or accessible during the initial

defatting process. a semi-automated segmentation

technique, supported by manual correction of the

threshold results was followed. during this multi

threshold segmentation, the mean grey-scale within

the image is calculated and sensitive edge detection

filters are employed (Rathnayaka et al., 2010);

(Canny, 1986), to distinguish the apparent tissue

types. The reconstructed sample consisted of

porosity ranging from 68.52 to 91.38%. All the

determined characteristics of the sample are in

agreement with data found in literature (Baroud et

al., 2004).

Table 1: Volumetric data obtained from the measurements,

reported as mean ± standard deviation.

Gender BMD ρ

[g/cm

] BV/TV

Male 0.62±0.07 3.23±0.62 0.24±0.05

Female 0.49±0.11 2.47±0.89 0.21±0.06

The length of the specimens was approximately

25mm and upon scanning, a sphere within the

cylinder was isolated, as illustrated in figure 1. the

position of the sphere was carefully selected in order

to represent the centre of the femoral head for each

of the samples. This allowed direct comparison of

the samples in terms of cross-sectional bone area.

Figure 1: Extraction direction of the harvested specimens.

In our study eight different orientations,

representing directions perpendicular to a specific

region of the femoral head were examined, as

demonstrated in figure 2. the medial region (reg-1)

which is located in fovea capitis femoris, the inferior

region (reg-2), the medial-superior region (reg-3),

the superior region (reg-4), the anterior region (reg-

5), the anterior-medial region (reg-6), the posterior-

medial region (reg-7) and the posterior region (reg-

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8). The 8 different sites represent regions subjected

to different amounts of loading in vivo (Thomas and

Daniel, 1983); (Hodge et al., 1986).

Figure 2: Trabecular sphere and cross-sectional area.

3 RESULTS

There were significant topographical variations of

trabecular bone strength in different subchondral

bone regions.

In the medial region (1) and the inferior region

(2) the mean cross-sectional area was the lowest,

wherease the highest were registered in the superior

(4) and medial-superior region (3). The anterior

region (5), anterior-medial (6) and posterior-medial

region (7) exhibited average values, which were

slightly enhanced in the posterior region (8). The

results are symmetrized for both male and female

donors in figure 3.

Figure 3: Mean cross-sectional areas registered for the 8

primary subchondral regions.

According to these results, the 2D trabecular

bone density in the superior and medial-superior

regions is higher and thus bone strength is predicted

to be elevated in this region which is expected to

compare favourably to the fovea capitis femoris. The

bone density is lower in the orientation of the fovea

capitis femoris which should prove more susceptible

to compression, whereas anterior and posterior

regions reflect similar cross-sectional areas.

No significant difference in anisotropy was

found between male and female samples, although

the absolute values were greater in males.

4 DISCUSSION

In this study eight different regions of the femoral

head were examined. The eight sites represent

regions subjected to different amounts of in vivo

loading. The superior region being the most heavily

loaded, posterior and anterior partially loaded and

medial and inferior being the least loaded. Our

findings are in agreement with loading distribution

described in other studies (Thomas and Daniel,

1983); (Hodge et al., 1986) and converge

exceptionally with a recent study (Tsouknidas et al.,

2012c), investigating the energy required for

osteopenetration in the aforementioned sites, as

indicated in figure 4. The figure represents the mean

values of penetration energy required throughout the

tested specimens.

Figure 4: The mean values of penetration energy in the 8

regions.

The motivation behind our study is to correlate

the attained values to architectural metrics which can

be measured through 3D distance transformation

techniques

(Bevill and Keaveney, 2009). These

characteristics can be easily obtained through

porosity analysis modules, integrated in the

visualisation software of contemporary CT devises,

DirectionalVariationofTrabecularBoneintheFemoralHead,aμ-CTbasedApproach

239

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providing physicians with a valuable and non-

invasive assessment tool for bone quality and

strength.

An independent verification of our results is of

course required and foreseen. in this context micro

finite element simulations will be setup, to correlate

our image analysis to compressive strength of the

specimens. a verification of these models will be

based on uniaxial compressive experiments of the

harvested samples, conducted only in the primary

direction (1), due to the destructive nature of the

tests.

Brown et al. (1980) and Martens et al. (1983)

performed compression tests by loading in three

directions: anterior-posterior, superior-inferior and

medial-lateral. They shown that anisotropy was

evident and increase in stiffness was found in the

regions traversed by the primary trabecular system

(Brown et al., 1980); (Martens et al., 1983).

However, in their studies bone specimens were

obtained from nonspecific regions of the entire

femoral heads. in our study we examined eight

different but well defined orientations with similar

positioning within the femoral structure. Sugita et al.

(1999) examined the differences in anisotropy in

osteoporotic bone in the primary compressive group

of the femoral head. They found increased values of

compressive stiffness in the parallel loading group

compared with the perpendicular loading group, but

the anisotropic behaviour of cancellous bone is

reduced, and the femoral head became isotropic as

the bone density decreased (e.g. in osteoporosis).

The anisotropy of vertebral bodies was also

examined in the literature. Mosekilde and Viidik

(1985), found that bone strength was greater in the

vertical than in the transverse direction.

Conclusively, we examined geometrical

anisotropy of trabecular bone and found this to

represent an important characteristic of this severely

inhomogeneous structure. the conversion of our

results with previous experimental findings,

strengthens our hypothesis that micro scale imaging

of the femoral head, at limited spatial resolution,

may be used as an indicator of both, bone strength

and anisotropy.

ACKNOWLEDGEMENTS

The authors would like to thank Dr. A. Tsouknidas

of the Aristotle University of Thessaloniki for his

contribution during the setup and preparation of this

manuscript, as well as Dr. K. Anagnostidis for

providing the bone samples.

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