Injury | International Journal of the Care of the Injured - Volume 47 - Supplement 2

Bone mechanical properties and changes with osteoporosis

Georg Osterhoff

, Elise F. Morgan

, Sandra J. Shefelbine

, Lamya Karim

, Laoise M. McNamara

e,f

, Peter Augat

Division of Orthopaedic Trauma, Department of Orthopaedic Surgery, University of British Columbia, Vancouver, British Columbia, Canada

Department of Mechanical Engineering, Boston University, Boston, MA 02215, USA

Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115, USA

Center for Advanced Orthopedic Studies, Beth Israel Deaconess Medical Center and Department of Orthopedic Surgery, Harvard Medical School, Boston, MA 02215, USA

Centre for Biomechanics Research (BMEC), Department of Biomedical Engineering, NUI Galway, Galway, Republic of Ireland

National Centre for Biomedical Engineering Science (NCBES), NUI Galway, Galway, Republic of Ireland

Institute of Biomechanics, Trauma Center Murnau, Murnau, Germany and Paracelsus Medical University Salzburg, Salzburg, Austria

A B S T R A C T

This review will define the role of collagen and within-bone heterogeneity and elaborate the importance of

trabecular and cortical architecture with regard to their effect on the mechanical strength of bone. For each of

these factors, the changes seen with osteoporosis and ageing will be described and how they can compromise

strength and eventually lead to bone fragility.

K E Y W O R D S

Bone resorption

Bone loss

Bone fragility

Collagen

Biomechanics

Introduction

Osteoporotic fractures occur spontaneously or as a result of minimal

trauma from day-to-day activities [1]. In 90% of all hip fractures, the

leading mechanism of trauma is a simple fall, [2

–

5] indicating bone

fragility in these patients. Early detection of an impaired quality of

bone is crucial in the prevention of osteoporotic fractures. Previous

studies suggest broad under-diagnosis of osteoporosis [6], and the

opportunity to start bone modulating therapies before the occurrence

of an osteoporotic fracture is missed in up to 84% of osteoporotic

fracture cases [7].

The assessment of bone mineral density (BMD) as a surrogate

marker of bone strength using non-invasive methods like dual-energy

X-ray absorptiometry is widely regarded as the gold-standard for

diagnostic screening and as a guide prior to therapeutic decisions [8].

However, BMD accounts for only 60% of the variation in bone fragility

[9], because it is unable to depict differences in bone material

composition and structural design. Both characteristics influence

bone strength to a large extent [10].

The unique mechanical properties of bone reflect the need to

provide at the same time strength and lightweight design, stiffness and

elasticity, the ability to resist deformation and to absorb energy [11].

This is possible because of the complex arrangements in compositional

and micro-architectural characteristics of bone as well as continuous

adjustments over time in response to dynamic extrinsic and intrinsic

factors. Ageing and other factors like estrogen deficiency can affect

these components and eventually result in decreased bone strength

and fracture toughness [12]. Osteoporotic fractures, therefore, are the

macroscopic result of a sequence of multiple nano- and micro-

structural events.

This review will define the roles of (1) trabecular and cortical

bone architecture, (2) structural and compositional heterogeneity

in trabecular bone, and (3) alterations in collagen in determining

mechanical integrity of bone. For each of these factors, the changes

seen with osteoporosis and ageing will be described and how they can

compromise strength and toughness, eventually lead to bone fragility.

Differences between trabecular and cortical bone

Macroscopically, the two most apparent structural features of bone

are those of trabecular and cortical bone. Cortical bone forms a solid

osseous shell around the bone and consists of dense and parallel,

concentric, lamellar units

–

the osteons. Each is surrounded by a layer

of cement-like substance, forming the so called cement line. The

osteons are nurtured and interconnected by a system of Haversian and

Volkmann

’

s canals as well as canaliculi [11]. On its outer surface,

cortical bone is covered by an envelope of connective tissue, the

periosteum; and on its inner surface it is covered by the endosteum.

In contrast, trabecular bone shows a characteristic network of

lamellar bone plates and rods that presents with less density, less

homogeneity, and a lesser degree of parallel orientation. The trabecular

Corresponding author at: Institute of Biomechanics, Berufsgenossenschaftliche

Unfallklinik, Murnau Prof.-Kuentscher-Str. 8, D-82418 Murnau am Staffelsee, Germany.

Tel.:

+49 8841 484563

; fax:

+49 8841 484573.

E-mail address:

biomechanik@bgu-murnau.de

(P. Augat).

Injury, Int. J. Care Injured 47S2 (2016) S11

–

S20

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