The Material Properties of Ovine Femoral Periosteum
1
1
McBride, S H; 1Evans, S; +1Knothe Tate, M L
Case Western Reserve University, Cleveland, OH
[email protected]
INTRODUCTION
The material properties of ovine femoral periosteum are not well
characterized. An understanding of these properties is critical to predict
the environment of pluripotent and osteochondroprogenitor cells that
reside within the periosteum and that have been shown recently to
exhibit a remarkably rapid capacity to generate bone de novo [1,2].
Furthermore, as the periosteum serves as the bounding envelope of the
femur, anisotropy in periosteal properties may play a key role in
modulating bone growth, healing and adaptation, in health, disease and
trauma.
Our approach was to quantify shrinkage upon release from underlying
bone as measure of pre-stress in the periosteum and to measure the
modulus of elasticity in diaphyseal periosteum loaded in the axial and
circumferential directions.
Axial
MATERIALS AND METHODS
Animal Model: Ovine femoral periostea were harvested
from skeletally mature sheep (2 year old ewes, 3/4
hampshire & 1/4 dorset, 62.2 - 79 kg, Ohio Agricultural
Research and Development Center, Wooster, OH) within 5
hours of euthanasia. The acute studies were carried out in
accordance with Case Western Reserve University Animal
Resource Center guidelines under IACUC approved
protocols (2010-0014).
Fig. 1. Ovine femur. Periosteum sampling sites, testing
direction (axial - green, circumferential - grey). Adapted from [3].
Shrinkage Studies: In situ evaluation of periosteal pre-stress
necessitated development of protocols to prevent artifacts observed
during pilot studies. First, squares of periosteum from the femoral
diaphysis (n=3 bones), measuring circa 25% of the diaphysial
circumference, were removed from the anterior, medial, posterior, and
lateral aspects. Samples were submerged in 3 mL of Dow Corning 200
fluid (Univar USA, Strongsville, OH), a 100 centistoke silicon oil that
mitigates swelling and dehydration during testing. Then, the percent
change in area, axial length, and circumferential length was measured
over time (16.5 hours) using high-resolution imaging (8.13 pixels/µm,
Leica DM IRE2, Leica, Wetzlar, Germany) and processing (MatLab,
Natick, MA) methods. Hypotheses related to these outcome measures
were tested using a one-way ANOVA test with pair-wise comparisons
(α=0.05). Data from each aspect were treated as a cohort. To determine
differences in length change for each aspect, we analyzed each time
point using a paired t-test of the differences between change in
circumferential and axial length (α=0.05) (MatLab).
Mechanical Testing: To measure the elastic modulus of the periosteum
in the axial (Fig. 1) and circumferential directions, a circa 7×1 cm strip
of diaphyseal periosteum, centered at the anterior aspect, was removed
in either an axial (n = 6, Fig. 1, green overlay) or circumferential (n = 4,
Fig. 1, grey overlay) orientation. The periosteum was fixed to a template
during resection to maintain the in situ length (7 cm) while allowing
limited width (1 cm) relaxation until the strip center was secured in the
testing grips. Samples were stretched, at a rate of 0.004 mm/sec
(Enduratec ELF 3200, Bose Co, Eden Prarie, MN), to failure (> 0.15 N
drop in force). Stress was calculated from the measured force and the
cross sectional area of the relaxed specimen. The associated tissue strain
was measured optically (Sony DC-HC52 video, Sony Co, New York,
NY with digital image correlation, MatLab). The elastic moduli of the
respective axial and circumferential toe and elastic regions were
measured from the slope of the resulting stress-strain curves. Statistical
analysis of differences between elastic moduli of each region was
carried out using the one-way ANOVA test with pair-wise comparisons
(α=0.05) (MatLab). Each region was treated as a cohort.
Fig. 2. Average per cent change in area and length. Change is
measured from initial condition in situ, for periosteal area and length.
Percent of Initial
We hypothesize that
•
the periosteum is pre-stressed in situ and
•
periosteum exhibits anisotropic material properties, e.g. the elastic
modulus of the periosteum depends significantly on the direction
of loading (axial, circumferential, Fig. 1).
RESULTS
Shrinkage Studies: Periosteum experiences an average of 40-55%
decrease in area immediately upon removal from the underlying bone
(Fig. 2). At any timepoint, the change in area is not significantly
different between aspects. The overall length shrinks 40-50% in axialoriented samples and 2-20% in the circumferential-oriented samples.
The change in length is significantly different in the axial and
circumferential directions for all aspects of the femur except the lateral
aspect.
Length
Area
X Axial
O Circumferential
Mechanical
Testing:
The
periosteum exhibits a high degree of
anisotropy (Fig. 3, Table 1). When
strained in the axial direction, an
initial compliant toe region is
followed by a much stiffer linear
elastic region. When strained in the
circumferential direction there is no
toe region and the tissue behaves
approximately twice as stiff as the
compliant axial toe region until
failure.
Figure 3. Periosteal stress-strain curves. Significant anisotropy is
observed between elastic regions of axial and circumferential specimens.
Table 1. Measured elastic moduli.
Sample Region
Elastic Modulus
(MPa)
Axial Toe Region
(n=6)
1.9
Axial Elastic Region (n=6)
25.7
Circ. Elastic Region (n=6)
4.4
95% Confidence
Interval
(0.8, 3.1)
(18.8, 32.5)
(3.2, 5.6)
CONCLUSIONS
The periosteum is a pre-stressed, anisotropic material. Based on the
results of this study, the periosteum is pre-stressed in the axial direction,
resulting in twice higher shrinkage upon release from underlying bone
compared to the circumferential direction. Furthermore, the elastic
modulus in the axial direction is more than ten fold greater than that in
the circumferential direction. These disparate material properties are
expected to exert a profound influence on femur mechanobiology during
development, growth, healing and aging, in health and disease [2]. These
studies may have important implications with regard to the development
of implants to replace periosteum when the amount of healthy
periosteum is a limiting factor, e.g. after tumor resection as well as
battlefield or other high impact, high energy injuries [4].
REFERENCES [1] Knothe Tate et al. J Bone Jt Surg 89(2):307-16,
2007. [2] Knothe et al. J Biomechanics, 2010. [3]http://en.wikibooks.org
/wiki/Anatomy_and_Physiology_of_Animals/The_Skeleton [4] Knothe
Tate et al. Trans ORS 35:0384.
ACKNOWLEDGEMENTS AO Research Fund F-07-99K, NIH/
NIAMS 2T32AR007505, Nick LaFrenz, Matt Kirsch.
Poster No. 2212 • ORS 2011 Annual Meeting