Elastic Modulus and Strength of Emu Cortical Bone

Abstract

The emu (Dromaius novaehollandiae) shows potential as a unique animal model for replicating the femoral head collapse process seen in end-stage human osteonecrosis. Since the collapse phenomenon (and interventions to prevent it) involve mechanical processes, it is important to elucidate the similarities and differences of emus versus humans in terms of hip joint biomechanics. A first step for comparison is the intrinsic mechanical properties of the respective bone tissues, as reflected in cortical bone flexural stiffness and strength. In four-point bending, emu cortical bone was found to have an elastic modulus of 13.1 GPa. Its yield stress was determined to be 113 MPa and the ultimate strength was 146 MPa. Emu cortical bone's elastic modulus was similar to that of other avian species, and falls approximately 25% below that of the human (17.3 GPa).

INTRODUCTION

Femoral head osteonecrosis remains an important unsolved problem in hip surgery, largely because reliable techniques are lacking to prevent mechanical collapse of the structurally compromised epiphyseal cancellous lattice. To examine the pathology of osteonecrosis, various animal models have been used in the past, including dogs, rabbits, mice, goats, pigs, miniature swine, and horses.^5,6,7,9,11 While these species have mimicked various histological attributes of the human condition, none has shown good concordance with late structural collapse of the femoral head, the aspect of pathogenesis that is of primary clinical concern. We earlier hypothesized that this lack of collapse might be due to biomechanical factors, since all of these model species are quadrupeds and can therefore load-protect the affected limb. The emu (Dromaius novaehollandiae), a large, flightless, bipedal bird (Figure 1), appeared to offer a means to overcome that key difficulty. In a recent exploratory series of 19 emus with cryogenically induced femoral head necrosis, 16 progressed to lameness from femoral head collapse and structural failure, at an average time point of 11.7 weeks.⁴

Figure 1.

Emu (Dromaius novaehollandiae)

Emus are members of the ratite family, along with ostriches, kiwis, cassowaries, and rheas. Native to Australia, emus have been widely farmed commercially in the United States since the early 1990's. Because of their relatively large size (150 cm in height and 450 N in body weight) and their bipedal gait, emus could potentially be used to objectively test conservative and/or surgical treatments of osteonecrosis, on a near-human size scale. Unfortunately, emus presently are an unfamiliar species for laboratory musculoskeletal investigation. Fully exploiting the utility of this new model's bipedality for osteonecrosis research will depend on rigorous understanding of the comparative biomechanics of emu versus human hips. A first step toward that goal is to document the intrinsic mechanical properties (elastic modulus, yield, and ultimate strength) of emu cortical bone. There is relatively little literature on the mechanical properties of even avian bone in general, but such evidence as exists suggests it to be substantially weaker and more compliant than its mammalian counterpart. ^3,8,10,12 In the case of emus, however, the habitual functional stimulus (terrestrial bipedal ambulation) suggests the possible evolution of much more mammal-like mechanical properties than would be appropriate for flighted species. Therefore, mechanical data for emu cortical bone, relative to mammalian cortical bone, would provide key point-of-reference information, upon which it will be possible to build. It enables the development of (CT Hounsfield number) density-based regressions of femoral head cancellous bone mechanical property distributions, for purposes of stress analysis via continuum finite element modeling. Of course, the mechanical properties of emu compact bone are also of interest in their own right, since realistic structural representation of proximal femoral cortices is necessary for hip joint stress analysis. More broadly, the emu's bipedality is potentially relevant for addressing a wide range of musculoskeletal research questions involving the biomechanics of lower limb diaphyses.

MATERIALS AND METHODS

Thirty-nine rectangular beam specimens of diaphyseal cortical bone from the femurs of skeletally mature young adult emus (age 24 to 36 months), oriented parallel to the bone's long axis (Figure 2), were taken from 5 separate femurs of freshly-sacrificed animals. Comparable numbers of specimens came from each side of the bones (anterior, posterior, medial, lateral, and from proximal versus distal stations), allowing exploration of mechanical property dependence on sampling location. Specimens were milled to a nominal length, width, and depth of 56.6 mm, 7.4 mm, and 1.6 mm respectively. Individual exact dimensions were documented by micrometer. During both the milling and subsequent testing processes, all samples were kept copiously irrigated with 0.9% saline.

Figure 2.

Specimen harvest region, within which samples were indexed according to circumferential (A, P, M, L) and longitudinal (Pr, D) site.

Modulus of elasticity, yield, and ultimate strength were characterized via a four-point bending test (Figure 3). Data collection was performed using an MTS Bionix 858 test machine (MTS Corp., Eden Prairie, MN). The fixture used had a support span of 25.4 mm and a loading span of 8.45 mm, to correspond to a support span-to-thickness ratio of 16:1, as recommended by ASTM D790.¹ The crosshead speed was adjusted to impose a longitudinal strain rate of 0.0005 sec^-1 at the specimen surface. Elastic modulus (E) was computed from:

E=(1/I)*(5/12)*(P/y)*c³

, where P/y is the slope of the visually apparently linear portion of the load-deflection curve, I is the cross-sectional area moment of inertia, and c is the loading span distance.² Slope (P/y) was computed using Microsoft Excel to curve-fit the linear portion (Figure 4) of the load-deflection curve. In all specimens, P/y in the curve-fitted region was almost perfectly linear, with R² > 0.99. A stress-strain curve was also plotted for each test. Specimen surface stress σ at a given load P was defined as σ= 3Pc/bh². Maximum strain ε at a given deflection y was defined as ε= 3yh/5c² where c is the loading span distance, b is the width of the specimen, and h is the specimen thickness. (This definition of strain, based on Hooke's Law (ε = σE), is restricted to the linear elastic region). Yield stress was defined as that stress at which the stress-strain curve intersected a line having the same slope as the fitted elastic modulus, but offset by 0.2% strain. Ultimate strength was the stress at the point of maximum load acceptance.

Figure 3.

Four-point bending test set-up

Figure 4.

Typical stress-strain curve terminating in brittle fracture for a trial.

Bone is, of course, viscoelastic. In order to appreciate the influence of strain rates on apparent elastic properties, eight specimens were loaded at three separate strain rates (0.000167 sec^-1, 0.00033 sec^-1, and 0.0005 sec^-1). The slow and medium speed tests were performed in the elastic region only, up to a maximum force of 35 N (ε<0.25 %). Each specimen was then tested to failure using the fastest strain rate (which was the same as that used for the other 31 specimens).

RESULTS

The stress-strain curves (Figure 4) generated by the four-point bending tests can be divided into two portions-the linear (elastic), and the non-linear (plastic) region. At low strains (typically 0% to 0.6%), emu cortical bone exhibited elastic behavior; this portion of the curve was used to calculate the modulus of elasticity. At higher strains the bone experienced yield and plastic deformation. The elastic modulus values of emu cortical bone ranged from 5.62 GPa to 19.83 GPa (Table 1), with an average of 13.05 GPa and a standard deviation of 3.94 GPa. Yield stress values ranged from 50.5 MPa to 191.3 MPa, with an average of 113.1 MPa and a standard deviation of 29.2 MPa. Ultimate stresses ranged from 66.3MPa to 231.2 MPa, with an average of 146.9 MPa and a standard deviation of 32.2 MPa. With α=0.05, there was no significant dependence of elastic modulus, yield, or ultimate stress on donor femur (p=0.0928 for elastic modulus, 0.1246 for yield stress, and 0.1844 for ultimate stress, all determined using repeated measures ANOVA). Neither was there a significant dependence of elastic modulus, yield, or ultimate stress on specimen harvest site. Again using repeated measures ANOVA, anterior versus posterior elastic modulus was found to have p=0.9447, yield stress had p=0.4845, and ultimate stress had p=0.9927. Medial versus lateral samples had elastic modulus, yield, and ultimate stress p values of p=0.2640, 0.4587, and 0.4026 respectively. For proximal versus distal comparisons, the corresponding p values were p=0.4405, 0.2445, and 0.3878, respectively.

Table 1.

Elastic modulus E (GPa), yield y and ultimate ult strength σ (MPa) and yield and ultimate strain ε (%), as a function of harvest site, donor, and loading rate. Parenthesized values are standard deviations.

Variable, N	E	σy	σult	εy	εult
Anterior, 18	12.53 (3.09)	111.81 (26.79)	148.46 (23.49)	0.73 (0.18)	1.41 (0.46)
Posterior, 13	12.27 (5.01)	104.11 (32.59)	142.35 (44.63)	0.71 (0.12)	1.32 (0.16)
Medial, 14	11.22 (3.81)	104.86 (29.63)	136.12 (30.46)	0.78 (0.16)	1.31 (0.31)
Lateral, 17	13.23 (3.78)	112.54 (28.81)	153.81 (34.53)	0.72 (0.22)	1.35 (0.41)
Proximal, 16	13.03 (4.10)	120.84 (20.74)	158.77 (31.05)	0.74 (0.19)	1.34 (0.40)
Distal, 23	13.06 (3.92)	108.28 (28.94)	139.53 (31.26)	0.73 (0.17)	1.26 (0.34)
Bird #1,10	11.33 (4.83)	115.29 (33.63)	139.92 (35.78)	0.87 (0.22)	1.31 (0.35)
Bird #2, 11	12.66 (3.83)	111.96 (28.94)	153.63 (37.14)	0.70 (0.20)	1.35 (0.34)
Bird #3, 10	12.95 (2.92)	101.11 (21.70)	140.45 (25.74)	0.67 (0.08)	1.42 (0.42)
Birds #4 and #5, 8	15.86 (2.99)	126.54 (29.87)	154.6 (29.84)	0.68 (0.08)	1.03 (0.20)
Slow Load,8	13.05 (2.51)	N/A	N/A	N/A	N/A
Medium Load, 8	14.14 (2.57)	N/A	N/A	N/A	N/A
Fast Load, 8	15.86 (2.99)	126.54 (29.87)	154.6 (29.84)	0.68 (0.08)	1.03 (0.20)
Whole Series, 39	13.05 (3.94)	113.11 (29.15)	146.93 (32.20)	0.73 (0.18)	1.29 (0.36)

Using repeated measures ANOVA, increasing strain rates showed a significant (p=0.0063) linear trend towards increasing the modulus of elasticity (regression coefficient, R = 0.412), but an insignificant quadratic trend (p=0.1817). The mean elastic moduli for each strain rate group were also significantly different (Wilk's Lambda p=0.0283).

DISCUSSION

Relatively little is known about the mechanical properties of bone from ratite species. The presently measured apparent elastic modulus of emu cortical bone, 13.05 GPa, is similar to that reported for ostrich, geese, and "domestic fowl" (Table 2), despite emu adaptation for terrestrial bipedal gait rather than flight. The load-deformation curves typical of emu specimens were generally similar to those of human cortical bone. Each curve clearly had an elastic (linear) portion at low strains, and a plastic (non-linear) portion at higher strains. All specimens were tested destructively to determine the bone's ultimate strength. Two distinct types of failure occurred. Roughly half of the specimens experienced a brittle failure (i.e. snapped in half), while the other half lost all internal strength, but did not abruptly fracture into two distinct fragments. In the latter category, ultimate strength was defined as the stress at the point when force began to decrease, i.e. a negative slope in the force-deflection curve. Tests were terminated after ultimate stress had been reached, since elastic modulus and yield stress could be obtained from the previous portion of the curves. Average ultimate stress was 146.9 ± 32.15 MPa, and average ultimate strain was 1.29 ± 0.36%. There was no significant difference in elastic modulus, yield, or ultimate stress between those specimens which fractured into two distinct fragments versus those which did not (p=0.655, p=0.274, and p=0.318, respectively).

Table 2.

Comparison of emu and other animal bone elastic modulus

	Emu	Turkey10	Ostrich12	"Domestic Fowl"12	Goose8	Horse12	Pig12	Human3
E (GPa)	13.05	16.95	13.62	13.43	13.2	24.99	14.60	17.5
% larger than emu	-	23.1	4.3	2.9	1.2	47.8	10.7	25.5

Due to the small specimen dimensions, machining flaws might be suspected to have potentially affected the apparent mechanical properties. Therefore, several representative specimens were stained with thionin and examined under a microscope to identify any flaws introduced during machining. No such flaws were detectable. Additionally, a flawless specimen was tested to a strain of 0.35%, scratched twice to a depth of about 0.2 mm with a scalpel (far greater damage than seen on any of the machined surfaces), and re-tested. The elastic moduli calculated before vs. after scratching differed only by 0.3%.

The present data, while not ideally tightly clustered, show that the intrinsic elastic modulus of emu cortical bone is about 25% smaller than for human bone. This suggests that, at least in terms of intrinsic mechanical behavior, emu bone is not grossly dissimilar from its human counterpart. Of course, there are many morphological differences that need to be addressed in future work. The emu femoral cortex is relatively thinner than that of humans (Figure 5). Additionally, the trabecular architecture is relatively coarser and perhaps differently oriented. The subchondral plate thickness, metaphyseal and diaphyseal cortical bone distributions, and the presence of a prominent antetrochanter also distinguish emu femurs from those of humans. Habitual functional loading, stress transmission pathways, and articular contact mechanics also are all in need of investigation.

Figure 5.

Comparison of emu and human proximal femur cross sections.