Effect of orientation and age on the crack propagation in cortical bone

1. Introduction

Bone is the basic structural material in all vertebral species. It is a natural composite material found in nature. The bone is made of water, organic and inorganic materials [1]. Quantitatively the bone contains up to 70% of mineral materials in the form of hydrooxyaptite, 22% of proteins in the form of collagen and 8% of water by weight [2,3]. Hip bone is an important bone in the bovine and human body. It is located in the lower part of the human body. Hip bones have furthermore four major parts, i.e. femoral head, acetabular, acetabulum ventra rim and acetabulum doral rim [4], that are interconnected with each other to form the hip joint. Compact bone mainly consists of osteons that are basically the lamellae or concentric layers of compact bone tissue around haversian canals as shown in Fig. 1. The orientation of these osteons play a very important role in the compact bone fracture toughness [5]. Nalla et al. (2003) investigated the effect of osteon orientation in bone specimens on the bone properties. They examined how the crack interacts with the microstructure of bone to explain that the crack propagation and properties vary with the orientation. The cortical bone anisotropy in the toughness properties can be justified by the toughness mechanicsm (crack bridging) induced by bone microstructure features [7]. These microcracks in front and behind the notch accelerate the main crack that originates from the notch and propagates under a continous loading as shown in Fig. 4. The microcracks amplify the local stress concentration at the crack tip [8].

Koester et al. reported that for a cracking of 500 μm, the driving force for propagation of a crack is five times more for the transverse (breaking) orientation than the longitudinal (spliting) orientation [9] as shown in Fig. 5.

Behiri et al. (1989) performed compact tension experiments on bovine cortical bone at different orientations, i.e. 0°, 15°, 30°, 45°, 75° and 90°. Behiri reported from his work that the crack resistance (critical stress intensity factor) increased from 3.2 MN m^−3∕2 to 6.5 MN m^−3∕2 (average) for 0° to 90° [10]. The crack propagation direction is shown in Fig. 6.

2. Material and methods

For specimen preparation the fresh bovine male hip joints were collected from different ages groups (5 months to 4 years) from different butcher shops in the local market. The compact tension specimens for the fracture toughness test according to ASTM E-399 [11] were cut from the hip bone joint both in the longitudinal and transverse direction to the fiber or osteons orientation. A joint of a 3 year old male calf is shown in Fig. 7. A total of 48 specimens were cut, 24 in the longitudinal direction and 24 in the transverse direction to the fiber orientation in the bone. The samples were reduced to the exact dimension of ASTM using a file. The samples' geometry according to ASTM E-399 are shown in Fig. 8.

The bone specimens were painted using black and white paint spackle pattern in two steps. In the first step the bone samples were painted with white paint using a spray. In the second step the samples were sprayed with black paint from a distance of two feet to obtain black spackles, as is shown in Fig. 9.

To observe the crack propagation and fracture toughness of the bone specimens both in the longitudinal and transverse specimens, the compact tension testing experiments were performed using a universal testing machine with special gripers for compact testing. The specimens were clamped with the help of the gripers in the machine as shown in Fig. 10. The load was applied at the rate of 1mm/minute cross head speed. The PC connected to the UTM recorded the applying load and displacement.

The crack tip and crack propagation were recorded via a camera attached to the UTM. The experiment was stopped when the crack started growing as shown in Fig. 11.

The data from the universal testing machine was used to calculate the fracture toughness K for the bone specimens using the below formulas.

The fracture toughness K was determined by using equation [16]: K = P c Y / B W 0.5 where P c is the critical load at fracture, B is the specimen thickness and W is the specimen width. Y = 29.6 ( a / w ) 0.5 − 185.5 ( a / w ) 1.5 + 655.7 ( a / w ) 2.5 − 1017 ( a / w ) 3.5 + 638.9 ( a / w ) 4.5 where the term Y is a constant found by the geometry and dimensions of the test specimen and a is the crack length.

3. Discussion

Due to different orientation of the osteons in the longitudinal and transverse direction the crack propagation direction is also different. In the longitudinal specimen the crack propagates straight because the initial crack is parallel to the orientation of the osteons but in the transverse specimens the crack propagated at an angle because of the perpendicular orienation of the osteons as shown in Figs 11 and 12. The crack propagation speed and length is also higher in the longitudinal specimens as compared to the transverse specimens due to the osteons orientation and crack bridging. After calculating the fracture toughness for all the specimens, the results show that the fracture toughness in bovine hip bone increases from 1 year to 3 years and then started decreasing from 3 years to 5 years. This trend is higher in transverse direction as compared to longitudinal direction. It is observed that the toughnes of the transverse specimen was high as compared to the longitudinal specimen due to the different orientation of the osteons in these specimens, as was the case in crack propagation. Similarly, Youshinori et al. performed experiments on whole long bones by applying torsion and lending loads. Youshinori et al. reported that on bending a large direct force is required to preduce transverse fracture while spiral fracture is produced by relatively smaller torque [23,24]. Overall the fracture toughness in aging bone is decreasing both in the longitudinal and transverse oriented bone specimens. The discussion can be summarized as below.

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In longitudinal specimens, crack propagatation is straight and sharp because the initial crack is parallel to the orientation of the osteons. In the transverse specimens, the crack propagates at an angle because of the perpendicular orienation of the osteons to the initial notch (see Figs. 2--3). Nalla et al. (2003) [7], Vashishth et al. (2000) [8], Koester et al. (2008) [9], Behiri et al. (1989) [10] experimentally reported the same results.

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The crack propagation speed in the longitudinal specimens is high as compared to the transverse specimens. Chen et al. 2008 [17] experimentally achieved that same behavior.

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The experimental results show that the force required for transverse compact test specimens frature is higher as compared to the force required for longitudinal compact test specimens failure. Literature review also showed that fracture is higher for transverse compact test specimens as compared to longitudinal compact test specimens [13–15]. The variations in the values of the above properties with respect to age were more sensitive in the case of bovine bones as compared to the human bone behavior. This is due to the short life span of the bovines in comparison to humans.

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The bone toughness consistently changes with age. Bone fracture toughness first increases up to 3 years and then decreases from 3 years onward as shown in Table 1. The previous research by Nalla et al. (2004) [14], Ural and Vashishth (2006) [18], Ural and Zioupos (2011) [19], Vashishth and Behiri (1997) [20], Wu and D. Vashishth (2002) [21], and Kulin et al. (2011) [22] reported the same trend in bones.

4. Conclusion

From this experimental research we learned that the transverse compact test specimens fracture toughness is higher in comparison to the longitudinal compact test specimens fracture toughness due to the presence of resisting osteon fibers acting as a crack bridging for the same age. The fracture toughness showed an increasing trend from 5 months to 3 years and then started decreasing from 3 years onwards. Our results showed that fracture toughness starts decreasing both in the longitudinal and transverse compact test specimens from 3 years onwards. Previous research on bones showed that transverse fractures required larger force in the plain fracture toughness test and bending test as compared to longitudinal and spiral fracture in the plain fracture toughness test and torsion test, which supports our experimental research work. 

Conflict of interest

None to report.