Cortical bone compression

The mechanical properties of cancellous bone are dependent upon the bone density and porosity, and the strength and modulus are therefore much lower than those for cortical bone. The axial and compressive strength are proportional to the square of the bone density, and moduli can range from 1 to 3 GPa. 

A follow-up study showed the relation between the composition and the redistribution of stress [47], Cortical bone was loaded with external compression and tension, and the stress stored within the apatite crystals was assessed via the shift in phosphate Vi band in two regions a collagen-rich area and an apatite-rich area. In the collagen-rich areas, stress was released under external tension, but localized stress intensification occurred under external. In apatite-rich areas, both tensile and compressive stresses were observed. 

The mechanical properties of cortical bone (specifically, the femur, tibia, humerus, and radius) of various species (specifically, horse, cattle, pig, and human) in tension, compression, and torsion are listed in Table I. It should be noted, for example, that human femur tensile strength (namely, 124 MPa) (Yamada, 1970) is in the same order of magnitude to that of cast iron (170 MPa) (Beer and Johnston, 1981) but, surprisingly, low in weight (Kaplan et al., 1994 Fung, 1993). These unique properties of bone are a direct consequence of the synergy of its molecular, cellular, and tissue arrangement. 

Mechanical Properties of Cortical Bone in Tension, Compression, and Torsion"... 

Raman photoluminescence piezospectroscopy of bone, teeth and artificial joint materials has been reviewed by Pezzotti (2005) with emphasis placed on confocal microprobe techniques. Characteristic Raman spectra were presented and quantitative assessments of their phase structure and stress dependence shown. Vibrational spectroscopy was used to study the microscopic stress response of cortical bone to external stress (with or without internal damages), to define microscopic stresses across the dentine - enamel junction of teeth under increasing external compressive masticatory load and to characterise the interactions between prosthetic implants and biological environment. Confocal spectroscopy allows acquisition of spatially resolved spectra and stress imaging with high spatial resolution (Green etal., 2003 Pezzotti, 2005 Munisso etal., 2008). 

Figure 6. Surgical placement of the flexible hinge finger joint implant. The metacarpal head is removed to create an appropriate joint space and the intramedullary canals are then prepared to accept the implant stems. When the implant is placed in position the stems fit securely in the intramedullary canals with the flexible hinge permitting 90° active motion. Joint space is maintained by transfer of the compressive forces of joint motion across the implant to cortical bone. Careful attention to reconstructions of tendons, ligaments, and joint capsules and postoperative therapy are very important in this procedure.

FIGURE 8.4 Typical stress-strain behavior for human cortical bone. The bone is stiffer in the longitudinal direction, indicative of its elastic anisotropy. It is also stronger in compression than in tension, indicative of its strength asymmetry (modulus is the same in tension and compression). From Ref. 9.)... 

FIGURE 8.14 Compressive load-unload-reload behavior of human vertebral trabecular bone. Similar to cortical bone tested in tension, an initial overload causes residual strains and a reloading curve whose modulus quickly reduces from a value similar to the intact modulus to a value similar to the perfect damage modulus. (From Ref. 105.)... 

The application of force will generate stresses within a bone. These may be either compressive, tensile or shear. Cortical bone will tolerate compressive stresses better than tensile or shear. 

Compressive forces in children can result in cortical bone buckling. These fractures, which are commonly also referred to as buckle or torus injuries, are incomplete and the cortex is intact (Fig. 8.6a,b). Torus is derived from the Latin meaning a protuberance or knot and typically involves both cortical surfaces, while a buckle fracture may only involve a... 

Trabecular or cancellous bone is spongy in nature and occupies about 20% of the total bone. Cancellous bone is lighter, less dense, has higher porosity (pores diameter varies from a few micrometers to millimeters), and a higher concentration of blood vessels than compact bone (also called cortical or dense bone) (Fig. 2). The porous architecture of cancellous bone is easily visible under the microscope or even with the naked eye because it contain very large pores. Cortical bone, which has less porosity and thus a lower concentration of blood vessels, occupies about 80% of the total bone. Due to its lower porosity, its porous architecture is not visible to the naked eye. The diameters of pores are 10-20 pm and mostly separated by 200-300 pm intervals. Spongy bone acts mainly in compression, whereas compact bone acts mechanically in torsion, tension, and compression. 

In addition to differences in rates of bone loss in the postmenopausal period, there are differences in the sites of loss and the type of bone affected. There is generally a rapid loss of trabecular bone, particularly during the first 5 yr, which predisposes to compression of the vertebrae, the commonest site of osteoporotic fracture. There is also a progressive, but slower, loss of cortical bone arising from resorption at the endosteal surface and resulting in thinning of the cortex. This is the cause of frequent fractures of the neck of the femur. Loss of one type of bone may ensue relatively independently of loss of the other. 

Bone -Compression -Cortical anisotropic, cancellous isotropic -Ultimate stress = 130-220 MPa (compression), 80-150 MPa (tension) -Compressive modulus = 0.1-30 GPa... 

A critical limitation for long-term performance of CPCs is their relatively low mechanical strength. Because of the self-setting reaction and the absence of high sintering temperatures, compressive strengths of CPC are up to 10 times lower than sintered CaP compounds (Komath and Varma 2003). However, the compressive strength of CPCs is still comparable to that of cortical bone (88-164 MPa) (Nissan et al. 2008). 

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