ovariectomized rats.
6.4. Discussion 1 Testing method
6.4.4. Effect of morphology on the energy needed to break segments
Localized toughness testing allowed for the examination of the relationship between bone area and the amount of energy (J) needed to break the femoral segments. The current work showed that there was a significant relationship between bone area and the amount of energy needed to fracture segment B in the rats that received GOAT OVX ALD. In contrast, no relationship was found between bone area and energy needed to fracture segment B in rats that received cow’s milk either with or without Alendronate. No overall relationship was found between total cross sectional area (an index of structural size) and the amount of energy needed to fracture femoral segment B for rats that received either of the milk diets with or without alendronate. Taken together the results from bone area and total cross sectional area suggest that the increase in the amount of energy needed to fracture segment B of GOAT OVX ALD rats is due to changes in the toughness of the bone material, and not the cross sectional size of the femur. Therefore, the smaller bone areas of the femoral segments B of GOAT OVX
6-22 ALD rats were perhaps tougher than the larger sized bone segments of the COW OVX ALD rats. The effect of morphology on the behaviour of fracture propagation has been examined in the micro-structure of human cortical bone by Yeni et al (1997) (27). They found that osteon morphology (both size and density) and the degree of cortical bone porosity accounted for 49% - 68% of the variation in fracture toughness of milled notched samples taken from expired elderly human femurs and tibias (27). However, on the macro scale, previous work has suggested that the fracture properties, Kc and Gc are not influenced by morphological properties such as bone thickness (28, 29). It remains unclear in the current study as to why the GOAT OVX ALD rats had smaller bone areas compared to the COW OVX ALD rats. It may be that the alendronate reduced periosteal apposition and at the same time allowed for a more complete mineralisation of the bone composite compared to the COW OVX ALD rats. In contrast further investigation of the rats fed the non-milk diet showed that there was a relationship between the size of the bone and the energy used to fracture the bones in the proximal femoral shaft of the CON OVX rats. While it did not result in tougher bones it did indicate that, as the size of the proximal shaft in the CON OVX rats got bigger, more energy was needed to break the bone segments. This was also apparent in the mid-shaft of CON OVX rats, except that the relationship between impact energy and bone size was reflected in significantly tougher bone compared to the CON OVX ALD rats. There was some difference in toughness between the SHAM rats and the CON OVX ALD rats in the mid-shaft. However, that difference may have been offset by the larger sized bones found in the CON OVX ALD rats.
6.4.5. Conclusion
A complementary effect was found between the milk based diets and alendronate in the proximal region of the femoral shaft resulting in tougher bone material per unit of area. The co-administration of goat milk and alendronate appeared to have a complementary effect in increasing the toughness of the bone composite in the mid sections of the femurs of 10 month old ovariectomized rats. The localisation of this effect in a limited region of the femur emphasises the importance of evaluating regional variation of toughness within the femur, when assessing the effect of dietary and medical therapies in animal models of osteoporosis. It is uncertain why supplementation with alendronate caused a different response when rats were fed goat’s milkcompared to cow’s milk, and further investigation would be required.
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6.5.
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