| Backround:Due to stress shielding of rigid internal fixator, Cortical resorption and resultant mechanical weakening are late effects of fractured bone. A significant number of refractures has been reported after removal of rigid internal fixator, as a result of this alteration in bone physiology. In osteoporotic fractures, the effects of stress shielding will be more obvious because of the lower rigidity of bone,longer healing time and corresponding longer time of fixation. However, little attention has been done on the effects of nail rigidity on bone healing of osteoporotic fracture. Previous research has failed to consider the effects of the strength of fixator at the same time, which is proportionate to rigidity in the same fixator. Flexible fixator has not gained acceptance in trauma care, as the material strength to maintain fragment apposition and alignment of the bone has been questioned and so far not been adequately documented. The combination of good mechanical properties and biochemical compatibility makes titanium alloys a desirable class of implant materials for orthopedic applications. However, the elastic modulus of bio-titanium alloys (Ti-6Al-4V,110GPa) currently in use is still not ideal compared with that of human bone(10-20GPa), which may lead to the obviously effects of stress shielding, especially, in osteoporotic fracture. Ti-24Nb-4Zr-7.9Sn is a newβ-type titanium alloy developed recently for biomedical application. It present higher strength(800-900MPa) than that of Ti-6Al-4V(600MPa),and with relatively low elastic modulus(33GPa).Objective:Our objective was to investigate the influence of different less-rigid nails (made in Ti–24Nb–4Zr–7.9Sn or Ti-6Al-4V) on bone healing of osteoporotic fracture and the effects of stress shielding in rats.Methods:52 female S-D rats, aged 3 months, were selected, OVX was performed in 46 rats and 6 rats received a sham-operation .Three months after OVX procedures, all animals were assessed for BMD using DEXA and confirmed the usefulness of the OVX group as an osteoporosis model.6 SHAM rats and 6 osteoporotic rats were sacrificed at the end of the 12-week experiment to provide biomechanical data. the open osteotomy model of Utv?g was chosen.. the fracture of left femur in 40 rats was stabilized with the high-rigid titanium alloy pin Ti-6Al-4V,elastic modulus 110GPa and the right with the low-rigid (Ti-24Nb- 4Zr-7.9Sn,elastic modulus 33GPa). 6 rats of every group were killed at two, four, six, and twelve weeks after fracture and the bony calluses were observed by X-ray, histology, micro-CT. The parameters of micro-CT included BV,TV,BVF. A 5 mm region of interest (ROI1) between 11 and 16 mm from the top of the greater trochanter, corresponding to the callus region, and total femur (ROI2) was measured. Subsequently, 6 rats of every group, 9 rats at twelve weeks ,were killed, and the bones were tested mechanically by a three-point bending test.Result:1.X-ray examination: X-ray film of 2,4,6,12weeks after operation of fracture showed the bone callus in low-rigid groups were more plentiful than in the high-rigid groups. 2.Histological examination of callus: There were no significant differences in the absorption, organization of hematoma, the time of the formation of trabeculae and matrix at 2 weeks,4weeks and 6 weeks, but compared with the high-rigid groups,the low-rigid groups got more collagen and callus formed, bone trabeculae were thicker, lamellar bone was more regular at 12 weeks.3.Micro-CT:In the low-rigid groups, TV(ROI1) increased obviously at 2 weeks,4 weeks,6 weeks and 12 weeks, and BVF (ROI1) was lower than the high-rigid groups at 2 weeks,4 weeks and 6 weeks, but BVF (ROI1) increased obviously at 12 weeks with significant differences (P<0.05).There were no significant differences (P>0.05) in BV(ROI1) at 2 weeks,4 weeks and 6 weeks. But at 12 weeks ,BV,BVF (ROI1) of the low-rigid groups was higher than the high-rigid groups and there was significant differences(P<0.05). There were no significant differences (P>0.05) in BV(ROI2) BVF (ROI2)at 2 weeks,4 weeks and 6 weeks, At 12 weeks the BV(ROI2) BVF (ROI2) were significantly increased in the low-rigid group(P<0.05).4.Biomechanics test: At 6 weeks the maximum load of the low-rigid group was lower than the high-rigid groups but there was no significant differences between two groups. At 12 weeks the maximum load significantly increased in the low-rigid group and higher than the high-rigid groups.Conclusion:In our study, the fixator manufactured from Ti-24Nb-4Zr-7.9Sn, compared with Ti-6Al-4V , permits more load to be transmitted through a fractured bone during healing for stimulating external callus formation ,results in less bone resorption and stronger enough to provide sufficient stability to maintain alignment and position in osteoporotic fractures. In conducting this study we were encouraged by the use/results of this newβ-type titanium alloy nails in the treatment of osteoporotic fractures and believe that products made by Ti-24Nb-4Zr-7.9Sn probably be a new method to treat osteoporotic fractures and sure to be a bright prospect of orthopedic applications.
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