Abstract
BACKGROUND:TiNi shape memory alloy (TiNi-SMA) distractor can be buried completely, there is no time delay and it is different from other distractors previously used. Therefore, it is necessary to reevaluate the osteogenetic time and quality.
OBJECTIVE: To investigate the quality of the alveolar ridge augmented by TiNi-SMA distractor by bone mineral density (BMD) and biodynamics tests of new-formed bone.
DESIGN: Randomized controlled observation.
SETTING: Department of Stomatology of General Hospital of Chinese PLA.
MATERIALS: Twelve healthy adult male mongrel canines (ranged from 21 to 26 kg) were provided and bred by Medical Experimental Animal Center of General Hospital of Chinese PLA. All the animals were accorded with the requirements of the quarantine and animal ethnics. Self-made distractors were used in the study. XR-36 Dual Energy X-Ray Absorptiometry (Norland, USA) and 858 Mini Bionix Ⅱ Biomaterial Testing Machine (MTS, USA) were used in the study.
METHODS: The experiments were performed at the Medical Experimental Animal Center of General Hospital of Chinese PLA from July 2000 to May 2004. The animals were grouped into group Ⅰ(3 months group) and group Ⅱ (6 months group) randomly with 6 animals in each group. The left mandible was used as experimental side and the right was used as control side. The canines were killed after anaesthesia at 3 or 6 months later in both groups. After the mandible samples were cut, the BMD of three points in the distraction gap, above the distraction gap and below the distraction gap were measured by dual energy X-ray. Compressive biomechanics test was performed in the middle of the distraction area of the experimental side and the corresponding area of the control side to detect compressive stress and the modulus of elasticity.
MAIN OUTCOME MEASURES: ①BMD of the distraction area of the experimental side and the control side in the group Ⅰ and group Ⅱ. ②Compressive strength and modulus of elasticity of the mandibles of the experimental side and the control side.
RESULTS: All of the 12 experimental canines were included in the final analysis, no drop-out. BMD determination: BMD was lower in above the distraction gap of the mandibles of the experimental side than in the control side in the group Ⅰ (t=-2.898，P < 0.05). There was no significant difference in BMD between both sides in the group Ⅱ (P > 0.05). Compressive biomechanics test: Compressive stress and the modulus of elasticity of the mandibles on the experimental side were lower than on the control side in the group Ⅰ (t =-3.274，-3.534, P < 0.05). All indexes increased by some degree in the group Ⅱ compared to the group Ⅰ (P > 0.05).
CONCLUSION: The new bone is strong enough for later repair such as implantation after distraction osteogenesis using TiNi-SMA distractor.

INTRODUCTION

Previously, alveolar ridge augmentation was achieved mainly by bone transplantation, vestibular extension and other methods. Recently, the study on the alveolar ridge augmentation by using distraction osteogenesis technique has got great progression [1-16]. Distraction implant, expansion-bolt and other half-buried devices are commonly used. However, TiNi shape memory alloy (TiNi-SMA) distractor can be buried completely and is different from other distractors in the surgical procedure and other aspects [17-18]. By measuring the bone mineral density (BMD) and the compressive biomechanics of the new-formed bone, this study evaluated the time and quality of the distraction osteogenesis to provide experimental basis for the late repair of implantation and so on.

MATERIALS AND METHODS

Materials
Experiments were performed at the Medical Experimental Animal Center of General Hospital of Chinese PLA from July 2000 to May 2004. Twelve healthy adult male mongrel canines (ranged from 21 to 26 kg) were provided and bred by Medical Experimental Animal Center of General Hospital of Chinese PLA. The serial number was given, and then random number for the animals was produced with statistical software STATA 4.0. The animals were grouped into group Ⅰ(3 months group) and group Ⅱ (6 months group) randomly with 6 animals in each group. The left mandible was used as experimental side and the right was used as control side. All the animals were certificated to accord with the requirements of the quarantine and animal ethnics. Self-made distractors were used in the study. XR-36 Dual Energy X-Ray Absorptiometry (Norland USA) and 858 Mini Bionix Ⅱ Biomaterial Testing Machine (MTS, USA) were employed in this study.

Methods
Surgical procedure: the operation was performed on the animals using the method set in the model establishment [17-18]. One month after having pulled out all premolars and the first molars of the mandibles of the canines, box-shaped osteotomies were carried out in the posterior teeth above the
mandibular canal of the left mandibles with some cortical bone on the tongue side reserved after the fine boring of the lateral bone cutting line and 2 TiNi-SMA distractors in ‘S’ shape were buried. The right mandibles were used as self-control and osteotomy was not performed on them. The canines in the group Ⅰand the group Ⅱ were killed after anaesthesia 3 months and 6 months after surgery respectively.
BMD measurement: The mandible samples were cut, the soft tissues were removed from them and each of them was sawed along the middle line into two pieces. The samples were put flatly on the scanning table and scanned with XR-36 Dual Energy X-Ray Absorptiometry (Norland, USA) [19]. The BMD of three points in the distraction gap, above the distraction gap and below the distraction gap were measured in the middle piece of the distraction area of the experimental side and the control side. Each sample was examined three times and the mean value was gotten.
Compressive biomechanics test: A piece of the bone with 7.0 mm in width was cut from the middle of the distraction area of the experimental side and the corresponding area of the control side. The height (H) and the width (W) were measured and recorded with a vernier caliper with 0.02 mm error. The bone was packaged in a plastic bag after wrapped with gauze dipped with normal saline and saved in refrigerator at –20 ℃. The samples were measured at room temperature with 858 Mini Bionix Ⅱ Biomaterial Testing Machine (MTS Company, USA). The samples should be naturally thawed completely and fixing bases should be made with self-coagulated plastic for the upper and lower ends before the measurement was made. Firstly, precompression with 300 N was made to remove the influence of the viscoelasticity from the bone tissue. The loading speed during the measurement was 1 mm/min. A computer was used to record load-distortion curve automatically. The precisions were 1×10-8 N and 1×10-10 mm for load measurement and distortion measurement, respectively. The load was increased until the test pieces broke. Fracture and distortion occurred in the load-distortion curve and then stopped the test. As the middle parts of the test piece are spongy bone and mandibular canal, the load is mainly supported by the peripheral cortical bone. The test piece will be broken down when the compressive stress exceeds the strength limit of the cortical bone of some area. Thus, the compressive stress is correlated with the area of the cross section (A) of the broken cortical bone. The thickness of the cortical bone of the mandible on the cheek side (T1) and the tongue side (T2) respectively and the compressive stress area A=W×(T1＋T2). Based on load-distortion curve, compressive stress (MPa) = compressive load/compressive stress area. The load-distortion curve data were processed with statistical software SPSS 10.0 and the compressive stress-distortion curve of the bone was drawn. Then the modulus of elasticity will be the maximal slope of the curve in the elastic strain region and can be calculated based on the graph.
Statistical analysis: Statistical data were analyzed using statistical software SPSS 10.0 by the first author. After the test of normality and homogeneity test for variance, contrast analysis was performed on the BMD, the compressive stress and the modulus of elasticity in the distraction side and the control side by t-test of pairing designed data. The data from the group Ⅰand the group Ⅱ were compared by t-test of two-group completely randomized designed data. The BMD at the upper, middle and lower regions of the mandible of the same side were compared with one-way analysis of variance.

RESULTS

Quantitative analysis of experimental animals
All of the 12 experimental canines were included in the final analysis, no drop-out.

General observation
All animals survived and were healthy until the scheduled observation time. During surgery, after the distractor was set, the piece of the bone distracted rose and there appeared 1.10-3.48 mm gap on the cheek side. Along with time extension, the piece of the bone rose gradually and reached the maximal value 3-5 day after the surgery. One month later, the average height of the alveolar process of the experimental side increased by 6.73±1.53 mm (4.64-8.82 mm) and thereafter, the height of the alveolar process changed little (Figure 1).

BMD measurement
Three months after the surgery, the BMD on the distraction side was lower than on the control side. There was significant difference in BMD between the distraction gap and the corresponding region on the control side (t=-2.898，P < 0.05) (Table 1). There was no significant difference in BMD of the three regions on the same side and in BMD between the corresponding regions on both sides six months after the surgery (P > 0.05) (Table 2). The BMD of the distraction gaps in the 6-month group compared to the 3-month group increased but they were not significantly different (P > 0.05).

Compressive biomechanics test: All test pieces were broken at the middle region above the mandibular canal. There was significant difference in the compressive stress and the modulus of elasticity of the mandibles on the experimental side and the control side 3 months after the surgery (t=-3.274，-3.534, P < 0.05). All indexes increased 6 months after the surgery, which was not significantly different from the control side (P > 0.05) (Table 3).

DISCUSSION

Strength of the bone depends on the density, the structure, the volume, the metabolic factors and so on, of which the density has the greatest influence on it. BMD reflects the content of the mineral substances in the bone. The BMD in three regions on the distraction side was lower than on the control side three months after the operation, but there was no significant difference in the BMD among the distraction gap, the distracted piece of the bone above it and the base bone below it. It is because that the operation initiated new bone formation, as well as the repair and reconstruction of the bone after the fracture, and the bone formation and absorption process, which interferes the deposition of mineral substances and lowered the BMD of the distraction side. Six months after the surgery, with the deposition of mineral substances, the BMD of the distraction gap as well as the whole distraction side reached normal level, but the repair and reconstruction of the bone will continue. Additionally, it indicates that the reference is stable and the data of the distraction side are comparable, by which there is no significant difference in the BMD of the three testing regions of the control side and between the group Ⅰand the group Ⅱ.
In the biomechanics test, the samples were often broken due to the shear stress 45° to the load direction produced during the compression. The broken regions located the weakest and stress focused parts. The fragmentation normally occurs at the middle part for homogeneous test piece. The test piece from the mandible is not regular in the structure and has a vacuous mandibular canal. The upper part of it locates at the center of the test piece, abuts upon the spongy bone above, bears concentrated stress, has a weak structure and is easy to be broken. The transverse cutting line of the distraction side is just round this position and the zone of ossification is above it. The breakage did not occur in the zone of ossification, but in the same place as on the control side in the compression test. This indicated that there was cutting line above the mandibular canal where was still the weakest place and at the same time indicated that the distracted new formed bone had certain support strength exceeding the weakest region of the whole piece of the bone.
Compressive strength and the modulus of elasticity are common indexes in the biomechanical performance test of the bone. They reflect the intrinsic property and the hardness of the bone, which are independent of the size and geometric shape of the bone. Like normal bone healing, bone formation and absorption occur at the same time in distracted bone formation. With time extension, the modulus of elasticity and the maximal compressive strength increased. The biomechanical performance of the bone in the distraction region was lower than in normal control 3 months after the surgery, but the compressive strength and the modulus of elasticity nearly reached normal level 6 month after the surgery.
The results of the test show that the strength and the quality of the new-formed bone by distraction with full-buried TiNi-SMA distractor are suitable for late repair such as implantation. Half-buried distraction device was presently recommended for normal implantation from the third to forth month after the distraction and for false tooth repair from sixth month after the distraction [2]. However, if TiNi-SMA distractor is used, the new-formed bone has certain strength and can bear implantation 3 months after the surgery, and the strength reaches that of normal mandible and can be used in false tooth repair 6 months after the surgery.

REFERENCES

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钛镍记忆合金牵张成骨的骨密度及生物力学测试**☆

谢 旻1，胡 敏2，刘洪臣2，姚 军2，周宏志2
1解放军总医院第三○九临床部口腔科，北京市 100091；2解放军总医院口腔科，北京市 100835
谢 旻☆，男，1971年生，河南省新乡市人，汉族，博士，副主任医师，2004年解放军军医进修学院毕业，主要从事口腔软硬组织修复研究。
国家自然科学基金资助项目（30171017）*；北京市自然科学基金资助项目（7042058）*
摘要
背景：钛镍记忆合金牵张器采用保留舌侧部分骨皮质的截骨方式，没有间歇期，与以往其他牵张器方法有所不同，因此对该方法成骨时间及质量都需要进行重新评价。
目的：通过对新生骨密度和生物力学测试观察钛镍记忆合金牵张器牵张成骨效果。
设计：随机对照观察。
单位：解放军总医院口腔科。
材料：选用雄性健康成年杂种犬12只，体质量21~26 kg，由解放军总医院医学实验动物中心提供并饲养。实验过程中对动物的处置符合动物伦理学标准。实验用牵张器为自行研制。XR－36型双能X线骨密度仪为美国Norland公司生产。858 Mini Bionix II型生物材料实验机为美国MTS公司生产。
方法：实验于2000-07/2004-05在解放军总医院医学实验动物中心完成。①采用统计软件随机法将实验犬分为牵张3个月组和牵张6个月组，每组各6只。两组实验犬左侧下颌骨均为实验侧，右侧下颌骨为自身对照侧。②牵张3个月组和牵张6个月组分别于牵张完成后3，6个月麻醉后处死实验犬，切取下颌骨标本，对牵张间隙上方、牵张间隙及牵张间隙下方进行双能X线骨密度测量，同时切取实验侧牵张侧区域中段及对照侧相应部位进行压缩生物力学测试，计算抗压强度、弹性模量。
主要观察指标：①牵张3个月组和牵张6个月组实验犬牵张侧及对照侧下颌骨密度测量结果。②实验侧牵张侧区域中段及对照侧相应部位抗压强度、弹性模量。
结果：纳入实验犬12只，12只均进入结果分析，无脱落。①下颌骨密度测量结果：牵张3个月组实验犬下颌骨牵张侧牵张间隙上方骨密度低于对照侧，差异有显著性意义(t=-2.898，P < 0.05)。牵张6个月组实验犬两侧各区域间的骨密度差异均无显著性意义(P > 0.05)。②压缩生物力学检测结果：牵张3个月组实验犬下颌骨牵张侧抗压强度、弹性模量均低于对照侧，差异有显著性意义（t=-3.274，-3.534, P < 0.05）。牵张6个月组实验犬两指标较牵张3个月组均有增加，差异无统计学意义（P > 0.05）。
结论：使用全埋置的钛镍记忆合金牵张器牵张成的新骨有足够的强度和质量，可以进行种植等后期修复。
关键词：牙槽突； 牵引器； 骨密度； 生物力学
中图分类号: R318 文献标识码: A 文章编号: 1673-8225(2008)17-03385-04
谢旻，胡敏，刘洪臣，姚军，周宏志.钛镍记忆合金牵张成骨的骨密度及生物力学测试[J].中国组织工程研究与临床康复，2008，12(17):3385-3388
[www.zglckf.com/zglckf/ejournal/upfiles/08-17/17k-3385(ps).pdf]
（Edited by Zhang Sh/Qiu Y/Wang L）