Stress analysis between implant prostheses with different moduli and surrounding bones

doi:10.12307/2023.671

Abstract

Abstract: BACKGROUND: Stress shielding can lead to the failure of implant prostheses to repair bone defects, mainly due to the fact that the elastic modulus of the implant is greater than that of the surrounding bone tissues.
OBJECTIVE: To explore methods for eliminating the stress shielding by analyzing the influence of the elastic modulus of the implanted prosthesis on the stress distribution.
METHODS: The bone tissue models of experimental canine and human bone tissues were obtained by CT scanning, and then were optimized. The gradient assignment was performed to establish a reliable bone mechanics model, and the finite element simulation was performed after the combination with the implanted prosthesis. First, the finite element simulation of experimental canine and human bone tissues and the corresponding implanted prosthesis was performed to simulate the distribution of stress and displacement of prosthesis with different elastic moduli after prosthesis implantation. Second, the reasons for the stress shielding even with small elastic modulus differences were analyzed. The bone and implant prosthesis models were established, and a method for assigning material properties was established. Finally, the feasibility of the model and material property assignment method was verified, and the influence of the relationship between the elastic modulus of the implanted prosthesis and the elastic modulus of the bone on the formation of stress shielding was quantitatively analyzed by randomly selecting the stress points.
RESULTS AND CONCLUSION: The mechanical properties of bone models of experimental canine and human bone tissues established by gradient assignment method are close to the actual bones. The finite element simulation mechanics test proved that the implantation of different elastic modulus had less effect on the relative displacement between the prosthesis itself and the surrounding bones. Moreover, quantifying the effects of elastic moduli on the stress distribution after implantation of the prosthesis into the bone will contribute to the research in the future.

Key words: stress shielding, bone model, biomechanical simulation, gradient assignment, elastic modulus

CLC Number:

Shao Yixin, Guan Tianmin, Zhu Ye, Lin Bing, Guo Chongyang, Pan Ting. Stress analysis between implant prostheses with different moduli and surrounding bones[J]. Chinese Journal of Tissue Engineering Research, 2023, 27(29): 4612-4619.

Figures/Tables 4

2.1 比格犬骨骼模型植入假体应力分布及位移变化对比分析此文建立了精度加高的实验犬骨骼模型并进行更为详细的赋值，另外进行仿真实验观察犬日常行走过程中腿部的应力变化，通过施加轴向力载荷分析植入假体和骨骼连接部位因此产生的影响，由于在行走过程中水平方向的应力较小，因此主要研究骨骼在竖直方向的载荷作用下位移和应力的变化。如表3所示，以LF2号骨骼的位移变化和应力分布为例，可以看出，不同弹性模量的植入假体对受到载荷后植入假体模型与骨骼模型间的位移变化有一定影响。为更有效地比较其影响的大小，制作如图8所示的柱状对比图，当植入假体弹性模量增大后，整个骨骼模型位移变化减小，植入假体弹性模量变小后，整个骨骼模型位移变化增大。但应考虑到，不同弹性模量的植入假体对术后假体与骨骼间发生位移变化的影响较小，并且在植入假体修复骨缺损手术过程中，常采取螺钉辅助固定的方式保证假体与骨骼间连接的稳定性，因此建议可以暂时不考虑植入假体弹性模量造成术后假体因位移差而脱落的可能性。应力分布如表3所示，虽然通过颜色观察没有办法确定最大应力所在位置，但可见不同弹性模量的植入假体对受到载荷加载后应力分布有一定影响，其中植入假体弹性模量大于骨骼弹性模量时，集中在植入假体模型上的应力增大；植入假体弹性模量小于骨骼弹性模量时，集中在植入假体模型上的应力减小，但与植入假体模型附近的骨骼模型应力增大了，并且该影响相较于植入假体弹性模量对位移变化的影响要明显很多。由表3可知，虽然植入假体的弹性模量只是略大于骨骼的弹性模量，且仍在骨骼弹性模量范围内，但应力仍明显集中在假体上，这很可能导致应力遮挡现象的产生，进而导致患者术后康复不好，甚至导致因植入假体周围骨质疏松而出现的假体脱落；反之，即使植入假体的弹性模量只是略大于骨骼的弹性模量，且仍在骨骼弹性模量范围内，仍会出现应力集中在植入假体周围的骨骼上，这可能导致患者骨骼的二次损伤。"

该有限元仿真结果不仅说明了植入假体与骨骼间弹性模量差对整体位移及应力分布的影响，同时还建立了比较可靠的比格犬骨骼模型及植入假体模型，并确立了材料属性赋予的方法，该方法赋予的材质属性较细致，与骨骼模型的力学性质比较接近。 2.2 人体骨骼模型植入假体应力分布对比分析通过与获取比格犬骨骼模型相类似的方式获取更为精细的志愿者骨骼模型并进行模型的优化、划分网格及赋予材质，截取模型上一小部分模拟植入假体力学仿真，分别赋予不同弹性模量的植入假体，对人体骨骼及植入体仿真人体直立行走过程中的受力过程，得到如表4所示结果。由于比格犬骨骼有限元仿真结果已经表明，在有骨骼约束的前提下，加之植入假体与骨骼间的接触较为紧密，位移变化并不明显，所以人体骨骼模拟植入假体有限元仿真着重对应力分布进行了分析。由于之前3组有限元仿真应力分布(如表3所示)可以看到植入假体所受应力相对集中，但无法清晰对比具体受力数值，为此，选取植入假体上的点和周围骨骼上对应的点进行应力数值获取，分别取植入假体上、下、左、右4个方向及与之相接处的人体骨骼上的点，每个方向取2个点，具体取点如图9A所示，其中a为植入假体上取的8个点，b为人体骨骼上对应的8个取点。通过ABAQUS软件对男性和女性的骨骼模型和植入假体进行各3组有限元仿真，并将这16个点的实验结果制成图表，图9A为男性志愿者的骨骼受力分布，图9B为女性志愿者的骨骼受力分布，其中A0a、A0b代表男性骨骼实验组中的植入假体和人体骨骼受力分布，A1a、A1b代表男性骨骼对照组A中的植入假体和人体骨骼受力分布，A2a、A2b代表男性骨骼对照组B中的植入假体和人体骨骼受力分布；图9B中命名方式与图9A相类似，受力点1-8分别代表图9A中1-8对受力点。从结果中可以很清晰地发现，当植入假体的弹性模量与人体骨骼相一致的时候，虽然无法做到受力对应点受力完全一致，但是由图9中A0a和A0b、B0a和B0b可知，当植入假体与人体骨骼弹性模量相一致的时候，应力分配较为平均，较少出现应力集中的情况；当植入假体的弹性模量与人体骨骼不一致时，将会出现应力分配不平均的情况；由图9中A1a和A1b、B1a和B1b可知，植入假体弹性模量大于人体骨骼时，应力集中在植入假体上，出现应力遮挡现象；由图9中A2a和A2b、B2a和B2b可知，植入假体弹性模量小于人体骨骼时，应力集中在人体骨骼上，容易出现二次骨折情况，并且即使植入假体弹性模量符合人体骨骼弹性模量范围，其造成的应力集中仍是非常明显的，甚至会出现应力差异在100%以上的情况，这对于植入假体修复骨损伤的术后康复是非常不理想的。所以，只有植入假体的弹性模量与植入区域的骨骼相一致才能防止上述情况的出现。同时该仿真结果也与植入假体修复骨损伤过程中出现应力遮挡现象和当植入假体弹性模量小于周围骨骼弹性模量时出现二次骨折现象相符合，由此可以证明，该仿真过程中建立的骨骼和植入假体模型及材料属性赋予方法是可行的。"

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