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SCIENCE D I R E C T ' d Journal of Bionic Engineering 3 (2006) 155-160

Design and Mechanics Simulation of Bionic Lubrication System of Artificial Joints

S. H. Su, Z. K. Hua, J. H. Zhang

School of Mechatronics Engineering and Automation, Shanghai University, Shanghai 200072, P. R. China

Abstract We propose a new structure for artificial joints with a joint capsule which is designed to overcome the drawback of current

prostheses that omit many functions of the lubricant and the joint capsule. The new structure is composed of three components: lubricant, artificial joint and artificial joint capsule. The lubricant sealed in the capsule can not only reduce the wear of the arti- ficial joint but also prevents the wear particles leaking into the body. So unexpected reactions between the wear particles and body can be avoided completely. A three-dimensional (3-D) finite element analysis (FEA) model was created for a bionic knee joint with capsule. The stresses and their distribution in the artificial capsule were simulated with different thickness, loadings, and flexion angles. The results show that the maximum stress occurs in the area between the artificial joint and the capsule. The effects of capsule thickness and the angles of flexion on stress are discussed in detail. Keywords: artificial joint, bionic joint capsule, 3-D FEA

Copyright 0 2006, Jilin University. Published by Science Press and Elsevier Limited. All rights reserved.

1 Introduction

A major cause of failure in hip and knee prostheses is the wear of the ultra high molecular weight poly- ethylene and osteolysis caused by the resulting debris. The osteolysis can allow the joint to loosen, which greatly shortens 'the lifetime of the artificial joint"'21. Late loosening of the prosthesis is associated with ex- cessive amounts of wear debris which result in adverse cellular reactions '39 'I. In order to improve the life span of the artificial joints, many researchers have focused on the development of new materials '5-71, surface coatings [*I and design '91. But the results are not satis- factory and problems related to wear debris and oste- olysis still exist. So it is important to look for better solutions to reduce wear and prolong the lifetime of artificial joints. Therefore, we propose and have de- signed a novel bionic lubrication system for artificial joints.

Corresponding author: J. H. Zhang E-mail: jhzhang@staff.shu,edu.cn

2 Structure design of novel bionic lubrication system

Inspired by animal joints, the novel bionic lubrica- tion system is composed of three components: a thera- peutic lubricant, joints and a joint capsule 'lo'. The most important innovation of this system is the joint capsule which not only to seals the lubricant, but also prevents debris from leaking into the body liquid. Thus adverse cellular reactions can be completely avoided.

Two kinds of prostheses were designed: for knee and hip. Schematic views of the two structures are given inFig. 1

1 Bionic joint capsule I

Bionic joint capsule

Bionic joint capsule Joint cavity

Tibia , Joint cavity Cotyle

.Ball

'I li (a) Knee prosthesis (b) Hip prosthesis

Fig.1 Schemes of the proposed structures.

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such as loadings, capsule thickness and flexion angle. 3 Mechanics simulation

Stress analysis of artificial joints is very important and many methods have been applied. These include elastic solutions "11, theoretical prediction '''I and finite elements

Preliminary study indicates that the artificial joints carry major loads while the joint capsule cames much less. Despite loads mainly depending on the joints, the capsule still supported loads up to 0.026 MPa standing when a 1 mm thick capsule and 3 kN loads were applied as constraints. Therefore, it is necessary to know the stresses and their distribution for optimum design and material selection of the capsule.

[11.13,141

3.1 Model and materials A simplified 3D model knee with a joint capsule

was built based on the anatomical knee joint using commercially available software (ANSYS 8.0). The joint head was simplified as a ball and the acetabulum was simplified as a spherical socket. The joint capsule was fixed to the femur and tibia at the contact area. The joint and capsule assembly is shaped like a hammer.

The following materials were used in the simula- tion: Ti-13Nb-13Zr alloy for the femur and the ball, ultra-high-molecular-weight polyethylene (UHMWPE) for the acetabulum, and medical silicone gel for the joint capsule. All the materials were taken to be linear elastic, continuum and isotropic. The values of the Young's modulus ( E ) and Poisson's ratio (v) of the materials in the model are listed in Table 1. The values for the TiNbZr alloy and UHMWPE were obtained from ref- erence [9], and the values for silicone gel were provided by the CuSil Silicone Technology Co., Ltd.

Table 1 Values of Young's modulus ( E ) and Poisson's ratio (v ) of materials used in the model

Materials E (MPa) V Ti-13Nb-13Zr alloy 79 000 0.36 UHMWPE 1950 0.43 Silicone gel 3.4 0.48

The finite element method (FEM) was used to cal- culate and simulate the magnitude and distribution of the stresses on the joint capsule under different conditions,

3.2 Mesh and constraints The model was partitioned by working-plane in

order to reduce the misshapen mesh appearance before meshed. A combination of free and mapped meshing was used. With the joint capsule 1 mm thick, the meshed model produced 40 737 elements and 10 235 nodes (Fig. 2). The constraints were applied at the distal end of the tibia. All the degrees of freedom were com- pletely restrained at the distal end of the tibia.

Fig. 2 The meshed model and constraints.

3.3 Stress analyses Because of the symmetry of the model with respect

to the working-plane, only half of the model was built for simulation. The ball head and acetabulum contacted each other. The contact has highly nonlinear behavior. Here, TARGE170 element and CONTA174 element were used to simulate the target area and contact area, respectively, and created area-area contact pairs. The friction coefficient was assumed as 0.02 between the contact pairs.

4 Results and discussion 4.1 Motion model

Multi-sub step and small time step were taken un- der static analysis solution. With a capsule 1 mm thick and a load of 750 N applied on the top section of the femur, the simulation when standing show that the maximum stress in the capsule is 0.0103 MPa, located at the contact zone between the tibia and the capsule. When the person is active, such as running on the flat or up and down stairs, the maximum loads on the joint reach five times body weight [l", so a 3000 N load (four times 750 N)

S. H. Su et al.: Design and Mechanics Simulation of Bionic Lubrication System of Artificial Joints 157

was applied again. The maximum stress was 0.0289 MPa and occurred in the same area as with the 750 N load. Whatever the load applied, the maximum stress was far smaller than the strength of the material, 9.97 MPa (Fig. 3) .

Fig. 3 Stresses and their distribution in the capsule when standing (thickness of capsule= 1 mm).

When the person is moving, both heavy load (3000 N) and angular displacement were applied on the femur. Fig. 4 shows the stress values and distribution at different angles of flexion. The maximum stress was 1.62 MPa (Fig. 4d) occurred at the contact zone between fermoral and the capsule.

Thus the maximum stress occurs at the connection between the capsule and joint, but the precise position depends on the type of motion. When stationary, the maximum stress occurs at the interface between the tibia and capsule; when the person is moving it occurs at the interface between the femur and capsule. But no matter what conditions were simulated, the maximum stress did not exceed the strength of the material.

Fig.4 Magnitude and distribution of stress in the capsule at different flexion angles.

158 Journal of Bionic Engineering (2006) Vo1.3 No.3

4.2 Effects of capsule thickness The capsule thickness has significant effect on the magnitude of the stress and distribution. To investigate this effect, simulations with different capsule thickness, 0.5, 1, 1.5 and 2 mm were carried out (Fig. 5, Fig. 6). When standing with a load of 3000 N, the maximum stress decreases with increase in the thickness of the

capsule. When moving the maximum stress and its dis- tribution depend on the thickness of the capsule and flexion angle (Fig. 6). In Fig. 6a and b, the maximum stress was tensile; in Fig. 6c and d, the maximum stress was compressive. As the capsule becomes thicker, the area of low stress decreases.

Fig. 5 Stress values and distribution of capsule with different thickness under standing state (Load: 3000 N).

Fig. 6 Stresses and their distribution with capsules of different thickness, person moving (Load: 3000 N, flexion angle: 30').

159 S. H. Su et al.: Design and Mechanics Simulation of Bionic Lubrication System of Artificial Joints

(c) 1.5 mm

5 Conclusions

(d) 2.0 mm Fig. 6 Continued.

A new structure for artificial joints with a capsule was proposed and designed to overcome the problems of current prostheses that omit many functions of lubrica- tion and the joint capsule. The stress magnitude and distribution were analyzed using finite elements.

(1) The maximum stress occurred at the interface between the capsule and tibia when stationary. When moving, the maximum stress occurred at the interface between the capsule and the femoral. The maximum stress did not exceed the strength of the material.

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Biloxi, USA, 1997, 283-284. Tipper J L, Hatton A, Nevelos J E, Ingham E, Doyle C, Streicher R, Nevelos A B, Fisher J. Alumina-alumina artificial hip joints. Part 11: Characterisation of the wear

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(2) The capsule thickness has significant effect on 141 Mosleh M, . Wear paaicles Of polyethylene biological systems. Tribology Transactions, 1996, 39, the magnitude and distribution of stress in the capsule

under different motions. The magnitude of the maximum stress and its position were dependent on the motion model.

Acknowledgement carbon. Biomaterials, 2001,22, 1507-1514.

843-848. [5] Vesa Saikko, Tiina Ahlroos, Olof Calonius, Jaakko

Kerhen. Wear simulation of total hip prostheses with polyethylene against CoCr, alumina and diamond-like

This work has been financially supported by the National Science Foundation of China through the grant number of 50105014, Bionic Lubrication System of Artificial Joints. The corresponding author, Dr. J H Zhang, would also like to acknowledge the financial support provided by the Science and Technology Committee of Shanghai under the grant number of 04 QMX1442 and partially financial support by Shanghai Leading Aca- demic Discipline Project, Project Number: Y0102.

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