Development of Asian hip stem 4 – Experimental validation for stress distribution and micromotion

Abstract

s / Osteoarthritis and Cartilage 23 (2015) A82eA416 A118 measure representing the knee function, and be used for follow-up of OA patients, to understand how OA affects the knee kinematics. 162 DEVELOPMENT OF ASIAN HIP STEM 4 e EXPERIMENTAL VALIDATION FOR STRESS DISTRIBUTION AND MICROMOTION M. Baharuddin yz, S.-H. Salleh x, M. Lee x, A. Mohd Noor x. yCyberjaya Univ. Coll. of Med. Sci., Cyberjaya, Malaysia; zUniv. of Malaya, Kuala Lumpur, Malaysia; xUniversiti Teknologi Malaysia, Skudai, Malaysia Purpose: Stress shielding and micromotion are two major issues which determine the success of newly designed cementless femoral stems. The correlation of experimental validation with finite element analysis (FEA) is essential to evaluate the stress distribution and fixation stability of the stem within the femoral canal. Methods: The experiment was performed to validate the finite element analysis of stress distribution and micromotion. A small left fourth generation composite femur was used because it mimicked an actual human femorawith Asian hipmorphology. The femur was loaded at the center of the femoral stem ball using an advanced material testing systemmachine at the rate of 1 kN/min, and constrained distally using a custom designed jig positioned at the base of the machine. The cyclic axial loading was set from 0 to 2000 N using a 5 kN load cell for 50 cycles. Micromotion was measured using two linear variable direct transducer (LVDT) proximally and distally, while strain distributionwas measured using four tri-axial rosettes medially and laterally at the metaphyseal region. Two holes of 4 mm diameter were drilled 10 mm below the osteotomy level for the proximal, and 10 mm above the femoral stem tip for the distal. The LVDT were fixed firmly at the extra cortical femora with the spring tip touching the steel pin. Elastic micromotion was computed using the difference between the peak and trough for each cycle. The rosette used in this study had three grid orientations 0 (e1), 90 (e2) and 45 (e3). The result for FEAwill focused on the equivalent von Mises stress and micromotion with similar loading condition and material properties as experimental testing. Figure 1. Experimental validation (a) Loading condition, (b) micromotion, (c) strain, (d) strain gauge orientation. Results:Micromotionwas found to be higher during the first load cycle compared to later cycles. The experimental and FEA results demonstrated abrupt changes between the first cycle and subsequent cycles. Elastic micromotion initially decreased for both locations before it stabilized during Cycles 7 and 8. Distal micromotion was initially 17.9 mm before it stabilized somewhere between 10 and 11.5 mm. On the other hand, proximal micromotion started at 4 mm before it stabilized in the range of 1.5 to 1.8 mm. The finite element analysis (FEA) discovered that themicromotion of proximal regionwas between 20 and 30 mm,while at the distal region ranged from 30 to 40 mm. These FEA results correlated well with the experimental results although the predicted micromotion value from FEA was slightly higher (z 10 20 mm). Although, the experimental testing was well correlated with FEA, the result from FEA were slightly higher due several limitations such as the friction coefficient (m 1⁄4 0.4) used in FEA for the implant bone interface, simplified boundary conditions, loading configurations, and materials properties (inhomogeneous). On the other hand, the maximum equivalent von Misses stress value was determined using FEA at locations similar to those used in experimental testing and was 15-20 MPa (A), 20-30 MPa (B), 5-10 MPa (C), and 15-20 MPa (D). The mean value for experimental testing was 14.26 ± 12.00 MPa (A), 11.68 ± 9.74 MPa (B), 6.14 ± 4.95 MPa (C), and 12.22 ± 9.81MPa (D). The femoral stem in this study distributed stress normally in both medially and laterally, which would prevent stress shielding from occurring and prolong the lifespan of the implant. Figure 2. Elastic Micromotion. Figure 3. FEA for stem micromotion. Figure 4. Strain distribution.Ă Table 1. Outcome measures for operated (OP) and non-operated (NOP) leg in ACL-patients during maximal unilateral counter movement jump testing. ACL patients (n 1⁄4 24) OP-limb NOP-limb Limb-to-limb asymmetry (%) P-value