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Research Papers

# Statistical Analysis of Interfacial Gap in a Cementless Stem FE Model

[+] Author and Article Information
Youngbae Park, DonOk Choi

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, KAIST 3022, Science Town, Daejeon 305-701, South Korea

Deuk Soo Hwang

Department of Orthopedic, Medical School, Chungnam National University, Daesadong 640, Jung-Ku, Daejeon 301-721, Korea

Yong-San Yoon1

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, KAIST 3022, Science Town, Daejeon 305-701, South Koreaysyoon@kaist.ac.kr

1

Corresponding author.

J Biomech Eng 131(2), 021016 (Dec 18, 2008) (8 pages) doi:10.1115/1.3005176 History: Received November 05, 2007; Revised August 25, 2008; Published December 18, 2008

## Abstract

In cementless total hip arthroplasty, a fair amount of interfacial gap exists between the femoral stem and the bone. However, the effect of these gaps on the mechanical stability of the stem is poorly understood. In this paper, a finite element model with various interfacial gap definitions is used to quantify the effect of interfacial gaps on the primary stability of a Versys Fiber Metal Taper stem under stair climbing loads. In the first part, 500 random interfacial gap definitions were simulated. The resulting micromotion was approximately inversely proportional to the contact ratio, and the variance of the micromotion was greater with a lower contact ratio. Moreover, when the magnitude of the micromotion was compared between the gap definitions that had contact at a specific site and those that had no contact at that site, it was found that gaps located in the proximal-medial region of the stem surface had the most important effect on the micromotion. In a second trial, 17 gap definitions mimicking a gap pattern that has been observed experimentally were simulated. For a given contact ratio, the micromotion observed in the second trial was lower than the average result of those in the first, where the gaps were placed randomly. In either trial, when the contact ratio was higher than 40%, the micromotion showed no significant difference (first trial) or a gentle slope ($−0.24μm∕%$ in the second trial) in relation to the contact ratio. Considering the reported contact ratios for properly implanted stems, variations in the amount of interfacial gap would not likely cause a drastic difference in micromotion, and this effect could be easily overshadowed by other clinical factors. In conclusion, differences in interfacial gaps are not expected to have a noticeable effect on the clinical micromotion of this cementless stem.

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## Figures

Figure 3

The sectional view of the FE model, in this case, the one with 67% contact ratio in Trial 2. (Left) von Mises stress (MPa). (Right) Contact pressure (MPa).

Figure 1

Experimental setup. The motion of the stem relative to the outer surface of the composite femur was measured with a system comprising six linear variable differential transducers, as described in a previous study (23).

Figure 2

Definition of stem surface zones used in the FE model. Note that the zones corresponded to the radiographic Gruen zones and that they were numbered accordingly.

Figure 4

Micromotion results from Trial 1. (Left) Stem micromotion versus contact ratio for the 500 FE gap definition cases. (Right) Average micromotion for each 10% increment of contact ratio (with standard deviation).

Figure 5

Micromotion results from Trial 1, showing that the results are roughly distributed around an inverse proportional trendline

Figure 6

Stem micromotion versus contact ratio for Trial 2, compared with the approximate trendline from Trial 1 (as shown in Fig. 5)

Figure 7

Effect of the bone-implant contact ratio on the total portion of the stem surface likely to be prone to ingrowth, i.e., having contact with bone as well as stability (micromotion<28μm)

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