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Technical Briefs

# Development of a Testing Methodology to Quantify Bone Load Transfer Patterns for Multiple Stemmed Implants in a Single Bone With an Application in the Distal Ulna

[+] Author and Article Information
Rebecca L. Austman, Cheryl E. Quenneville, Brendon J. Beaton

Biomechanical Testing Laboratory, Department of Mechanical and Materials Engineering, The University of Western Ontario, London, ON, N6A 5B8, Canada

Graham J. King

Biomechanical Testing Laboratory, Department of Surgery,  The University of Western Ontario, London, ON, N6A 5B8, Canada

Karen D. Gordon

School of Engineering, University of Guelph, Guelph, ON, N1G 2W1, Canada

Cynthia E. Dunning1

Biomechanical Testing Laboratory, Department of Mechanical and Materials Engineering, The University of Western Ontario, London, ON, N6A 5B8, Canadacdunning@eng.uwo.ca

1

Corresponding author.

J Biomech Eng 130(2), 024502 (Mar 28, 2008) (4 pages) doi:10.1115/1.2899572 History: Received October 31, 2006; Revised September 06, 2007; Published March 28, 2008

## Abstract

Optimal parameters for many orthopaedic implants, such as stem length and material, are unknown. Geometry and mechanical properties of bone can vary greatly amongst cadaveric specimens, requiring a large number of specimens to test design variations. This study aimed to develop an experimental methodology to measure bone strains as a function of multiple implant stem designs in a single specimen, and evaluate its efficacy in the distal ulna. Eight fresh-frozen cadaveric ulnae were each instrumented with 12 uniaxial strain gauges on the medial and lateral surfaces of the bone. The proximal portion of each ulna was cemented in a custom-designed jig that allowed a medially directed force to be applied to the distal articular surface. An implant with a finely threaded stem was cemented into the canal by an experienced upper extremity orthopaedic surgeon. Six loads $(5–30N)$ were applied sequentially to the lateral surface of the prosthetic head using a materials testing machine. Testing was repeated after breaking the stem-cement bond, and after removing and reinserting the stem several times into the threaded cement mantle. Near the end of the testing period, the initial stem was reinserted and data were collected to determine if there was any change in bone properties or testing setup over time. Finally, a smooth stem was inserted for comparison to the threaded stem. Strain varied linearly with load $(R2≥0.99)$ for all testing scenarios. Bending strains were not affected by breaking the stem-cement bond $(P=0.7)$, testing durations up to $18h$$(P=0.7)$, nor the presence of threads when compared to a smooth stem $(P>0.4)$. Furthermore, for all gauges, there was no interaction between the effect of the threads and level of applied load $(P>0.1)$. This methodology should prove to be useful to compare stem designs of varying lengths and materials in the same bone, allowing for a direct comparison between implant designs for the ulna and other bones subjected primarily to bending loads. Furthermore, it will minimize the need for large numbers of specimens to test multiple implant designs. The ultimate goal of using this protocol is to optimize implant stem properties, such as length and material, with respect to load transfer.

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## Figures

Figure 4

Comparisons between a finely threaded implant stem to a smooth implant of the same material, length, and average diameter. (a) Example of microstrain output for one representative specimen, for Gauge pair 3. A linear response to load was observed for all trials. (b) Average microstrain and standard deviations for all eight specimens, with a 20N load applied. No significant difference was found (P=0.9).

Figure 3

Average microstrain and standard deviations for all eight specimens comparing the original stem inserted (bonded), and then retested after breaking the bond with the cement mantle (broken bond). A 20N load was applied in all cases. No significant difference was found (P=0.7).

Figure 2

After cement curing, the initial threaded implant was removed, leaving a threaded cement mantle that could accept various stems with the same diameter and threading.

Figure 1

An ulna was potted in custom-designed jig using dental cement. Strain gauges were applied to the bone, and loads were applied to a prosthetic ulnar head attached to a threaded stem. The inset shows the implant before it is inserted in the bone.

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