Abstract
INTRODUCTION
Recently there have been case reports of component fractures and elevated metal ion levels potentially resulting from the use of cobalt-chrome modular necks in total hip arthroplasty. One potential cause that has been suggested is fretting corrosion caused by micromotion at the taper junction between the modular neck and the femoral stem. The objective of the current study was to investigate the effects of various impaction and loading methods on micromotion at the modular neck-femoral stem interface in a total hip replacement system.
METHODS
A femoral stem was potted using dental acrylic and displacement transducers were inserted to measure micromotion in the modular neck pocket (Figure 1a). An 8° varus, long, cobalt-chrome, modular neck and 28 mm XXL cobalt-chrome femoral head were inserted in the femoral stem using various assembly techniques (a) hand assembly, (b) impaction loads: 2, 3, 4, 6, 16.4 kN and (c) in- vivo simulated impaction loads (constructs were placed on top of a block of ballistic gel (Clear Ballistic LLC, Fort Smith AR) and impacted): 2, 4, and 16.4 kN (Figure 1b). Impaction was obtained by placing the construct in a drop tower and impacting them. All constructs were oriented in 10/9 as per ISO 7206-6 and tested in an MTS machine with a sinusoidal load of 2.3 kN for 1,000 cycles in air at frequency of 10 Hz (Figure 1a). Micromotion data was recorded. To simulate the loading experienced with heavier patients and/or higher impact activities, selected constructs (as shown in Table 1) were sinusoidally loaded with 5.34 Kn load. Three samples were tested for all methods described above.
RESULTS
Micromotion decreased as impaction forces increased (Table 1). There was a significant reduction in micromotion for impaction forces of 4, 6, and 16.4 kN when compared to hand assembled constructs. There was also a significant difference between 16.4 kN and each of the other impaction methods. The presence of ballistic gel to simulate in-vivo impaction did not significantly affect micromotion for any of the impaction forces. Increasing the loading force to 5.34 kN significantly increased micromotion for each of the assembly methods.
DISCUSSION
Modular necks assembled by hand generated nearly twice as much micromotion as those assembled with 16.4 kN impaction force. There was significantly less micromotion following impaction with 16.4 kN than all other impaction forces, which reinforces the manufacturer's recommendation of impacting the neck with 3 firm mallet blows (∼ 17 kN). To the authors' knowledge this is the first study to simulate in-vivo impaction using ballistic gel. The use of ballistic gel did not result in statistically significant increases in micromotion. This suggests the recommendation of three firm mallet blows is still appropriate during in-vivo impaction. As expected, increased loading forces resulted in greater micromotion. This implies that apart from assembly impaction forces, increased load forces present in heavier patients or due to higher activity levels may result in higher levels of micromotion.
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