Abstract
Statement of Purpose
Mechanically assisted crevice corrosion of modular tapers continues to be a concern in total joint replacements as studies have reported increases in local tissue reactions1. Two surgical factors that may effect taper seating mechanics are seating load magnitude and orientation.
In this study 12/14 modular taper junctions were seated over a range of loads and loading orientations. The goals of this study were to assess the effects of load magnitude and orientation on seating load-displacement mechanics and to correlate these to the pull-off load.
Methods
Ti6Al4V 12/14 tapers and CoCrMo heads were tested axially at four seating load levels (n=5): 1-, 2-, 4- and 8- kN. Three orientation groups were tested at 4 kN (n=5), 0°, 10° and 20°. The load-displacement behavior during testing was captured using data acquisition methods and two non-contact eddy current sensors fixed to the neck, targeting head-neck relative motion (Micro-Epsilon).
Loads were ramped (200 N/s) with a servohydraulic system from 0 N to peak load and held for 5s (Instron). Off-axis test samples were oriented in an angled fixture. Displacement and load data were recorded in LabView. Seating displacement was the distance traveled between 50 N and thepeak load.
Axial tensile pull-off loads (5 mm/min) were applied until the locking ability of taper junctions failed.
Statistical analysis was performed using ANOVA test (P<0.05).
Results
Axial tests
Seating load-displacement behavior at different seating loads (Fig. 1) show consistent characteristic behavior. Displacements rise parabolically to the peak load reflecting elastic deformation and rigid-body motion. Unloading is elastic and the y-intercept of the unloading curves reflects seating displacement.
Displacement and work of seating for the axial tests (Fig. 2a and 2b, respectively) show that both increase with seating magnitude. Displacement increases approximately linearly, while work increases parabolically with seating load. All groups are significantly different (P<0.05).
Pull-off loads (Fig. 3) increase linearly with seating load. Pull-off loads are approximately 44% of the seating load. All loads are statistically different (P<0.05).
Off-axis tests
Measured seating displacements were comprised of rigid-body and elastic motion. A stiffness-correction method removed elastic motion2. Displacement, work and pull-off at the different seating load orientations were not significantly affected (P>0.05, data not shown).
Discussion
Static seating load magnitude and orientation effects on seating mechanics and pull-off loads simulating surgical assembly were quantitatively studied in an instrumented seating test method. Increased seating loads increased work of seating, seating displacement and pull-off load. Such seating plots may assist in better understanding of the design, material, and surgical factors associated with taper locking mechanics. Seating orientation to 20° offset did not significantly affect seating mechanics or pull-off loads. Pull-off loads were about 50% of the seating loads for all cases which is consistent with other work3.
Conclusions
Increased seating load magnitude increased seating displacement, work and pull-off loads in 12/14 tapers. Load orientation had no significant effect.
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