Original ArticlesMorse-type tapers: Factors that may influence taper strength during total hip arthroplasty*,**
Section snippets
Taper description
Typically, Morse tapers are described by the angle that the taper surface makes with respect to the longitudinal axis of the stem component. In this study, 4 commercial brands of modular THA implants were tested (Prosthesis A, Zimmer, Inc., Warsaw, IN; Prosthesis B, Smith & Nephew Richards, Memphis, TN; Prosthesis C, Wright Medical Technology, Arlington, TN; Prosthesis D, Johnson & Johnson–DePuy, Warsaw, IN). All of the prostheses consisted of titanium alloy stems and cobalt-chrome alloy heads
Force of impaction
The force imparted by the weight on the femoral head impactor was determined at a variety of drop heights. A linear relationship existed between the drop height and the force of impaction (Fig. 1).For each prosthesis tested, linear regression techniques were used to calculate the proportionality constants between the distraction and the impaction forces. A drop height effect was observed illustrating that an increased impaction
Discussion
Many variables affect the disassembly strength of Morse-type tapers. As we showed in a previous report, the design of these tapers varies considerably among manufacturers [4]. These design features, such as taper angle, taper mismatch, and surface roughness, play a large role in determining the strength of the taper when impacted with a standardized force [4]. There are many variables, however, that affect the mechanical strength of the taper that are under the surgeon's control, including the
Conclusion
We can provide surgeons with some guidelines for the optimal use of modular femoral prostheses that employ a Morse-type taper. First, because the disassembly strength of the taper is a linear function of the impaction force, the greatest possible impact should be delivered to the taper when it is assembled. Although this hypothesis was not tested formally, the implication is that the axis of impaction should be in-line with the axis of the taper. Off-axis blows reduce the vector component of
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No benefits or funds were received in support of this study.
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Reprint requests: Andrew H. Schmidt, MD, Department of Orthopaedic Surgery, Hennepin County Medical Center, 701 Park Avenue, Minneapolis, MN 55415. E-mail: [email protected]