Abstract
Objectives
Modularity in total knee arthroplasty, particularly in revisions, is a common method to fit the implants to a patient's anatomy when additional stability or fixation is needed. In such cases, it may be necessary to employ multiple points of modularity to better match the anatomy. Taper junction strength at each of these levels is critical to maintain the mechanical stability of the implant and minimize micromotion. This effect of distributed assembly loads through multiple tapers and the resulting strength of the construct have not been previously evaluated on this revision tibial implant. The purpose of this study was to evaluate the possible dissipation of impaction force through multiple taper connections as compared to a single connection.
Methods
Two different constructs representative of modular implants were studied: a construct with a single axial taper connection (Group A; representing implant-stem) was compared to a construct with an adaptor that included two, offset, modular taper connections (Group B; representing implant-adapter-stem). For Group A, the stem taper was assembled and impacted through the stem. For Group B, the two tapers of the adapter and stem were hand assembled with the mating components and impacted simultaneously through the stem. Assembly load for each construct was recorded. As shown in Figure 1, the constructs were then fixed in a mechanical test frame and an axial distraction force was applied to the end of the stem at a constant displacement rate of 0.075 mm/sec until taper separation or mechanical failure occurred. Force and displacement data were recorded at 50 Hz. Disassembly force was normalized to assembly force for each component. Minitab software was used to analyze the data using a t-test.
Results
Separation occurred at the implant-stem interface for all components of Group A, without component fracture. Separation occurred at the adapter-stem interface for all components of Group B. There were neither component fractures nor separation of the implant-adapter interface. As seen in Figure 2, the disassembly force for Group A was greater than Group B (p = 0.456), likely due to the higher assembly forces. However, the ratio of these forces (disassembly/assembly) was greater for Group B than Group A, but not statistically significant (0.794 vs. 0.754 with p = 0.657).
Discussion
There are no applicable standards for required taper disassembly strength. However, the results of this study indicate that the addition of a second taper connection and offset does not significantly change the static ratio of disassembly to assembly force of the studied constructs. Axial distraction forces vary with activity and constraint afforded by a particular prosthesis. Future work to further characterize the behavior of these constructs could include varying loading rate to determine if that affects the location of taper disassociation. A period of fatigue testing could also be added to evaluate the effect of cyclic loading on the taper connections.
