Abstract
Introduction
Modular acetabular liners offer surgeons the flexibility of using various bearing materials and sizes to accommodate the patient's needs. The need for a robust locking mechanism to ensure the long term successful performance of the implant is critical due to the hardship a revision surgery would have on the patient. The traditional method to evaluate torque resistance by using epoxy to affix a roughed femoral head to the acetabular liner has been acceptable to this point. However, efforts to design an acetabular liner that is resistant to high torque failures have shown this method to be inadequate to evaluate the performance of the lock detail, as failures only occur between the femoral head and the liner. Therefore a test method that would ensure failure of the lock detail was needed.
Materials and Methods
In the current study the performance of prototype vs. production acetabular liners and shells were evaluated. Aluminum test shells were provided and a combination of production acetabular liners and prototype liners were provided by the Prototype Department at MicroPort Orthopedics. The traditional method was followed. A custom holding fixture was attached to the load cell plate of the test machine, and a roughed femoral head was attached to the actuator. The appropriate shell and liner combination was selected and assembled using three firm hammer blows with a two pound surgical hammer. Once assembled, the test construct was affixed to the holding fixture mounted to the test machine. Devcon 5 minute epoxy was mixed per the instruction and approximately 10 cc was placed into the cup of the liner. The femoral head was then brought into place using load control until contact was made. After the epoxy had cured torque was applied via the femoral head at a rate of 0.417° per second until failure of the epoxy or the lock detail was observed. In every trial the epoxy failed before the lock detail. A new method was devised. A paddle fixture was fabricated and attached to the actuator of the test frame (See Figure 1). The interior of the cups were modified to receive the paddle fixture. The test was repeated using the new fixation method and failure of the lock detail was achieved.
Results and Discussion
Table 1 and Figure 2 below shows the results of the torque testing for the group using the epoxy and the group using the mechanical lock fixturing. The development of higher torque resistant polyethylene liners shows a need to develop a fixation method that will ensure the proper evaluation of the lock detail.
For figures/tables, please contact authors directly.
