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. 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.Introduction
Materials and Methods
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. 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) INTRODUCTION
METHODS
Modular necks allow intra-operative adjustment of neck length, offset, and version, enabling the surgeon to better match leg length and accommodate anatomical differences. However, there have been recent reports of early fatigue failures of the neck initiating from the neck/stem taper, and some retrieved components exhibit severe fretting corrosion.1 Fatigue testing according to ISO 7206-6 (10/9 orientation) has been shown to replicate the clinical fatigue failures, but results in relatively minor fretting and corrosion. The purpose of this pilot study was to evaluate techniques for accelerating fretting corrosion with the goal of replicating the most severely corroded clinical retrieval cases. Constructs tested in this study consisted of a single stem and neck design (PROFEMUR modular, Wright Medical Technology). The worst case long varus neck design was evaluated in two materials: Ti6Al4V and wrought CoCr. Introduction:
Methods:
