Clinical wear depends on several factors such as implant specific factors (material, design, and sterilization), surgical factors/techniques, and patient-specific factors (weights and activities). The load magnitude for wear testing in the standard protocols (i.e., 2 kN as per ASTM F1714 or 3 kN as per ISO 14243-3) represent an average patient weight and does not address the other “what-if”’ scenarios (i.e., wear vs. patient weights, activities, duration, etc.,). The results from Wear factor was first evaluated using actual wear testing conducted on metal on cross-linked polyethylene bearings along with well-established Dowson's wall bridge equation. As per Dowson-Wallbridge, volumetric wear is V=2.376·KNWR+C or K=V/(2.376·NWR) where V is the volumetric wear in mm3, K is the wear factor in mm3/Nmm, N is the number of cycles, W is the load in Newtons, R is the bearing radius in mm, and C is the creep (assumed to be negligible, i.e., C=0 in this model. 28 mm simulator wear was first used to evaluate wear factor, but since simulator wear presented as a mass loss, these results were converted to volumetric wear using the equation
(m is the wear in mg and r is the density of XLPE in mg/mm3 (=0.923). The Dowson-Wallbridge equation was then validated for predictive accuracy against actual wear testing on the predecessor THR system. The wear factor thus obtained was used to compute the theoretical-wear for other sizes (i.e., 42 and 46 mm bearings). The theoretical-wear was then compared to simulator wear for predictive accuracy.Objective
Methods
Published simulator studies for metal/UHMWPE bearings couples showed that increasing the femoral head diameter by 1 mm increases wear by approximately 10% due to increased contact area. Therefore, there are concerns about increased wear with dual mobility hip bearings. The purpose of the study was to compare wear from dual mobility hip bearings to that with traditional fixed bearings. In addition, for the dual mobility bearings, the effect of femoral head material type on the liner wear was also evaluated.Background
Purpose of the study
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
Laser marking of implants surfaces is necessary in order to provide traceability during revisions which will help identify product problems more quickly, better execute product recalls and improve patient safety. There are several methods of marking employed within the medical field such as chemical etching, electro pencil marking, mechanical imprinting, casting of markings, marking with vibratory type contact, ink jet, hot foil and screen printing. However, these methods have various drawbacks including marking durability or addition of potentially toxic chemical compounds. As a result laser marking has become the preferred identification process for orthopedic implants. Laser marking is known for its high visual quality, good reproducibility and precision. However there are concerns about the laser marking potential to affect fatigue life of a device. There is a limited number of research papers that studied the effect of laser marking on fatigue life of implants. The objective of the current study is to investigate the effects of laser marking on the fatigue life of titanium alloy material. Two groups of four point bend specimens were used to investigate the effect of laser marking on the fatigue life. The first group comprised of the specimens without laser marking while the second group comprised of specimens with laser marking currently utilized for the implant surfaces. Prior to conducting the fatigue testing, a non-destructive X-ray diffraction (XRD) residual stress analysis was conducted to determine if the laser marking had introduced any residual stresses. Imaging analysis was also conducted to examine any potential surface damage on the test sample's surface. A servo-hydraulic test machine was used for the fatigue four point bend testing regime where the inner and outer spans were 30 mm and 90 mm respectively. All testing was conducted at a frequency of 10 Hz, a stress ratio R=0.1, and sine-wave loading in air. Testing was stopped at 10 Million cycles or at failure of the specimen.Introduction
Material 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
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. 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.Objectives
Methods
Total knee replacement (TKR) implant designs and materials have been shown to have a significant impact on tibial insert wear. A medial-pivot (MP) design theoretically should generate less wear due to a large contact area in the medial compartment and lower contact stresses. Synovial fluid aspiration studies have confirmed that a first generation MP TKR system (ADVANCE®, MicroPort Orthopedics Inc., Arlington, TN, USA) generates less wear debris than is seen with other implant designs articulating against conventional polyethylene (CP). The objective of this study was to evaluate the Introduction
Objectives
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:
Large diameter metal-metal bearings possess several clinical advantages over small bearings including greater joint stability, improved range of motion, and lower wear due to improved lubrication. Simulator wear tests were conducted to assess the effects of thermal processing on the wear behavior of large diameter metal-metal hip bearings. Three groups of high carbon, cast 54 mm hip bearings with different thermal processing histories were tested. Two groups of bearings were manufactured to identical specifications and subjected to either no heat treatment(as-cast) or to typicall thermal processing prior to testing. The third group was comprised of commercially available as castbearing systems. Wear tests were performed on a Shore Western orbital bearing wear test machine. A simulated gait profile (triple-peak Paulprofile) with a maximum force of 2000N was applied to the bearings at a frequency of 1 Hz. The bearings were tested in the inverted position (headabove, shell below). The general wear behavior of all three groups of bearing couples was similar to that previously reported for metal-metal bearings. All couples exhibited a run-in wear phase followed by a low-wear steady-state phase. For all bearing couples tested the heads demonstrated more wear than the shells. The appearance of the worn surfaces of all the bearing couples tested in this study were consistent with that of previously reported in-vitro wear testing as well as metal-metal hip bearing retrieval studies. There was no statistical difference among the three groups tested in the run-in or steady-state wear rates, although the heat treated bearings tended to wear less on average. The results of this study indicate that thermal processing has no adverse effect on the wear of large diameter metal-metal hip bearings.
