This work uses a mathematical method to correlate the forces calculated to push-on and pull off a femoral head from a stem and correlate the results of in vitro testing. This work aimed to mathematically model the force needed to disassemble the THR unit for a given assembly load. This work then compared these results with the results of an in vitro experiment. The research presented aimed to determine the assembly forces necessary to prevent movement of the head on the stem through friction. By assessing the forces necessary to push the head onto the stem securely enough to prevent any movement of the head through friction, it is likely that the fretting and corrosion of the head taper interface will be reduced.Summary
Introduction
Vitamin-E has been introduced into highly-crosslinked polyethylene liners to reduce the oxidation potential of the material while maintaining low wear rates. However, little has been reported on adverse testing of the material with one test on diffused vitamin-E polyethylene [1] and no adverse tests of vitamin-E blended polyethylene reported. Adverse testing of crosslinked polyethylene has focused on the use of large diameters, the incorporation of third body particles, roughening of the counterface or severe activity [2–4]. This investigation considers the wear of vitamin-E blended highly-crosslinked polyethylene under standard and adverse conditions articulating against uncoated and chromium nitride (CrN) coated metal heads. Seven metal heads were tested against prototype ϕ52 mm 0.1 wt% vitamin-E blended highly-crosslinked polyethylene liners (Corin, UK). Three heads remained as cast double heat treated metal (MoP) while four, of similar metallurgy, were coated with CrN via electron beam physical vapour deposition (CrNoP) (Tecvac, UK) and polished to a similar surface finish. Tests were conducted for 5 million cycles (mc) under conditions described in ISO 14242–3: 2009. Alumina particles (mean size 2.4 μm) at concentrations of 0.15 mg/mL were added to the lubricant for 1 mc to consider the effect of severe head damage. Testing continued for a further 1 mc without the presence of the particles and then 3 jogging intervals (14,400 cycles each) were conducted at slow, medium and fast speeds [3]. Wear volume was determined gravimetrically for the heads and liners and fluid collected throughout the testing was analysed for cobalt concentration using graphite furnace atomic absorption spectroscopy.Introduction
Methods
Bearing surfaces of metal-on-metal (MoM) hip resurfacing devices and total hip replacements (THRs) are a known source of metallic debris. Further, large diameter heads and the high friction of a MoM joint are thought to lead to fretting and corrosion at the taper interface between modular components1. The metal debris generated can cause significant problems on the joint area2. This paper investigated fretting and corrosion of femoral head-neck junctions. Variables of the head-neck junction which may have an effect on fretting and corrosion were identified with the aim of determining the key drivers so that their risk on fretting and corrosion could be reduced through design. Additionally, a Chromium Nitride (CrN) coating was assessed to determine the effect on fretting and corrosion of coating the stem (male), head (female) or both trunnion interfaces. As there is currently no standard specification for a head-neck trunnion interface and trunnion designs vary significantly across the market, this work may lead to a positive change in the design and materials used in head-neck taper interfaces for all THR devices. Suitable head and stem combinations were identified to enable individual variables such as; coating, medial-lateral (M-L) offset, head offset and taper angle to be isolated (Figure 1 and Figure 2). For the coated components a 3 μm CrN coating was applied to trunnion using electron beam physical vapour deposition (Tecvac, Cambridge, UK). Fretting and corrosion testing was carried out in accordance with ASTM F1875-98 (2009) method II procedure B3 following assembly of the components under a 2 kN load.Introduction
Methods
Patients suffering from finger joint pain or dysfunction due to arthritis and traumatic injury may require arthroplasty and joint replacement. Single-part silicone-based implants remain the material of choice and most widely used option, although reports on their long-term clinical performance are variable. For trauma indications, patients have a high expectation of functionality necessitating the use of materials with high wear resistance and mechanical performance. A new proximal inter phalangeal (PIP) joint designed by Zrinski AG (Wurmlingen, Germany), comprising a self-mating carbon fibre reinforced polyetheretherketone (CFR-PEEK) coupling, may provide a suitable alternative. Here we describe the wear performance of the CFR-PEEK components in a PIP joint wear simulator and subsequent characterisation of the wear particles. Four proximal and distal PIP components were milled (Zrinski AG) from CFR-PEEK (Invibio Ltd, UK) and subjected to wear testing (Endo Lab ® GmbH, Germany). The test was conducted at 37°C over 5 million cycles in 25% bovine serum (refreshed every 0.5 million cycles). The load was a static force of 63N applied at a frequency of 1Hz with a flexion/extension angle of ±40°. Wear rate was determined by mass loss from each component. Pooled serum samples from the wear simulator were subjected to protein digest and the remaining particulate debris isolated by serial filtration through 10μm, 1μm and 0.1μm filters. Particle size and morphology was subsequently determined by scanning electron microscopy (SEM) (Continuum Blue, UK).Introduction
Methods
Particulate debris created during a fiber-filled PEEK material (MOTISTM) rubbing on a ceramic femoral head in a hip wear simulation study was characterized. The particles were cleaved from the protein lubricant with a double enzymatic protocol and then sized using two different techniques. The sizes obtained were verified using an AFM imaging technique. Many metal-on-UHMWPE joints ultimately fail due to late aseptic loosening. This occurs due to the particulate debris built up in the periprosthetic area. The body’s natural immunological response leads to bone resorption, the prosthesis becomes loose and severe pain can then necessitate revision. It is therefore important to characterize the wear particles of novel materials in order to understand their biological impact. Particles were generated in a Durham hip wear simulator from a MOTISTM acetabular cup articulating against a ceramic femoral head for 25 million cycles A double enzymatic protein cleavage protocol was used as it was shown to be the least harmful to the particles. A bi-modal distribution of sizes was seen with a large number of particles of 100 nm and a large number at the two micron size range. AFM results verified the size of the particle distribution and also showed that the smaller particles were round to oval and the larger particles were long and thin. No carbon fibers were evident in the AFM images. Although the wear rate over the 25 million cycles1 remained low and linear, the average particle size tended to increase over the 25 million cycles whilst the volume of the particles decreases over the period. Howling This method only allowed around 100 particles to be imaged at a time, no size distribution was given. Ctyotoxicity was also tested using U937 monocytic cells indicating that MOTISTM has no cytotoxic effects such as necrosis.
In designing artificial joints the main criteria are to reduce the wear-rate of the material and the reaction of the body to the wear particles produced. These can be achieved by using harder materials (metals, ceramics) or by reducing the chances of producing a wear particle by the choice of material and design. This paper will look at combinations of PEEK-OPTIMA against different counterfaces with the aim of reducing the wear-rate of artificial hips and knees. Pin-on-Plate Studies Twenty-six different sets of experiments combining PEEK-OPTIMA in different formulations and against different counterfaces were conducted to evaluate the lowest wear combination. The lowest wear-rate combination was CFR-PEEK PAN against low carbon CoCrMo alloy (K=0.144×10-6 mm3/Nm) which is only about 1/8th of the wear of UHMWPE against stainless-steel (1.1×10-6 mm3/Nm). Gamma radiation sterilisation did not seem to affect the PEEK wear-rate. Hip Simulator Studies A 25 million cycle wear study has been conducted on the Durham Hip Simulator using 54 mm diameter alumina heads against CFR-PEEK thin-walled acetabular cups (MITCH). Five joints were in active stations and one acted as a loaded control. Wear was measured gravimetrically. Particles were analysed using a NanoSight LM10 instrument at 0.5, 10 and 25 million cycles. Also an Atomic Force Microscope (AFM) was used to look at particles above 2μm which is the limit of the NanoSight instrument. The wear-rate was linear over the whole 25 million cycle test at 1.16 mm3/ million cycles (range 0.811–1.392 mm3/million cycles). As the test progressed, the number of particles reduced and the dominant particle size increased from about 40nm to circa 200 nm. The AFM showed some particles as large as 3μm to exist also. No fluid film lubrication was observed to be generated in these joints so the low wear-rate was due to the inherent low-wear properties of the material combination. Knee Simulator Studies CFR-PEEK was moulded into the interpositional bearings for experimental lateral and medial unicompartmental knee designs and tested for 5 million cycles using 5 pairs of active joints and one pair of loaded controls in the Durham Knee Simulator. Wear was measured gravimetrically. Whilst the CFR-PEEK components gave a total wear-rate (both medial and lateral) of 2.72 mm3/million cycles, UHMWPE inserts in a similar application
The generation of particle debris from ultra high molecular weight polyethylene (UHMWPE) against metal hip joints has been shown to cause osteolysis leading to joint loosening in the medium term. This is known as late aseptic loosening since infection is absent In an attempt to reduce the volume of wear debris, attention has moved to metal-on-metal prostheses as the total volume of wear debris is less. However, the size, shape and number of the particles are important as well as the total volume as these affect the biological response of the body leading to aseptic loosening. The Durham Mk I Hip Joint Simulator was used to generate CoCrMo wear particles in a series of tests. Four simulator tests took place in succession, initially 50 mm Birmingham hip replacements were tested where both the head and the cup were as-cast CoCrMo alloy. A second test was conducted where the components were 38 mm and both head and cup were as-cast CoCrMo. A third test using 50 mm components was completed where both head and cup were double heat treated CoCrMo alloy and a final test took place where both components were 50 mm the head was as-cast and the cup was as-cast which had been pre-worn to 5 million cycles. Bovine serum with a concentration of 17 g/l of protein was used as a lubricant and particles were sampled every half million cycles. The volumetric wear was also obtained gravimetrically. A double enzymatic protocol was used to cleave the proteins from the particles taking great care to minimise any effect on the particles. Finally the particles were suspended in distilled, de-ionised water to enable them to be analysed using a NanoSight LM10 particle analyser. This yielded the size distribution of the particles. This was then confirmed by placing an aliquot of the suspended particles onto a carbon coated copper grid and drying them under a lamp. These particles were then imaged in the TEM. Energy Dispersive X-ray analysis was also used to obtain the chemical composition of the particles. The results indicated a strong correlation between the gravimetric wear and the number of particles. All the as-cast CoCrMo alloy tests showed a consistent particle modal average size. The double heat treated particles were shown to be smaller, with occasional large flake like particulates which were identified under the TEM. This particle data correlates extremely well with previous data from simple material testing pin on plate experiments. Previous studies have used microscopy to investigate the size and morphology of the particulate debris
