Abstract
Diamond like carbon (DLC) coatings have been proven to be an excellent choice for wear reduction in many technical applications. However, for successful adaption to the orthopaedic field, layer performance, stability and adhesion in physiologically relevant setups are crucial and not consistently investigated. In vitro wear testing as well as adequate corrosion tests of interfaces and interlayers are of great importance to verify the long term stability of DLC coated load bearing implants in the human body. DLC coatings were deposited on articulating lumbar spinal disks made of CoCr28Mo6 biomedical implant alloy using a plasma-activated chemical vapor deposition (PACVD) process. As an adhesion promoting interlayer, tantalum films were deposited by magnetron sputtering. Wear tests of coated and uncoated implants were performed in physiological solution up to a maximum of 101 million articulation cycles with an amplitude of ±2° and −3/+6° in successive intervals at a preload of 1200 N. The implants were characterized by gravimetry, inductively coupled plasma optical emission spectrometry (ICP-OES) and cross section scanning electron microscopy (SEM) analysis. It is shown that DLC coated surfaces with uncontaminated tantalum interlayers perform very well and no corrosive or mechanical failure could be observed. This also holds true in tests featuring overload and third-body wear by cortical bone chips present in the bearing pairs. Regarding the interlayer tolerance towards interlayer contamination (oxygen), limits for initiation of potential failure modes were established. It was found that mechanical failure is the most critical aspect and this mode is hypothetically linked to the α-β tantalum phase switch induced by increasing oxygen levels as observed by X-ray diffraction (XRD). It is concluded that DLC coatings are a feasible candidate for near zero wear articulations on implants, potentially even surpassing the performance of ceramic vs. ceramic.
Highlights
Owing to the aging process and related factors, some major joints of the human physiology are subject to decay during their lifetime and eventual replacement by artificial solutions
The aim of this work is to investigate the qualification of Diamond like carbon (DLC) coatings for long-term application in bearing implants using the example of lumbar spinal disks regarding mechanical stability, corrosion resistance as well as defect tolerance and third body wear
In direct comparison to an uncoated metal-on-metal part, the generation of high wear on the latter following a run-in phase up to 7 million cycles is evident. This effect has been linked to roughening of the uncoated cobalt chrome molybdenum (CCM) surface [18] by third-body wear particles and consequent deterioration of the lubrication regime, which correlates to observations on explanted metal-on-metal joints [19]
Summary
Owing to the aging process and related factors, some major joints of the human physiology are subject to decay during their lifetime and eventual replacement by artificial solutions. This mostly affects hip, knee, spine and shoulder, each of which generate more than one million replacement procedures each year. To ensure a long lifetime of an artificial joint replacement it is crucial to know all possible failure mechanisms including wear mechanisms. These wear mechanisms differ depending on the joint materials and on the geometry of the implant. The question is not just one of reducing the amount of wear, and how the size, shape, and surface chemistry of released wear particles differ among bearing surface combinations, since these factors may influence the biologic reaction and subsequent tendency for adverse body reactions (adverse local tissue response and systemic effects)
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