Introduction
In the recent past metal-on-metal hip arthroplasty has attracted the renewed interest of the orthopedic world. When polyethylene wear and subsequent osteolysis were recognized 
as important causes of failure of hip implants designers and developers of orthopaedic implants looked at the past, and noticed excellent long term results of some original metal-on-metal prostheses without any obvious wear and without osteolysis.
Total Hip and Resurfacing Hip
With the revival of metal-on-metal articulation the old dream of "hip resurfacing" became a reality. With thin metal shells a minimum of bone resection is required at the acetabular and femoral side. The larger diameter of the femoral head provided the hip joint with improved stability and range of motion.
As a result of the early success of surface replacement hip arthroplasty also total hip systems were designed with larger femoral heads.
Wear DebrisSo far so good. However soon after the introduction of new metal-on-metal hip prostheses concerns about the presence of metal degradation products in or near the hip joint or disseminated into the body were expressed in the literature. It became known that metal-on-metal articulations, despite the fact that they generate less volumetric wear, may produce more wear particles. Metal wear particles, much smaller than polyethylene wear particles, could become biologically active. Swelling of the soft tissues around the hip were seen.
Metal Ion Release Metal surfaces could also undergo corrosion and produce metal ions, which may combine with proteins and cause allergic reactions. Hypersensitivity reactions of surrounding tissues were described. High blood levels of chrome and cobalt were found in patients with metal-on-metal prostheses and changes in red and white blood cells were detected. Implant Design
Transfer of knowledge gathered from decades of small headed implants simply to large headed monoblock implants may proof to be incorrect. Lack of fixation of the acetabular component was a frequent complication of the original metal-on-metal hip design and is a remaining problem today. Today with improved manufacturing facilities clearances are usually small enough to allow fluid film or mixed lubrication at the dome of the articulation and wide enough to prevent equatorial impingement
Most acetabular cups are spherical. These spherical cups run the considerable risk of being pushed out of the acetabulum due to forces around the full surface of the cup after impaction, called the rebound effect. Press fit fixation by equatorial over-sizing as seen in cups with a three radial design is enhanced by compression forces only and is probably more reliable.
Ceramic Engineered SurfacesThe ACCIS hip prosthesis system for conventional total hip arthroplasty and surface replacement arthroplasty is specifically designed to overcome the possible disadvantages of metal-on-metal articulations. The ACCIS components are like the components of other metal-on-metal prostheses made of a chrome-cobalt-molybdenum alloy, but are different. The surfaces of the components are modified with the ceramic Titanium-Niobium-Nitride (TiNbN) in order to minimize metal particulate wear and prevent corrosion.
Pre-clinical tests and clinical follow-up, including blood metal ion measurements, of patients with the ACCIS total hip prosthesis and the ACCIS prosthesis for surface replacement arthroplasty has demonstrated that :
wear was considerably reduced by the ceramic coating
the metal components do not corrode: metal ions did not increase up to 7 years postop.
Karel J. Hamelynck, orthopaedic surgeon
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All Accis components are manufactured and distributed by the manufacturer: Implantcast GmbH of Buxtehude, Germany