Total Hip Arthroplasty options 
The most commonly used hip implants are of the metal-on-polyethylene or ceramic-on-polyethylene type. They consist of a metal femoral stem on which a metal or ceramic head is placed. These metal stems are made of either Chromium-Cobalt-alloy or from Titanium-alloys The head of the implant articulates within a polyethylene acetabular component with or without a metal outer shell. The acetabular components can be either of a cemented version or has a special surface structure for biological fixation (bone ingrowth)
polyethylene wear
At the articulation between the head of the implant and the polyethylene of the acetabulum cup wear occurs. Polyethylene wear is now as the major cause of implant failure. The amount of wear is influenced by materials used, design of the implant, the weight and the activity level of the patient.Especially in young patients with a more active lifestyle, polyethylene may be the cause of the need for revision surgery. Alternative Bearings 
To prevent excess wear as seen in polyethylene other bearing surface have been developed. The Ceramic on Ceramic articulation has been in use over 15 years clinically and offers an acceptable alternative. Full Ceramic implant components require a certain thickness to obtain the fracture resistance needed in total hip arthroplasty
cross linked polyethylene 
Recently it was found that radiation of the polyethylene with gamma rays, caused cross linking of the material, This cross linked polyethylene offers promising reduction of the wear of the material addding to its longevity, Whether this promisses are met will be seen when long term clinical results become available.
Metal-on-Metal
Another way of
avoiding
the use of polyethylene is the Metal-on-Metal articulation. In the early days of total hip replacement, the McKee Farrar Hip implant was of the metal-on-metal type (from 1953) and bearing wear and osteolysis were no major causes of failure. However the limitations of the manufacturing process caused impingement between head and acetabular component, high friction subsequently causing acetabular loosening. rates of the metal-on-metal bearing, the implant may not be subject to dislocation and loosening and the procedure may be more durable than conventional THR in comparable patient groups,
Large Head arthroplasty Smaller Heads (22mm, 28mm or 32mm ) are used because of the 
limitationts of the materials. Polyerthylene needs a certain thickness and thus the heads of an implant cannot be as big as found in the normal anatomy. This has not prevented hip replacement from becoming so succesfull. However a smaller head was also a smaller "way" to luxate and postoperative dislocation of the hip is a complication that occurs too often. With the number polyethylenesm that may allow a thinner wall and especially the metal-on-metal articulation allows the use of head sizes that are more physiological. Large head arthroplasty is an option to prevent, but also to treat recurrent dislocations.
ceramic engineered surfaces 
To overcome the possible negative aspects of metal-on-metal; articulation (wear and metal ion release) the Accis system was design , The articulating surfaces are engineered with a ceramic (Titanium-Niobium-Nitride) to reduce wear and prevent metalion release (continue to read)