الفهرس 
ACKNOWLEDGEMENTOnly 14 pages are availabe for public view
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Abstract
The purpose of hip arthroplasty is to restore function by restoring normal anatomy and thus normal hip biomechanics. ‘‘Normality’’ applies to the position of the center of rotation of the hip, anteversion (femoral and acetabular components), overall offset and leg length. These parameters will be considered individually, but they are all inter-related. Large femoral heads represent a main option available to address hip instability. Femoral head sizes greater than 36 mm have been designed to provide increased stability through a rather basic concept: by increasing the range of motion before impingement through an improved head-neck ratio and by increasing the jump distance. It is also successfully used in revision applications in patients with a history of recurrent dislocations. Most of the clinical advantages reported with the use of large diameter head are: -increased stability and lower dislocation rate by increase the head to neck ratio and more importantly increase the translation distance. -increased range of motion (ROM) by reducing component-on-componen impingement -accelerated rehabilitation protocol . Large head metal-on-metal devices also benefit from decreased wear when compared with smaller-head metal-on-PE designs. At this time there is no universal consensus among experts as to the best articular bearing surface for routine use. The four alternatives of 1) conventional metal ball on polyethylene cup articulation 2) metal head on the new cross-linked polyethylene cup 3) ceramic on ceramic articulation and 4) metal on metal each have drawbacks. With conventional polyethylene, wear over time is a certainty.With new cross-linked polyethylene wear appears to be slowed, but still present. In addition cross-linked polyethylene is new to clinical use and the apparent advantages over the previous polyethylene generation have yet to be clinically aproved. Ceramics, while exhibiting almost no wear and being very well tolerated by the human body, are brittle and there is concern regarding the theoretical risk of implant fracture. In addition due to implant design constraints there is a greater risk of hip dislocation with use of ceramic on ceramic implants. Metal on metal articulations appear to solve the problems of the above alternatives. However the metal ions that are generated by these implants have created concerns regarding the potential for the development of secondary health problems.