Hip replacement surgery has gone through tremendous evolution since the first procedure in 1840


Hip replacement surgery has gone through tremendous evolution since the first procedure in 1840. In the past five decades the advances that have been made in technology, advanced and smart materials innovations, surgical techniques, robotic surgery and methods of fixations and sterilization, facilitated hip implants that undergo multiple design revolutions seeking the least problematic implants and a longer survivorship. Hip surgery has become a solution for many in need of hip joint remedy and replacement across the globe. Nevertheless, there are still long-term problems that are essential to search and resolve to find the optimum implant. However, most of these implants only last for 10-15 years, and one of the most common problems for both patients and doctors is implant failure. Observed in long-term, implant loosening is the main cause of failure. Occasionally, dislocation or bending of implants may occur. Fatigue fracture and wear were identified as the main problems related to loosening of implants, stress shielding, and final implant failure. There are several factors which contribute to implant integrity, including material and design, implant positioning, cementing technique and patient characteristics. To improve integrity and life of hip implant by using finite element analysis, two major factor should be considered, design and biomaterial. By using this method there are various parameters that should be define, including complex geometry of the bone and an implant, biomaterial properties, and specifics boundary conditions, i.e. contact between the hip prosthesis and the bone depending on fixation method. In orthopaedic biomechanics finite element method was first used for the purpose of determining the stresses in human bones. Since then, this method has been more and more frequently applied in determining of stress state in bones and prostheses, as well as fracture fixation devices, including hip implants. The high rates of revision surgery are considered to be affected by prosthetic hip design, materials and fixation methods, as well as the patient’s age and activity. To decrease the number of revision surgeries, new technical innovations for the prosthesis have been created
This project aims to study the effects of a set of variables within which an optimal prosthesis design can be made by means of finite element analysis. It includes study of factors influencing design process and structural integrity of the hip implant.

Keywords: Biomaterials, Finite Element Method, Hip Prosthesis, Total Hip Joint Replacement

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INTRODUCTION
Total Hip Replacement (THR) surgery or Total Hip Arosthroplasty (THA), is a surgery in which the hip joint is replaced with a prosthetic implant. Hip replacement surgery was first performed in 1840 in New York, by Dr. Carnochan who was the first surgeon to insert a wooden piece between the diseased hip joint. In recent years the THR surgery is being considered by most in the field to be a successful elective major procedure especially in restoring mobility to patients. Worldwide, there are approximately one million implants performed annually. The surgery has become a routine with a minimum rate of early complications, offering patients great relief of pain when medications fail. It further offers tremendous improvement in function and consequently quality of life. Therefore, it is one of the most satisfying operations to both patients as well as surgeons. THR is performed as a solution to several degenerative and traumatic processes which affect the hip joint; it is a reconstructive surgical procedure.
In the past five decades, advances that have been made in technology; innovations in materials, surgical techniques, and methods of fixation and sterilization of prostheses, facilitated hip joint implants go through multiple design evolutions seeking better results and longer survivorship for implants. This resulted in the breakthrough of hip surgery; nevertheless, not all implants were able to withstand the test of longevity. Although the risks of the surgery are low, there are long term complications that contribute to the failure of the prosthesis; the most problematic is the limited life span of the prosthesis. Ten per cent of the implanted prostheses have a revision surgery within ten years; the average life-span of the prosthesis is fifteen years, which is a crucial problem to the growing number of young patients under forty years of age. These young patients usually are more active and therefore they will require multiple revision surgeries throughout their life-time.
Revision surgery is substantially more difficult than primary THR surgery due to the involvement of removal of existing implant and fixations, complexity of the procedure, and longer anaesthesia and operative time. Preparation of the operating site has to be performed prior to insertion of the new prosthesis with the use of any necessary fixation methods or reinforcement. Revision surgery has a higher rate of intraoperative fractures than primary THR. Postoperatively, patients are prone to have more complications and rehabilitation takes longer. Therefore, it is more risky, costly and inconvenient to the patient than primary surgery.
The high rates of revision surgery are considered to be affected by prosthetic hip design, materials and fixation methods, as well as the patient’s age and activity.
The prosthesis typically consists of a femoral stem, a femoral head (ball) that attaches to the stem, an acetabular cup and a fixation agent to secure the stem into the femur and the acetabular cup into the acetabulum in the pelvis, Fig. (1).

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