Hip endoprosthesis are considered to be the greatest successful progress in orthopedic surgery in the last 100 years. Over 1 000 000 surgeries are conducted every year, over 300 000 – only in the USA.
Hip arthroplasty is an operation involving the replacement of a diseased hip joint with an artificial one. Each endoprosthesis consists of an acetabulum, acetabular insert, stem and metal head. The artificial cup is mounted in place of the natural acetabulum. Most often it is made of titanium, and inside it there is an insert made of polyethylene or ceramics. A metal or ceramic head is attached to the titanium stem of the prosthesis. In this way, the metal head and acetabular insert form a new joint, capable of making movements, allowing to perform human physiological gait.
Although hip joint prostheses have been in use for about 100 years, they only perform without any faults properly for 12 – 15 years. A human takes about 1,000,000 steps a year. During this period, there is a constant wear of its components, especially the ultra-high molecular weight polyethylene (UHMWPE) cup used in approximately 90% of hip joint endoprostheses.
When selecting hip replacement implants for particular group of patients it is vital to ensure the smallest contact strength in the endoprosthesis at the initial stage of use. The primary objective is to minimize the wear of prosthesis components, the speed of which is largely dependent on the contact pressures present. However, the issue of reliable assessment of contact pressure in endoprostheses is still not fully resolved. This is due to the lack of relatively simple and effective calculation methods ensuring the possibility of their estimation depending on the compressive force, endoprostheses size, radial clearance and type of materials used. Various numerical methods used since 1985 to assess contact pressures are used on a small scale by engineers – designers of endoprostheses and doctors that perform arthroplasty.
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