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Hip replacement is a medical procedure during which the diseased hip joint is replaced with an artificial implant so that the function of the hip would be retained. Usually, the term Hip replacement means total hip arthroplasty, a replacement of the whole hip joint, whereas hemiarthroplasty is a replacement of only one articulation surface of the joint, while the other remains unchanged.
First attempts to replace a hip joint were carried out with implants made of ivory in the late 19th – early 20th century. In the next few decades a large number of completely artificial implants, varying in comprising materials, design, application were introduced which gradually led to an increasing overall improvement of hip replacement outcomes and patients’ quality of life. Today it is one of the most common and routine orthopedic procedures.
The variety of different hip implant types makes it quite clear that none of them is yet perfect. And what would be a perfect hip implant? The answer is both simple and complex – it would mimic the natural structures of the joint and surrounding bones as much as possible, in terms of stability, durability, elasticity and low deterioration. Unlike artificial materials, from which implants are made, bone is an ever changing tissue, renewing its structure and readjusting to forces acting on it. In this way bone sustains its qualities throughout the most part of a lifetime.
Furthermore, articulate surfaces of the joint are covered with cartilages and lubricated with synovial fluid so as to reduce friction, to absorb some of the impact force and to allow normal and fluent movements. This makes repetition of the joint function a hard task. Firstly, implants should withstand very high forces, be resistant to impact-caused microchanges in the structure, get along well with surrounding tissues and provide a wear-and-tear resistant articular interface.
The design and biomechanics of modern day implants have not changed much since the beginning of hip replacement treatment. What really changed was the material used. Implants consist of two main parts – acetabular cup which acts as an artificial hip socket, or acetabulum, and femoral component, replacing the femoral head and neck. Acetabular cup and femoral component are commonly anchored into bone using bone cement mass which hardens almost immediately after application and forms a strong bond with the bone. Some newer implants today have a porous coating that mimics bone trabeculae. Additional fixation is then formed, as native bone grows into this coating.
Here are some examples of most commonly used hip implants:
This type of implants has been used since the early 1960s. Now with improved, so-called “highly crosslinked” polyethylene plastic liners, a much lower wear of the implant is achieved.
Ceramic heads of femoral components are harder and more scratch resistant than ones made of metal. Together with plastic cup liners they offer greater wear resistance compared to implants with metal heads.
Metal-on-metal implants have no liners and the heads are bigger which has a benefit of increased motion range and a lower incidence of hip dislocation. Furthermore, metal-on-metal implants offer greater wear resistance compared to plastic components and cause less inflammatory reactions. However, there is an ongoing discussion, that metal wear debris, generated from metal-on-metal implants, may be associated with increased metal ion concentrations in the blood. Whether this actually affects the health of a patient is not yet known.
This type of implants is considered to be one of the most reliable. With their longevity and durability fully ceramic implants are especially suitable for younger, more active patients. Since ceramics is the hardest material used in implants, it has an extremely low wear rate, measured about 0,0001 mm per year. Ceramic surfaces are also the smoothest which has an effect on wear rates as well. However, long-term data on their reliability is still limited, as this type of implants is relatively new.
Hip resurfacing is an alternative to total hip replacement, when the proximal femoral bone is conserved and only the head of femur is “resurfaced” – a hollow metal cap is placed over it. Hip socket is replaced just as it would be in total hip replacement. Hip resurfacing offers lower risk of hip dislocation and relatively fast recovery after the procedure. Since much less bone is removed, any further revision surgeries of hip joint are made easier. But this procedure should only be performed in patients with adequate bone mass and low risk of femoral neck fracture.
Notable hip implants manufacturers are: Biomet, Inc., DePuy Orthopaedics, Stryker Orthopaedics, djo surgical, Wright Medical Technology, Inc, Zimmer Orthopaedics and others. Usually, all of them produce several types of implants. Such variety can be confusing but it is also useful as every patient is unique, has his own pace of life, health condition, activities and movement capacities, so an implant must be chosen individually. Well selected implant will last longer and serve better.
The clinic NordOrthopaedics offers you a variety of hip implants. Each individual case is carefully evaluated in order to select the implant that suits one’s needs the best.