The Artificial Hip

Artifical hip joints are broadly of two types; those that require bone cement to anchor it in the human body and those that do not. There are important design differences between the two varieties. Since a hip joint has two components to it - the ball and the socket, it is possible to have a third type of artifical hip that uses cement on the ball side and none on the socket side. (It is quite unusual to use a cemented socket with an uncemented ball).

The precise choice of implant is made by the specialist taking into account your age, lifestyle, how active you are, whether the hip replacement is being done for the first time or is a replacement for a previously carried out artifical hip and also the specialists own experience and training. Whatever design is chosen, it is very likely to have a bearing made of a metal ball moving inside a plastic cup. While newer implants using other materials such as ceramic are available in the market, Mr. Trakru does not use them as the long term results with these are not as well known as with the other older types.

The socket: The artificial socket is used to replace the natural cup of the hip joint and to house the ball portion of the joint. The type for use with cement is made of a special type of polyethylene which is very tough and slippery particularly when wet. It has ridges on the outside and a wire marker so that its position can be seen on the X-Ray. The ridges are designed to improve the fixation of the cup by the cement. It is implanted into the natural socket by first filling the cavity with the cement and then pushing in the artificial cup which is held still while the cement sets.

The socket for use without cement is made of metal. It has a specially designed surface on its outer side that looks and feels like sandpaper. It is designed to "fool" the body into mistaking it for bone and causing it to join up with the socket much like broken bone heals. However, it is important that the artifical socket is held firmly in place while the healing process is going on and this can take 6-12 weeks. The initial fit is achieved by machining the natural socket accurately and using an artifical socket that is precisely 1-2 mm bigger. The metal socket is then forcibly "jammed" into place and further fixation can be obtained if necessary by using additional screws. The next stage is to place a plastic insert within the metal shell against which the artifical ball will form the joint.

The ball: The ball portion of the artificial hip consists of a metal stem or rod on top of which a metal ball is attached at an angle to mimic the shape of the top end of the human thigh bone. The component for use with cement is fairly smooth on it outer aspect. It is inserted into the thigh bone after first filling the marrow cavity with bone cement and holding it still while the cement sets.

The implant for use without cement has a specially fabricated outer surface as in the case of the uncemented socket. The surface feels porous like a sponge and encourages bone to grow into it. This process takes about 6-12 weeks to complete. It is inserted after first machining the marrow cavity of the thigh bone with special drills and then jamming in a component that is slightly larger.

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