WHAT IS A TOTAL HIP PROSTHESIS (DEVICE)

The artificial hip joint replaces the hip joint that was removed because of disease. A device that replaces a removed (amputated) part of the body is called prosthesis.

So it is appropriate to call an artificial hip joint also a total hip prosthesis  (THP).

The term "prosthesis", however, may sound awkward for some people, the term device sounds better for some, and we may also speak about a total hip device.

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OUTLINE  OF  A TOTAL  HIP PROSTHESIS

A total hip prosthesis is composed of two  components: the femoral (thighbone)  component and the cup component.

Schematic picture of the total hip device (Click on the icon for a full size picture)

The acetabular component, a cup, is implanted into the acetabulum. Acetabulum is the anatomic name of the  hip socket in the pelvic bone

The "femoral or stem component" consists of  a long metal stem. The lower part is  placed into the marrow cavity of the femoral (thigh) bone. On the upper end the stem  has a neck. In some models of total hip device there is a collar between the neck and the stem, a disc that rests on the sawed off femoral bone. On the neck is attached the "ball", a metal or ceramic ball component that replaces the removed femoral head. That ball articulates with the cup. The ball is attached to the  neck section  either   directly in the factory, surgeons call this type for monobloc construction as in this picture (Harris  2DTotal Hip TM, Howmedica 1980), or  the surgeon chooses the proper size of the ball at the operation and attaches the ball component on the neck  after testing it during operation - the so called modular construction. For more details see the chapter Morse taper.

There have been many models of total hip device on the market, the number is > 1 000. For only the minority of them (about 20%) are the results, the failure rates, known. Moreover, new models appear steadily and are advertised as decisive improvement. Categorization of the many models may be difficult and the reader may be bewildered.

Moreover, the large stem component actually is a ballast.The shaft component is basically a big piece of metal – about 16 centimeters long shaft that keeps a metallic ball with about 5 centimeters diameter.

Show Picture: Stem component – an unnecessary ballast.

The only part of the whole stem component that has any function at all is the ball. It replaces the damaged and removed femoral head. Look at the picture. The whole stem component is there only to keep the relatively small ball component fixed to the skeleton. The stem component is big, it engages about one third of the whole thighbone. It is a ballast. When the total hip fails, the one third of the thighbone skeleton round this big ballast suffers, it is damaged or destroyed. Is this ballast, the whole stem component, necessary to fix the metallic ball to the skeleton? Many surgeons say: no, it is not.

 

 

 

 

 

 

 

 

 

 

 

An obvious way to dispense off the stem ballast  is to remove only the damaged surface of the femoral head and put on a thin metallic hemisphere instead of it. This is the Surface replacement hip. (Click here if you wish to know more about it)

Then there are at least two total hip models that use much smaller stem components that do not engage the shaft of the thighbone.

Show pictures:  Thrust and Osseo-integrated Total Hip models. 

  (Click on the icon for a full size picture)

	The one model is the so called Thrust plate total hip. The model has an ordinary femoral ball. The shaft is, however, a short thick spike going only through the femoral neck. The construction rests with the so called thrust plate on the sawed off femoral neck; the femoral neck receives the loads from the patient’s body weight through this "thrust plate".
Thrust plate total hip
	Another model is the so called Osseo-integrated total hip, developed by Swedish surgeons. Again, this total hip model has ordinary femoral ball. The shaft is, however, a screw that is screwed into the femoral neck skeleton. The screw is a copy of the successful Swedish screw system used for fixation of artificial teeth. Although it looks like the thrust plate model, the Osseo-integrated total hip relies on fixation of the whole screw, not just on the small plate at the upper end of the screw. The preliminary results with this model of the osseo-integrated total hip are good.
Osseo-integrated total hip

See also the chapter Prostheses for other hip replacement operations for details

Early after the operation, the ball is kept in place in the cup by the tension of the muscles and soft tissues around  the hip in the first place; later forms a new capsule around the new hip joint, which helps to stabilize the artificial hip's components.

The artificial hip cups are made from UHMWPE (Ultra High Molecular Weight PolyEthylene), metallic alloys, or ceramic materials.

The balls of the total prostheses are made either from metallic alloys or ceramic materials.

The shafts of total hip prostheses are made from metallic alloys.

Relevant issues:

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