Bipolar replacement, or arthroplasty, is one type of partial hip replacement that is frequently done for fractures, but can also be done for arthritis, avascular necrosis, and other problems about the hip. It is an option when there is disease of the femoral head, but the acetabulum or socket is not involved. Bipolar arthroplasty provides a way to replace the femoral head without having to ream or put an implant in the socket. It is a more conservative procedure than total hip arthroplasty. In effect, it is a partial hip replacement.
Original partial hip replacements were unipolar or monoblock. The first of these, designed in 1942 by Dr. Austin Moore, consisted of a large round femoral head attached to a stem which fit inside the normal femoral canal. This became known as the Austin Moore prosthesis and was a standard in fracture treatment for many years. Later designs updated and refined the concept of a unipolar prosthesis.
It was thought, however, that the large metallic head pressing against the natural articular cartilage of the acetabulum caused excess friction and wear. Many times this led to pain and a less satisfactory result. The bipolar prosthesis was devised as a way to limit friction and wear against the acetabular surface.
The bipolar prosthesis was first developed by Dr. James Bateman in 1974 in a design he called the universal proximal femur. In the past 30 years, the concept of a bipolar prosthesis has been updated and refined by many implant manufacturers.
A bipolar prosthesis is a “ball within a ball” or a “head within a head.” It has two mobile articular surfaces rather than one. A small metallic femoral head sits on top of the normal femoral stem. This is the prosthetic femoral head. A second layer, or inner bearing, sits on top of the femoral head. The inner bearing is made of the same plastic or polyethylene used in an acetabular component. This creates the same metal on polyethylene bearing sur-face found in a total hip. An outer metallic shell called the outer bearing fits on top of the plastic inner bearing. The outer bearing then articulates with or makes a joint with the normal acetabulum.
Like a unipolar component, the large metallic outer bearing rests against the normal articular cartilage of the socket and in a similar way allows for normal movement of the hip joint.
There is also movement, however, between the small femoral head and the plastic inner bearing surface. Thus there are two layers of movement and two bearing surfaces in bipolar arthroplasty. The addition of a second bearing surface may increase range of motion of the hip, but more importantly it may also reduce the amount of friction and wear on the acetabular side. This will lead to less pain and greater longevity of the prosthesis. Though multiple studies have been done, it is not clear how much movement occurs at each bearing surface or whether the outer bearing surface becomes fixed in the acetabulum over time.
The diameter of the outer bearing is typically 43 mm or greater and is much larger than the femoral head in a total hip arthroplasty, which is 22–36 mm. For this reason, a bipolar prosthesis tends to be more stable than a standard total hip arthroplasty. It is less likely to dislocate and come out of the joint.
A bipolar arthroplasty has the same femoral stem component as a total hip. It can be converted to a total hip arthroplasty if necessary. Bipolar replacement is most commonly done for fractures. The main indication is a displaced fracture of the femoral neck just below the head of the femur. In this circumstance, the ball of the femur has broken off and is not likely to heal.
Accordingly, the treatment is to replace the femoral head with a bipolar prosthesis. This is routinely done for subcapital or femoral neck fractures in elderly patients. A bipolar prosthesis may also be used to treat a femoral neck fracture that has first under-gone pinning but then failed to heal. Bipolar arthroplasty may be done as a primary procedure for problems that cause hip pain but only involve the femoral head. Often this is a young patient with avascular necrosis who has hip pain and collapse of the femoral joint surface but no wear on the acetabulum. In younger patients, this will relieve pain and preserve bone in the socket.
A bipolar prosthesis may also be used for other disease processes such as a bone cyst or tumor, which involve only the femoral head and not the acetabulum. Some of the possible problems of a bipolar prosthesis include groin pain, long-term wear of the acetabular joint cartilage and protrusion of the large metal head into the pelvis. Indeed, some studies show better results for total hip rather than bipolar arthroplasty for reconstructive problems. Other options such as hip resurfacing and limited hip resurfacing are now some-times used in younger patients.