Bicentric bipolar hip prosthesis: A radiological study of movement at the interprosthetic joint

Abstract

Background: The bipolar hip prostheses after some time functions as a unipolar device. There is a need to change the design of bipolar hip prostheses to make it function as a bipolar device over a prolonged period of time. A bicentric bipolar hip prosthesis was used as an implant for various conditions of the hip. We evaluated the movement of this newly developed prosthesis at the interprosthetic joint radiologically at periodic intervals.

Materials and methods: Fifty two cases were operarted with the Bicentric bipolar prosthesis for indications like fracture neck of femur and various other diseases of the hip and were followed up with serial radiographs at periodic intervals to evaluate, what fraction of the total abduction at the hip was occurring at the interprosthetic joint.

Results: In cases of intracapsular fracture neck of femur, the percentage of total abduction occurring at the interprosthetic joint at 3 months follow-up was 33.74% (mean value of all the patients), which fell to 25.66% at 1.5 years. In indications for bipolar hemireplacement other than fracture neck of femur, the percentage of total abduction occurring at the interprosthetic joint at 3 months follow-up was 71.71% (mean value) and at 1.5 years it was 67.52%.

Conclusion: This study shows the relative preservation of inner bearing movement in the bipolar hip prosthesis with time probably due its refined design. Further refinements are needed to make the prosthesis work better in patients of intracapsular fracture neck femur.

Keywords: Bicentric bipolar prosthesis; inner bearing; interprosthetic joint.

Figure 1

Bicentric bipolar hip prosthesis

Figure 2

(a) Left anterioposterior radiograph in the neutral position showing angles A and B. (b) Radiograph shows measurements A¹ and B¹, with the operated limb in maximum abduction

Figure 3A

Preoperative (a), postoperative (b) and (c) six years followup anteroposterior radiograph of pelvis with both hips of a case of monoarticular rheumatoid arthritis of left hip

Figure 3B

Preoperative (a), immediate postoperative neutral (b) and abduction (c) and 5 years postoperative radiograph (d) of a 60 year old patient in whom Bicentric hip prosthesis was used for fracture neck of femur

Figure 4A

Graph depicting change in the movement at the built-in joint and that between the acetabulum and the prosthesis, expressed as percentage of total movement, has been plotted against time in patients of group 1 (intracapsular fracture neck femur)

Figure 4B

Graph depicting change in the movement at the built-in joint and that between the acetabulum and the prosthesis, expressed as percentage of total movement, has been plotted against time in patients of group 2

Figure 5

Diagrammatic representation of neck cup impingement (which has been corrected in the present BHU hip prosthesis by staggering in the cup)

Figure 6

Cross-section of the head showing the small 22 mm head that allows greater thickness of ultra high molecular weight polyethylene to be put in to reduce friction at the inner bearing

Figure 7

Clinical photograph of a woman with the bipolar prosthesis in situ able to sit cross-legged and squat comfortably