The great benefit of bipolar prosthesis over conventional unipolar prostheses lies in its interprosthetic motion, which would supposedly delay, or prevent, acetabular erosion (a recognized complication of the unipolar models).5
It does this by reducing wear due to sliding motion in the acetabular socket. The high molecular weight polyethylene (HMWPE) articulation absorbs some of the impact forces during gait. Although the success of bipolar models in this regard has been borne out by many studies,5–7
there have been some reports in the literature that cast doubt on the continuing function as a bipolar prosthesis. This was suggested by radiographic studies in which the prosthesis was imaged in various non-weight-bearing and static weight-bearing positions. It has been suggested that after a variable period, the interprosthetic motion stiffens up and the prosthesis acts as a unipolar unit.8,1
There are various methods of estimating the fraction of movement that is occurring at the interprosthetic joint in a bipolar prosthesis. Phillips2
used radiograph in supine position, first in neutral and then in maximum abduction and adduction. Verberne1
used image intensifier to do radiological examination. We followed the method of plain radiographs, as described by Bochner et al
Drinker and Murray8
reported that although some inner motion occurred in most implants, it was less than predicted. In a study of femoral neck fractures, Verberne1
noted minimal movement at the inner joint 3 months after surgery. Phillips2
found that the prosthesis functioned largely as a unipolar device, with movement occurring primarily at the outer joint in 75% of the fracture group.
Studies in the literature that quantify the fraction of hip motion occurring at built-in joint are only few. Verberne,1
in his study of 20 patients using the Variokopf prosthesis (made by Interplanta GmbH) of femoral neck fractures fitted with bipolar prosthesis (femoral head size 24-32 mm), reported that the built-in bearing was hardly functional after 3 months. The percentage of total abduction occurring at the built-in joint at 3 months follow-up in patients of intracapsular fracture neck femur in our study has been 33.74% compared with 16.9% reported by Verberne1
in his study, and the difference is statistically significant (P
-value >0.05 using the Student T-test).
We propose the following reasons for prolonged preservation of movement at the interprosthetic joint of the bicentric bipolar prosthesis. The seating angle between the stem and the collar has been modified so that after seating of the prosthesis there is preservation of the calcar by quarter to 1 cm. In the conventional bipolar (Bateman type), this angle is so low that the calcar has to be sacrificed for seating and hence on load bearing it sinks if cement is not used.3
The ultra high molecular weight polyethylene (UHMWPE) lining has a bevelled margin. Thus, at the extremes of movement, the neck impinges on the metal cup and not on the UHMWPE lining therefore reducing wear and debris formation.9
Neck cup impingement is a very important factor for the failure of bipolar prosthesis. To tackle this problem, the cup has been modified in a staggering fashion and the neck of the prosthesis has been made trapezoidal like the Charnley type neck . This change in design has provided overall gross increase in movement at the inner articulation and avoids the neck cup impingement, a very important cause for the failure of the bipolar prosthesis.9
Diagrammatic representation of neck cup impingement (which has been corrected in the present BHU hip prosthesis by staggering in the cup)
The smaller diameter head provides a larger thickness of UHMWPE for better survivorship of the prosthesis. UHMWPE wear in a bipolar is 0.7 mm/year.10
For this very reason, a 22 mm Charnley head diameter has been used in the bicentric bipolar prosthesis3
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
The benefit of positive eccentricity has been provided by reducing the head diameter of the inner ball to 22 mm. This also provides a greater range of movement between the metallic head and the UHMWPE thus providing greater percentage of movement at the inner bearing for a longer time, adding longevity to the system.3
The prosthesis is available in 37–55 head sizes in 1 mm increments, which prevents use of inappropriate head sizes and subsequently early failure of the prosthesis. This avoids the Hertzian contact stress.11
For social and cultural reasons, sitting and squatting is the integral part of daily life style of patients of the Indian subcontinent. The bicentric bipolar prosthesis provides the same without limiting patient range of movement at the hip .
Clinical photograph of a woman with the bipolar prosthesis in situ able to sit cross-legged and squat comfortably
Thus, in our series, appreciable motion continued to occur at the inner bearing 5 years after operation in group 2. This clearly shows that in the patients with diseased and eroded acetabular cartilage, because of the increased coefficient of friction between the acetabular cup and the acetabulum, the inner bearing moves much greater than the outer bearing. The outer bearing moves mostly at the extremes of motion when the neck impinges over the cup margin.
The relative motion of the inner versus the outer bearings in the bipolar prosthesis is dictated by the principle of static and sliding friction at the bearing surfaces. Too large a femoral head will lead to neck impingement, and this may account for the poor inner bearing motion found in the Bateman prosthesis by Drinker and Murray.8
In osteoarthritis, lack of smoothness and lubrication increase static friction at the bone-metal articulation, and may partly account for the good mechanical performance of the prostheses in osteoarthritis.
Thus, it can be seen that there are several factors affecting components of motion in the bipolar prosthesis, ranging from head size, inner bearing size, quality of UHMWPE that influences debris formation, whether the hip is loaded or unloaded, condition of the acetabular cartilage and indication for the hemiarthroplasty procedure.
Factors that could contribute to the decreased movement at the inner bearing are smaller size of the outer cup compared with the size of the acetabulum; low quality and inadequate thickness of UHMWPE, leading to an increased coefficient of friction and frictional torque, greater volumetric wear and accumulation of debris; neck cup impingement; adhesions formed at the inner head and UHMWPE liner; toggling present at the interface, which may produce debris of the UHMWPE and cause stiffness.