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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.
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.
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%.
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.
Since the end of the 1960s, prosthesis with an inner bearing is being used for hip hemireplacement arthroplasty.1 In bipolar prosthesis the movement of the hip should occur mainly in the built-in bearing (inner bearing or interprosthetic joint or built in joint) because of the low friction rather than in the joint between the prosthetic head and the acetabulum thus sparing the acetabulum. Well known examples include the Christiansen prosthesis (Christiansen 1969) with a built-in trunnion bearing and the prostheses described by Bateman (1974), Gilberty (1974) and Monk (1976), all of which feature a ball and socket joint within the prosthesis.1 The key issue in the value of bipolar femoral head replacement is whether it functions as a bipolar or unipolar device.2
There have been reports in the literature that state that the bipolar prosthesis becomes a unipolar prosthesis functionally after few months.1,2 Therefore, there is need for further refinement of the prosthesis design to overcome this limitation. In this study, we have introduced further changes in the design of a bipolar prosthesis and patented it under the name of bicentric bipolar prosthesis.3 We conducted a radiological evaluation of movement at the interprosthetic joint on follow-up in bipolar arthroplasty done for various indications of the hip.
The present prospective study was carried out in 52 cases of various hip conditions during the period March 2003 to January 2011. Bicentric bipolar prosthesis [Figure 1] has passed through various stages of stem design, which were tried on cadaver bones. Prototypes were found to be unsatisfactory. The final design was approved and national patent was obtained and has been used in this study.
All patients in whom this prosthesis was used were divided into the following two groups: Group 1: Fracture neck of femur; Group 2: Non-traumatic hip pathologies that severely hampered the quality of life of the patients due to pain at hip and restriction of mobility. These included healed tuberculosis hip, monoarticular rheumatoid involving the hip joint, ankylosing spondylitis and non-traumatic avasular necrosis (osteonecrosis) of the femoral head with secondary osteoarthrosis involving the hip. Patients who had undergone any surgical intervention (like screw fixation for neck femur fractures, core decompression for osteonecrosis femoral head, etc.) were excluded from the study.
In all cases uncemented type of hip hemireplacement arthroplasty was performed. The post-operative protocol was to encourage the patients to perform quadriceps strengthening exercises from post-operative day 1, and partial weight bearing walking was allowed from 3 days after the operation and full weight bearing was allowed at 6 weeks from the date of operation.
Anteroposterior plain radiographs of both hips were taken with the patient supine in the neutral position, (both in terms of abduction/adduction and rotation,) and then in maximum abduction of the operated limb, maintaining neutral rotation with the non-involved limb still in a neutral position (adduction/abduction and rotation). On the X-ray in the neutral position, a line drawn tangential to the most inferior aspects of the ischia was used as a reference line and angle A was defined by the intersection with this line of a line drawn along the inferior margin of the acetabular component. Angle B was formed by the intersection of the ischial reference line with a line drawn along the center of the long axis of the femoral stem. The exercise was repeated on the anteroposterior radiograph taken with the operated limb in maximum abduction position and angles A1 and B1 were plotted [Figure 2].
The difference B2 between angle B and B1 represented the change in the amount of abduction of the hip and the change A2 between A and A1represented the amount of motion taking place between the acetabular component and the acetabulum. The difference between B2 and A2 represented the amount of abduction taking place at the inner bearing.4 During this method, the limbs were maintained in neutral rotation. Thus, the total amount of abduction and its outer and inner components were calculated and tabulated for each patient at each follow-up.
Follow-up studies were carried out postoperatively at 6 weeks; at monthly intervals in the first year, at 6 months interval upto 4 years and yearly thereafter. At each follow-up, supine radiographs in neutral and abduction were carried out as described above and tabulated [Tables [Tables11 and and2].2]. The arithmetic mean of the sample population at each follow-up was calculated.
35 patients in group 1 and seventeen patients in group 2 were operated. There were 28 males and 24 females. The patients were aged between 48 and 86 years. The maximum number of cases was in the 40–60 years age group.
The mean of percentage of movement occurring at the interprosthetic joint and at the acetabulum in each follow-up [Figures [Figures3A3A and and3B]3B] has been tabulated in Tables Tables11 and and22 and also depicted in the graphs [Figure [Figure4A4A and and4B4B].
As shown in Figure 4A among patients in group 1 at the initial (6 weeks post-operative) follow-up, the movement at the hip between the acetabulum and the prosthesis was 63.49%, and only 36.51% of the total movement was occurring at the interprosthetic joint. The movement at the interprosthetic joint fell to 25.66% and 22.67% at followup of 1.5 years and 6 years respectively. The fraction of the movement occurring at the built-in joint gradually [Figure 4] decreased up to 1.5 years, and stabilized thereafter. There was no statistically significant difference between the 1.5 year and the 6 year observations (P-value <0.05).
Among the patients in group 2, at the initial follow-up (6 weeks), 76.04% of the total prosthetic movement was occurring at the inner bearing while only 23.96% of the motion was occurring between the acetabulum and the prosthesis. At 1 year followup, the movement at the interprosthetic joint had steadily decreased to 68.39% of the total movement. The outer bearing movement was 31.61%. At the end of 5 years, the movement at the interprosthetic joint had gradually decreased to 55.91% of the total movement.
As depicted in Figure 4B in patients of group 2, the fraction of movement at the interprosthetic joint gradually decreased up to 2.5 years and then stabilized as opposed to patients of group 1, where the same value had stabilized by 1.5 years.
Results of the roentgenographic study show that the movement at the interprosthetic joint continues to be more than the movement between the prosthesis and the acetabula even after 5 years in cases of group 2. Those with normal actetabulum (group 1) had an initial fall in motion at the inner bearing during the first year. But, the inner motion was found to maintain a plateau (about 20%) at 6 years follow-up.
One of the intraoperative complications was splitting of the anterior femoral cortex during hammering of the prosthesis into the femoral canal in one patient. There was no episode of dislocation of the prosthesis.
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.4
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 [Figure 5]. 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
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 [Figure 6].
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 [Figure 7].
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.
This study shows the relative preservation of inner bearing movement in the bipolar hip prosthesis with time probably due its refined design. However, a follow-up time of 6 years is not sufficient to comment on the superiority of the prosthesis because several complications like wear, prosthesis loosening and protrusio are not seen in the early follow-up. Further studies need to be done on the effect of weight bearing on the inner bearing movement. Further refinements are needed to make the prosthesis work better in patients of intracapsular fracture neck femur. The bicentric bipolar hip prosthesis has been shown to be a good option for both traumatic and atraumatic hip disorders.
Source of Support: Nil
Conflict of Interest: None.