BICENTRIC BIPOLAR PROSTHESIS IN NON-TRAUMATIC HIP PATHOLOGIES : MAXIMUM 10 YEARS FOLLOW UP Affiliation

This longitudinal study was done in 30 cases of various hip condi ons, during the period of March 2002 to March 2012. All pa ents had secondary osteoarthri s of hip with abnormal acetabulum following advanced tubercular arthri s, rheumatoid arthri s, ankylosing spondyli s and avascular necrosis of femoral head. Primary bipolar hemi-replacement arthroplasty was done in all the cases using Banaras Hindu University (BHU) bicentric bipolar hip prosthesis. Each pa ent was assessed radiologically for movement and frac on of it occurring at various components of prosthesis and clinically for func onal outcome using modified Harris hip score for the maximum of 10 years.


Objec ve
To assess clinical and radiological outcome of BHU bicentric bipolar hip prosthesis in treatment of non-trauma c hip pathology.

Methodology
This longitudinal study was done in 30 cases of various hip condi ons, during the period of March 2002 to March 2012.All pa ents had secondary osteoarthri s of hip with abnormal acetabulum following advanced tubercular arthri s, rheumatoid arthri s, ankylosing spondyli s and avascular necrosis of femoral head.Primary bipolar hemi-replacement arthroplasty was done in all the cases using Banaras Hindu University (BHU) bicentric bipolar hip prosthesis.Each pa ent was assessed radiologically for movement and frac on of it occurring at various components of prosthesis and clinically for func onal outcome using modified Harris hip score for the maximum of 10 years.

Results
There were 17 males and 13 females (n=30), aged between 31-60 years.Three and half years following surgery, (n=12) 100% pa ents were able to squat and sit crossed legged.At four and half years follow-up, (n=7) 100% pa ent had excellent result when assessed by modified Harris hip score.Radiologically, by the end of 10 years (n=1) all the movements of hip occurred at inner bearing while the outer bearing was fixed to acetabulum.

Conclusion
This intermediate term follow-up study suggests that the use of BHU bicentric bipolar hip prosthesis for bipolar hemireplacement is associated with be er clinical outcomes, in non-trauma c hip pathologies.

INTRODUCTION
Bateman and Giliberty introduced bipolar implant for hip reconstruc on, principally in fracture neck of femur, avascular necrosis of femoral head and non-union of femoral neck in an a empt to alleviate the problems of 1,2 conven onal metallic proximal femoral endoprosthesis.The bipolar hip prosthesis has a ball and joint socket, and the majority of mo on is supposed to occur between the small inner metallic head and the ultra-high molecularweight polyethylene (UHMWPE) socket.The UHMWPE socket is bonded to an outer stainless steel shell, and this outer shell ar culates with the acetabulum.Various other authors have advocated use of bipolar prosthesis in posttrauma c condi ons and avascular necrosis of femoral [3][4][5] head.With success of bipolar prosthesis in fracture neck of femur, its indica on was extended to various other hip pathologies like advanced osteoarthri s of hip, rheumatoid arthri s, avascular necrosis of femoral head or even cases with acetabular dysplasia, which were previously being 3 treated by total hip arthroplasty.The unique design of bipolar prosthesis helped to reduce the shear stresses and decrease the incidence of protrusio acetabuli and stem 4,5 loosening.The impact loads are be er absorbed by the bipolar prosthesis, thus providing further protec on for the (RA), ankylosing arthri s, tuberculosis (TB) of hip.However, mid to long-term follow-up demonstrated unacceptably high rates of complica ons like pain, migra on of prosthesis, osteolysis, and the need for revision [10][11][12][13] to total hip arthroplasty.The objec ve of this study was to analyze the long-term outcomes of Banaras Hindu University (BHU) bicentric bipolar hemiarthroplasty applied to these indica ons including the inner and outer bearing movements of the prosthesis.

METHODOLOGY
It was a longitudinal study conducted between March 2002 to March 2012, where 35 bipolar hemi-replacement arthroplas es were performed for the treatment of various non-trauma c hip pathologies causing hip OA secondary to healed TB hip, RA, AVN and Ankylosing arthri s of hip.Five pa ents lost to follow up during the period of me, hence remaining 30 were included in the study.All the operated pa ents had preopera ve modified Harris hip score of less than 50.Pa ents with age less than 30 and more than 60 or have had previous hip surgery were excluded from the study.
A patented bipolar prosthesis by name "BHU bicentric bipolar prosthesis" designed by the second author was used for the study.First and second author were involved in the surgery and case management.Second author was involved from the beginning of the study hence had maintained all the records of the pa ents whereas first author was involved in the last three years of study.
It is a bipolar hip prosthesis with some design modifica on, compared to the conven onal bipolar hip prosthesis.BHU bicen c bipolar prosthesis has passed through various stages of stem design and modifica ons, which were tried on cadaver bones.The final design was approved and na onal patent was obtained and has been used in this study a er ethical approval from the ins tu onal review commi ee (Figure 1).
All the surgeries were performed by senior surgeons with pa ents under general anesthesia or spinal anesthesia, a er taking wri en informed consent.Pa ents were posi oned in lateral decubitus and Harding lateral approach was used.In all the cases uncemented type of hip hemi-replacement arthroplasty was performed.The postopera ve protocol was to encourage the pa ents to perform quadriceps strengthening exercise from postopera ve day 1 and par al weight bearing walking was rd allowed from 3 day a er the opera on and full weight bearing was allowed at 6 week from the date of opera on.The pa ents were followed at 6 weeks, 3 months, 9 months, 12 months than 6 monthly for 4 years and then yearly ll last follow up.At each follow up, radiological assessment of movement at the inner and outer bearing of prosthesis was done by plain anteroposterior radiograph of bilateral hip in neutral posi on and then in maximum abduc on.Addi onally, clinical assessment was done by self-administered modified Harris hip score based on pain, func on, entry in public transporta on, absence of deformity, range of mo on, si ng crossed legged (Table 1).Grading of modified Harris hip score was as follows: 1. Poor: 0-69 2. Fair: 70-79 3. Good: 80-89 4. Excellent: 90-105.

Figure 1 : BHU bicentric bipolar hip prosthesis
Jha SC et al On the x-ray in the neutral posi on (Figure 2), a line drawn tangen al to the most inferior aspect of the ischium was used as a reference line and angle A was defined by the intersec on with this line of a line drawn along the inferior margin of the acetabular component.Angle B was formed by the intersec on 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 AP radiograph taken with the operated limb in maximum abducted posi on and angle A1 and B1 was plo ed.The difference B2 between angle B and B1 represented the change in the amount of abduc on of the hip and the change A2 between angle A and A1 represented the amount of mo on taking place between the acetabular component and the acetabulum.The difference between B2 and A2 represented the amount of 14 abduc on taking place at the inner bearing.During this method, the limbs were maintained in the neutral rota on.Total amount of abduc on movement occurring at outer and inner component were calculated and tabulated for each pa ents at each follow up.
Sta s cal analyses were performed using STATA 12 so ware.Descrip ve sta s cs were conducted based on the collected data from 30 pa ents.pa ents in this series.Modifica on of sea ng angle between the stem and the collar has allowed preserva on of the calcar by quarter to 1 cm a er implanta on of the prosthesis.Due to low sea ng angle in conven onal bipolar (Bateman type), the calcar has to be sacrificed during the surgery and hence on weight bearing the prosthesis tends 17,18 to sink if cement is not used.

RANGE-OF-MOTION SCALE:
The ultra-high molecular weight polyethylene (UHMWPE) lining has a beveled margin (Figure 3).Thus, reduces wear and debris forma on 17,18 at extremes of the movement.The neck of the prosthesis has been made trapezoidal like the Charnley type neck and the cup has been modified in a staggering fashion to decrease neck cup impingement.This modifica on in design has grossly improved movement at the inner ar cula on and avoided the neck cup impingement, thereby increasing the life of the bipolar prosthesis.UHMWPE wear in a bipolar prosthesis is 0.7 mm/year.A 22 mm Charnley head diameter has been used in the BHU bicentric bipolar prosthesis, so that thicker UHMWPE can be provided in outer bearing, thereby increasing the survivorship of the 17,18 prosthesis.
Moreover, smaller sized head further provides increased movement at the inner bearing for a longer me, adding longevity to the prosthesis.BHU bicentric bipolar prosthesis is available in 37-55 mm head sizes in 1 mm increments, which provides appropriate head size for replacement and subsequently prevents early failure of the prosthesis, by avoiding Hertzian contact 17,18 stress.
The BHU bicentric bipolar prosthesis allows crossed legged si ng and squa ng without limi ng pa ent range of movement at the hip (Figure 4).from 46 to 86 in pa ents treated with bipolar hemiarthroplsty for AVN hip, but at final follow-up of average 11.4 years, 20 (42%) hips had radiographic failures, 12 (25%) hips were revised and groin symptoms were present in 20 (42%) hips.Hence they no longer recommend bipolar prosthesis for the treatment of osteonecrosis of femoral head.
Vazquez-vela et al. no ced in all pa ents with protusio and thin medial wall there was no increase in protusio, rather in majority of pa ents there was significant thickening of the medical acetabular wall.⁹In none of the pa ents, bone gra was used.Incidence of ectopic para-ar cular bone forma on following total hip arthroplasty is variable in literature.However, func onal impairment due to this 25,26 occurs in 3-10% cases of THA.
Incidence of this complica on is remarkably less in cases of bipolar hip arthroplasty since this is a rela vely simple procedure where minimal capsular exposure is required and acetabular prepara on is either not required or is minimal.Bhan et al. in their study of bipolar hip arthroplasty in ankylosing spondyli s concluded that bipolar arthroplasty gives excellent results in OA hip secondary to AS, since it is technically easy, has lower risk of ini a ng heterotopic ossifica on and provides excellent func onal recovery.²⁷In a study by Rai et al.where they used BHU bipolar prosthesis in treatment of intra-capsular fracture neck of femur they no ced gradual increase in movement at outer bearing, which was 80.51% at last follow-up.¹⁷Similarly, the movement at the inner bearing gradually decreased from a mean of 7.53 (31.4%) to 4.5 (19.49%).They concluded that there was a slight loss of movement at the inner bearing over the months on subsequent follow up as compared to the first follow up, whereas this difference was not sta s cally significant.

CONCLUSION
The BHU bicentric bipolar prosthesis has been shown to be a good op on for non-trauma c hip disorders, as it is associated with be er clinical outcomes.

RECOMMENDATION
It may be a viable alterna ve to the costly total hip replacement arthroplasty for the poor Indian subcon nent pa ents.

LIMITATION OF THE STUDY
The findings of this study should be interpreted in line with the following limita ons.First our sample size was small with only one of the pa ent having 10 years follow-up.Second, 5 pa ents were lost to follow-up.

2 acetabulum.
Added advantage of greater stability and reduced likelihood of disloca on was a ained with the 2, 6 bipolar hip prosthesis.Several authors have reported good to excellent short-term and intermediate-term results with the use of bipolar prosthesis for osteoarthri s (OA) of the hip, avascular necrosis (AVN) of the femoral head, rheumatoid arthri s 7-9

Figure 2 :Figure 3 :
Figure 2 : Method of assessing component movement by Drinker and Murrays method

Figure 4 :
Figure 4 : Eight year follow up : Pa ent squa ng and si ng crossed legged

Table 1 : Modified Harris hip Scoring System RESULTS
Pa ents based self-assessment was done by modified Harris hip score ques onnaire at each follow up (Table3), At 6 months follow up (n=30) 14.28 % pa ents had good outcome, 61.91% had fair outcome and 28.57% had poor outcome.While by 1 years (n=21) 14.28 % had excellent outcome, 7.14 % had good outcome, 42.86% had fair outcome and 35.72% had poor outcome.By the end of 4.5 years (n=7) all the pa ents had excellent outcome on modified Harris hip score (Graph 2).

Table 2 : Abduc on movement and frac on of it occurring at various componentsTable 3 : Analysis of Modified Harris Hip Score
We propose changes in the design of BHU bicentric bipolar prosthesis as men oned by Rai et al. for the prolonged preserva on of movement at inter-prosthe c joint, and good to excellent modified Harris hip score in majority of DISCUSSIONTotal hip arthroplasty is already an established and standard modality of treatment for severe OA of hip secondary to rheumatoid arthri s, avascular necrosis of femoral head and is being performed all over the world.Although the early and intermediate outcomes were very successful and promising with longer follow up surgeons are tackling complica ons like acetabular component loosening, instability, disloca on and difficulty in revision.Rela vely simple and gra fying procedure of bipolar hip arthroplasty could be a good and cheaper alterna ve in both aged and younger pa ents with hip OA.Almost 80% of pa ents showed good to excellent results a er bipolar hip arthroplasty in study conducted byDudani etal.Bateman in his series of 760 degenera ve hip has shown good results with improvement of Harris hip score from 51 to 87 a er bipolar hip arthroplasty.¹⁵,⁷McConville et al. found bipolar hip arthroplasty as viable op on against THA, in their series of 100 consecu ve pa ents of degenera ve arthri s of hip.⁸At an average follow-up of 65.7 months Pandit et al. no ced 93-96 % hips had good to excellent results a er bipolar hip arthroplasty, which was comparable to our study where at end of 4.5 years all 7 pa ents had excellent results.¹⁶