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ACETABULAR RECONSTRUCTION IN PAPROSKY TYPE III DEFECTS

ABSTRACT Objectives: Severe pelvic deficiency presents a difficult problem in hip arthroplasty. Specifically, the goals are to restore the pelvic bone stock, place the acetabular component in the correct anatomical position, and optimize joint stability. Currently, many surgical techniques have been developed for prosthetic revision surgery for acetabular complex defects, but no consensus has been reached on the best treatment. The objective of this study was to review mid-term cases of severe bone defect (Paprosky type III) treated with a bone allograft and ring Bursch-Schneider anti-protrusion cage (BSAC). Methods: A retrospective consecutive series review of the first 23 complex acetabular reconstructions performed between 2006 and 2011 was conducted. The series included the learning curve of the procedure and a minimum 5-year follow-up. Conclusion: Our study confirmed the efficacy of using a frozen morselized allograft combined with a metal ring-type BSAC for acetabular reconstruction. The anatomical location of the center of rotation of the hip must be recovered for long-term success. In massive loosening cases, the anatomical center of rotation can only be restored by bone density reconstruction using a graft protected by a ring to improve the centering of the head. Level of Evidence IV, Case Series.


INTRODUCTION
In recent years it has been producing an increase the number of surgeries revision total hip arthroplasty (THA). This is because the indications for THAs are expanding include younger patients with more planned activity and demands. 1 Charnley in 1979 suggested that revision prosthetic surgery would be the main cause of concern in the THA. Nowadays, THA loosening rates have been increasing so that doubled revision surgery each 10 years. 2 In prosthetic revision surgery, where a significant acetabular loss is there, must perform an acetabular reconstruction. The purpose of the acetabular reconstruction is to obtain a stable, permanent fixation of a new acetabular component to restore the center of rotation and, if possible restoring bone stock. During revision surgery simple acetabular defects, either cavitary or segmental, can be solved with conventional cementless THA (hemisferic acetabular components). However, combined segmental and cavitary defects are more difficult, especially those with no upper acetabular coverage. 3,4 Currently there are many surgical techniques for prosthetic revision surgery in acetabular complex defects, but there is no consensus on the best treatment. The impacted bone graft is an attractive treatment option for restoring severe acetabular bone defects (Paprosky type III) in total revision of the THA; 5 however, it requires the placement of a metal frame for attaching a primary stabilization. 6,7 The objectives of this paper is to review medium term cases operated by presenting a severe bone defect (Paprosky type III) treated with bone allograft and ring Bursch -Schneider ant protusio cage (BSAC). This review technique is attractive because it addresses the three objectives of the acetabular reconstruction: providing a stable and lasting fixation for a new acetabulum, restore the center of rotation; and achieve the restoration of bone stock.

METHODS AND MATERIALS
A retrospective consecutive series review was undertaken of the first 23 complex acetabular reconstructions performed between 2006 and 2011 (Table 1). This series includes the learning curve of the procedure and has a minimum 5-year follow-up. All cases were performed by the senior author (JM), a consultant orthopaedic surgeon with an interest in revision hip arthroplasty surgery. All patients were operated using cancellous allograft bone crushed with BSAC and a cemented cup. The average time between the initial total hip replacement (THA) and the revision acetabular surgery was 111 months (range, 12-228 months). Acetabular bone status was classified during the operation according to the criteria of Paprosky et al. 3 as Grade 3A (15 hips) and Grade 3B (8 hips). Different cemented (3 hips) and cementless (20 hips) THAs were revised in this series. Each revised cup was individually assessed; 16 (69,56%) only underwent acetabular revision, whereas 7 (30,44%) underwent revision of both components. In all cases where a revision was done, an uncemented stem was used (Conelock-Biomet). The cohort included 10 men (43.47%) and 13 women (56,52%) with an average age at the time of revision surgery of 77.04 years (range, 68-88 years). The average number of procedures performed before the present acetabular revision was 1.78 (range, 1-3). Diagnosis at the time of cemented cage reconstruction was as follows: aseptic loosening/mechanical failure (n = 19), instability (n = 1), reimplant for infection (n = 3). Preoperative evaluation according to the scale of MD Score average was 6.6 (range 5-9). The surgical planning, in cases where a intrapelvic penetration of the acetabular loosened component existed, included performing a digital subtraction angiography (DSA) arteriogram, to locate the femoral artery ( Figure 1). The primary objective was to restore the center of rotation of the hip.
All procedures were performed by the same surgeon, in supine position and using an anterolateral Watson-Jones aproach. The acetabular component was removed with the required extractional instrumentrs, preserving as much bone as possible. Cystic areas were aggressively debrided. Then, the acetabulum was inspected to identify osteolysis areas, determine the type of bone defects, and evaluate the presence of pelvic discontinuity. The acetabular bone bed was reamed with hemispherical reamers. Acetabular margins were defined, and the cup was always placed at the nearest of the center of rotation of the hip. In revision surgery with large bone defects, the obturator foramen is always a good reference for positioning the cup down and out. In all cases we use fresh frozen allograft femoral head, got in the bone bank. Bone was morselized with a bone mill, or manually using a rongeur. Then placed the BSAC ring, pinning the inner pin on the ischium and the other flange bolted to the ileum with 5 screws, checking its stability ( Figure 2). Then put a cemented polyethylene cup size to the size of the corresponding ring. Table 1. Summary of the 23 consecutive acetabular impaction graftings performed by using impaction grafting of a defect that has been contained with a metallic Bursch-Schneider antiprotusio cage, followed by implantation of a cemented polyethylene cup.

Case Gender Age
Merle 'Aubigne score   In all cases, low-molecular-weight heparin was used as an anticoagulant during the first month after surgery, and 2 g of Cefazoline was used in the initial anesthesia in no septic revision. Following our normal hospital procedure, surgical bleeding was controlled with tranexamic acid. During the postoperative period, ambulatory re-education was carried out. Patients were evaluated clinically using the Merle d'Aubigne score (MD). Radiological assessment was carried out by means of a standard AP X-ray of the pelvis and lateral hip, checking for migration, osteolysis and signs of radiolucency in the three DeLee acetabular zones.

RESULTS
We had no intraoperative complications. In all cases has gotten a good stability in the rings anclage and acetabular components have been chosen. We had no infections nor vascular injuries or nerve complications. Postoperative prosthetic dislocation has been done in 4 cases (17.39%), all were resolved with bloodless reduction. Mean blood of these patients was 650cc (300-800cc range), therefore we can say that they are bleeding interventions that have had to resort to transfusion in 14 cases (60.86%). From the clinical point of view, the mean MD at the end of the follow-up was 16 points (range [14][15][16][17]. At the final of the follow-up has not been any reoperation in these patients. The radiographic assessment does not demonstrate mobilization components, although the assessment of the incorporation of the grafts is difficult to assess because of the multitude of metallic artefact (Figure 3). No new intervention has been made during the follow-up of our series.

DISCUSSION
Our study has important limitations. This is a retrospective study done without a control group. Rather, the study was performed with a single observer, limiting how bias in interpreting results. This study supports our surgical approach and highlights the benefit of correctly position the center of rotation, which led us to use the ring BSAC. The restitution of the lost bone stock is one of the biggest challenges of acetabulum revision surgery. Classification systems for the acetabular defect allow unification in the defect and can give guidelines for treatment. The AAOS classification system described by D'Antonio et al. 9 grades the acetabular defect in five types: segmental, cavitary or combined deficiencies, pelvic discontinuity, and arthrodesis. The Paprosky classification system 3 stratifies the degree of bone loss based on radiographic parameters to guide the identification of reconstructive options. In four patients of our series, we could see a dislocation of the prosthesis. This happened at the beginning of the cases. All were resolved without the need for new surgeries. These dislocations made us think that the use of a hip stabilizing orthosis in abduction would be appropriate. For this reason, we have used this type of orthosis for three months after the operation. This prevented new dislocations. In grade III Paprosky acetabular defects the first major problem we face is the loss of bone stock that exists and therefore we have to get an acetabular regeneration. Some authors when faced with loosening and significant bone loss 10,11,12,13 have chosen to use a cemented cup of the next larger size and fill the defect with cement. This approach is still used despite Sofcot 1988 report 14 that highlighted a repeated rate of 33.3% of loosening with this technique and potential rate of 25% at 5 years detachment. These data support the need to rebuild and restore acetabular bone stock in revision arthroplasty by placing bone graft. Many surgeons have used the structural allograft in B A combination with a cemented cup, without using a reinforcing ring or without cemented cup. 15,16,17,18,19,20 After promising initial results, 15 was reported failures after 5 years 21 (20% rate of loosening), 10 years 22 (loosening rate 47%) and then at 16 23 (66% rate of loosening). Nowadays, the morselized frozen allograft is giving excellent results. Graft incorporation occurs in two phases: initial phase with partial resorption of the graft and repopulation phase where new bone is formed in the graft host. 19,20,24,25 The incorporation of frozen morselized allografts has been verified in animals 26,27 and confirmed in vivo. 28,29 The use of isolated bone graft when faced with significant bone loss does not solve the associated problems such as mechanical stability and integration of the graft. Therefore, it seems appropriate to use a metal protective ring to stabilize these allografts and improve integration. There are two types of metal reinforcing rings: one type only provides a proximal fyxing (Müller ring type) and the other provides dual fixing through a hook (Ganz ring, cross Kerboull) or screw (BSAC). Some authors believe that the use of proximal fixation alone is not enough and that many of the failures can be attributed to the lack of primary stability. 1,30 Gerber et al. 31 adds that most failures occurred due to lack of primary stability of the ring, which led to graft lysis after loosening. For that reason in our series we have always resorted to using BSAC ring with good results. There are many advantages with the combination of a reinforcement ring with a graft. First, the center of rotation of the hip is more likely to be restored 31,32,33,34 which avoids the "high" center of rotation of the hip advocated by some surgeons. We have managed to restore the center of rotation of the hip in all cases with the technique used. However, the elevation of the center of rotation is an acceptable alternative if several conditions are met: limited bone defects, without combined effect of rotation center. Second, the graft is protected and stabilized, which is essential for the integration of the graft. 30,31 Few studies have compared the use of different reinforced rings. Bonnomet et al. 32 found the ring BSAC to be better than the Muller ring in loosening cases with severe bone loss. Our series with a minimum follow-up of 4 years confirmed the good long-term results described in other published studies. 1,3,35,36,37,38,39 Patients who have Paprosky grade 3 acetabular defects are in a very unfavorable clinical situation with great functional limitations. The clinical results achieved at follow-up demonstrated a marked improvement ( Figure 4). These patients would be, in case of don't realise this surgery, candidates to a Girdlestone resection arthroplasty. Radiological evaluation of bone graft resorption is difficult after using allograft bone impacted with cement and metal rings with an acetabular revision, but the stability of cemented cup gives us an important tranquillity. In most cases there is a uniformity of the graft. But be cautious, because the results may deteriorate beyond 10 years. Allografts have been known resorptions later (after 10 years), therefore we must continue to monitor the long term this type of reconstruction. 37 Radiological evaluation of bone graft absorption is difficult after using allograft bone impacted with cement and metal rings in an acetabular revision, but the stability of cemented cup gives us an important tranquillity. In most cases there is a uniformity of the graft. But it's important to be cautious, because the results may deteriorate beyond 10 years. When allografts, it has been known absorptions later (after 10 years), therefore we must continue controlling long term this type of reconstruction. 37

CONCLUSION
Our study confirms the efficacy of using a frozen morselized allograft combined with a metal ring type BSAC during acetabular reconstruction. It is necessary to recover the center of rotation of the hip on its anatomical location for long term success. In massive loosening cases, the anatomical center of rotation can only be restored by the reconstruction of bone density using a graft protected by a ring which improves the centring of the head.