Acetabular reinforcement rings in revision total hip arthroplasty

KOTSOVOLOS E, ZALAVRAS CH, ARNAOUTOGLOU C, MANDELLOS G,
MITSIONIS G, XENAKIS TH
Department of Orthopaedic Surgery, University of Ioannina

ABSTRACT
Introduction: Reconstruction of the acetabulum in revision total hip arthroprasty may constitute a challenge for the orthopaedic surgeon, especially when bone defects are present. Metal rings, methyl-methacrylate cement and bone grafts have been used as reconstruction modalities. We present our experience from use of acetabular reinforcement rings to solve this difficult clinical problem.
Material and methods: From 1987 to 1998, acelabular reinforcement rings were utilized in 61 revisions of total hip arthroplasties in 56 patients, 54 women and 2 men, of age from 32 to 81 years (mean 60 years). In 47 hips Muller rings were used and in 14 hips Burch-Schneider rings. For the existing defects of the acetabulum, morsellized allografts from frozen femoral heads and bone substitutes as Lubboc and Collapat were used. The rings were stabilized with 2-5 screws. The polyethylene was secured inside the ring with methyl-methacrylate cement. The patients were evaluated clinically with the Merle d' Aubigne-Postel scale and radiologically.
Results: Follow-up ranged from 2 to 13 years (mean 6 years). Clinically improvement of the patients was significant. Range of motion improved from 3.6 to 5.3 points, pain from 2.7 to 5.5 points and walking ability from 3.1 to 5.3 points. Incorporation of the bone grafts took place in all cases at a mean time of 4 months. Acetabular reconstruction was successful in 57 of 61 hips (93.5%). Failure of the rings occurred in 4 of 61 hips (6.5 %) and reoperation was required. Two of the 14 Burch-Schneider rings failed (14%) with loosening and displacement and two of the 47 Muller rings (4%) after dislocation of the polyethylene cup. Complications included dislocation of the prosthesis in 2 cases, superficial infection in 1 and pubis rami fracture in 1 case.
Discussion: Reinforcement rings in our opinion can be of valuable help in reconstruction of the bone deficient acetabulum. Simultaneous use of bone grafts is necessary, especially in the presence of large defects, in order to fill them and to provide satisfactory contact of the ring with the acetabulum. The Muller rings appear to have better results than the Burch-Schneider rings, but we must consider the fact that the Muller rings were used in less extended defects and their follow-up time was shorter (5.5 vs 7 years). Although optimal placement of the ring is technically difficult and not always achieved, reinforcement rings combined with bone grafting can provide a satisfactory solution in reconstruction of the acetabulum in revision hip arthroplasty.

Key words: Revision of the acetabular component, reinforcement rings in acetubular component revision.

INTRODUCTION
Total hip arthroplasty (THA) constitutes the most common adult reconstructive hip procedure, performed at increasing number of patients. Moreover, younger and more active individuals are nowadays submitted to the procedure[11]. As a consequence, revision THA is frequently needed to address the problems of loosening, which despite the accumulating knowledge on its pathogenesis, has not yet been solved.
The use of cemented acetabular components has raised concern regarding high rates of loosening and early failure[5,14]. More favorable results have been reported for uncemented components[23,24]. However, achievement of the necessary stability is not always possible, as in cases of bone deficient acetabula. Therefore, failure is likely to occur if bone defects, that preclude contact of most of the component's surface with native bone, are present.
Acetabular bone deficiencies can be attributed to a variety of causes. Bone defects may be preexisting ones, as in cases of dysplasia or acetabular fractures, not managed at the primary procedure. They may be created during previous surgery, in an effort to accommodate the implants. Bone defects often result from osteolysis, due to reactions initiated by wear particles, due to instability because of loosening of the prosthesis or due to infection. Finally, they may be inadvertently caused during revision surgery, at removal of the previous implants or cement[11].
Consequently, the new component will not be placed on bone of sufficient strength and quality, as is native bone, secure fixation and stability will not be achieved and failure will be highly probable4. Thus, the presence of acetabular defects constitutes a challenging problem at acetabular reconstruction and the best possible solution should be provided by the surgeon.
Acetabular reinforcement rings (ARR) have been designed and utilized to address the complex problem of acetabular bone defects and we retrospectively present our experience with use of these devices in the reconstruction of the bone deficient acetabulum, at revision THA.

Figure 1 A. The Burch-Schneider antiprotrusio cage. B. Removed modified Muller ring
(The case of figure 3B).

MATERIAL - METHODS
Study population
From 1987 to 1998, we utilized ARR for the reconstruction of 61 bone deficient acetabula, during revision THA in 56 patients. 54 patients were female and 2 male of a mean age of 60 years, ranging from 32 to 81 years. In the 32 right and 29 left hips, the etiology of the revision procedure was failed primary THA due to aseptic loosening in 48, failed revision THA in 12 cases and persistent dislocation of the prosthesis in 1 case. Revision THA took place 1 to 25 years (mean 12 years) after the previous THA. All cases were characterized by the presence of type III bone defects, according to the Paprosky classification18.

Acetabular reinforcement rings
We have used two different types of ARRs. The Burch-Schneider antiprotrusio cage (Protek AG, Bern, Switzerland) was implanted in 14 cases and the Muller ring or it's modification, Ganz ring (Protek AG, Bern, Switzerland), in 47.
The Burch-Schneider ring, originally designed by Burch and modified by Schneider, is made of rough-blasted titanium and can provide fixation to both ilium and ischium, by means of its 2 malleable flanges.
The Muller ring, made of grit-blasted titanium, has a superior flange for fixation to the ilium and the modified Muller ring (Ganz ring) has, in addition, a malleable hook that, when placed beneath the teardrop, can assist for the proper positioning of the implant but adds little to the stability.
Fixation of the superior flanges of both types of rings is accomplished by screws, whereas the inferior flange of the Burch-Schneider ring can be either driven into the ischium, or stabilized by screws.

Figure 2 A. Preoperative radiograph of a 32 year-old man with history of Legg-Calve-Perthes disease just before revision THA. B. Postoperative radiograph. C. At 5.5-years follow-up.

Operative technique
All procedures were performed by the senior author (T.X.). A posterior approach was used in all cases. The failed acetabular component, cement and the existing membrane were carefully removed and the integrity of the acetabulum was assessed. The extent and location of bone defects was evaluated in order to confirm indicative preoperative radiological findings and the decision to implant an ARR was validated.
Residual bone surfaces were roughened with a reamer, when possible, taking care not to compromise further the deficient bone stock. Osteophytes were removed from the superior lip to improve contact of the ring with the floor of the acetabulum. We used morsellized cancellous allograft for filling of the existing bone defects in all cases. Bone allografts were obtained from femoral heads, stored at 32[o] C at the bone graft bank of our department, after testing the grafts for culture of microorganisms and screening the donor patients for the presence of hepatitis B, C and human immunodeficiency viruses. One to two femoral heads were needed, which were supplemented by bone substitutes in 9 cases.
Implantation of the ring followed. The sizes that we used were 46-58 mm for the Muller rings and the standards 44 mm and 50 mm for the Burch-Schneider rings. We always tried to achieve the best possible contact of the ring with the reconstructed acetabulum and then, the implant was secured to the native bone by screw fixation of the superior flange to the ilium. We used 2-5 cancellous, fully threaded, 6.5 mm screws to stabilize the ring in place by decreasing shearing forces. Screw placement should be undertaken with caution to avoid injury to vital intrapelvic structures and should take place at the posterosuperior or posteroinferior quadrant25, which, in addition to being more safe, offers better bone stock. In cases of Ganz rings the hook was placed beneath the teardrop, whereas in Burch-Schneider rings we tried to fix the inferior flange either by screws or by inserting it inside the ischium. However, this was not always possible.
Having established firm anchorage of the ARR to the pelvis, we proceeded with implantation of the polyethylene socket, which was cemented into the ring. This allowed for optimal orientation of the socket, regardless of the relative ring position, which may be influenced by the acetabular bone deficiency.
Postoperatively, prophylactic antibiotics were administered intravenously for 2 days, as well as an anticoagulation regimen of low-molecular-weight heparin subcutaneously for 3 weeks, followed by low-dose aspirin per os for 2 months. Mobilization of patients was variable, according to the intraoperative findings and the stability of the reconstructed acetabulum, as assessed by the surgeon. Usually, patients were instructed to walk with minimal weight bearing, supported by 2 crutches. Gradual increase of the weight led to full weight bearing after an average period of 4 months.

Follow-up
All patients have been followed-up for a minimum period of 2 years. Mean follow-up time was 6 years (2 to 13years) and was longer for patients with Burch-Schneider rings compared to the ones with Muller rings (7 years vs. 5.5 years).
Patients were evaluated both clinically and radiologically at 1.5, [3, 6] 12 months postoperatively and yearly thereafter. The clinical outcome was assessed with the Merle d'Aubigne-Postel scale16, which evaluates the parameters of hip range of motion, pain and walking ability, that determine the functional condition of the patient. Each parameter is graded in a 1 to 6 point scale, with the best results given a score of 6.
Radiological examination comprised plain anteroposterior and lateral radiographs of the operated hip. These were evaluated for presence of radiolucent lines at the ring-bone or polyethylene cup-cement interface of width greater than 2 mm, migration of the ring or the cup and mechanical failure of the ring or screws. In addition, incorporation of the bone grafts was assessed.

Figure 3 A. Radiograph at 13-years follow-up with Burch-Schneider ring. This patient has a total score of 17 according the Merle d'Aubigne-Postel scale. B. Complication: dislocation of the polyethylene cup of Muller ring.

RESULTS
Clinical evaluation showed marked improvement in all the parameters assessed. Specifically, range of motion improved from 3.6 to 5.3 points, pain from 2.7 to 5.5 and walking ability from 3.1 to 5.3 points.
Radiological examination revealed aseptic loosening of 2 Burch-Schneider rings 28 months and 1 year after revision arthroplasty. The former, with severe medial migration, was temporally transformed to a hemiarthroplasty in combination with filling of the acetabular medial bone defect with morsellized cancellous allograft, but till now the patient denies new operation. The latter was revised with the use of a Muller ring. We also encountered in 2 patients dislocation of the polyethylene cup. Both cups were fixed into Muller rings, one of them, 17 months postoperatively, required cemented fixation of a new cup plus a smaller head (22 mm). The other one, 7 years postoperatively, was also managed with cemented fixation of a new cup but after 2 years, because of another dislocation of the polyethylene cup a new acetabular reconstruction was undertaken with Symbios ring. Two dislocations occurred one in a case of a Muller ring after a fall and the other, 2 years after revision with Burch-Schneider ring, which were treated conservatively with reduction under anesthesia.
Thus, after a mean time of 6 years, 57 of 61 acetabular reconstructions (93.5%) were considered successful, with an accompanying satisfactory clinical outcome.
Intraoperative complications consisted of an ischium fracture during insertion of the inferior flange of a Burch-Schneider ring to the bone and a femur fracture. No injury to nerves or vessels took place, except for two sciatic nerves neurapraxias that resolved completely. Traction during the revision procedure was responsible for the neurapraxias. There was one superficial infection that resolved with antibiotic administration. No deep vein thrombosis or pulmonary embolism was apparent. Graft incorporation took place in all cases at an overage time of 4 months.

DISCUSSION
The principles of acetabular reconstruction comprise restoration of the center of rotation to the anatomical position, establishment of normal joint mechanics and achievement of adequate containment and rigid initial fixation of the component to host bone. Acetabular bone defects may preclude the above, according to their extent and location. Therefore, reestablishment of structural integrity of the acetabulum is of paramount importance.
Treatment options that have been proposed include filling of the defect with cement, bipolar prostheses, elevation of the hip center of rotation, massive bulk allografts and ARR. However, no single technique appears to provide a solution to the full spectrum of possible acetabular defects.
Filling of the defect with cement has been associated with unacceptably high rates of failure. Jasty and Harris[12] reported a 75% loosening rate at less than 7 years, when a 1 cm or larger medial wall defect was present. Reports on bipolar prostheses used with bone grafts showed high rates of migration, up to 85% at 6.5 years and failure[3,17]. Elevation of the hip center of rotation to improve coverage of the component carries increased risk of aseptic loosening of both the acetabular and the femoral component, higher rates of dislocation and poor abductor mechanics[26].
Massive bulk allografts may provide initial support, but their long-term durability is uncertain. Justy and Harris13 reported a 32% failure rate at 6 years, which further increased to 58%, when grafting exceeded two thirds of the socket, and concluded that structural allografts provide only a short-term solution. Pollock and Whiteside[21] found that only 35% of uncemented acetabular components placed against structural allografts did not change position after 2-5 years and, based on the technically easier revision on a previously placed allograft, stated that these cases should be viewed as 2-stage procedures. Paprosky and Magnus[20] noted that, when superior migration of the previous component was more than 2 cm and the teardrop and Kohler line were not intact, failure was likely at 5 years. Moreover, Paprosky et al[19] observed a 100% failure rate at 3-9 years, in cases where the cup rested primarily on the allograft. Interestingly, in the series of Garbuz et al7 the best results were achieved when allografts were combined with ARR and this combination was recently proposed by another study as well[2].
By comparison, ARR have provided consistently satisfactory results in most series. Fuchs et al[6] report a 3% loosening rate at a 2-5 year follow-up and no re-revisions. Rosson and Schatzker[22] had a failure rate of 10% at 5 years and Haentjens et al[10] of 7% at almost 8 years. At a same follow-up time, Gill et al[8] had further revised only 5 of 63 Burch-Schneider rings. In another study[27] the probability of 10-year survival of acetabular reconstruction with Muller rings was calculated to be 80%.
In our patients we observed a 93.5% survival of the acetabular reconstruction with substantial clinical improvement. Moreover, authors who utilized ARR in other clinical conditions, such as primary THA for congenital dislocation of the hip9, or in mixed series of primary and revision THA15 report similarly good results for the acetabular reconstruction achieved.
The good outcome, reported in all these series, is the result of the satisfactory mechanical stability that can be achieved by screw fixation of the ring to the pelvis, when biological fixation of the implant to host bone is unlikely. The ARR retains the center of hip rotation to the anatomical position and joint biomechanics in optimal condition. In addition, it diverts mechanical stresses away from the underlying bone graft and distributes them to the periphery, therefore facilitating incorporation of cancellous bone and protecting allografts from early mechanical failure and leading to augmentation of the bone stock.
At early stages of development of the technique, cement was utilized to fill the existing bone defects and provide additional fixation of the ring to the pelvis. Nowadays, screw fixation alone is preferable and bone grafts are used for the bone defects present. Although Berry and Muller[1] found no difference between the 2 methods, Rosson and Schatzker[22], on the other hand, conclude that bone grafting under the acetabular ring gives better results than cement, with a failure rate of 6% versus 13% and presence of circumferential radiolucencies in 2% compared to 39% of the surviving hips. Gill et al9 characteristically state that, when an acetabular ring is used the only role for cement is fixation of the cup to the metal ring. In our cases we had 2 dislocations of the polyethylene cup, both in reconstructions with Muller ring. None were combined with differently oriented components, an explanation given by Haentjens[10]. In our opinion, this is probably due to the smooth surface of Muller ring and the insufficient cement mantle. We also prefer the bone grafting technique. In our series, incorporation was complete in all cases and this contributed to the successful outcome.
However, aspects of the operative technique should be taken into consideration. Reconstruction of the bone deficient acetabulum is demanding and necessitates extended experience in hip surgery. Optimal placement of the ring is not always possible, especially with the Burch-Schneider one. The greater dimensions of this ring permit reconstruction of extended defects, however, the deformation of the malleable superior and inferior flange to achieve good contact with the ilium and ischium, respectively, may lead to fatigue and failure of this titanium-made implant. In addition, fixation of the inferior flange may not always be possible. Rosson and Schatzker[22] made no attempt to secure it to the bone, whereas Berry and Muller[1] inserted it into the ischium and warned for attention to the sciatic nerve. Placement of the screws should begin close to the central portion of the ring and not at its periphery. Otherwise, the ring may be tilted and intimate fitting of the ring to the grafted acetabulum may be not possible or the superior flange may be submitted to increased stress.
As far as choice of the implant is concerned, we observed better survival rates with the Muller ring: 96% versus 86% for the Burch-Schneider one. However, the results of these 2 devices cannot be directly compared. The Burch-Schneider ring was the one to be used initially in our practice. Therefore, there is a longer follow-up time of 7 years, compared to 5.5 years for the Muller rings and the effects of the learning curve cannot be excluded.
Long-term studies are awaited to evaluate in a more reliable way the performance of the ARR in the reconstruction of acetabular bone defects and answer questions about their fate after long follow-up periods. Moreover, new generation devices, which may be hydroxyapatite or porous-coated, will allow for additional biologic fixation to improve results.
In conclusion, ARRs can be of value in the reconstruction of the bone deficient acetabulum, in revision THA, with satisfactory mid-term results.
Careful preoperative planning, meticulous surgical technique and continuous effort to improve implants will aid in the management of the increasing number of patients with the complex problem of acetabular bone defects at revision THA.

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Mailing address:
Xenakis Theodoros
Professor in Orthopaedics
Department of Orthopaedic Surgery,
University of Ioannina
Ioannina