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Int Orthop. 2009 June; 33(3): 659–664.
Published online 2008 June 26. doi:  10.1007/s00264-008-0569-0
PMCID: PMC2903106

Language: English | French

Long-term results of the Wagner conical screw cup

Abstract

Thanks to its high primary stability, the concept of threaded cups has gained wide acceptance. The aim of this study is to report the long-term results of the Wagner conical screw cup. Between January 1993 and December 1994, 212 conical screw cup were implanted; 118 implants were clinically and radiographically followed up after a mean of 12.1 years. The Merle d’Aubigné score improved from a preoperative mean of 8.8 to 16.6 postoperatively. Four revisions were performed for heterotopic ossification. Three revisions were done to treat deep infection. Two isolated cup revisions and three more complete total hip joint revisions were required due to aseptic loosening. The overall survival rate at 12 years in this study was 93.2% (110/118); for aseptic loosening alone it was 95.8% (113/118). The Wagner conical screw cup by yields very good long-term results.

Résumé

Du fait de leur stabilité primaire, le concept des cupules vissées a été largement utilisé. Le but de cette étude est de rapporter les résultats à long terme de la cupule vissée conique de type Wagner. Matériel et méthode: entre 1993 et 1994, 212 cupules de ce type ont été implantées, 118 implants ont été revus radiographiquement après un délai de 12,1 ans. Résultat: le score de Merle d’Aubigné a été amélioré passant de 8,8 en pré-opératoire à 16,6 en postopératoire. 4 révisions ont été réalisées pour ossifications hétérotypiques. 3 révisions ont été réalisées pour des problèmes d’infection. 2 révisions isolées de cupules et 3 de hanches totales ont été nécessaires du fait d’un descellement aseptique. Discussion, le taux de survie globale à 12 ans dans cette étude est de 93,2% (110/118). Si l’on considère comme référence le descellement aseptique, ce taux est de 95,8% (113/118). La cupule de Wagner conique vissée permet de bons résultats à long terme.

Introduction

In the 1970s, the limitations of cemented hip replacement in young and active patients became gradually known. Thus, the goal was to find some kind of “biological fixation”, which led to the introduction of cementless prostheses. Two of the first cementless prostheses were the Lord and the AML prostheses. Thanks to its high intraoperative primary stability, the concept of the threaded cup has gained wide acceptance in German-speaking regions. Between 1970 and 1980, cementless threaded cups with a smooth surface were widely used. By the end of the 1980s, however, some of the systems of the first generation showed weaknesses which entailed a high rate of aseptic loosening [219]. Threaded cups of the first generation are characterised by untreated or smooth surfaces (ceramic, polyethylene, cobalt-chrome or titanium). Second-generation threaded cups, such as the Wagner, have rough, structured surfaces, and they are usually made from titanium. Threaded cups of the third generation use modular metal, ceramic and modified polyethylene (PE) inserts [2, 3]. In addition to the implant type used, the long-term success of threaded cups is also dependent upon the cup position. The correct position is decisively influenced by the screw-in behaviour of threaded cups, which is dependent on the thread shape.

Until now, almost no long-term results for threaded cups have been available [1, 4, 68, 10, 19]. The aim of this study is to report the long-term results of the Wagner conical screw cup.

Material and method

Between 1 January 1993 and 31 December 1994, 212 conical screw cups (Fig. 1) were implanted—for the most part by the surgeon who designed this implant, Prof. Heinz Wagner. To date, 118 cases have been clinically and radiographically reviewed after a mean in situ time of 12.1 years (min.: 10, max.: 14). The average age at the time of surgery was 50.9 years (min.: 26, max.: 70). The patient cohort included 38 men and 80 women. The indications for the implantation of the conical screw cup were dysplastic coxarthrosis (45), secondary arthrosis of the hip (29), rheumatoid arthrosis (5), necrosis of the femoral head (3), revision arthroplasty (32) and conversion from a hemi-endoprosthesis to total hip replacement (4).

Fig. 1
Wagner conical screw cup from above (a) and in cross section (b)

On the femoral side, 64 cases were treated with the cementless Wagner cone prosthesis, 27 with an cementless CLS stem, 15 with an cementless Wagner revision stem and 12 with a cemented Müller straight stem. The bearing surfaces were polyethylene/ceramic in 52 cases, metal-on-metal in 46 cases and polyethylene/metal in 20 cases; 82 cups were inserted with additional screw fixation, and 36 were anchored without the use of screws.

Previous operations included 5 Chiari pelvic osteotomies, 19 intertrochanteric corrective osteotomies, 4 Wagner-type spheric acetabular osteotomies, 17 resurfacing procedures, 15 total hip replacements, 4 hemi-endoprostheses and 9 displacements of the greater trochanter.

During their stay at the hospital, all patients were treated with low molecular weight heparin and compression stockings as prophylaxis against deep vein thrombosis. For 12 weeks, partial weight-bearing of 20 kg with the support of lower arm crutches was required. A clinical and radiographic follow-up examination was performed 3 months and 12 months after surgery, and, annually thereafter.

The clinical evaluation was rated according to the Merle d’Aubigné score. Standard X-rays in the anteroposterior (AP) position as well as in a Lauenstein position were taken to assess the radiographic results. Radiolucencies in the acetabulum were rated according to DeLee and heterotopic ossification according to the Brooker classification.

Results

The Merle d’Aubigné score improved from a preoperative mean score of 8.7 (min.: 3, max.: 14) to a postoperative mean score of 16.6 (min.: 7, max.: 18).

Early postoperative complications noted were a temporary lesion of the femoral nerve and the lateral cutaneous nerve of the thigh, and a recurring dislocation.

Four revisions were performed for heterotopic ossification that impaired joint function. Three implants were removed without substitution in order to treat deep infections. Two isolated cup revisions and three more complete total hip replacements were performed for aseptic loosening. The radiographic evaluation of the remaining conical screw cups revealed four radiolucencies in DeLee zone I, two each in zone II or III and three in zones I–III. Thirty heterotopic ossifications were observed (I: 15, II: 3, III: 5, IV: 7).

There was no clinical or radiographic difference between conical screw cups with or without additional screws.

Figure Figure22 shows the 14-year postoperative course of a conical screw cup after loosening of a Wagner resurfacing cup.

Fig. 2
a The AP X-ray of the left hip of a 49-year-old patient shows the pronounced loosening of a Wagner resurfacing cup with destruction of the acetabulum. b The X-ray examination 1 week after revision surgery shows the reconstruction of the acetabulum ...

Figure Figure33 represents the postoperative 11-year course of a CLS stem and a Wagner conical screw cup after necrosis of the femoral head.

Fig. 3
a The AP X-ray of the right hip of a 44-year-old patient shows advanced necrosis of the femoral head. b The AP X-ray 1 year after total hip replacement: CLS stem and Wagner conical screw cup with a PE/ceramic articulation. c On the AP X-ray after ...

Discussion

In the 1970s, after initial euphoria, it became evident that cemented hip replacement in young, active patients has its limitations. Thus, the goal was to find some kind of “biological fixation”, which led to the introduction of cementless prostheses. Two of the first cementless implants were the Lord and the AML prostheses [7].

Thanks to its high intraoperative primary stability, the concept of the threaded cup has gained wide acceptance in German-speaking regions. However, by the end of the 1980s, the first weaknesses of some of the first-generation systems started to show [10].

The screw-in behaviour and primary stability of threaded cups is dependent on the geometry of the thread. The individual thread geometries of threaded cups are very diverse and can be combined in variable manner. Current threaded cup models have a sharp thread or a flat thread with a depth of up to 3 mm, up to four thread pitches and a gradient of approximately 5 mm. When screwed into place, the sharp thread generates expansion or compression in the bone which leads to tension at the level of the thread. Effenberger was able to demonstrate that a sharp thread yields a high torsional moment and tilting stability. Locally, pretension on both sides is produced. The material as well as the surface influence osseous integration [2, 3].

Based on their clinical and radiographic analysis of 60 cementless threaded Lord cups after a mean of 6 years, Fernandez-Gonzales et al. [5] concluded that smooth threaded cups are not a viable alternative to cemented implants. Five cups were revised for aseptic loosening, and six more required revision, i.e. 18% of the implanted cups were found to be aseptically loose. They divided the stability of the threaded cups into stable, fibrous-stable and unstable; 26 cups (43%) could be classified as fibrous-stable.

For the threaded Lord cup, Grant and Nordsletten [7] published a cumulative Kaplan-Meier survival rate of 65% after 17.5 years. In their study of 479 Link type V threaded cups and 110 modified Link type V threaded cups “Bad Bramstedt”, Fink et al. [6] achieved similar results. The Link type V threaded cup showed migration of more than 3 mm and tilting of the cup by more than 5° in 73% of the cases and in 39% of the cases treated with the modified cup. The cumulative survival rate of the Link type 5 threaded cups after 15 years was 70.2%. It must be noted that the Lord as well as the Link type V cup are threaded cups from the first generation.

In their work about the threaded Weill ring or the Mecring, Aldinger et al. [1] reported a survival rate after a mean follow-up period of 12.4 years of 63.8% for the Mecring and 76.2% for the Weill ring.

Another first-generation threaded cup is the Parhofer-Mönch cup. Hendrich et al. [10] published a 10-year survival rate of 71.4%. The failure of threaded cups of the first generation—in spite of their high primary stability—is attributed to the lack of secondary osseointegration.

Thus, threaded cups of the so-called second generation have rough surfaces, either corund-blasted or structured. Epinette et al.’s work [4] is one of the few studies that document the long-term results of threaded cups of the second generation. They examined a series of 418 hydroxyapatite-coated threaded cups of the type Arc2f (Osteonics, Allendale, NJ, USA). After a mean of 10 years, 304 cups were available for evaluation. The cumulative survival rate was 99.4%.

In 2006, Reikeras and Gunderson published a cumulative survival rate of 91% after 16 years for a hydroxyapatite-coated threaded cup [16]. Pellengahr et al. [15] obtained similar results for the threaded ring Munich II model (second generation); they noted a significant improvement of the clinical and radiographic results in comparison to the threaded ring Munich type I (first generation).

The cementless Zweymüller threaded cup also belongs to the second generation. Grübl et al. [8] reported a survival probability after 15 years of 85% for this type of cup. In their study of the JRI-Furlong threaded cup, Robertson et al. [17] demonstrated that the concept of threaded cups of the second generation with rough surfaces yields good results. This type of threaded cup is hydroxyapatite-coated and has a polyethylene liner. The 14-year survival rate for this cup in combination with a PE inlay and a ceramic head was 98.0%. From this study, Robertson et al. concluded that this cup with the articulation partners mentioned is a good implant choice for young patients.

Havelin et al. [9] pointed out that the probability for the revision of threaded cups is dependent on the prosthetic bearings. After 8 years, the revision rate for the Tropic threaded cup was 6.1% for those with a metal insert and 2.8% for those with a ceramic insert.

In our own study, the survival rate for septic and aseptic loosening of the Wagner conical screw cup after a mean of 12.1 years was 93.2% (110/118), and isolated for aseptic loosening alone 95.8% (115/118). Three different prosthetic bearings were used. Polyethylene/ceramic was used in 52 cases, metal-on-metal in 46 cases and polyethylene/metal in 20 cases. This reduces the comparability of our study with the work done by others.

Another detail of the Wagner conical screw cup should be carefully analysed. The cup has a number of drill holes which can be filled without difficulty with bone grafts after implantation, for example in cases of protrusion coxarthrosis with its associated centrally located bone defects. The problem of backside wear is often criticised in this type of implant design.

In 1999, Volkmann et al. [18] described a revision rate of 4.7% after 6–8 years for the threaded cup Munich I type. Bone grafts in this cup type were inserted through a centrally located drill hole. At the level of this drill hole, the polyethylene inlay was in direct contact with the acetabulum, thus resulting in destruction of the bone and loosening. For this reason, the design was changed in 1991, and the central hole was sealed off.

In a recent study, Nieuwenhuis et al. [13] also examined the problem of drill holes in threaded cups. Their analysis of the Omnifit hip implant with a mean follow-up period of 60 months revealed a high incidence of acetabular osteolysis of 43%. They postulated a direct correlation between backside wear and the number of screw holes in a threaded cup. The osteolysis was generally localised in the area of the central hole of the threaded cup. As a consequence of their poor results, Nieuwenhuis et al. abandoned this cup design.

In our own work, the radiographic evaluation of the remaining threaded cups reviewed to date showed few radiolucencies (four in DeLee zone I, two each in zone I or II and three in zones I–III). In particular, we found no osteolyses in the area of the drill holes. As a second-generation threaded cup, the Wagner conical screw cup yields very good long-term results. The design as well as material of this cup type have proven their value and, with the proper indication, can be recommended for use.

We see the indication for this type of threaded cup in revision arthroplasty, dysplastic arthrosis of the hip and protrusion coxarthrosis. In revision arthroplasty, the Wagner conical screw cup is indicated if the bone defect after revision of a loose implant is less than 25% of the circumference of the acetabulum. The threaded cup can also be recommended with a good record of success for the treatment of dysplastic hips. Additional fixation with screws may increase primary stability; however, from our point of view, it is not mandatory. In cases of protrusion coxarthrosis, filling of bone defects through the screw holes does not pose any difficulties.

To assess the reliability of an implant, long-term studies are indispensable. The knowledge acquired by such work is vital for the successful development of new implants. In the meantime, the Wagner conical screw cup is no longer commercially available; the encouraging clinical and radiographic results with regard to cup design and material should be taken into account for the development of future threaded cups.

References

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