The Dual-Mobility POLARCUP: First Results From a Multicenter Study

Abstract

Dislocation is a leading cause of revision after total hip arthroplasty (THA). To address this risk, dual-obility technology was developed, which features a mobile polyethylene liner locked onto a femoral head and articulating in a metallic acetabular shell. This study reports clinical outcome data after implantation of the third-generation POLARCUP Dual-Mobility System (Smith & Nephew Orthopaedics AG, Rotkreuz, Switzerland). Primary THA procedures were performed in 150 patients. At 7.1 years, cumulative cup survival according to Kaplan-Meier was 97.4%. The mean Postel-Merle d’Aubigne score improved from 8.9 to 17.1 during the investigation. Two cups were revised at 5.4 and 6.4 years because of aseptic loosening. No dislocations were observed during follow-up. The current results confirm excellent early to midterm clinical outcomes for the POLARCUP Dual-Mobility System.

Total hip arthroplasty (THA) has been established as an extremely successful procedure for the treatment of advanced hip osteoarthritis. However, limitations in contemporary surgical technique and implant design continue to yield less than optimal clinical success. In particular, postoperative hip dislocation remains a leading cause of hip revision.^1,2This complication is reported to occur in upward of 3.9% of all primary THA patients,^1,3,4with rates as high as 14.4% after revision THA.^1,5

To address dislocation risk after THA, the dual-mobility concept was introduced by Bousquet and colleagues in 1976.⁶ With dual mobility, a freely mobile polyethylene liner articulates within a metallic acetabular shell. Moreover, the design of the liner allows it to lock onto the femoral head, limiting dislocation. The femoral head rotates within the liner as with traditional designs, with the exception that during extended range of motion (ROM), the liner is able to rotate within the cup. This configuration allows for the reduced dislocation risk and improved ROM often limited to large femoral heads, while simultaneously supporting low friction and reduced implant wear.^6-11

Long-term outcomes for early-generation dual-mobility cups have been promising. In a retrospective study reporting 10-year outcome data for 106 hips, Philippot et al⁷ have reported a 94.6% survival rate. Similar results were obtained by Farizon⁸ et al, who calculated a survival rate of 95.4% after 12 years in their cohort of 135 cases operated between 1980 and 1981. In a multicenter retrospective study reporting 17-year follow-up data for 438 hips, Philippot et al¹²reported a prosthesis survival rate of 89.2%, with a cup survival rate of 96.3%. In that study, there were 13 cases of aseptic loosening, 23 intraprosthetic dislocations, and 7 polyethylene revisions due to wear. Given the relative clinical success, the use of dual-mobility cups has been recommended for primary THA, particularly for patients at risk of postoperative hip instability.^7,8,12

Despite excellent clinical results with early-generation dual-mobility hips, there are still design limitations. When the implant moves through full ROM, contact is made between the polyethylene liner and the neck of the femoral stem. This contact has the potential to erode the mobile liner, leading to revision.⁶To address this complication, the POLARCUP Dual-Mobility System (Smith & Nephew Orthopaedics AG, Rotkreuz, Switzerland) was developed with a true third articulation, consisting of a mirror-polished femoral stem that makes contact with a chamfered liner adapted to the shape of the stem.⁶

Although this third-generation dual-mobility design is quite promising, clinical outcome data are necessary to evaluate its clinical performance. Therefore, the purpose of our investigation was to assess the safety and efficacy of the POLARCUP Dual-Mobility System, as determined by clinical, radiologic, and survivorship analyses. To our knowledge, this is one of the first complete reports of midterm clinical outcome data for this implant.

Materials and Methods

Beginning January 1, 2001, 150 consecutive primary THA procedures were performed by the investigators. The study was a multicenter retrospective analysis of this initial patient cohort. Functional performance of the hips was measured with the use of the Postel-Merle d’Aubigne score, and the radiographic outcome was assessed according to DeLee & Charnley on standard anteroposterior (AP) and lateral images. Ectopic ossifications were classified according to Brooker. Survival of the cups was studied using the method of Kaplan-Meier, with a 95% confidence interval and cup revision due to aseptic loosening as endpoint.

Figure 1: POLARCUP Dual-Mobility Hip Prosthesis.

The cohort included 61 men and mean patient age at the time of surgery was 69.0 ± 11.2 years (range, 40.3-90.5 years). Mean body mass index (BMI) of the patient sample was 27.2 ± 5.2 (range, 15.6-43.6). Of the patients, 131 had a diagnosis of primary osteoarthritis (87%), with 13 patients presenting with femoral head necrosis (8.7%) and 6 patients with secondary osteoarthritis (4%). Eighty-three patients had a preoperative Charnley classification of A, whereas 56 were classified as B, and 11 classified as C. Preoperative Postel-Merle d’Aubigne score was 8.9 ± 2.3 (range, 3-15).

All patients were implanted with POLARCUP Dual-Mobility acetabular cups (Figure 1) made of stainless steel (ORTHINOX ISO 58-32-9) with a hydroxyapatite (HAP ISO 13779-1 et 2) coating. A posterior surgical approach was used in all cases. Of the cups, 140 were fixed without the use of screws, with 10 requiring the use of screws. One hundred seven hips were implanted with metal-on-polyethylene articulations, and 43 utilized a ceramic-on-polyethylene coupling. Size 28 femoral heads were implanted in 81 patients, and size 22 heads were implanted in the remaining 69 patients. The cups were combined with noncemented stems in 106 hips and with cemented stems in 44 hips. The Corail stem (Depuy Orthopaedics, Warsaw, Ind) was used in 46 cases, with the Académie (Sanortho, Wilmington, North Carolina) used in 26, the Aura II (Biomet, Warsaw, Ind) in 24, the Symetric (Aston, Saint Etienne, France) in 22, the Viso (Serf, Decines, France) in 12 cases, and a Zweymuller stem in 8 cases. Twelve patients received various stems. After implantation of the POLARCUP, AP position was found to be between 40º and 50º for 125 patients, greater than 50º for 23 patients, and less than 40º for 2 patients. Intra- and perioperative complications included 6 cases of deep-vein thrombosis, 2 cases of edema, and 1 case each of embolism, neurologic problems, and phlebitis.

Results

At time of the investigation, 20 patients of the original cohort had died from causes unrelated to the intervention. Seven patients were lost to follow-up after surgery, and 2 cups had been revised, leaving 121 patients in the study. Mean follow-up for patients analyzed was 6.2 ± 0.8 years (range, 3.3 -7.1 years). The Postel-Merle d’Aubigne score improved during the investigation to 17.1 ± 1.2 (range, 12-18) at follow-up. In regard to radiographic results, there was no evidence of periprosthetic osteolysis or radiolucencies around the cup (Figure 2). Moreover, cup position remained unchanged for all patients during follow-up. One hundred (100) hips had class 0 Brooker classification, whereas 16 hips had class 1 and 5 hips had class 3.


Figure 2: A AP radiographs of A, POLARCUP fixed with one screw through the flange without the pegs in combination with a cemented stem and B, POLARCUP fixed without screws or pegs in combination with an uncemented POLARSTEM.

During follow-up, there were 4 observed complications, including 1 case each of infection, nerve paralysis, pelvic fracture, and femoral revision due to persistent thigh pain. Two cups were revised for aseptic loosening after 5.4 and 6.4 years, respectively, the first occurring after the patient had undergone chemotherapy for cancer.

With cup revision due to aseptic loosening as an endpoint, Kaplan-Meier survival rate at 7.1 years was 97.4% (95% CI: 89.4-99.4) (Figure 3).

Figure 3: Kaplan-Meier survival analysis

Figure 3: Kaplan-Meier survival analysis at 7.1 years with cup revision due to aseptic loosening as endpoint: 97.4% cumulative survival (95% C.I.: 89.4-99.4).

Discussion

This investigation reports midterm clinical outcome data after THA using the POLARCUP Dual-Mobility Hip System. At a mean follow-up of 6.2 years postoperatively, the current third-generation dual-mobility design appears to be performing favorably.

As indicated previously, the primary design feature of dual-mobility cups is reduced risk of dislocation during full ROM. The results of our investigation support this feature because there were no observed dislocations during follow-up. This finding supports data published previously for the POLARCUP. After the implantation of 450 prostheses for osteoarthritis, Fiquet and Noyer⁶reported a final dislocation rate of 0.22%. These results are particularly favorable when compared with published dislocation rates for THA using conventional cups, which can be upward of 3.9%.^1,5

In addition to reducing dislocation risk and improving ROM, dual-mobility has been reported to support low friction and reduced implant wear.^6-11 It is well understood that the generation of polyethylene wear particles is positively correlated with periprosthetic osteolysis.^13,14 Although there was no direct measurement of wear in our investigation, the presence of osteolysis would suggest premature wear as a primary factor. However, at follow-up, there was no evidence of osteolysis or implant loosening for any of the cups. As such, the generation of wear particles after implantation of the POLARCUP may be relatively low. Moreover, overall radiographic results were excellent for the current patient cohort. Although these data do suggest low wear, additional research is necessary to support this observation.

Both clinical function and implant survival rate in this study compare favorably with previous reports for dual-mobility systems. The mean Postel-Merle d’Aubigne score improved from 8.9 to 17.1, and similar improvements were reported by Philippot et al⁷ (7.1 to 15.8 at final follow-up at 10 years). At a follow-up of 7.1 years, implant survival in this study was 97.4%. Retrospective studies published by Farizon et al ⁸ and a more recent series by Philippot et al ⁷ report survival of early-generation dual-mobility cups of 95.4% after 12 years and 94.6% at 10 years, respectively. Although Farizon did not report aseptic loosening, Philippot reports 2 cup revisions due to aseptic loosening in the initial 106 cases. These data suggest that outcomes for the third-generation POLARCUP implant compare favorably with early-generation dual-mobility systems.

The results of this investigation suggest excellent early to midterm clinical outcomes for the POLARCUP Dual-Mobility System. After an assessment of midterm follow-up, the use of this device can be recommended as an excellent option for primary THA. Additional research will be performed to assess long-term clinical safety and efficacy of the POLARCUP.

References

Langlais FL, Ropars M, Gaucher F, Musset T, Olivier Chaix. Dual mobility cemented cups have low dislocation rats in THA revisions. Clin Orthop Relat Res. 2008;466:389-395.
Brytom S, Espehaug B, Furnes O, Havelin LI. Femoral size is a risk factor for total hip dislocation (a study of 42987 primary THR from the Norwegian Arthroplasty register). Acta Orthop Scand. 2003;74:514-524.
Bourne RB, Mehin R. The dislocating hip: What to do, what to do. J Arthoplasty. 2004;19(4);S111-S114.
Masonis JL, Bourne RB. Surgical approach, abductor function and total hip arthroplasty dislocation. Clin Orthop Relat Res. 2003;405:46.
Phillips CB, Barrett JA, Losina E, et al. Incidence rates of dislocation, pulmonary embolism, and deep infection during the first six months after elective total hip replacement. J Bone Joint Surg Am. 2003;85:20-26.
Fiquet A, Noyer D. “Polarsystem” dual mobility hip prosthesis and “minimally invasive surgery” (MIS). Int Surg. 2006;22:1-5.
Philippot R, Adam P, Farizon F, Fessy MH, Bousquet G. Survival of cementless dual mobility sockets: 10-year follow-up. Rev Chir Orthop. 2006;92:326-331.
Farizon F, De Lavison R, Azoulai JJ, Bousquet G. Results with a cementless alumina-coated cup with dual mobility. Int Orthop. 1998;22:219-224.
Guyen O, Prabhakar PP, Schultz FM, Carret JP, Bejui-Hugues J, An KN. Tripolar versus conventional total hip implants: range of motion to impingement test. Poster presented at: 71st Annual American Academy of Orthopaedics Surgeons Meeting; March 24-28, 2004; San Francisco, Ca.
Guyen O, Chen QS, Shultz F, Berry DJ, Bejui-Hugues J, An KN. A tripolar hip implant dislocation test. Poster presented at: 72nd Annual American Academy of Orthopaedics Surgeons Meeting; March 23-27, 2005; Washington, DC.
Adam P, Farizon F, Fessy MH. Dual articulation retentive acetabular liners and wear: surface analysis of 40 retrieved polyethylene implants. Rev Chir Orthop. 2005;91:627-636.
Philippot R, Farizon F, Camilleri JP, et al. Survival of dual mobility socket with a mean 17 years follow-up. Rev Chir Orthop. 2008;94:43-48.
Orishmo KF, Claus AM, Sychterz CJ, Engh CA. Relationship between polyethylene wear and osteolysis in hips with a second-generation porous-coated cementless cup after seven years of follow-up. J Bone Joint Surg Am. 2003;85:1095-1099.
Ingham E, Fisher J. The role of macrophages in osteolysis of total joint replacement. Biomaterials. 2005;26(11):1271-1286.

Authors

Dr Bauchu is from Polyclinique des Minguettes, Vénissieux, France; Dr Bonnard is from Clinique du Tonkin, Villeurbanne, France; Dr Cyprès is from Clinique du Renaison, Roanne, France; Dr Fiquet is from Infirmerie Protestante, Caluire, France; Dr Girardin is from Centre Hospitalier Général, Montbrison, France; and Dr Noyer is from Cabinet Médical “le Caducée,” Bourgoin-Jallieu, France.

Drs Bauchu, Bonnard, Cyprès, Fiquet, and Noyer are consultants for Smith & Nephew Orthopaedics AG. Dr Philippe Girardin has no relevant financial relationship to disclose.

Correspondence should be addressed to A. Fiquet, MD, Infirmerie Protestante, 1-3 chemin du Penthod, 69641 Caluire Cedex, France.