S 04/10 Polar Cup

S 04/10 "Mittelfristige klinisch-radiologische Ergebnisse nach Polar-Cup Implantation. Birgt der Einsatz eines Dualen Mobilitätssystems in der Primärversorgung der Coxarthrose Nachteile?"

Dr. med. Wolfram Steens, Klinikum Vest/Paracelsus Klinik Marl

(abgeschlossen 11/2012)

A dual-mobility acetabular component consists of a large, fixed, porous-coated acetabular component and a bipolar femoral component. This configuration provides a stable, well-fixed implant platform against bone and 2 articular interfaces, a large polyethylene surface directly apposed to a highly polished metal implant, and a standard-sized (28- or 32-mm) femoral head captured within polyethylene. The concept has extensive laboratory and clinical support and the modular dual-mobility shell offers surgeons substantial flexibility to avoid the issue of hip instability in primary arthroplasty especially in elder patients.
As the long-term durability of these implants is unknown, wear rates of a dual-mobility design with the current generation of highly cross-linked polyethylene might result in higher wear rates if the biomechanical principle does fail. The purpose of this retrospective study is to analyze if a higher rate of osteolysis as a consequence of debris can be seen 5 years after surgery.
Patients and Methods
129 of 250 primary total hip replacements with use of a Polar Cup in 2005/6 were included. 7 cases with exchange of the acetabular component in the meantime were excluded from the study. 2 Patients refused the xray examination and were also excluded from the study. 8 Patients had died in the meantime. 38 Patients had a change of residence and were lost for follow- up. 66 Patients declined a follow-up examination basically. There were 41 male and 83 female patients with a mean age of 67.3 years at the time of surgery. The mean follow- up was 4.98 ± 0.6 years.
15 cases had a cemented and 114 had a cementless Polar Cup. 24 cases had a cemented and 105 had a cementless femoral component. The right hip was replaced in 77 cases and the left hip in 52. 5 cases had a bilateral replacement.
Follow- up examination included a standard view of the pelvis, a Lauensteins view of the replaced hip and a physical examination. In addition patients were asked to answer 3 different clinical scores (Harris Hip Score, WOMAC Score, SF-36 Score).
Radiological analysis of the prostheses showed a significant increase (p=0.013) of periarticular ossifications according to Brooker. 2 cups showed an osteolysis of less than 1 mm in all 3 zones of the DeLee- Charnley Score. 8 cups had an osteolysis in 2 zones of less than 1 mm. 5 cups had an osteolysis in zone 1 less than 1 mm.
Radiological analysis of the uncemented stems showed a bone ingrowth in 96 of 105 cases and a fibrous- stable ingrowth in 9 cases according to Engh. 16 stems showed a stress shielding stage I. 20 cemented stems had no signs of loosening and 4 had a pistoning of the cement mantle stage 1b according to Gruen. One of the latter had had an intraoperative fracture. Radiographic follow-up revealed neither evidence of component loosening nor migration or polyethylene wear.
The mean result of the Harris Hip score was 95.9 (SD 4.79) points. The WOMAC Score revealed a mean result of 43.3 (SD 36.0) points.
The Short Form Health Survey (SF-36) had a mean score of 73.94 (SD 19.33).
7 Patients had a luxation within the time of follow-up. In 2 cases a luxation occurred twice.
Use of the bipolar component was successful in achieving stability in primary hip replace- ment. It has to be observed carefully if there is any disadvantage of this device because of higher polyethylene wear in the long-term follow-up. Proponents of the dual-mobility liner cite improved wear characteristics over conventional THA; however, few studies support this proposition. Retrieval studies have reported that the dual-mobility liner does not avoid wear or osteolysis. Theoretically, it is inconceivable that wear would be diminished with an additional articulation with a huge surface area, where the differential hardness has been reversed to a soft-on-hard bearing. So far this mid-term study does not give any evidence of a higher wear rate leading to increasing osteolyses or implant loosening. The clinical outcome of this study does not reveal a situation over conventional THA.