DURIBILITY OF A NON-CEMENTED, HEMISPHERICAL, RIM-FIT, HYDROXYAPATITE COATED ACETABULAR COMPONENT

Introduction Acetabular fixation is one of the major factors affecting long-term longevity and durability of total hip arthroplasty (THA). Limited data exist regarding mid-term performance of modern non-cemented rim-fit cups with HA coating. The aim of this study was to assess the minimum 5 year clinical and radiographic performance of PSL cups. Therefore we retrospectively analyzed results of this component in patients that had adequate followup from a prospective institutional database. Materials and Methods A retrospective analysis of a prospective database was performed to identify patients that underwent non-cemented THA between 2003 and 2007. 223 primary THA (210 patients) were performed by single surgeon via posterolaeral approach using a grit-blasted, HA coated rim-fit design and highly cross-linked polyethylene and were followed with minimum 5 years. The mean age was 62.5 years ± 10.8. The majority of the stems were non-cemented (87%) and the majority of the femoral heads were metal (75%), 22- or 28-mm diameter. 72% of the cups were solid and 28% were multi-hole. Clinical assessment included the Hospital for Special Surgery (HSS) hip score [18] at final follow-up, and Kaplan-Meier survivorship. All patients received pre- and post-operative anteroposterior (AP) weight bearing pelvis radiograph as well as a false profile view of the hip. Cup positioning was analyzed using the EBRA software (Einzel-Bild-Roentgen-Analysis; University of Innsbruck, Innsbruck, Austria) for functional abduction angle, anteversion, and cup migration. Osseointegration was assessed on the DeLee and Charnley's zones on both AP and false profile views. Osseointegration was defined based on the following characteristics: 1. presence of Stress Induced Reactive Cancellous Bone (SIRCaB), where new bone condensation (not apparent on preoperative radiographs) was present at the load bearing area of the cup (Figure 1) 2. presence of radial trabeculae that project in continuum from the shell into the pelvis, suggesting integration of the trabecular bone onto the metal surface at the load bearing area, (Figure 2) 3. absence of radiolucency. Radiolucency was determined by radiolucent lines that were at least 1–2 mm wide and were seen in sequential radiographs, not apparent on the initial postoperative radiograph. Linear and rotational migration was defined as > 3 mm or > 5°change in the cup position, respectively, as measured on serial radiographs. Any changes in cup position or presence of circumferential radiolucencies were considered as loosening. Results The average duration of follow-up was 6.2 ± 1.1 years (5 – 10 years). The mean HSS score was 34.8 ± 5.0 (19 – 40). There was an overall revision rate of 3.6% (8 cases) with Kaplan-Meier survivorship for all causes of 96.4% (95% CI: 0.92 – 0.98). There was one periprosthetic femur fracture. One stem was revised for fracture at the truniun/neck junction. There were 2 dislocation (0.9%); in one hip the cup was revised and the other was treated with a constrained liner. In 3 THAs (1.3%), stems were revised for loosening/failure of osseointegration (2 non-cemented stems, 0.9%) and osteolysis (one cemented stem, 0.4%). One THA (0.45%) underwent two stage revision for treatment of periprosthetic infection. There were no revisions for cup loosening or osteolysis or ceramic head fractures. The Kaplan-Meier survivorship for cup revision for any failure was 99% (95% CI: 0.96 – 0.99) and for mechanical failure was 100% (95% CI: 0.97 – 1). In radiographic analysis, the average functional cup abduction angle and anteversion were 41.7° ± 5.2 (range, 30 – 52) and 16.8° ± 6.1 (range, 4 – 30). 96% of the cups were within the safezone of Lewinnek. There were no migration or change in cup position in any cases. Presence of SIRCaB and radial trabeculae in all 3 zones were seen in 47% and 93% of cups, respectively; both were most prevalent in Zone 1. The absence of radiolucent line was observed in 96% of cases. In 161 THAs (72%), no screws were used due to excellent initial stability. Detail radiographic osseointegration assessment in the non-screw fixation group (as compared to the THAs with screw fixation) showed significantly higher incidence of SIRCaB (49% versus 39.7%, p=0.05) and radial trabeculae (97.5% versus 94.7%, p=0.001). There was also significantly less radiolucent lines in the non-screw fixation group (p=0.001). Discussion No evidence of radiographic failure to osseointegrate was found in this study as evidenced by absence of radiolucency, evidence of radial trabeculae, and a reactive condensation of new bone to the well-fixed acetabular shell. Interestingly, we found that the solid designs had significantly better osseointegration when compared to multi-hole designs. In this single surgeon series with mid-term follow-up reiterates that the HA-coated hemispherical rim-fit acetabular component has excellent radiographic osseointegration, clinical outcomes and high survivorship for mechanical failures.