Comparison of canal fill and radiolucent line formation between two fully coated, hydroxyapatite tapered stems: a 2-year follow-up after total hip arthroplasty

Introduction: Comparison between fully hydroxyapatite (HA)-coated stems with differing geometry are lacking in the total hip arthroplasty (THA) literature. This study aimed to compare femoral canal fill, radiolucency formation, and 2-year implant survivorship between two commonly used, HA-coated stems. Methods: All primary THAs performed with two fully HA-coated stems (Polar stem, Smith&Nephew, Memphis, TN and Corail stem, DePuy-Synthes, Warsaw, IN) with a minimum 2-year radiographic follow-up were identified. Radiographic measures of proximal femoral morphology based on the Dorr classification and femoral canal fill were analyzed. Radiolucent lines were identified by Gruen zone. Perioperative characteristics and 2-year survivorship were compared between stem types. Results: A total of 233 patients were identified with 132 (56.7%) receiving the Polar stem (P) and 101 (43.3%) receiving the Corail stem (C). No differences were observed with respect to proximal femoral morphology. Femoral stem canal fill at the middle third of the stem was greater for P stem patients than for C stem patients (P stem; 0.80 ± 0.08 vs. C stem; 0.77 ± 0.08, p = 0.002), while femoral stem canal fill at the distal third of the stem and presence of subsidence were comparable between groups. A total of six and nine radiolucencies were observed in P stem and C stem patients, respectively. Revision rate at 2-year (P stem; 1.5% vs C stem; 0.0%, p = 0.51) and latest follow-up (P stem; 1.5% vs C stem; 1.0%, p = 0.72) did not differ between groups. Conclusion: Greater canal fill at the middle third of the stem was observed for the P stem compared to the C stem, however, both stems demonstrated robust and comparable freedom from revision at 2-year and latest follow-up, with low incidences of radiolucent line formation. Mid-term clinical and radiographic outcomes for these commonly used, fully HA-coated stems remain equally promising in THA despite variations in canal fill.