Canal fill and radiographic comparison analysis of novel fully hydroxyapatite coated, variable triple-tapered geometry stems: one-year follow-up after direct anterior approach total hip arthroplasty.

Background: Optimal femoral stem alignment and femoral canal fill have been associated with improved osteointegration in patients undergoing cementless total hip arthroplasty (THA). Direct anterior approach (DAA) has become more popular among new surgeons, and the ability to achieve appropriate stem alignment and canal fill due to limited surgical exposure may pose added risks. To mitigate these concerns, several modern tissue sparing stem designs have been designed. This study aimed to compare implant canal fill and alignment between two of the leading DAA-friendly femoral stems available.

Methods: This was a multi-center, retrospective study of patients who underwent DAA THA with either A (n = 149) or B stem (n = 85) between 2021 and 2023 and had a minimum one-year follow-up. Radiographic measures of proximal femoral morphology, including canal calcar ratio (CCR), Morphological Cortical Index (MCI), and Dorr Class (based on the CCR), as well as postoperative measures including femoral canal fill ratio (CFR), signs of osteointegration and stem alignment were analyzed and compared between stem types.

Results: Proximal femoral morphology was similar between groups, as suggested by the CCR (p = 0.600), MCI (p = 0.489) and Dorr class distribution (p = 0.516). Patients who received stem A had improved CFR at the level of the lesser trochanter compared to patients who received stem B (0.73 vs. 0.70, respectively; p = 0.042), with similar CFR at the tip of the stem (0.76 vs. 0.77, p = 0.562). Similarly, there was a trend towards improving stem alignment for stem A patients (2.16 vs. 2.47, p = 0.082). Spot weld formation was significantly higher in stem A group (59.7% vs. 37.6%, P = 0.001).

Conclusion: For patients with similar femoral morphology undergoing DAA THA, the stem A group demonstrated superior anatomical fit in the metaphyseal region, as evidenced by CFR, spot weld formation and implant positioning. Further longer follow up research is needed to elucidate these findings and their correlation to clinical outcomes.