Long-term results of the metaphyseal-loading anterolaterally flared anatomic femoral stem for total hip arthroplasty.

Background: Patients with secondary hip osteoarthritis due to developmental dysplasia of the hip (DDH) often have abnormal femoral morphology, making stem design critical for long-term outcomes. The FMS-anatomic stem previously demonstrated favourable mid-term results. Its successor, the Anatomic Fit stem, was developed with a reduced hydroxyapatite-coated area to enhance proximal load transfer and a narrower lateral flare to facilitate insertion. This study aimed to analyze the long-term clinical and radiographic outcomes of these stems.

Methods: The one-hundred seventy-eight hips underwent total hip arthroplasty using either the FMS-anatomic or Anatomic Fit stem. After applying exclusion criteria, 119 hips (36 FMS-anatomic and 83 Anatomic Fit) were analyzed. Clinical and radiographic outcomes were assessed, and stem survival was evaluated using Kaplan-Meier analysis with stem revision as the endpoint.

Results: The 15- and 20-year survival rates were 97.2% and 94.4% for the FMS-anatomic stem, and 98.8% for the Anatomic Fit stem at both time points, with no significant difference. Spot welds were observed in nearly all cases, but their distribution differed significantly: zones 2 and 6 in the FMS-anatomic group and zones 1 and 7 in the Anatomic Fit group (P < 0.05). Stress shielding of Grade 3 or higher occurred in 16.5% of FMS-anatomic stems and 33.7% of Anatomic Fit stems (P < 0.05).

Conclusions: Both stems showed excellent long-term survival and proximal fixation. However, the Anatomic Fit stem did not reduce stress shielding, despite its modified design intended to improve load transfer.