Early clinical outcomes of a newly designed Zweymüller cementless stem in total hip arthroplasty: a retrospective cohort study

The Promontory stem, a newly developed Zweymüller cementless femoral stem with a proximal fixation-enhancing fin, was introduced in Japan in 2022. This study aimed to evaluate its early clinical and radiological outcomes. This retrospective cohort study included 56 patients who underwent primary total hip arthroplasty using the Promontory stem between August 2022 and October 2023. Clinical scores (JOA, EQ-VAS, HOOS-JR, FJS-12) and radiographic findings (stem alignment, stress shielding, radiolucent lines, cortical hypertrophy) were assessed at a mean follow-up of 22.8 months. Multivariate logistic regression was used to identify predictors of stress shielding around the stem. The mean patient age was 66.0 years and 71.4% were female. JOA scores improved significantly from pre-to post-operative time points in all domains; the total score increased from 42.9 to 82.3 ​at 2 years ( p ​< ​0.001). Mean EQ-VAS, HOOS-JR, and FJS-12 scores at final follow-up averaged 77.3 ​± ​17.6, 79.8 ​± ​15.7, and 63.5 ​± ​25.6, respectively. Radiologically, stress shielding was observed in 66.1%, with 17.9% having grade 2 or higher. Radiolucent lines were noted in 69.6% and cortical hypertrophy in 17.9%. No cases of aseptic loosening occurred. Complications included stem subsidence (3.6%), dislocation (3.6%), and periprosthetic fracture (1.8%). The 2 year implant survival rate was 98.2%. Multivariate logistic regression identified older age, female sex, and pre-operative lower bone mineral density of femoral neck as predictors of stress shielding. The Promontory stem showed favorable early outcomes with no major complications and high survivorship, despite frequent stress shielding and proximal radiolucent lines. • A newly designed Zweymüller-type stem was evaluated in a Japanese cohort undergoing THA. • The implant demonstrated high early survivorship of 98.2% over a mean follow-up of 22.8 months. • Stress shielding occurred in 66.1%, with grade ≥2 observed in 17.9%. • Radiolucent lines were frequently observed in Gruen zones 1 and 7. • Age, sex, and pre-operative femoral bone mineral density were independently associated with stress shielding.