Matching lines to designs: A novel radiographic index for shorter femoral stems in total hip arthroplasty

Introduction

Matching stem design and femoral morphology to balance stability and osseointegration with complication risk is critical in total hip arthroplasty (THA). Several historical radiographic measurements have assisted surgeons in selecting the best stem design for a patient. As these measurements were developed for longer stems (>150 mm) they have less utility for shorter stems (<120 mm). This study evaluated a set of new radiographic measurements tailored to shorter stems and the relative radiographic performance of several medial-to-lateral (ML) and metaphyseal-filling (MF) stems.

Methods

Retrospective cohort study on all elective primary THAs between January 2011 and August 2020 at a single institution. Patients received a short metaphyseal-engaging stem and a minimum six-month follow-up with standardized radiographs. Six radiographic indices were created based on validated reference points. Postoperative radiographs were evaluated for stable fixation by bone ingrowth and lucent zones.

Results

There were 748 cases, with a mean age of 63 (±12.2) years. Radiolucencies were evident in 13.6 % of stems. Metaphyseal-filling stems had a lower incidence of radiolucencies than ML stems (9.1 vs 19.3 %, p < 0.001). A Metaphyseal Flair Index (MFI) below 0.6 and a Lesser Cortical Index (LCI) above 0.3 significantly predicted higher rates of radiolucency (p < 0.001). Patients with an LCI of 0.3 or higher had a 93 % probability of developing radiolucencies when using ML stems (p < 0.001).

Conclusion

Two indices (MFI and LCI) predicted areas of osseointegration in modern short femoral stems. Metaphyseal-filling stems have lower radiolucency rates than ML stems. Due to the higher probability of radiolucencies, surgeons should consider avoiding ML stems in patients with LCI values above 0.3.