Migration of the uncemented Echo Bi-Metric and Bi-Metric THA stems: a randomized controlled RSA study involving 62 patients with 24-month follow-up

Background and purpose — Despite the good results after total hip arthroplasty (THA), new implants are continuously being developed to improve durability. The Echo Bi-Metric (EBM) THA stem is the successor to the Bi-Metric (BM) THA stem. The EBM stem includes many of the features of the BM stem, but minor changes in the design might improve the clinical performance. We compared the migration behavior with radiostereometric analysis (RSA) of the EBM stem and the BM stem at 24 months and evaluated the clinical outcome.Patients and methods — We randomized 62 patients with osteoarthritis (mean age 64 years, female/male 28/34) scheduled for an uncemented THA to receive either an EBM or a BM THA stem. We performed RSA within 1 week after surgery and at 3, 6, 12, and 24 months. The clinical outcome was evaluated using Harris Hip Score (HHS) and Oxford Hip Score (OHS).Results — At 24 months, we found no statistically significant differences in migration between the two implants. During the first 3 months both the EBM and the BM stems showed visible subsidence (2.5 mm and 2.2 mm respectively), and retroversion (2.5° and 2.2° respectively), but after 3 months this stabilized. The expected increase in HHS and OHS was similar between the groups.Interpretation — The EBM stem showed a migration at 24 months not different from the BM stem, and both stems display satisfying clinical results.

To improve the longevity of total hip arthroplasty (THA) new designs are continuously being developed. The introduction of new implants should optimally be done by phased stepwise introduction (Malchau 1995, Nelissen et al. 2011) including radiostereometric analysis (RSA) of implant migration.Some subsidence of hip stems is generally accepted within the first 3 months, but after that osseointegration and stability should have occurred. Mean subsidence of up to 1 mm of the stem at 24 months has been reported (Nysted et al. 2014, Weber et al. 2014, Hoornenborg et al. 2018, Sesselmann et al. 2018, Kruijntjens et al. 2020).This study investigates by RSA potential differences in migration at 24 months, between 2 different designs of porous-coated uncemented hip prosthesis; the Bi-Metric Full Proximal Profile THA stem (BM) and the Echo Bi-Metric (EBM) stem (Zimmer Biomet, Warsaw, IN, USA) (Figure 1). Both stems are press-fit titanium alloy stems with a proximal plasma spray porous titanium coating and a distal part with a roughened titanium surface. The BM has shown good clinical results and excellent stem survival in register studies since its introduction in 1984 (Jacobsen et al. 2003, Davies et al. 2010, Mäkelä et al. 2010, Lazarinis et al. 2011). The EBM is the successor to the BM and has 3 theoretical design improvements: a slimmer design of the neck to increase range of motion; a polished bullet-shaped distal tip to reduce distal stress; and an extended porous coating to support biological ingrowth proximally. Evaluation of adaptive bone remodeling and stress shielding will be addressed in another publication.Open in a separate windowFigure 1.The stems: on the left the Bi-Metric stem and on the right the Echo Bi-Metric stem.We hypothesized that the migration of the EBM was less at 24 months, compared with the BM stem.