The anatomical SP-CL stem demonstrates a non-progressing migra- tion pattern in the first year: a low dose CT-based migration study in 20 patients

Abstract

Background and purpose — RSA is the gold standard for evaluation of early implant migration. We report the results of a new CT-based method Sectra CT micromotion analysis (CTMA) applied to assess the migration pattern in 20 patients in the 1st year after surgery, both with and with- out the use of tantalum beads in the bone. The patients had an SP-CL anatomical stem that uses an S-shape, designed to better fit the curvature of the femur.

Patients and methods — 20 THA patients (mean age 61 years, 10 female) received SP-CL stems, tantalum mark- ers in the femur, and low-dose CT scans at 1 day, 3 months and 12 months postoperatively. In addition, precision as well as inter- and intra-observer variability of the 12-month migration was measured.

Results — The 3-month subsidence was median 0.5 mm (95% CI 0.3–1.0) and the internal rotation 1.8° (CI 0.9–2.6). At 12 months the corresponding values were 0.6 (CI 0.3–1.6) mm and 1.9° (CI 0.8–2.4). Precision was 0.1 to 0.3 mm and 0.1° to 0.4° at 3 and 12 months. Intra- and inter- observer variability yielded R-values averaging 0.96 and 0.98.

Interpretation — The migration mainly took place during the 1st 3 months, in line with other uncemented stems. The number of patients with subsidence over 2 mm in the first year (5) might be due to the design of the prosthesis with an anatomical shape. Alternatively, our results might indi- cate a challenge when choosing the correct size for these new anatomical stems. CTMA provided precise and highly repeatable measurements of migration without the need for tantalum markers.