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

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 without 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 markers 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 indicate 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.