Effect of symmetrical restoration for the migration of uncemented total hip arthroplasty: a randomized RSA study with 75 patients and 5-year follow-up

Abstract

Background: Inferior placement of a femoral stem is predictive for early loosening and failure, but does restoration of the original hip anatomy benefit the function and survival of a total hip replacement?

Methods: Seventy-five patients with primary unilateral hip osteoarthritis operated with an uncemented anatomical stem were randomized for either standard or modular stems. We used 50 ABG II stems with modular necks and 25 standard stems (control group). We measured the symmetry in hip anatomy between healthy and operated side. The anatomical restoration variables were anteversion, global offset, and femoral offset/acetabular offset (FO/AO) quota. We performed measurements using a CT-based 3D templating and measuring software. Migratory behavior of the stems was then measured postoperatively with repeated radiostereometry (RSA) examinations over 5 years.

Results: Both stem types showed an early (within 3 months) good stabilization after an initial slight rotation into retroversion and subsidence. There were no significant differences in RSA migration between modular and standard stems. Postoperative anteversion and FO/AO quota had no impact on stem migration. The standard stem tended to result in insufficient global offset (GO), whereas the modular stem did not.

Conclusions: The modular stem gave good symmetrical anatomical restoration and, like the standard version, a benign migratory behavior. Anteversion, GO, and FO/AO quota had no significant impact on stem migration. It therefore seems to be of no importance whether we choose a modular or a standard stem with regard to postoperative stem migration for this stem type. We overestimated the effect anatomical parameters have on stem movement; hence, we believe the study to be underpowered.

Trial registration: ClinicalTrials.gov identifier: NCT01512550. Registered 19 January 2012-retrospectively registered.

Keywords: 3D-CT; Anatomical restoration; RSA; Radiostereometry; THA; THR.

Conflict of interest statement

Dr. S. Kiernan and Dr. G. Flivik have both served as advisory consultants for Ortoma AB during software development. The other authors declare that they have no competing interests.

Figures

Fig. 1

ABG II system: standard and modular stems

Fig. 2

Reference points for 3D-CT measurement of anteversion. A proximal reference point in the center of the medullary canal at the lower level of trochanter minor and a second 3 cm more distal reference point in the center of the medullary canal formed the longitudinal axis of the femur. The perpendicular line (femoral offset) runs from the longitudinal axis of the femur to the center of rotation. The hip anteversion is the angle between this perpendicular line in relation to the posterior femoral condylar line

Fig. 3

Femoral offset (FO) is the distance between the longitudinal axis of the femur to the center of rotation. Acetabular offset (AO) is the distance between the center of rotation to the symphysis line. Global offset is the FO plus the AO

Fig. 4

Overview of statistical analysis and variables

Fig. 5

Line charts with 95% confidence intervals

Fig. 6

Line charts with 95% confidence intervals