Dynamic trial fitting by an expanding trial cup does not jeopardize primary acetabular component stability.

Background: Trial fitting of the acetabular component in uncemented total hip replacement is traditionally done by trial cups. Since trial cups do not resemble the real press-fit obtained by the definitive cup, a dynamic trial inserter, called the X-pander ®, was developed to mimic the real amount of press-fit. However, the concern is raised of losing the initial press-fit by using the X-pander® due to pre-expansion of the acetabulum. The purpose of this study was to assess if there is a difference in primary stability between both methods.

Methods: A biomechanical randomized study was performed with bovine calf acetabula, with randomization between either using the X-pander® or the traditional trial cups to assess primary stability. The primary outcome was the force needed to achieve lever out of the implanted cup (Anexys, Mathys or Trident, Stryker), measured in Newton meter (Nm) with a biomechanical testing set up.

Findings: In total, 54 cups (19 Anexys, 35 Trident) were inserted and tested after randomized trial fitting. Overall mean lever out was 45.1 Nm (SD 14.6) for the X-pander® group and 45.0 Nm (SD 14.5) for the trial cups group. After adjustment for potential confounders (cup size and type) mixed model analysis did not reveal a significant difference in lever out force between both testing devices (mean 1.0 Nm, 95%CI (-5.9; 8.0), p = .77).

Interpration: Initial press-fit of the implanted cup is not lost by pre-expansion as done with dynamic trial fitting with the X-pander®.