Initial stability of cementless acetabular cups: press-fit and screw fixation interaction--an in vitro biomechanical study

Abstract

Background: Press-fit and screw fixation are important technical factors to achieve initial stability of a cementless acetabular cup for good clinical results of total hip arthroplasty. However, how these factors affect one another in initial cup fixation remains unclear. Therefore, this study aimed to evaluate the mutual influence between press-fit and screw fixation on initial cup stability.

Methods: Foam bone was subjected to exact hemispherical-shape machining to diameters of 48, 48.5 and 49 mm. A compressive force was applied to ensure seating of a 48-mm-diameter acetabular cup in the foam bone prior to testing. Screws were inserted in six different conditions and tightened in a radial direction at the same torque strength. Then, the socket was rotated with a twist-testing machine, and the torque value at the start of axial rotation between the socket and the foam bone was measured under each screw condition.

Results: The torque values for the 48-mm-diameter reaming were >20 N m higher than those for the 48.5- and 49-mm-diameter reaming in each screw condition, indicating that press-fit fixation is stronger than screw fixation. Meanwhile, torque values for the 48.5- and 49-mm-diameter reaming tended to increase with increasing the number of screws.

Conclusions: According to our experiment, press-fit fixation of a cementless acetabular cup achieved rigid stability. Although the supplemental screws increased stability of the implant under good press-fit conditions, they showed little impact on whole-cup stability. In the case of insufficient press-fit fixation, cup stability depends on screw stability and increasing the number of additional screws increases cup stability.

Fig. 1

Titanium mesh-coated acetabular cup (Nakashima THA Cup; Nakashima Medical Co. Ltd, Okayama, Japan) with three screw holes and a diameter of 48 mm

Fig. 2

The screws are inserted in the following six conditions: no screw (No-S), one screw (SH-1 and SH-2), two screws (SH-1-2 and SH-2-3), and three screws (SH-1-2-3)

Fig. 3

The socket is rotated by a twist-testing machine (Mini Bionix; MTS Japan Ltd, Tokyo, Japan) under a rotational speed of 0.2 rpm and a pressure of 500 N; the torque value at the start of the axial rotation between the socket and the foam bone is measured

Fig. 4

Results of the twist testing for the 48-mm-diameter reaming; the additional screw fixation is effective, but it has little influence on the whole-cup fixation (a). For the 48.5- (b) and 49-mm-diameter (c) reaming, the shapes of the line graphs are different from those for the 48-mm-diameter reaming, indicating marked deterioration of the cup fixation. The cup fixation tends to increase with the increasing number of screws and the addition of screws to the outer side of the cup rather than the inner side

Fig. 5

The cup fixation tends to increase with the increasing number of screws and the addition of screws to the outer side of the cup rather than the inner side. The influence of the additional screws on the whole-cup fixation is 18.1, 58.6 and 50.0 % for the 48-, 48.5-, and 49-mm-diameter reaming, respectively [(SH-1-2-3 − No-S) ÷ No-S × 100]. *P < 0.05