Cement as a locking mechanism for screw heads in acetabular revision shells – a biomechanical analysis

In acetabular revisions, polyethylene (PE) liners are often cemented into metal shells on top of acetabular screw heads. This study investigates the possibility of using this technique to obtain “fixed-angle” acetabular screws – a concept that has not yet been reported in the literature.

Two groups of screws (n=8) were inserted into Trabecular Metal revision shells (Zimmer), into which PE liners were then cemented. Screws in Group 1, inserted in the shell’s pre-fabricated holes, were countersunk, whereas screws in Group 2 were inserted in custom-drilled holes that make their heads protrude into, and interdigitate with, the overlying cement mantle. Perpendicular loading was then applied to the screw shafts both statically to failure and cyclically.

A greater stiffness was observed for the protruding screws upon static loading; and while the countersunk screws all failed at the screw-cement junction (53.44 ± 8.33 N), the protruding screws all failed at the screw shaft (1049.79 ± 32.12 N) – a 20-fold difference (p < 0.05). Under cyclic loading, only the protruding screw head specimen did not fail, undergoing an overall displacement within the limits of osseointegration.

These results support the hypothesis that the protrusion of an acetabular screw head into an overlying cement mantle significantly increases its angular stability. Provided other variables are favorable, this “locking effect” may increase the initial stability of the whole implant, thus improving the ultimate success of complex acetabular revisions.