Locking versus standard screw fixation for acetabular cups: is there a difference?

Total hip replacement has been established as a valid treatment option for displaced subcapital fractures. However, insufficient primary fixation may be the reason for early loosening in these osteoporotic patients. Primary fixation of the cup is usually achieved by press-fit fixation that can be enhanced using screws. Locking the screws into their respective cups may seem to improve the primary fixation of the construct, as locked plates proved superior fixation for osteoporotic fractures. The study consisted of three groups: in each group, three cups were fixed into blocks of foam bone using press-fit technique. In the first group, no additional screws were used, in the second group two standard screws were inserted, while in the third group two acetabular screws were cemented into the cup to simulate locked screw fixation. Load was applied onto the rim of the acetabular component to cause shearing between the cup and the block. Cup fixation was examined by a loading machine that acquired load versus displacement. The stiffness (load vs. displacement) was calculated. Screws, either locked or non-locked, enhanced cup fixation by 26 % (p value < 0.01). No significant changes were found between the locking and non-locking screws groups. These experimental results indicate that acetabular screws enhance primary cup fixation. This may become significant in conditions where the acetabular bone stock is suboptimal, such as when performing total hip arthroplasty after displaced subcapital fractures. However, there is no superiority for locked screws over standard screw fixation.