Revising a loosened cancellous screw with a larger screw does not restore original pull-out strength - A biomechanical study.

Abstract

Cancellous screw fixation is often used in fracture fixation. When this screw is over-tightened, damage to the bone and other non-linear processes such as fracture and construct failure would be involved. The objectives of this study were (1) to determine the reduction in pull-out strength when a cancellous screw spins and (2) to determine how much pull-out strength can be restored by revising with a larger diameter screw. A biomechanical study using synthetic polyurethane foam (320 kg/m³) was performed to assess (1) the pull-out strength of a 6.5 mm cancellous screw, (2) the pull-out strength of a loosened 6.5 mm cancellous screw and (3) the pull-out strength of a loosened 6.5 mm cancellous screw revised with a 7.3 mm cancellous screw. The baseline pull-out strength of the 6.5 mm cancellous screw was 2213.91 ± 200.51 N. There was a 79.1% (463.79 ± 99.95 N) reduction in pull-out strength once spinning occurs (p = 0.027). When a spinning 6.5 mm cancellous screw was revised to a 7.3 mm cancellous screw, the pull-out strength increased to 1313.65 ± 93.23 N, 59.3% of the baseline pull-out strength (2213.91 ± 200.51 N) (p = 0.027). A loosened 6.5 mm cancellous screw results in a 79.1% reduction in pull-out strength. Revising a loosened cancellous screw by inserting a larger 7.3 mm diameter screw partially improves the pull-out strength to 59.3% of the baseline. Surgeons should consider the use of "two-finger tight" torque when inserting a screw to avoid stripping.