Biomechanical comparison between standard and inclined screw orientation in dynamic hip screw side-plate fixation: The lift-off phenomenon.

BACKGROUND

Common failure modes of dynamic hip screw are cut-out and lift-off. To minimize the latter, distal screws can be inserted in different orientations. However, the effectiveness remains controversial. The aim of this study was to biomechanically investigate the influence of distal screw orientation on construct stability.

METHODS

Thirty artificial generic long bones were assigned to three groups (n = 10) and fixed with two-hole dynamic hip screw-plates, inserting distal cortical screws with neutral parallel screw orientation (A), divergent screw orientation (B) or convergent screw orientation (C). Starting at 60 N, cyclic loading was applied to the implant tip perpendicular to the lag screw axis with progressive peak load increase at a rate of 0.002 N/cycle until failure. Parameters of interest were construct stiffness and machine actuator displacement after 250, 1000 and 5000 cycles, as well as cycles to failure.

RESULTS

Displacement after 250, 1000 and 5000 cycles was significantly higher in Group C than in Groups A and B,  < 0.01, whereas no significant differences were observed between Groups A and B,  = 0.20. Specimens in Group C failed after 11,584 [standard deviation (SD), 5924] cycles, significantly earlier than those in Groups A and B [A: 27,351 (SD, 12,509); B: 28,793 (SD, 14,764)],  ≤ 0.02. Cycles to failure were not significantly different between Groups A and B,  > 0.99.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Parallel or divergent distal screw insertion provides similar construct stability in terms of resistance to plate lift-off. In contrast, converging screw insertion leads to inferior stability and is not advisable from a biomechanical point of view.