Biomechanical analysis of the sliding characteristics of compression hip screws

We examined the effect of screw-plate angle on the sliding characteristics and jamming potential of four popularly used stainless-steel and cobalt-chromium-molybdenum compression hip screws. The actual coefficient of sliding friction for these alloys was measured in each device. The force on the screw required to overcome the static frictional force also was determined, by varying the lengths of screw engaged in the barrel under conditions of static equilibrium representing 130-degree and 150-degree screw-plate angles. For the 130-degree loading configuration, this force was significantly (p < 0.001) higher than that required for the 150-degree loading configuration for all four screw types. The actual coefficient of friction was relatively constant for each material, although slight variations due to differences in design between screw types were found. A positive correlation (p < 0.01) was seen between the apparent coefficient of friction (the ratio of sliding force to normal force) and the length of the screw extending from the barrel. All stainless-steel screws jammed in the 130-degree tests when not completely engaged in the barrel. None of the 150-degree tests produced jamming and none of the cobalt-chromium-molybdenum screws jammed in either the 130-degree or the 150-degree test. Examination of jammed devices by scanning electron microscopy showed galling on the superior surface of both the screw and the barrel.

Understanding the conditions that facilitate sliding of hip screws aids in ensuring their proper use. The higher the nail-plate angle, the easier it is to impact the hip-fixation device and thus allow bone impaction and stability at the fracture site. The potential for jamming a sliding hip screw is decreased by maximum engagement of the screw in the barrel. Differences in the material and design of sliding hip-fixation devices have relatively little effect on the sliding characteristics compared with the nail-plate angle and the engagement of the screw in the barrel.