The Journal of Bone and Joint Surgery, Vol 62, Issue 8 1308-1314, Copyright © 1980 by Journal of Bone and Joint Surgery, Inc
Biomechanical analysis of the sliding characteristics of compression hip screws
RF Kyle, TM Wright and AH Burstein
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. CLINICAL RELEVANCE: 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.
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