The Journal of Bone and Joint Surgery, Vol 76, Issue 1 95-103, Copyright © 1994 by Journal of Bone and Joint Surgery, Inc
Relative motion of hip stems under load. An in vitro study of symmetrical, asymmetrical, and custom asymmetrical designs
J Hua and PS Walker
University College London, Stanmore, Middlesex, United Kingdom.
Symmetrical, asymmetrical, and custom asymmetrical stems without collars
were manufactured and inserted without cement, in that order, into each of
eight femora. Displacement transducers were mounted at different locations
to measure relative motion between the stem and the bone. A load was
applied to simulate the forces acting on a hip joint in a single-limb
stance. A force of 1000 newtons was applied three times in order to seat
the prosthesis, and then a cyclical force of 1000 newtons was applied to
the head of the prosthesis; readings of relative motion were taken every
500 cycles, until 2500 cycles had been reached. The relative motion
diminished as the number of cycles increased, with most of the reduction
occurring within the first 500 cycles. The symmetrical stem had the least
relative axial motion, with an average of six micrometers, but the other
stems also had low values: eleven micrometers for the custom stem, and
nineteen micrometers for the asymmetrical stem. In terms of axial rotation,
however, the symmetrical stem had several times more motion than did the
other two stems. Motion of the stem tip toward or away from the canal in
the frontal plane and in the sagittal plane also was higher for the
symmetrical stem. Axial migration (permanent sinkage after 2500 cycles) was
very similar between all three types of stem. In a torsional loading test,
the symmetrical stem showed about ten times more relative rotational motion
than did the other two stems. Over-all, the custom stem and the
asymmetrical stem had similar results.(ABSTRACT TRUNCATED AT 250 WORDS)
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