| Summary: | Problem:
Interbody fusion is a surgical technique in which a bone or synthetic implant is inserted
between two vertebrae to support the spine following removal of diseased or damaged tissue.
One failure mode of these implants is subsidence, in which the implant sinks into the adjacent
vertebrae. This can lead to pain, deformity and nerve damage and can only be corrected through
secondary surgery. Surgeons would like to know if there is some way to reduce the risk of
subsidence, particularly in patients with low bone density, e.g. through implant design changes.
This study looks for the bone regions which provide the most resistance to subsidence. The
results could be used to improve upon current surgical techniques and may assist in the
development of better spinal implants.
Method:
To identify the strongest and stiffest regions in the upper and lower surfaces of the
vertebrae, indentation tests were done in fresh-frozen human vertebrae from the lower spine (L3-S1) using a 3 mm-diameter hemispherical indenter at 0.2 mm/s to a depth of 3 mm. The results
were used to develop structural property maps. The effects of spinal level, bone density, disc
degeneration and removal of the endplate (a thin shelf of bone covering the main load-bearing
component of the vertebra) were considered.
Results:
The upper and lower L3-L5 surfaces were stronger around the periphery than centrally
and stronger at the rear margin than in front. The rear half of the lower surface was stronger than
the corresponding region of the upper surface. The SI surface was strongest at the rear margin,
with a steady decrease in strength from the back to the front and no lateral variation.
Bone density and disc degeneration had no effect on stiffness, however strength was
proportional to bone density in L3-L5, and disc degeneration lowered the strength of the lower
L3-L5 surfaces. Endplate removal reduced the strength and stiffness by about 2.5 times. The
locations of the strongest bone were not affected by any of these secondary variables.
Implications:
Implant designs and/or placement could be altered to take advantage of the stronger bone
regions in the vertebral surfaces. Since degeneration did not change the locations of the
strongest bone regions, implants designed using the general structural property maps should
perform well in patients with a wide range of bone and disc conditions. Sparing the endplate
may reduce the risk of implant subsidence.
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