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Mapping the structural properties of the lumbosacral vertebral endplates Grant, Jenifer Pamela

Abstract

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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