The mechanical stability of a spine with degenerative disc disease that is corrected by the use of intervertebral cage-implants, employs the concept of distraction-compression as described by Baghy (1988). With the insertion of the implant, thevertebrae are distracted and the annulus stretched into tension providing a stiff and stable construct. A common method of implementing these cages is inserting them posterior to the spine, a procedure known as posterior lumbar inter-body fusion (PLIF).However, in the use of two cylindrical-type cages in a typical bilateral posterior insertion, Wong et al. (1997) showed that for the insertion of optimally sized cages that restore the disc height, the procedure is likely to require subtotal or totalfacet excision. The removal of the facets causes concern for the overall stability of the spine. This instability may not be significant though if the cage implants were large enough to provide enhanced tension in the annulus and increased stiffness inthe intervertebral joint. However, to our knowledge there has been no work done to establish how cage size affects the mechanical stability of the totally-facetectomised spine. The aim of this study therefore is to investigate the extent to whichcylindrical cages of progressively larger sizes can provide stability to the lumbar spine with its facets entirely removed for bilateral posterior lumbar inter-body fusion.
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