DISTRACTIVE PROPERTIES OF A THREADED INTERBODY FUSION DEVICE - AN IN-VIVO MODEL

Study Design. Twenty sheep underwent anterior lumbar interbody fusions with either a threaded titanium interbody fusion device (cage, n = 8) , autogenous iliac crest dowel graft (autograft, n = 6), or interbody decortication only (sham, n = 6). Two sheep had misplaced cages and we re excluded. Sheep were killed after 6 months. Objectives. To determin e whether this model is useful for examining the distractive and fixat ion properties of interbody fusion cages. Summary of Background Data. Interbody fusion cages are used in anterior lumbar interbody fusion pr ocedures to provide immediate intersegmental fixation and to distract and preserve interbody height. The process of physiologic anchorage by bone ingrowth into such devices is under investigation. Methods. Shee p were radiographed immediately after surgery and 2, 4, and 6 months a fter surgery. Interbody distraction and angulation were measured with a digital photo image analyzer at each time point. After the sheep wer e killed, stiffness to flexion, extension, and lateral bending moments were measured. Twelve untreated cadaver spines were also tested for c omparison. Results. After surgery, interbody distraction successfully occurred in cage and autograft-implanted sites. Loss of interbody heig ht ensued, however, in all groups during the first 2 months. Percentag e loss of height was lowest in cage sites. By 6 months, only cage site s remained distracted beyond normal. Fusions in all groups were stiffe r than untreated spines. Autograft sites were stiffer than cage sites to lateral bending. Sham sites were stiffer than cage and autograft si tes to flexion, but this likely resulted from complete intervertebral collapse. Conclusion. Despite early subsidence, interbody fusion cages successfully distracted and preserved interbody spaces. This model is useful for investigating methods of improving distraction and stabili zation.