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 were excluded. Sheep were killed after 6 months. Objectives. To determine whether this model is useful for examining the distractive and fixation properties of interbody fusion cages. Summary of Background Data. Interbody fusion cages are used in anterior lumbar interbody fusion procedures 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. Sheep were radiographed immediately after surgery and 2, 4, and 6 months after surgery. Interbody distraction and angulation were measured with a digital photo image analyzer at each time point. After the sheep were killed, stiffness to flexion, extension, and lateral bending moments were measured. Twelve untreated cadaver spines were also tested for comparison. Results. After surgery, interbody distraction successfully occurred in cage and autograft-implanted sites. Loss of interbody height ensued, however, in all groups during the first 2 months. Percentage loss of height was lowest in cage sites. By 6 months, only cage sites remained distracted beyond normal. Fusions in all groups were stiffer than untreated spines. Autograft sites were stiffer than cage sites to lateral bending. Sham sites were stiffer than cage and autograft sites 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 stabilization.