Background: Although interbody cages are widely used, there is
little histological documentation of the tissue within cages in the human
spine. The purpose of this study was to describe the contents of retrieved,
clinically failed, interbody cages from human patients, with special reference
to the influence of graft type on the viability of bone in the cages.
Methods: Seventy-eight cages that had been retrieved from
forty-eight patients were analyzed. There were eight carbon-fiber cages and
seventy threaded metal cages. Of the sixty-seven cages for which information
about grafting was available, fifty-six had been packed with autograft only,
six had local autograft mixed with demineralized bone matrix, four had
allograft, and one had demineralized bone matrix only. The indications for
cage retrieval included a failed fusion, malposition or migration of the cage,
trauma (a compression fracture at the fusion site), low-back pain, progressive
spondylosis, nerve-root impingement, and/or infection. The cages had been in
situ for an average of twenty-two months. Undecalcified sections through the
center of each plastic embedded cage were reviewed, and the approximate areas
occupied by viable bone, necrotic bone, fibrocartilage, hyaline cartilage,
fibrous tissue, and graft substitute were visually estimated. Debris particles
were estimated by a semiquantitative scoring system.
Results: Seventy-one of the seventy-eight cages showed evidence of
vascular ingrowth and areas of histologically viable bone, representing
incorporating bone graft. The average area occupied by viable bone was 44%
(range, 0% to 80%). In some cages, relatively large fragments of cortical bone
graft were associated with only minimal new-bone formation. Fibrocartilage
occupied up to 50% of the available area in these failed cages. Some cages
also contained small fibrocartilage seams connecting segments of bone in a
pattern that suggested motion in vivo. In thirty-one of the seventy-eight
cages, =5% of the available area was occupied by hyaline cartilage,
probably from vertebral end plates or facet joints.
Conclusions: While this study was not designed to test the efficacy
of cages or of bone graft, the prevalence of hyaline and fibrocartilage in
these failed cages illustrates the importance of graft and graft-site
preparation to maximize bone-graft incorporation.
Level of Evidence: Therapeutic study, Level IV (case
series [no, or historical, control group]). See Instructions to Authors for a
complete description of levels of evidence.