Do the adjacent level intervertebral discs degenerate after a lumbar spinal fusion? An experimental study using a rabbit model.
METHODS
A rabbit model of disc degeneration adjacent to a lumbar spinal fusion.


OBJECTIVE
To use a rabbit model to determine the long-term changes in the intervertebral discs at the levels above (cephalad) and below (caudad) 2 fused lumbar levels.


BACKGROUND
Lumbar spinal fusion is generally carried out to eliminate motion at a specific lumbar level. However, it is commonly thought that by eliminating motion at a level, one increases the motion at the adjacent levels cephalad and caudad the fused levels. There have been studies that have reported on degeneration occurring at the cephalad and caudad levels adjacent to the fused levels.


METHODS
A total of 9 New Zealand white, female rabbits: 4 rabbits in the control group and 5 rabbits in the experimental group. The 5 rabbits in the experimental group underwent a posterolateral 2-level lumbar spinal fusion from L3 to L5. The changes in the lumbar discs were assessed using radiographs, magnetic resonance (MR) images, and histology at 6 months and 12 months.


RESULTS
The results at 6 months are less clear than those at 12 months. The results at 12 months for the experimental group are (1) the intervertebral disc height decreased at the caudad adjacent level and to a lesser extent at the cephalad adjacent level; (2) the MRI scores for the discs at the caudad and cephalad adjacent levels showed severe loss of signal intensity as compared to the discs at the same levels in the control group. This loss was more pronounced at the caudad level where the loss of signal intensity was similar to that seen at the fused levels; (3) the histologic analysis showed severe degenerative changes with a lack of live cells in the nucleus pulposus and in the endplate at the caudad adjacent level. At the cephalad level, live cells were apparent (albeit few) in the nucleus pulposus, and there was a more normal looking endplate with live cells.


CONCLUSIONS
The intervertebral discs at both the cephalad and the caudad levels adjacent to the 2 fused lumbar levels in this rabbit-model experiment carried out over 12 months after surgery showed degenerative changes asassessed using disc-height measurements, MR images, and histology, and the effect was more severe at the caudad adjacent level.
Department of Orthopaedics, University of Tokushima, Tokushima, Japan.
Kosaku
K
Higashino
Takahiko
T
Hamasaki
Jin Hwan
JH
Kim
Motohiro
M
Okada
S T
ST
Yoon
Scott D
SD
Boden
William C
WC
Hutton
D000818:Animals; D004195:Disease Models, Animal; D005260:Female; D007403:Intervertebral Disc / Q000473:pathology* / Q000601:surgery; D055959:Intervertebral Disc Degeneration / Q000209:etiology / Q000473:pathology*; D008159:Lumbar Vertebrae / Q000473:pathology* / Q000601:surgery; D011183:Postoperative Complications / Q000473:pathology*; D011817:Rabbits; D013123:Spinal Fusion / Q000009:adverse effects* / Q000379:methods