Preventing Disc Degeneration After Spinal FusionChronic low back pain caused by an unstable spinal segment can be treated with spinal fusion. The condition most likely to create this type of instability is called spondylolisthesis. In spondylolisthesis, a defect in the supporting column of bone allows the main body of the vertebra to separate and shift forward or slip over the vertebra below it. This shift narrows the spinal canal where the spinal cord is located. The result is pressure on the spinal cord with pain and sometimes even more serious symptoms.
It makes sense that if a spinal segment is fused and no motion is allowed at that level, there's a change in the way stresses and forces applied to the spine are transmitted. Loss of motion at one level means greater movement and pressure at the segment above and/or the segment below the fused level. The result can be degeneration of the disc in between called adjacent segment disease (ASD).
Adjacent segment disease (ASD) is a problem potentially created by the treatment for the first problem (fusion for the spondylolisthesis). That's not good -- it means the patient may need another surgery. An important question is: what factor or factors are the most important in this condition? Are there some patients who are more likely to develop ASD? Who might that be and could it be prevented?
One theory is that people who already have some degenerative changes in the adjacent segment are at increased risk for ASD. Another theory suggests that the type of spondylolisthesis present makes a difference in outcomes. To find out if either of these ideas is correct, a group of surgeons from Korea compared MRIs before and after surgery for patients who had lumbar fusion for spondylolisthesis. They were carefully selected and did not have any sign of segmental degeneration before the fusion operation.
Two groups of patients were included: group one had isthmic spondylolisthesis. Isthmic spondylolisthesis refers to a slip that occurs as a result of a defect of the supporting bone called the pars interarticularis. A tiny fracture develops that fails to heal causing a defect in the bone. This defect could be something that was present at birth or developed over time as a result of a hyperextension injury. Athletes involved in gymnastics, ballet, and football seem to be affected most often.
Group two had degenerative spondylolisthesis. As the name suggests, degenerative spondylolisthesis is age-related and can affect more than one vertebra. Women ages 50 and older seem to develop this problem. Everyone in the study (in both groups) had spondylolisthesis at the L4-L5 lumbar spine level. Fusion was done at that level using pedicle screws in a procedure called interbody fusion.
This type of fusion holds the segment still all the way around -- it provides a 360-degree fusion. Interbody fusion can be done from an anterior (front of the spine) approach called an anterior lumbar interbody fusion (ALIF) or from the back called a posterior lumbar interbody fusion (PLIF).
MRIs and X-rays were used to measure before and after results. Types of information collected from these imaging studies included: disc height, motion and translation of the vertebral body, presence of bone spurs called osteophytes, and endplate sclerosis (hardening of the disc where it attaches to the bone). They also measured the angle of the lumbar spine at the fused segment. This measurement is expressed in degrees and is called the L4-L5 segmental lordotic angle.
Remember these patients were all selected because they didn't have any signs of disc degeneration at the L3-4 or L5-S1 segments. So any changes in the adjacent segments after surgery are especially important. What they found was a fairly equal rate of adjacent segment disease (ASD) between the two groups.
When degenerative changes did occur, it was more likely in the degenerative group. The isthmic groups were able to go longer before adjacent segmental changes were observed. MRIs were the best tool for diagnosing ASD; X-rays were not as accurate. Some patients who had ASD didn't have any symptoms whereas others had back and leg pain that was worse when walking and better when bent forward or sitting.
There wasn't a significant difference in who developed ASD based on whether they had an anterior (ALIF) fusion or a posterior (PLIF) approach. What was most significant was the lordotic angle (curve). Patients who had a fusion that placed the lumbar segment at less than 20-degrees of lordosis were more likely to develop ASD.
In summary, this is the first study to report on the occurrence of adjacent segmental disease (ASD) after a rigid (360-degree) lumbar fusion at the L4-L5 level using before and after MRIs and in a group who did not have preexisting adjacent disease before surgery. The authors' analysis of the data do not support age, gender, or body mass index as risk factors for the development of ASD. Surgeons should pay close attention to the angle of fusion and provide at least a 20-degree lordotic angle (or greater) to assist in preventing ASD.
Kyeong Hwan Kim, MD, PhD, et al. Adjacent Segment Disease After Interbody Fusion and Pedicle Screw Fixations for Isolated L4-L5 Spondylolisthesis. In Spine. March 15, 2010. Vol. 35. No. 6. Pp. 625-634.
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