Factors May Predict Disc Degeneration Following Spinal Fusion for Adolescent Idiopathic Scoliosis
Commentary by Noriaki Kawakami, MD, DMSc and Christopher M. Bono, MD
Lumbar disc degeneration following spinal fusion in the treatment of adolescent idiopathic scoliosis (AIS) tends to occur in patients with greater L4 tilt angle and fewer mobile segments in the lower spine, according to a retrospective study in Spine Deformity.
“As long as adolescent idiopathic scoliosis surgery is done as a fusion surgery during the adolescent period, the effect on the adjacent segments, particularly on the distal lumbar segments cannot be ignored,” said coauthor Noriaki Kawakami, MD, DMSc, Director of the Spine Center at Meijo Hospital, Nagoya City, Japan. “The decision making process for choosing the fusion area should be based on not only the short-term outcomes but also on the long-term follow-up data. Although many investigators have been studying which segment is the best choice for the patient’s clinical outcome, we still have many unsolved issues that make the choice difficult,” said Dr. Kawakami, who also is Clinical Professor of Orthopedic Surgery at Nagoya University School of Medicine.
Unknown Aspects of Surgery
“The first of such unsolved issues is related to the upper/lower instrumented vertebra (UIV/LIV). When the fusion covers a greater area, better correction can be achieved. At the same time, however, it is a known fact that the lower the LIV placement is, the higher the rate of disc degeneration and the breakdown of distal unfused segments,” Dr. Kawakami explained.
Another unresolved aspect is whether the residual curve magnitude of the scoliosis affects outcomes. “The magnitude of the lower lumbar curvature such as L4 tilt or L3 tilt, seems to negatively affect the clinical outcome during the follow-up period,” Dr. Kawakami said. “This raises the question, of whether greater residual curvature with more unfused segments on the lower lumbar area after fusion results in a better clinical outcomes compared with a smaller residual curvature with fewer unfused segments, Dr. Kawakami added.
To begin to answer these questions, the researchers examined influence of spinal balance on lumbar degenerative changes at distal unfused segments in 93 patients with AIS. The patients average at the time of surgery was 15.2 years and the average follow-up time was 154 months.
At follow-up, nearly half of the patients (48%) developed degenerated discs, with L5/S1 being the most common location (40%). The prevalence of disc degeneration increased continuously with lower placement of fusion, from a low of 27% in patients with fusion in the thoracic spine to a high of 50% in patients with placement at L4.
Patients with disc degeneration were more likely to have undergone spinal fusion at an older age, have a greater L4 tilt preoperatively and at 10 years postoperatively, and have a greater number of mobile segments compared with patients who did not develop disc degeneration (Table).
The authors speculated that surgical correction of scoliosis may be beneficial when performed before patients develop a greater L4 tilt, and that fusing at L2 may reduce disc degeneration.
Clinical Implications Are Limited, Expert Says
“The clinical implications at this time are somewhat limited,” commented Christopher M. Bono, MD, President of the North American Spine Society. “What the authors have demonstrated is that at a minimum of 10 years following a corrective surgery for AIS, degeneration of the unfused discs below the instrumentation construct undergo degeneration in 48% of patients. They further describe different frequencies of degeneration with differing lower instrumented vertebrae.”
“While they have described these changes and noted some relationships, the most important finding of the study was a negative one,” noted Dr. Bono, who also is Associate Professor of Orthopedic Surgery at Harvard Medical School and Chief of the Orthopedic Spine Service at Brigham and Women’s Hospital, both in Boston, Mass. “There was no significant difference in the prevalence of low back pain in the patients with and without new-onset degeneration below the construct. Consistent with the body of literature regarding adjacent level disease and disc degeneration in non-deformity patients, there seems to be little correlation between the radiographic changes and clinical symptoms.”
“Thoughtful and honest evaluations such as this one, examining a single disorder treated in a uniform manner, can give us useful prognostic information as it implicitly controls a multitude of covariables by nature of the study design,” Dr. Bono said.
Authors Call for Future Research
“Our article is just the first step. As mentioned in the article, one of the limitations is that a 10-year follow-up period is still not enough to know the influence of the curve magnitude and the placement of LIV on disc degeneration of the unfused segments,” Dr. Kawakami said. “Twenty and 30 years follow-up data down the road with a greater number of patients may be required to fully address this research question,” Dr. Kawakami added.
“In future studies, we will focus on the disc degeneration of the unfused segments with the different type of curvature, approach, scoliosis magnitude, etc. On the other hand, when we look at the cranial segments, the question can be replaced to the relation of possible cervical degeneration and clinical neck symptoms with the placement of UIV on the upper thoracic segment,” Dr. Kawakami concluded.
“Future studies should probably focus on more long-term radiographic and clinical follow-up,” Dr. Bono concurred. “It is possible that, if followed long enough, the patients with degeneration may eventually have a higher frequency of clinical symptoms. It would be important to know if, and when, that becomes an issue.”