Prevalence and Patterns of Degenerative Disk Disease in the Whole Spine: Hierarchical Cluster Analysis of Pan-Spine MRI Images of 785 Subjects
Authors: Rajasekaran S. , Kanna R. , Kothari A. , Shetty J. 
Published: June 2012

Introduction The study presents the results of whole spine analysis of disk degeneration and a comparison of the incidence and patterns of degeneration between controls and patients for the first time in literature. Disk degeneration is the most common cause of back pain and although well investigated, the exact etiology is still elusive. There are proponents for both the mechanical and genetic basis of the disease and the general consensus is that it is multifactorial in most patients. A salient deficit of studies on disk degeneration so far is the fact that they have been restricted to the lumbar spine. Mechanical effects on disk degeneration would be confined to areas of spine under biomechanical stress whereas a genetic influence on the disk degeneration would be revealed by a more extensive involvement of the whole spine. Whole spine analysis would also allow comparison of the incidence of degeneration between regions under more and less biomechanical stress. The literature is also inadequate on comparison of the incidence and difference in patterns of degeneration between patients and controls with no history of back pain. The above knowledge is vital not only for proper understanding of etiology of disk degeneration but also for planning preventive and treatment aspects of the disease. We report the results of whole spine in a large cohort of 785 subjects where the incidence and pattern of involvement are analyzed by hierarchical clustering. Materials and Methods Total 640 patients who presented to our out-patient department with a history of significant low back pain for the last 3 months (requiring medical consultation), from January 2010 to August 2011 and who had undergone whole spine MRI scans, were included for the study. There were 347 men and 293 women with a mean age of 47.03 ± 14.1 years. Total 44 patients were excluded from the study as they were found to have associated structural diseases. As a part of controls, 189 healthy volunteers who were otherwise asymptomatic and had no history of significant back pain or neck pain were included in the study. There were 45 men and 165 women with a mean age of 33.87 ± 8.44 years. All the MRI scans were performed with a 1.5 Tesla MRI scanner (TR 4000 msec, TE 100 msec; slice thickness 5 mm; gap 1 mm; matrix 384 × 224; field of view 300 × 300 mm; number of excitations 3; echo train 16). Sequences studied were whole spine T2-weighted sagittal section and lumbar sagittal sections. Lumbar disk degeneration on MRI was graded using Pfirrmann grading. After analysis of whole spine DD pattern, hierarchical clustering program was used to obtain cluster pattern of degenerated disks. In hierarchical clustering, groups with common characteristics (like similar DD patterns) are clustered and then joined to the next closest cluster with similar characteristics. Results Five distinct patterns of degeneration were observed in the lumbar spine. Single-level involvement was the most frequent pattern and was seen more commonly at the levels of L4 to L5 (8.9%) and L5 to S1 (13.6%). The involvement was almost equal between the two levels of L4 to L5 and L5 to S1. Contiguous involvement of two levels was the next common pattern seen and was again more common at L4 to L5 and L5 to S1 (11%). Skip lesions with intervening normal disks was seen in 3% of the cases; upper lumbar was seen in 1% and pan lumbar was seen in 2% of the cases. None of the patients with history of back pain had a normal MRI. Comparing patients and controls, the total number of lumbar disks degenerated was significantly different between the two groups ( p ). Patterns of Disk Degeneration in Whole Spine Whole spine analysis showed marked difference in the incidence and pattern of disk degeneration between controls and patients. Lower lumbar disk degeneration was observed in 66.6% in patients compared to only 10.5% in controls ( p ). Hierarchical Clustering Hierarchical clustering allowed identification of four patterns of disk degeneration in patients—lower lumbar, lower cervical spine, upper lumbar, and lower thoracic in order of involvement. Lower lumbar disk degeneration was found to be strongly associated with lower cervical degeneration (58%) whereas upper lumbar cluster was more commonly clustered with thoracic degeneration (18%). In contrast, cervical degeneration was more common than lumbar degeneration in controls and the above-clustered pattern was absent. Conclusion This study analyzes the incidence and patterns of disk degeneration in the whole spine and has revealed the following interesting results. Disk degeneration is a pan spinal disease and not restricted to lumbar spine in patients with low back pain. Patients with low back pain have a significantly higher incidence of asymptomatic cervical disk degeneration compared to controls. The incidence of disk degeneration at all levels of lumbar spine was significantly more in patients compared to controls implicating disk degeneration as an important cause of low back pain. Pan lumbar disk disease was observed only in patients and not in controls. Thoracic disk degeneration was more commonly seen in controls than patients. In controls, cervical degeneration was more common than lumbar degeneration. Hierarchial clustering showed that the most frequent pattern of disk degeneration association was between lower lumbar and lower cervical regions—the two regions with the greatest mobility and mechanical stress in the spine. The distinct clustering pattern of patients was not identifiable in controls. I confirm having declared any potential conflict of interest for all authors listed on this abstract Yes Disclosure of Interest None declared