One of the largest studies to investigate lumbar spine disc degeneration found that adults who are overweight or obese were significantly more likely to have disc degeneration than those with a normal body mass index (BMI). Assessments using magnetic resonance imaging (MRI) show elevated BMI is associated with an increased number of levels of degenerated disks and greater severity of disc degeneration, including narrowing of the disc space. Details of this study now appear in Arthritis & Rheumatism, a journal published by Wiley-Blackwell on behalf of the American College of Rheumatology (ACR).
The World Health Organization (WHO) reports that obesity - one of the most preventable risk factors for a number of diseases - has more than doubled since 1980. According to WHO, in 2008 roughly 1.5 billion people aged 20 and older were overweight, with more than 200 million men and close to 300 million women considered obese. In the U.S., studies estimate one in three children is overweight or obese and excess weight could lead to more severe obesity in adulthood.
Moreover, previous research has linked higher BMI to low back pain, which is often debilitating and can limit function, impact psychological well being, diminish overall quality of life, and is associated with substantial socioeconomic and health-care costs. Experts suggest that disc degeneration is one cause of low back pain, and therefore, BMI could be involved in the development of degenerative disc disease. To expand the knowledge of this important health concern, a team of researchers led by Drs. Dino Samartzis and Kenneth M.C. Cheung at the University of Hong Kong in Hong Kong investigated the association between elevated BMI and presence, extent, and severity of lumbar spine disc degeneration on MRI in adults.
The team recruited 2,599 participants aged 21 and older from Southern China between 2001 and 2009. Participants were from diverse social and economic backgrounds and were recruited regardless of whether they had lower back pain or not. The study group included 1,040 men and 1,559 women who had a mean age of 42 years. Researchers conducted radiographic and clinical assessments, and MRIs of the lumbar spine were obtained for all subjects.
Study findings reveal that 73% of participants displayed disc degeneration, with men (76%) having a significantly higher prevalence of degeneration than women (71%). Not surprisingly, increasing age was found to increase the prevalence of disc degeneration. BMI assessments of the study group show that 7% of subjects were underweight, 48% were in the normal weight range, 36% were overweight, and 9% were obese.
Descriptions of treatment for low back pain (LBP) date to Hippocrates (460-370 BCE), who reported joint manipulation and use of traction. Onset of LBP often is associated with bipedal ambulation. Theories propose that this transformation in the mechanics of locomotion is the inciting evolutionary event that made the lumbar spine susceptible to degenerative disease. Degeneration is universal to structures that comprise the functional spinal unit, composed of 2 adjacent vertebral bodies and the intervertebral disk. The disk and 2 zygapophyseal joints at the same level function as a trijoint complex.
As humans age, they endure both macrotraumas and microtraumas and undergo changes in body habitus that alter and redistribute biomechanical forces unevenly on the lumbar spine. Natural progression of degeneration of the lumbar segment with motion proceeds with characteristic anatomic, biomechanical, radiologic, and clinical findings in lumbar degenerative disk disease (LDDD).
Traditionally, disk degeneration has been linked to mechanical loading. The importance of mechanical factors has been emphasized by experiments on cadaver spines with both a severe single event and relentless loading. Failure of disks is more common in areas where there are the heaviest mechanical stresses, such as the lower lumbar region. It has been suggested that mechanical factors produce endplate damage, the antecedent to disk degeneration.
The disk is metabolically active, and the metabolism is dependent on diffusion of fluid either from the marrow of the vertebral bodies across the subchondral bone and cartilaginous endplate or through the annulus fibrosus from the surrounding blood vessels. Morphologic changes in the vertebral bone and cartilaginous endplate, which occur with advancing age or degeneration, can interfere with normal disk nutrition and further the degenerative process. This disruption of the normal endplate results in deformation when under loading. This allows nuclear material to pass through the endplate, reducing intradiscal pressure with subsequent bulging and loss of height and adding more stress to the surrounding annulus. Compressive damage to the vertebral body endplate alters the distribution of stresses in the adjacent disk. Continual cyclic loading makes these changes worse. Diminished blood flow in the endplate initiates tissue breakdown first in the endplate and then in the nucleus. These altered stress distributions adversely affect disk cell metabolism. These changes then alter the integrity of the proteoglycans and water concentration, reducing the number of viable cells with subsequent alteration in the movement of solutes into and out of the disk.
The importance of normal blood flow to the homeostatic nutritional process in the intervertebral disk complex has been suggested to explain the association of atherosclerosis and aortic calcification with increased disk degeneration and subjective low back pain (10). As degeneration progresses, structures of the disk become more disarranged and greater stresses are placed on the annulus and facet joints. As increased forces are transmitted to the annulus, there may be fragmentation and fissuring. Disk degeneration involves structural disruption and cell-mediated changes in composition, but which occurs first is not clear. Biochemical factors can increase susceptibility to mechanical disruption, and this could adversely influence disk cell metabolism. Regardless of the initiating mechanism, these mechanisms would be interactive and additive, the end result being an altered functional ability of the disk to resist applied forces.
“Our research confirms that with elevated BMI there is a significant increase in the extent and global severity of disc degeneration. In fact, end-stage disc degeneration with narrowing of the disc space was more pronounced in obese individuals,” said Dr. Samartzis. The authors suggest that with weight gain, physical loading on the disc and/or a chronic low-grade inflammation from the fat cells may play a role in disc degeneration. Dr. Samartzis further added that “Since overweight and obesity are worldwide concerns whose prevalence continues to rise, our study’s findings have considerable public health implications. If these issues continue to plague society, they can further affect spine health leading to low back pain and its consequences.”
Mechanical load and injury
Abnormal mechanical loads are also thought to provide a pathway to disc degeneration. For many decades it was suggested that a major cause of back problems is injury, often work-related, which causes structural damage. It is believed that such an injury initiates a pathway that leads to disc degeneration and finally to clinical symptoms and back pain. Animal models have supported this finding. Although intense exercise does not appear to affect discs adversely and discs are reported to respond to some long-term loading regimens by increasing proteoglycan content, experimental overloading or injury to the disc can induce degenerative changes. Further support for the role of abnormal mechanical forces in disc degeneration comes from findings that disc levels adjacent to a fused segment degenerate rapidly.
This injury model is also supported by many epidemiological studies that have found associations between environmental factors and development of disc degeneration and herniation, with heavy physical work, lifting, truck-driving, obesity and smoking found to be the major risk factors for back pain and degeneration. As a result of these studies, there have been many ergonomic interventions in the workplace. However, the incidence of disc degeneration-related disorders has continued to rise despite these interventions. Over the past decade, as magnetic resonance imaging has refined classifications of disc degeneration, it has become evident that, although factors such as occupation, psychosocial factors, benefit payments and environment are linked to disabling back pain, contrary to previous assumptions these factors have little influence on the pattern of disc degeneration itself. This illustrates the tenuous relationship between degeneration and clinical symptoms.
The authors note that disc degeneration is a complex process involving structural and chemical changes of the disc. They recommend that future studies that investigate risk factors for disc degeneration should take into account the impact of overweight and obesity on the disease. Dr. Cheung concludes, “Deeper understanding of how elevated BMI contributes to disc degeneration and low back pain could aid in the development of novel interventions that can improve quality of life for those with these disabling conditions.”
Lumbar Degenerative Disk Disease
The sequelae of disk degeneration are among the leading causes of functional incapacity in both sexes and are a common source of chronic disability in the working years. Disk degeneration involves structural disruption and cell-mediated changes in composition. Mechanical, traumatic, nutritional, and genetic factors all may play a role in the cascade of disk degeneration, albeit to variable degree in different individuals. The presence of degenerative change is by no means an indicator of symptoms, and there is a very high prevalence in asymptomatic individuals. The etiology of pain as the symptom of degenerative disease is complex and appears to be a combination of mechanical deformation and the presence of inflammatory mediators. The role of imaging is to provide accurate morphologic information and influence therapeutic decision making. A necessary component, which connects these two purposes, is accurate natural history data. Understanding the relationship of etiologic factors, the morphologic alterations, which can be characterized with imaging, and the mechanisms of pain production and their interactions in the production of symptoms will require more accurate and reproducible stratification of patient cohorts.
Full citation: “The Association of Lumbar Intervertebral Disc Degeneration on MRI in Overweight and Obese Adults: A Population-Based Study.” Dino Samartzis, Jaro Karppinen, Danny Chan, Keith D.K. Luk and Kenneth M.C. Cheung. Arthritis & Rheumatism; Published Online: January 30, 2012 (DOI: 10.1002/art.33462).