Low back pain is a leading cause of disability worldwide,1 and drives a great deal of health care seeking and associated diagnostics. In the search for underlying pathology, intervertebral disc degeneration has long been a primary suspect in the development of painful spinal disorders. No clinical diagnosis more strongly conveys this belief than that of degenerative disc disease, a term that has become pervasive in spine care and related research. Weiner2 observed that a biomedical model of degenerative disc disease has dominated the care of low back pain for generations, with perceived pathomechanical issues treated with physical therapy (e.g., exercise), chiropractic, and surgical approaches (e.g., spine fusion, disc replacement surgery) and patho-chemistry considerations with medications. Most recently, stem cell, protein, and gene therapies have been presented as potential treatment strategies for cellular and molecular pathophysiology associated with degenerative disc disease.3 Yet, despite its widespread use and clinical implications, the term remains controversial.
Certainly, disc degeneration or failure plays a contributing role in some clinical conditions, such as lumbar spinal stenosis and disc herniation, but its importance in the vast majority of cases of back symptoms for which individuals seek care is unclear.4 As noted in a recent Lancet series on low back pain, while “some imaging and clinical findings increase the likelihood that pain is arising from the intervertebral disc … there is no widely accepted reference standard for discogenic pain.5” Failure to establish clear concepts, nomenclature, and standardized measures of disc degeneration and degenerative disc disease has been raised as a problematic issue in advancing related knowledge.6 Yet, little attention has been given to this rather fundamental issue.
Given its important clinical consequences, we systematically searched and reviewed the use of the term degenerative disc disease in the health sciences literature to elucidate its current usage and to serve as a basis for related clinical, research, and policy recommendations. Specifically, we sought to determine the consistency with which degenerative disc disease is defined or operationalized in the health sciences literature and to investigate factors, such as spinal region, discipline, and country of origin, which might explain heterogeneity in use of the term.
MATERIALS AND METHODS
We conducted a systematic search of publications using the term degenerative disc disease in the health sciences literature over the decade from January 1, 2007, through December 31, 2016. We were particularly interested in how the term was defined when it was the object of study or central to the research being reported, so we limited the primary sample from which definitions of degenerative disc disease were extracted to publications in which the term appeared in the title. Specifically, “degenerative dis∗ diseas∗” was searched as a title word and limited to 2007 to 2016 in Ovid MEDLINE (R), Embase, CINAHL, and Scopus. Duplicates were eliminated, as were books and book chapters.
Two investigators independently reviewed all publications in the primary sample and extracted, coded, and entered the data for each directly into an electronic database created for the review. The information extracted for each publication included year of publication, discipline or department of corresponding or lead author, country of origin, publication type (e.g., full journal article, proceedings), study type (e.g., cohort study, randomized clinical trial), the context in which degenerative disc disease was being studied (e.g., surgery, basic science), and the spinal region being studied (e.g., cervical, lumbar). Finally, the definition of degenerative disc disease used in the publication was noted and placed in one of eight broad categories or “other” (Table 1).
After data were extracted, coded, and entered into the electronic database, discrepancies were identified and errors were corrected. In cases where discrepancies in definition codes occurred, the publication and definition were re-evaluated and consensus was reached. Descriptive statistics were used to summarize the findings of the review.
Finally, Scopus was queried to provide a broad indicator of trends from 2007 to 2016 for increasing or decreasing use of the term, degenerative disc disease, relative to spine-related publications, in general. A function in Scopus provides a frequency count of all publications for each year in which a term appears in the publication title, keywords, or abstract. Changes in the frequency of use of the term, degenerative disc disease, were compared with yearly frequency counts for the broader terms, lumbar spine, and cervical spine, over the 10-year timeframe.
The search limited to 2007 to 2016 for the term degenerative disc disease specifically in the title of publications resulted in 437 publications after initial duplicate removal. The final primary study sample for review comprised 402 publications after 13 book chapters, 11 remaining duplicates, 5 retracted or withdrawn articles, 3 erratum, 1 sound recording, and 2 unavailable articles were omitted. The majority of the publications were full-length research articles (69.9%), followed by proceedings (18.9%), and the remainder comprised research newsletter articles, narrative reviews, other reports, and editorials and letters in indexed research journals.
The term degenerative disc disease was featured as the object of study in publication titles with increasing frequency in the scientific literature over the decade under review (Figure 1). Its appearance specifically in the title increased during the last 3 years of the decade by 89%, as compared to the first 3 years (Figure 1).
Countries and Disciplines of Origin
Using the primary sample (n = 402), the published work on degenerative disc disease emanated from 42 countries, with the largest contributors being the United States (39·8%) and China (13.2%). Fifteen of the 28 member states of the European Union together contributed 20.2% of the publications on the topic, with Switzerland contributing 4.5%, and Turkey 4.0%. All other identified countries of origin contributed less than 3% each to the total number of publications. The corresponding or lead authors were also from a wide variety of disciplines and organizations, but were most commonly orthopedic surgeons (41.8%), followed by neurosurgeons (23.6%).
Accordingly, degenerative disc disease was studied most commonly in the context of spine surgery (52.2%), followed by basic science (15.4%), interventions other than surgery (11.2%), and reports of occurrence rates and associations of various phenomena deemed relevant to degenerative disc disease as revealed in cross-sectional and descriptive studies (9.7%). The lumbar region was nearly three times more commonly the focus of papers on degenerative disc disease (61.0%) than was the cervical region (22.4%). The remaining papers studied multiple or unspecified spinal regions, or used animal models.
Degenerative Disc Disease Case Definitions
Degenerative disc disease was defined or operationalized in many different ways, with no one definition used in the majority of publications (Table 1). Of the eight broad definitional categories, most frequently the term was used without any further definition provided (30.1%), such as “degenerative disc disease for which surgery was performed” or simply “degenerative disc disease.” In other instances, case definitions specifically included radiculopathy or myelopathy, such as “one symptomatic, focal, contained, herniated disc ... radicular pain [must be] concordant with image findings ...,”7 or “symptomatic cervical disc disease causing intractable, debilitating radiculopathy”8 (14.4%), or only included axial pain (5.5%), such as “chronic back pain” accompanied by common but variable degenerative changes.9
Degenerative disc disease was often equated with disc degeneration (e.g., Pfirrmann grade or disc desiccation, narrowing or bulging) regardless of the presence or absence of symptoms (15.4%), or with other specific imaging findings (4.5%), such as disc herniation or lumbar spondylolisthesis with adjacent segment stenosis. In 12.7% of publications, degenerative disc disease was defined as disc degeneration as a cause of back or axial pain, with half of these defined specifically as a painful disc or discogenic pain (e.g., “positive discography at operated levels”).10 In another 7.2% of the publications, degenerative disc disease represented a mix of broad ranging degenerative or pathological findings and diagnoses. For example, one study cited evidence of degenerative disc disease as “radiographic evidence of mild to moderate osteophyte formation of vertebral endplates, loss of disc height …, herniated nucleus pulposus, loss of water content on MRI …, or vacuum phenomenon, associated with a history of back and/or radicular pain that is severe, ongoing, and recurrent.11” In another study, degenerative disc disease comprised “spondylosis, spondylolisthesis, stenosis, disc herniation, degenerative scoliosis, standalone DDD, or discogenic back pain, all with mention of degeneration.12” This variability was also seen within the countries from which the related research originated (Table 2).
Cervical Versus Lumbar Degenerative Disc Disease
The most notable differences occurred in the definition of degenerative disc disease and the context in which it was studied when comparing the full-length journal articles on cervical versus lumbar regions (Figure 2). The vast majority of articles on cervical degenerative disc disease were written in relation to surgery (82.8%), while this was not the case in the lumbar region (46.3%). Also, it was much more common for the operational definition of degenerative disc disease to specifically include radiculopathy or myelopathy in the cervical region (62.5%) than in the lumbar region (5.1%). Conversely, definitions involving disc degeneration, “discogenic” pain, and only axial pain were commonly used for the lumbar region (45.7%), but were rare for the cervical region (1.5%). Furthermore, none of the papers on cervical degenerative disc disease used a broad range of degenerative findings and diagnoses to define the condition, while this was the case for 18.3% of papers on the lumbar region.
Use of “Degenerative Disc Disease” in Orthopedics, Neurosurgery, and Other Disciplines
When looking at full-length journal articles coming from neurosurgery versus orthopedics, the majority by both groups focused on degenerative disc disease in the context of surgery (63.5% vs. 61.2%, respectively). However, a greater portion of the publications from neurosurgeons was of longitudinal outcomes studies (36.5%) and a lesser portion was randomized clinical trials (2.7%) than those from orthopedists (18.7% and 9.0%, respectively). The articles by neurosurgeons were more commonly of the cervical region than was the case for orthopedists (35.2% vs. 23.9%), and more commonly included radiculopathy in the clinical case definition (29.7% vs. 14.9%). Conversely, the case definition of “degenerative disc disease for which surgery was performed” was used in a higher proportion of articles led by orthopedists than neurosurgeons (26.9% vs. 14.9%). Differences in case definitions were more apparent when comparing surgical versus other disciplines (Table 3). In particular, surgeons were more likely to use the term degenerative disc disease without further explanation (40.5%) or specifically with radiculopathy or myelopathy (22.4%) than were other disciplines (18.8% and 5.7%, respectively).
Study Types or Designs
As an indication of the types of research being conducted on the topic of degenerative disc disease, the subset of full-length research publications was reviewed with respect to study type or design. Longitudinal or outcomes studies were most common (20·6%), followed by narrative reviews (15.7%), systematic reviews (12.1%), gene expression or association studies (10.0%), descriptive or cross-sectional studies (9.3%), clinical trials other than randomized controlled trials (8.5%), randomized controlled trials (6.1%), case reports (5.0%), nongenetic basic science studies (e.g., biomechanics/modeling) (4.6%), methodological studies (e.g., measurement) (2.5%), and others (5.7%).
Growth in Articles Including “Degenerative Disc Disease” Relative to Other Spine-related Articles
There was greater relative growth in the number of articles using the term, degenerative disc disease, from 2007 to 2016 when compared with growth in the number of other spine-related articles using the broad terms of cervical spine or lumbar spine (Figure 3). When comparing increases from the first 3 years to the last 3 years of the decade, the number of articles in which the term degenerative disc disease appeared in the title, abstract, or keywords increased by 74%, relative to increases of 23% and 24% for cervical spine and lumbar spine, respectively.
It has been said that the stage of development of a field can be judged by the clarity of its concepts. From this perspective, despite the longstanding and growing use of the term degenerative disc disease since its introduction as early as the 1940s,13 our study results reveal a grossly underdeveloped concept, with greatly varying, disparate definitions found in the health sciences literature. Such inconsistencies pose major challenges to clear, accurate communication in medicine and science, and to advancing related knowledge. They also create confusion and misconceptions among clinicians, patients, and the general public.
Definitions ranged from disc desiccation [low signal intensity on T2-weighted magnetic resonance imaging (MRI)], a typical finding in adults, regardless of the presence of symptoms, to specific symptomatic conditions, such as disc herniation with concordant radiculopathy. Moreover, there was no one definition that was used in the majority of publications. This variability was seen within the disciplines and countries from which the related research originated, with the greatest systematic differences observed when referring to degenerative disc disease in the context of cervical versus lumbar regions, as well as between surgical and other disciplines. While the term is used in many clinical and basic science disciplines, it is particularly common among spine surgeons and had become the most common rationale for lumbar spine fusion in the United States during the decade ending in 2008.14
A recent survey of experienced clinicians in the UK from an established spine research society also revealed controversies related to clinical use of the term degenerative disc disease.15 Of the participating clinicians, 18% stated that they did not use the term, with some proposing avoiding such labels that may engender fear. Others claimed patients need to be given a diagnosis and degenerative disc disease offers an alternative to “non-specific low back pain.” Indeed, degenerative disc disease, despite the lack of clarity of its definition, has served as a dominant, longstanding pathophysiological model of low back pain under a biomedical framework.2,16 Yet, limited progress has been made over prior decades in prevention and effective treatment of low back pain under either biomedical or biopsychosocial frameworks.17 Certainly, muddled concepts and the failure to create a common language in medicine and science have contributed to the stagnation. The case of degenerative disc disease is particularly notable because of its widespread use, the clear confusion as to its meaning, and the consequences of its use. However, similar problems with variable, nonstandardized definitions, nomenclature, and measurement are attracting increasing attention for other common spinal conditions as well, such as lumbar spinal stenosis, endplate pathology, paraspinal muscle changes, and low back pain in general.1,18–20
A strength of the current review of case definitions is the identification and inclusion of all publications with degenerative disc disease in the title from a systematic search of major health sciences search engines to sample definitions used. Similarly, identifying all publications from the Scopus search with the term in the title, keywords, or abstract provided a highly relevant indicator of temporal trends in growth in its usage relative to growth in spine-related articles in general. With respect to limitations, we placed definitions in categories reflecting the diversity observed during the review. However, other, perhaps more refined, categorizations could have been used.
The benefits and harms of clinical use of the term degenerative disc disease may be debated.6,15,16 However, there is little doubt that the use of the term has led to miscommunication and misconceptions among clinicians, patients, the general public, and policy makers. It has also clouded research on disc degeneration and other specific, diverse clinical phenotypes studied under the rubric of degenerative disc disease, and their role in symptomatic spinal conditions.6 Replacing the term with more precise case definitions or phenotypes under study would be a substantial advance to facilitate accurate communication and allow clearer comparisons and syntheses of related study results to move the field forward.
The Degenerative Spinal Phenotypes Group of the International Society for the Study of the Lumbar Spine, was co-chaired by Dino Samartzis, Joseph Assheur, Bradley Weiner and Michele Battié, with the goal of investigating and promoting common concepts, nomenclature, definitions, and core measures for degenerative spinal phenotypes to advance knowledge in the field. Elizabeth Dennett assisted with the literature search.
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