IntroductionA new classification system for degenerative disc disease of the lumbar spine based on magnetic resonance imaging, provocative discography, plain radiographs and anatomic considerations
Introduction
Prior investigators have developed classification systems for both lumbar disc herniations and degenerative disc disease (DDD) of the lumbar spine. Classification of lumbar disc herniations was first described in 1992 by Kim et al. [1]. They reported a case series of 28 patients predicting types of lumbar disc herniations through magnetic resonance imaging (MRI), with 80.6% accuracy. In 1995, Kramer [2] described a more complex classification for lumbar disc herniations encompassing the size of the herniation as well as the direction of the extruded material to formulate a consistent surgical intervention model. Despite the work of Kim and Kramer, Milette [3] made another attempt at classifying lumbar disc herniations, recommending the use of computed tomography (CT) scans and discography as a complement to MRI. Although these three papers are helpful in discussing the pathological anatomy of lumbar disc herniations, they neither address nor classify the pathological anatomy of lumbar degenerative disc disease.
Thompson et al. [4] first proposed a classification system for DDD of the lumbar spine in 1990 based solely on histological analysis. The five-point classification reflected the relationship between age and degree of degeneration. In 2002, Boos et al. [5] also classified lumbar disc degeneration using human cadaver specimens. Their histological system classified discs into eight groups based solely on age and described the general structure of the discs in each age group.
In 2000, Southern et al. [6] were the first to classify DDD with MRI, using a four-point scale using human cadavers. They correlated their MRI data with quantitative discomanometry, which directly assesses the biomechanical/functional integrity of the end plate–disc–end plate complex through the injection of fluid into the disc and measurement of intradiscal pressure. MRI assesses disc morphology and hydrogen content based on signal intensity. Although the focus of this paper was largely biomechanical, Southern et al. showed that MRI could be used reliably to classify DDD, which correlated directly with the biomechanical function of the segment. In 2001, Pfirrmann et al. [7] advanced Thompson's work by basing their system largely on Thompson's gross morphology classification but used MRI as a primary means of classification. They used a five-point scale and rated an excellent validation of their system through intraobserver and interobserver reliability.
In 2001, a combined task force from the North American Spine Society, the American Society of Spine Radiology and the American Society of Neuroradiology developed a nomenclature and classification system for lumbar disc pathology [8]. This system is thorough in its description of nomenclature but did not invest significantly in developing a classification system for lumbar DDD. The system does not address degrees of degeneration, nor does it recommend specific surgical intervention based on lumbar DDD.
To date, none of the classification systems take into account multiple diagnostic imaging modalities. One aspect of DDD not detectable on MRI is internal disc disruption, which can be reliably detected only through provocative discography. In addition, the intervertebral segment is a three-joint complex. Prior classification systems have completely ignored the state of degeneration of the facet joints. When fusion, either interbody fusion, instrumented posterolateral fusion or circumferential fusion of the degenerative segment, is the only method of surgical treatment, a comprehensive classification system requiring identification of internal disc disruption or facet joint degeneration may not be necessary.
However, fusion, through any approach, for the treatment of lumbar DDD is no longer the only surgical treatment available. Artificial disc replacement (ADR) in the lumbar spine has been performed in Europe for nearly two decades, and soon artificial disc technology will be available on a widespread basis in the United States. It is imperative that the spinal surgery community try to avoid the same mistakes made in the past by orthopedic surgeons during the transition from total joint fusion to total joint replacement. None of the classification systems described in the literature correlate MRI with discography and plain radiographs, which are the three primary radiographic diagnostic tests for lumbar degenerative disc disease. None of these systems includes the condition of the posterior elements at the level of degeneration, nor the condition of the end plates. These classification systems do not correlate a stage of disc degeneration with a particular surgical approach. A degenerative lumbar disc that is appropriate for treatment with interbody fusion may or may not be appropriate for ADR. Because the biomechanics of motion are much different from the biomechanics of fusion, interbody fusion and disc arthroplasty cannot be thought of as interchangeable surgical treatments for lumbar DDD.
Discs that have a significant loss of height, and sclerotic and irregular end plates are not ideal for disc replacement. The design of current artificial disc prosthesis technology is unforgiving in terms of central placement and end plate–to–implant contact. Discs of this type also have a reduced elasticity of the segment, which is different from the biomechanical models used to test these devices.
Early clinical data from an ADR Investigational Device Exemption in the United States (A Shelokov, MD, personal communication, 2002) suggest that patients with preexisting degeneration of their facet joints do quite poorly after an ADR because the facet joints become overloaded, leading to facet joint pain complex. Therefore, it is imperative that patient selection for ADR includes a thorough understanding of the degenerative state of a patient's posterior column.
These are significant issues as ADR becomes more widely available. Not every patient with lumbar DDD should have ADR with the technology of devices currently available. The spinal surgery community must better define which discs are appropriate for replacement and which discs should be fused. These questions cannot be answered until the intervertebral joint complex is accurately classified.
Section snippets
A new classification system for lumbar degenerative disc disease
A new classification system for lumbar DDD has recently been developed. The intervertebral segment is divided into two parts—the anterior column and the posterior column—based on MRI, discography and plain radiographs. The classification system uses MRI, provocative discography and plain anteroposterior and lateral radiographs. These three imaging modalities are combined to assess the following: MRI image appearance, lordotic angle of the intervertebral segment, the shape and condition of the
Discussion
In order for all stakeholders to discuss which interdiscal segments are and are not appropriate for ADR, a classification system with simple nomenclature must be validated and accepted, much like the King or Lenke classifications for spinal deformity. The language of lumbar DDD must be common for everyone if we are to move forward with the transition from fusion to ADR.
The indications for lumbar fusion and lumbar disc replacement overlap in a number of areas, but they are not identical.
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Support in whole or in part was received from DePuy Spine under grant #200240. Author TJA acknowledges a financial relationship (consultant for DePuy Spine), which may indirectly relate to the subject of this manuscript.