In recent years, the incidence of CSM has significantly increased.[12] Affected by environmental factors and growth of age, IDD, cervical small joint degeneration and formation of vertebral marginal osteophyte may cause spinal stenosis, chronic compression of the spinal cord, leading to neck pain, motor dysfunction and even paralysis.
At present, it is accepted that the pathological process of CSM mainly includes static, dynamic and ischemic mechanisms.[13, 14, 15] Cervical IDD is considered to be the trigger of static mechanism. It will lead to changes in the biomechanics of cervical spine, which may induce the formation of spur in the vertebral endplate. Meanwhile, the herniation of degenerative discs will squeeze the ligamentum flavum and make it penetrate into the spinal canal, causing spinal canal stenosis. When the structure of cervical spine is abnormal due to the static mechanism, the flexion and extension of cervical spine will precipitate the irreversible damage of the spinal cord.[16] If the cervical instability caused by IDD occurs in the motion segment, it will result in dynamic compression on spinal cord, along with the progress of pathological process, the stability and joint degeneration of this segment will gradually deteriorate, and the spinal canal will become increasingly narrower.[13] Briefly, IDD plays a leading role in the pathogenesis of CSM.
A recent study shows that non-surgical treatment is not suitable for moderate and severe CSM, for there is no evidence indicating non-surgical treatment can effectively inhibit or reverse the natural history of CSM, and the progression of the disease will bring serious consequence, such as deterioration in the quality of life, significant dysfunction and adverse impact on surgery efficacy, while the risk of secondary spinal cord injury or central syndrome is higher. Therefore, it is generally believed that once CSM is diagnosed, surgery should be performed as early as possible.[17] As mentioned in preceding part of the text, preoperative IDD largely determines surgical strategy, while postoperative IDD plays a decisive role in prognosis. With the continuous development of relevant grading system, treatment concepts and techniques, surgical decision-making of CSM have been further improved, providing important clinical value for standardized treatment.
In 2001, Pfirrmann et al.[18] developed the most well-known grading system based on MRI, dividing IDD into five grades according to disc signal intensity, disc structure, distinction between nucleus and anulus, and disc height. Though this classification has been widely accepted and proved to have excellent inter- and intra-observer agreement,[19] study[6] found that it did not demonstrate discriminatory when applied to evaluate IDD in the elderly spine, besides, on the basis of images and descriptions provided, there were ambiguities in grading IDD as one level or another. To address these deficiencies, Griffith et al.[6] proposed a modified Pfirrmann classification which increased the 5 grades to 8 (Table 1, Fig. 1), so as to improve its discriminatory power when evaluating the elderly spine and minimize ambiguity when selecting grades.
The establishment of modified Pfirrmann grading system not only gives spine surgeons a clear definition of IDD, but also provides an ideal treatment plan prediction of prognosis for patients with CSM. At present, JOA[20] and NDI[21] scoring systems are the most commonly used criteria to evaluate the treatment of patients with CSM, in particular, JOA system can divide CSM into three levels, mild, moderate and severe according to the score, in order to help physicians determine whether patients need surgery as soon as possible. It is worth mentioning that both scoring systems focus on patients, especially their functional status, but neither JOA nor NDI scores lay emphasis on the cervical spine, no matter vertebra, inter-vertebral disc, spinal canal or spinal cord. Hence the advantage of modified Pfirrmann grading system is obvious.
However, it must be emphasized that the cervical IDD is only one consideration for surgery. Other important related factors include: non-surgical treatment, cervical spine stability, operative segment, severity of spinal stenosis and spinal cord compression, prognosis, etc. Thus, modified Pfirrmann grading system can only be regarded as an important reference for surgery, and the most ideal treatment scheme can be formulated by combining JOA and NDI scores.
The results show that the twice inter-observer agreement (ICC: 0.76, 0.79; wκ: 0.82, 0.81) of modified Pfirrmann grading system are slightly higher than that reported by Griffith et al.[6], while the intra-observer agreement in this study is excellent ((ICC: 0.84; wκ: 0.87), similar to that of Griffith et al.[6], indicating that modified Pfirrmann grading system has a very good consistency. It is noteworthy that the evaluators involved in establishing modified Pfirrmann grading system were all radiologist (two musculoskeletal radiologists and a general radiologist). However, the six physicians in our study came from two specialties (three spine surgeons and three radiologists), thus we could have a multi-angle and more comprehensive understanding of the imaging manifestations of IDD, which may be one of the factors that caused the slight differences in results between two articles.
The current study has limitations which could be improved in some ways to better ascertain the inter- and intra-observer error of this grading system. Firstly, its relatively small sample size. Though the number of patients included in our study is more than that of Pfirrmann et al.[18] and Griffith et al. [6], further expanding our sample population will allow for more meaningful statistical testing on the agreement of these parameters. Secondly, recall bias from evaluators, namely the deviation of results for repeated assessments in all evaluators, as shown in Table 6. This deviation has been mentioned by Wang YX et al.[22] in their study, which indicated that there was no significant difference in repeated assessments performed on the same day by the same evaluator, but the deviation was obvious when the same evaluator made further assessments 8 months later. Thus, in any study setting, paired assessments should be conducted ideally in a short period of time. And 12-weeks interval still might be long in our study. Thirdly, the difference in specialty is a important factor. Though evaluators were from two specialties and the multidisciplinary team might increase the comprehensiveness of this study, we must point out that radiologists did not specialize in spine and lack deep understanding and profound insights of IDD or the grading system, which may affect the accuracy of final result. So, it may be valuable to repeat this study with overall senior spine surgeons to explore if higher skill level and specialization will cause a better agreement than that assessed by junior evaluators or multidisciplinary team. Finally, as mentioned above, postoperative IDD is the cause of poor prognosis and reoperation, but we excluded patients with presence of instrumentation in the cervical spine so as to make a better judgment of the inter-vertebral disc. On this issue, there was much controversy when we designed the study, after long discussions, we determined to eliminate all objective factors including fracture, tumor, infection, and presence of instrumentation. For postoperative IDD, we will lay more emphasis in our later study. Therefore, high-quality, large sample, and multicenter studies should be performed in our future clinical work to provide spine surgeons with the best evidence-based information.