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Kaidi and Kim: Reply to Commentary on “Classification(s) of Cervical Deformity”
See the article "Classification(s) of Cervical Deformity" in Volume 19 on page 862.
To the editor,
We thank Dr. Goel for his interest in our article and appreciate the comments made regarding our manuscript. As acknowledged in our original submission, the classification of cervical deformities is still a nascent field [1]. In recent years, significant progress has been made towards creating statistically validated classification schema for cervical deformities; however, there is still work to be done [2-4]. We agree with Dr. Goel that current classification schema, including the Kim Classification, do not address cases of craniovertebral junctional instability. We also acknowledge that this instability can be an important consideration when managing patients with complex cervical deformities. While Dr. Goel’s proposed classification schema is appreciated [5], the Kim Classification cannot reasonably be modified to include craniovertebral junctional instability given the rigorous statistical methodology used in the classification’s creation.
The Kim Classification was created utilizing a 2-step cluster analysis of dynamic spine radiographs (a combination of hierarchical and kappa cluster analyses). This machine-learning-based statistical analysis requires high sample sizes to adequately power conclusions. Given the rarity of craniovertebral junctional instability, these cases were excluded inoriginal analysis of Kim et al. [2]. If they were included, the small sample size would make any derived classification schema inherently unreliable. The exclusion of these cases should not signify that craniovertebral junctional instability was considered unimportant or irrelevant in surgical decision making.
Overall, the classification of cervical deformities is still an evolving field, and we acknowledge that the Kim Classification does not appropriately describe every case of complex cervical spine deformity. It does, however, serve as an additional tool for surgeons to utilize when describing and managing cervical spine deformities. Further, it is the first validated classification schema designed to guide surgical management of different cervical deformities [6,7]. As we develop more nuanced approaches to treating cervical deformities, we appreciate the continued discussion being had on this important topic.


Conflict of Interest

The authors have nothing to disclose.


1. Kaidi AC, Kim HJ. Classification(s) of cervical deformity. Neurospine 2022;19:862-7.
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2. Kim HJ, Virk S, Elysee J, et al. The morphology of cervical deformities: a two-step cluster analysis to identify cervical deformity patterns. J Neurosurg Spine 2019 Nov;15:1-7. doi: 10.3171/2019.9.SPINE19730. [Epub].
3. Koller H, Ames C, Mehdian H, et al. Characteristics of deformity surgery in patients with severe and rigid cervical kyphosis (CK): results of the CSRS-Europe multi-centre study project. Eur Spine J 2019;28:324-44.
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4. Ames CP, Smith JS, Eastlack R, et al. Reliability assessment of a novel cervical spine deformity classification system. J Neurosurg Spine 2015;23:673-83.
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5. Goel A. Goel’s classification of atlantoaxial “facetal” dislocation. J Craniovertebr Junction Spine 2014;5:3-8.
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6. Louie PK, Sheikh Alshabab B, McCarthy MH, et al. Classification system for cervical spine deformity morphology: a validation study. J Neurosurg Spine 2022;37:865-73.
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7. Kim HJ, Virk S, Elysee J, et al. Surgical strategy for the management of cervical deformity is based on type of cervical deformity. J Clin Med 2021;10:4826.
crossref pmid pmc

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