**4. Classification**

Currently there is ongoing focus on research to find a classification system that dictates treatment modality and predicts outcome. Ames et al. [37] (**Figure 2**) was built on basic deformity descriptors and five associated modifiers. Deformity descriptors differentiated deformity by type, ranging from sagittal to craniovertebral junction deformities, as well as regional location factoring thoracolumbar deformities. The selected modifiers accounted for various factors correlating with ACD and thoracolumbar deformity; Diebo et al. [38] described in his proposal of classification a two-step approach. Initially identifying the five most discriminate parameters are cSVA and T1 slope on lateral view, and maximum focal kyphosis, C2 slope and number of kyphotic levels on extension view. Those parameters were able to describe most of the deformity. On the second step his team proposed 3 distinct morphologies of sagittal cervical deformities based on lateral

#### **Figure 2.**

*Description of the CSD classification system, which includes a deformity descriptor and 5 modifiers. D = double; L = lordosis; N = none; T = thoracic.*

and extension radiographs. Overall, the current classifications remain limited to radiographic or clinical description.
