Sagittal parameters of cervical global balance. Normative values from a prospective cohort of asymptomatic volunteers

Objective. Our objective was to define reference parameters to analyse sagittal balance of the cervical spine in a population of asymptomatic volunteers.Material and methods. After having obtained approval from the bioethics committee, we conducted a prospective, transversal, single center study using a low-dose radiographic system (EOS Imaging, Paris, France). Absence of pain was assessed using the Oswestry Disability Questionnaire and a visual analogue scale. A total of 106 subjects were included (55.66% men). The parameters measured were: pelvic incidence (PI), pelvic version (PV), sacral slope (SS), thoracic and lumbar curves, the position of the C7 plumb line and the spino-sacral angle (SSA). The C7 slope, together with new parameters, was measured: cranial incidence, defined in relation to the McGregor line and the sella turcica, made it possible to define cranial slope and version. Finally, an intrinsic angle of measurement of the cervical spine was described: the spino-cranial angle (SCA).Results. The study evidenced a strong correlation between the C7 slope and the cranio-cervical system. Economic sagittal balance in a population of asymptomatic subjects was defined by a constant SCA angle of 83+/-9°. To maintain this balance, a spine with a marked C7 slope will present lordosis and vice versa. Cranial incidence is an anatomical parameter characteristic of the cranio-cervical complex which makes it possible to analyse the spatial positioning of the head and to predict the desired value of cervical lordosis which is strongly correlated to cranial slope.Conclusion. The C7 slope has a predictive value of the shape of the cervical spine in the sagittal plane. Our research has completely modified the notion of physiological cervical lordosis, since one third of the asymptomatic population had kyphosis of the cervical spine. The results of our study could be used to study sagittal balance before and after arthrodesis or cervical disc prosthesis because the reference position of the cervical spine should be integrated into a complete analysis of the spine.