Radiologically defining horizontal gaze using EOS® imaging – a prospective study of healthy subjects and a retrospective audit

Publication date: Available online 18 October 2017
Source:The Spine Journal
Author(s): Hwee Weng Dennis Hey, Kimberly-Anne Tan, Vivienne Chien-Lin Ho, Syifa Bte Azhar, Joel-Louis Lim, Gabriel Ka-Po Liu, Hee-Kit Wong
Background ContextAs sagittal alignment of the cervical spine is important for maintaining horizontal gaze, it is important to determine the former for surgical correction. However, horizontal gaze remains poorly-defined from a radiological point-of-view.PurposeTo establish radiographic criteria to define horizontal gaze.Study design/SettingThis study was conducted at a tertiary healthcare institution over a 1-month period.Patient SampleA prospective cohort of healthy patients was used to determine the best radiological criteria for defining horizontal gaze. A retrospective cohort of patients without rigid spinal deformities was used to audit the incidence of horizontal gaze.Outcome measuresTwo categories of radiographic parameters for determining horizontal gaze were tested: (1) the vertical offset distances of key identifiable structures from the horizontal gaze axis; and (2) imaginary lines convergent with the horizontal gaze axis.MethodsSixty-seven healthy subjects underwent whole body EOS® radiographs taken in a directed standing posture. Horizontal gaze was radiographically defined using each parameter, as represented by their means, 95% confidence intervals (CI) and associated two standard deviations (SD). Subsequently, applying the radiographic criteria, a retrospective audit of such radiographs (prior to the implementation of strict radioimaging standardization) was conducted. There were no sources of funding, and no conflicts of interest related to this study.ResultsThe mean age of our prospective cohort was 46.8 years while that of our retrospective cohort was 37.2. Gender was evenly distributed across both cohorts. The 4 parameters with the lowest 95% CI and 2SD were the distance offsets of the midpoint of the hard palate (A), and the base of the sella turcica (B), as well as the horizontal convergents formed by the tangential line to the hard palate (C), and the line joining the centre of the orbital orifice with the internal occipital protuberance (D). In the prospective cohort, good sensitivity (>98%) was attained when two or more parameters were used. Audit using criteria B+D yielded compliance rates of 76.7% – a figure much closer to that of A+B+C+D (74.8%). From a practical viewpoint, criteria B+D is most suitable for clinical use, and can be simplified to the "3-6-12 rule" as a form of cursory assessment. Verbal instructions in the absence of stringent postural checks only ensured that ~75% of subjects achieved horizontal gaze.ConclusionsFulfilment of criteria B+D is sufficient to evaluate for horizontal gaze. Further criteria can be added to increase sensitivity. Verbal instructions alone yield high rates of inaccuracy when attempting to image patients in horizontal gaze. Apart from improving methods for obtaining radiographs, a radiological definition of horizontal gaze should be routinely applied for better evaluation of sagittal spinal alignment.



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