The assessment of cervical foramina with oblique radiographs: the effect of film angle on foraminal area.

Study Design: Radiographic evaluation of cadaveric cervical spine specimens.

Objective: Assess reliability and reproducibility of foraminal dimensions obtained from cervical radiographs of varying obliquity and determine optimal angles for visualizing foramina at each cervical spine level.

Summary Of Background Data: Oblique radiographs may be considered to assess cervical foraminal dimensions and are generally obtained 45 degrees from the anteroposterior (AP) orientation. Previous reports have suggested that foraminal area observed on these radiographs may be influenced by changes in obliquity so that certain film angles may be better suited for accurately assessing foramen size, depending on which level is being evaluated.

Methods: Radiographs of 4 human cadaveric cervical spines were obtained at 5-degree increments from 20 to 70 degrees relative to AP orientation, using both left and right sides of each specimen. Foraminal area was estimated by measuring height and width of each foramen and also with a freehand area measurement tool. Reliabilities of both methods were calculated. At each level, foraminal area was plotted against film angle and quadratic best-fit curves were used to determine the maximum area observed and the optimal angle of obliquity for assessment.

Results: Assessment of foraminal area using the height and width values was associated with good interobserver reliability, whereas the freehand method exhibited excellent reliability. The optimal film angles for calculating foraminal area increased from 46.3 degrees for C2-C3 to 56.1 degrees for C7-T1. The ideal film angle that minimized the overall error of measurement across the entire cervical spine was estimated to be 52.4 degrees.

Conclusions: Optimal angles for visualizing lower cervical foramina are larger than those for upper cervical spine. To minimize overall loss of foraminal area throughout the entire cervical spine, oblique cervical radiographs should be obtained at an angle of approximately 52 degrees from the AP orientation.