The dimensions of the posterior arch of C2 for instrumented screw fixation. A radiological study in the Spanish population.

Abstract

To describe the feasibility of the posterior arch of C2 accepting two crossing screws in the Spanish population. One hundred and fifty patients who underwent a routine neck CT scan for non-cervical pathology were enrolled. Submillimeter slices (thickness 0.7 mm) every 0.4 mm were performed with a 64 multi-detector CT scan, which allows isometric measurements. We measured the length and height of the cortical and cancellous (endomedullar) region of the lamina and the spinous process, inclination, maximal screw length and spinolaminar angle. The average (standard deviation) measurements of the lamina were: width of the left cortical 7.2 (1.5) mm, right cortical 6.9 (1.3) mm, width of the cancellous part of the left lamina 4.8 (1.5) mm, right side 4.6 (1.4) mm. The mean left cortical height was 13.0 (1.5) mm and 13.1 (1.6) mm for the right. The mean height of the cancellous part was 9.0 mm for both sides. The average measurements of the spinous process were: cortical length 15.7 (3.5) mm, endomedullar length 12.5 (3.9) mm; cortical height 11.9 (2.2) mm, endomedullar height 8.4 (2.1) mm; spinolaminar angle 49º (4); the maximum screw length 3.18 cm, and the inclination angle 143º. A CT scan with submillimeter slices is necessary in order to avoid malpositioning of the screws. The outer cortical measurements are 2 to 4 mm bigger than the endomedullar ones. Taking into account the dimensions of the spinous process, 24% of the population would not be candidates for this crossing screw technique.